This article was downloaded by: [Aston University]On: 06 October 2014, At: 04:38Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK

Italian Journal of ZoologyPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/tizo20

Are the traditional classes of arthropods naturalones? Recent advances in palaeontology and someconsiderations on morphologyAlberto Mario Simonetta aa Dipartimento di Biologia Animale e Genetica “L. Pardi” , Università di Firenze ,Via Romana 17, I‐50125, Firenze, ItalyPublished online: 28 Jan 2009.

To cite this article: Alberto Mario Simonetta (2004) Are the traditional classes of arthropods natural ones? Recentadvances in palaeontology and some considerations on morphology, Italian Journal of Zoology, 71:3, 247-264, DOI:10.1080/11250000409356579

To link to this article: http://dx.doi.org/10.1080/11250000409356579

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”)contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensorsmake no representations or warranties whatsoever as to the accuracy, completeness, or suitabilityfor any purpose of the Content. Any opinions and views expressed in this publication are the opinionsand views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy ofthe Content should not be relied upon and should be independently verified with primary sources ofinformation. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands,costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly orindirectly in connection with, in relation to or arising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Any substantial orsystematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distributionin any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found athttp://www.tandfonline.com/page/terms-and-conditions

Ital. J. Zool., 11: 247-264 (2004)

Are the traditional classes ofarthropods natural ones? Recentadvances in palaeontology and someconsiderations on morphology

ALBERTO MARIO SIMONETTADipartimento di Biologia Animale e Genetica "L. Pardi",Università di Firenze, Via Romana 17, I-50125 Firenze (Italy)

ABSTRACT

The present paper considers the problems of evaluating someimportant morphological characters both in fossil and livingarthropods as examples of the difficulty of assessing the signifi-cance of all characters of the Arthropoda for the purpose of phy-logenetic reconstruction. As both the fossil and the molecular evi-dence point to a splitting of the main branches of the metazoanswell in the early Vendian or even earlier, and of the arthropodsduring the Cambrian, this must be carefully considered in the as-sessment of all sorts of evidence. Moreover the fossil evidenceclearly indicates that, as far as the arthropods are concerned,there was a slow acquisition of the arthropod characters in differ-ent lineages. I argue, on the evidence of a few selected and im-portant characters (such as basic morphology of the appendages,tagmosis, development of tracheal systems and of the overall evol-ution of the general morphology of selected groups) that conver-gent and parallel evolution was common and that, especiallywhen functionally important characters are concerned, extremelysimilar or even identical structures were repeatedly and indepen-dently evolved. Therefore, I argue that an assessment of the phy-logenetic significance of characters is indeed possible, but that itrequires the simultaneous assessment of all functionally relatedcharacters and apparatuses and of their adaptive significance, andfurthermore that no standard rules may apply, all the more so themore the characters considered are functionally important. Finally,just as a working hypothesis, I suggest a new phylogeneticarrangement of arthropod taxa.

KEY WORDS: Arthropoda - Phylogeny - Morphology - Adapta-tions.

(Received 12 May 2003 - Accepted 18 September 2003)

INTRODUCTION

During the last decade controversies on the phyloge-netic relationships among arthropods and among themand other phyla have been more lively than ever, asthere appears to be an increasing difficulty in the as-sessment of often contradictory evidence. In order toappreciate the extent of the controversial issues con-cerning the relationships of fossil and recent arthro-pods, one may just refer, among others, to Delle Cave& Simonetta (1975, 1981, 199D, Simonetta (1975), Si-monetta & Delle Cave (1977, Abstract in XIV Conv.U.Z.I., no. 81; 1981, Abstract in Boll. Zool. 48 (suppl.):103), Schräm (1978), Briggs (1983), Briggs & Fortey(1989), Smith (1990, Abstract in Am. arachn. soc. 14th

annu. conf.-. 41), Walossek & Müller (1990, 1998), Dallai(199D, Emerson & Schräm (199D, Ramsköld & Edge-comb (199D, Bergström (1992), Kukalova Peck (1992),Shear (1992), Williams & Carroll (1993), Wagele (1993),Bergström (1994), Kraus & Kraus (1994, 1996), Wills etal. (1994, 1995, 1998), Averof & Akam (1995), Pangani-ban et al. (1995), Zrzavy & Stys (1995), Budd (1996,2002), Fryer (1996), Moura & Christoffersen (1996),Arthur (1997), De Robertis (1997), Nielsen (1997),Rieger & Schultz (1997), Delle Cave et al. (1998),Wheeler & Hiyashi (1998), Minelli (1998), Brusca(2000), Simonetta et al. (2000).

As for the problems concerning the relationships be-tween arthropods, sensu lato, and other phyla, duringthe last years and mainly, but not only, on the evidenceof research on 18S RNA, several Authors (e.g. Eernisseet al., 1992; Aguinaldo et al, 1997; Adoutte et al, 1999;Giribet & Wheeler, 1999; Garey, 2002; Schmidt-Rhaesaet al., 2002) have argued that arthropods should begrouped with nematodes and with a few other taxa asEcdysozoa, rather than with the Annelida, within thetraditional Articulata. This proposal, being marginal tothe evidence discussed in the present paper, will beconsidered but briefly at the end of this contribution, asI think that it must be considered, at least, with muchreservation as it clashes with palaeontological, morpho-logical, embryological, and biochemical evidence.

The present paper considers different sets of prob-lems: A) the implications of the accruing evidence of apre-Cambrian, possibly Vendian, origin of most, if notall the major phyla; which, at that time, were represent-ed by animals which had not yet acquired some ormost of the characters that have been commonly con-sidered as diagnostic for the same phyla in later times,B) some of the more significant recent advances in theknowledge of the morphology and evolution of Cam-brian arthropods and related groups, and C) it exam-ines some problems of arthropod morphology with thepurpose of showing how several important structures,which have often been deemed to be of major signifi-cance for determining the affinities of these animals,may as well be the result of convergent or parallel evo-lution and, therefore, in spite of their great similarity

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

04:

38 0

6 O

ctob

er 2

014

248 A. M. SIMONETTA

cannot be considered as being homologous for the pur-pose of establishing phylogenies. Therefore, it seemsquite improbable that there may be any standardmethod to determine their phylogenetic significance, sothat decisions will necessarily be mostly based on thebalanced and necessarily, to some extent, subjective as-sessment of the overall morphology of each animalconsidered by the individual scholar.

Almost all the difficulties that we shall consider havebeen noticed by previous Authors and the consequentdoubts on the validity of the major taxa which are theconsequence of the problems discussed in this contri-bution have been advanced now and then in the past.However, the radical hypothesis that none of the tradi-tional classes of living arthropods is a monophyletic tax-on is, I think, new.

Many zoologists and palaeontologists, who pursue anideal of objectivity comparable with that of typical ex-perimental sciences, will probably be scandalised bythis stance and will label my attitude as nihilistic. I hopein the last section of this paper to show that my attitudeis nothing of the sort and that it is quite justified by thepeculiarities of evolutionary zoology as a science per-taining at least as much to the field of historical sci-ences as to that of experimental sciences.

EVIDENCE FOR AN EARLY RADIATION OF METAZOANSAND ITS IMPLICATIONS

As a sort of premise to the discussion of the evidenceconsidered further on, I think that any scholar attempt-ing to establish the relationships of two or more arthro-pod higher taxa must address the growing evidence,both palaeontological and molecular, that assumes thatthe metazoans are monophyletic, but that they musthave already split into most of their major branches wellbefore the age of the "Ediacaran" faunae (von Salvini--Plaven, 1981, 1985; Fortey et al, 1996; Conway-Morris,1997; Ayala et al, 1998; Martin et al, 2000), whether asearly as assumed by some molecular biologists or onlyby the early Vendian, is irrelevant for our purpose. Forinstance, the presence in the Vendian all over the worldof an unquestionable, albeit very primitive, mollusc (Fe-donkin & Waggoner, 1997; Fedonkin, 2001; Fedonkin &Simonetta, in preparation) implies that, though they hadnot yet evolved all their "diagnostic" characters, yet mostif not all the living taxa of traditional phylum level musthave been already present by early Vendian.

The fact must always be borne in mind that the grow-ing amount of palaeontological evidence concerningthe Pre-Cambrian and Cambrian radiation of at leastmost of the major phyla provides an unquestionableterminus ante quern the different taxa must have begunto acquire their distinctive morphology, but not neces-sarily such characters that we deem to be especially di-agnostic of their more recent descendants; this, as weshall see, in more than one instance makes inferencesbased on evidence other than fossils untenable.

ADVANCES IN THE UNDERSTANDING OF CAMBRIANARTHROPODS AND RELATED TAXA

Lobopods and related groups

We resume here a discussion that my collaboratorsand I have been developing for many years, as succes-sive discoveries and improvements in the understandingof already known taxa required a steady revision of itsconclusions, while the main principles could be re-tained (Simonetta, 1975; Delle Cave & Simonetta, 1975,1981, 1991; Simonetta & Delle Cave, 1977, Abstract inXIV Conv. U.Z.I., no. 81; Simonetta & Delle Cave, 1981,Abstract in Boll. Zool. 48 (suppl.): 103; Delle Cave etal, 1998; Simonetta et al, 2000).

Again, since our last general contribution (Delle Cave etal, 1998), new or better preserved fossil arthropods, es-pecially from Palaeozoic strata have been discovered,though it is regrettable that much of the new material isstill unpublished or has been the object of merely prelim-inary notes. Much the same is the situation concerningthe protonychophorans, which, as a group, must be con-sidered as the ancestors of the living onychophorans andare related with arthropods: new species have been de-scribed by Collins in a preliminary paper read at theThird International Conference on Trilobites and their Rel-atives, Oxford 2-6 April 2001 (D. Collins, in preparation)but, as yet, have not been formally described. Therefore,with regard to the evidence they provide, we shall men-tion them mainly on the basis of notes taken there.

All the available evidence supports the affinities oftypical arthropods with onychophorans, Tardigrada andPentastomida and only the true affinities of the last--mentioned group are controversial. Indeed the Ordovi-cian pentastomids so far discovered, as far as their gen-eral morphology is concerned, are clearly closer thanthe living ones to arthropods, though in some speciesthere is no mouth (Walossek & Müller, 1994; Walosseket al, 1994)! This obviously may support the inclusionof pentastomids among the arthropods, but it falsifiesthe suggestion by some scholars (e.g. Wingstrand, 1972;Abele et al, 1989) of their inclusion among the crus-taceans. This is, indeed, a typical instance in which wedeal with clearly contradictory evidence. While both themorphology of the sperms and molecular evidenceseem to point specifically to affinities with the Brachiu-ra, on the other hand the unquestionable widespreadoccurrence of fully parasitic pentastomids in the LowerOrdovician clearly points, as argued by Walossek &Müller (1994), to a Cambrian origin for the Pentasto-mids, when "true" crustaceans had not yet evolved (seefurther on the discussion of this point and the argu-ments pointing to an at least diphyletic origin for thetraditional Crustacea). This clearly falsifies the conclu-sion that the pentastomids may be the "sister group" ofany specific group of living crustaceans: they cannot bethe sister-group of a then not yet existing group of ani-mals. Moreover, the known early developmental stagesof the Lower Ordovician pentastomids appear to have

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

04:

38 0

6 O

ctob

er 2

014

MORPHOLOGY, PALAENTOLOGY AND ARTHROPOD SYSTEMATICS 249

no possible relationship with the development of mod-ern crustaceans, nor even, as far as known, of the Or-dovician "crustaceans". In this case to maintain the sig-nificance of the data pointing to a connection betweenthe pentastomids and the Brachiura requires such trulyunbelievable palaeontological acrobatics that it is muchsafer to conclude that such evidence, though it may wellbe highly significant in other cases, is irrelevant here(on the general implications of this fact, see further on).

As far as the onychophoran-protonychophoran assem-blage is concerned, there is some interesting new evi-dence. In a paper read in 1989, Delle Cave & Simonetta(1991) suggested that a curious animal, then known aftera single specimen, of which photographs had been pub-lished by Collins with a query as to its affinities, was in-deed a protonychophoran. Now several new specimenshave been discovered and were illustrated by Collins atthe Oxford meeting (Collins, 2001, Abstract in 3 rd int.conf. on trilobites and their relatives, Oxford Univ., p. 9)and, while some details of Delle Cave's & Simonetta'soriginal reconstruction must now be amended, yet theattribution to the Protonychophorans is fully confirmed.The animal (Fig. 1A) is interesting as it shows a certaindegree of differentiation between the appendages of thefore part of the body and the posterior ones.

Yet this is much more evident in two other newspecies, also discovered by Collins (Collins, 2001, Abstractin 3 rd int. conf. on trilobites and their relatives, OxfordUniv., p. 9), which are provided by a posterior group ofstrong, walking legs provided with large, hooked nails, fitto hang on some sort of branched perch, and a foregroup of legs which are much longer, thinner and provid-ed with a rich and long fringe of setae or thin spines (Fig.IB). It is plain, also from the preservation of some fossils,that these made up a meshwork of hairs, which clearlyfunctioned as a net to catch tiny planktonic organisms.

These last-mentioned animals are especially interest-ing as they show how, even in an early phase of mor-phological evolution of a, broadly speaking, very primi-tive group of organisms, some of them had achieved aconsiderable specialisation, both morphological andfunctional, of a group of appendages. Thus the compar-ative morphology of the Cambrian protonychophoransshows a variety of development in their appendagesthat exceeds that of their living relatives, while none ofthem shows any trace or even a hint of characters typi-cal of modern onychophorans such as the antennae, theventral rotation of the mouth, the special glands used toglue their preys, and the sclerified maxillae, these beingall features which appear to be correlated as necessaryadaptations to a terrestrial habitat. Equally, the very dif-ferent development of the tegumental sclerites, whichin the fossils range from none to extremely long andstrong spines, was clearly allowed by their aquatic habi-tat, while the velvety tegument of their modern relativesis clearly suitable for terrestrial animals usually crawlingin cracks and under rocks. Thus the fossil animals (Fig.2), while less advanced than living representatives ofthe phylum in some features, had a range of diverging

A

B

Fig. 1 - Reconstruction of two still formally undescribed protony-chophorans (original after notes from Collins (2001) oral presenta-tion and modified after Delle Cave & Simonetta, 1998).

specialisations considerably exceeding those of their liv-ing counterparts.

"True arthropods"

Yochelson (1996) justified with historical reasons Wal-cott's inclusion of the Burgess shale arthropods in theframework of the conventional classification of crus-taceans usual in his times against Gould's (1990) argu-ment that he had "shoehorned" the animals into an im-proper classification due to a somewhat ideologicalbias. Anyway, Walcott's tentative arrangement was, as itwas shown by later investigations, just as bad as, for in-stance the packing of all of them into the "Trilobitomor-pha" by St0rmer (1944).

I surmise that while there was some practical justifica-tion for Walcott's attitude, nowadays there is a tendencyto do just the opposite: people experiment with entirelynew formal classifications based on mechanically pool-ing together just a few lines of evidence of very hetero-geneous significance.

As a typical example, I shall discuss briefly here therecent paper by Budd (2002). This I do rather than tocriticise his conclusions, just for the purpose of pointing

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

04:

38 0

6 O

ctob

er 2

014

250 A. M. SIMONETTA

Fig. 2 - Various Cambrian protonychophorans (after Delle Cave &Simonetta, 1998).

to some methodological problems that it exemplifies, asmy objections might well apply to a number of othercladistic analyses: as a starting point, Budd takesAysheaia as an outgroup for his cladistic analysis, this isan arbitrary decision, as there is no reason to considerAysheaia being in any way either as particularly primi-tive or typical among the protonychophorans. As a mat-ter of fact, Aysheaia is the only protonychophoran hav-ing only the first pair of appendages specialised as agrasping organ; all other known genera either show al-most no hint of specialisation or have a whole group ofspecialised appendages (Fig. 2). Such a choice, howev-er, implicates consequences in the following cladisticanalysis. A similar objection may be raised with hischoice of Parapeytoia: Budd assumes as a feature ofanomalocarids the lack of segmentation of the dorsalside, now such a lack of distinct terga is a (debated)feature of the Burgess shale species, the dorsal surfacebeing unknown in all other species of the group. Thereis, indeed, some indication of distinct terga in Cassubia,and the Burgess Shale species do not have the kind ofappendages and the sternites occurring in Parapeytoia(indeed, the very mechanics of the appendages ofParapeytoia and the presence of strong sternites arerather an indirect indication that the dorsal side of thisgenus was well segmented). Thus he introduces in his

matrix some wholly gratuitous assumptions that will in-evitably bias the whole analysis. We might well go onlike that: for instance: we do not know the mouthparts,if any, of Leanchoilia, yet they are implicitly assumed inBudd's matrix and in the resulting cladogram. Again:Budd assumes that the traditional subdivision of livingarthropods into the classical classes is a natural one,which is by and large a petitio principii, and so on.

Naturally the choice of outgroups is an arbitrary deci-sion of the taxonomist, but as it necessarily impinges onthe results of the analysis, this is a further objection inprinciple, to the claim that cladistic analysis is in anyway more objective than other traditional methods.

As a matter of fact, as we said, Budd's paper exempli-fies the fact that the methods used are in no way objec-tive. Indeed, they imply a selection of taxa and of char-acters, which is necessarily either subjective to a consi-derable extent, or is dictated by the availability of evi-dence due to merely random circumstances, thus, bystarting from necessarily biased premises, even suppos-ing that the methods employed are reliable, the resultsare also necessarily biased. Obviously the results will beeven worse if the author assumes in his matrix charac-ters that, at least for some species, are completely un-known or, at least, disputed.

I suppose that any biologist assumes that, in princi-ple, all the particular phylogenies that may be recon-structed should eventually fit into a more comprehen-sive one, so that, theoretically, all of them should mergeinto an all-embracing one covering all past and presentliving beings. Therefore it is also necessary to considerthat any phylogeny assumed for a given group of ani-mals must be compatible, when considered in a broad-er perspective, with those of all other animals.

I am obviously aware that the inclusion into a cladis-tic matrix of a large number of unknown quantities,which will be unavoidable if we consider incompletelyknown animals, will make the whole unmanageable,but it is equally true that to exclude any evidence impli-cates a loss of information that must affect the results ofthe analysis, so that, to quote the traditional dictum "in-cidis in Scyllam, cupiens vitare Carybdim" !

We shall now consider some further typical examplesof the problems and facts that face us.

Given the length of this paper, which prevents theconsideration of all the relevant problems and evidence,we shall discuss just a few examples as representativesof the methodological problems that face us.

The evolution of the arthropod appendages and the problemof anomalocarids (Dinocarida)

A first subject which deserves discussion because ofits general implications is that of the evolution of thearthropod appendages both in arthropods and arthro-pod-like Cambrian animals.

Clearly we cannot be sure that all details of the recentreconstructions of the appendages in various genera are

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

04:

38 0

6 O

ctob

er 2

014

MORPHOLOGY, PALAENTOLOGY AND ARTHROPOD SYSTEMATICS 251

correct, but we may be certain that in several generaliving at the same time as others with completely seg-mented appendages, the proximal section, which wemay call the coxal region of the appendages, was eitherentirely unsclerified (e.g., Naraoia longicaudata, Retifci-ties, some trilobites such as Agnostus) or incompletelysegmented and covered by small irregular sclerites (Cu-cumericrus, if this is not an anomalocarid) (Fig. 3).Equally the outer branch of the appendages may wellbe incompletely separated from the first articles of theinner branch (Fig. 3) and in the Middle Cambrian Bran-chiocaris (Fig. 3) the whole of the inner branch is re-duced and fused along the whole of its outer marginwith the outer branch, so that the result is a structurebasically corresponding with that of a modern fillopodi-um. Moreover articulated legs, probably incompletelysegmented as in Cucumericrus, occur in the dinocarid(= anomalocarid) Parapeytoia (Fig. 4).

As we said, the hypothesis that complete arthropodis-ation of the appendage may have been attained repeat-edly and independently by different animals may besupported by the morphology of one of the Chenjianganomalocarids, Parapeytoia (Fig. 4).

We have here an unquestionable anomalocarid (I amusing the term informally), the morphology of whosemouth and of the 'big appendages' are typical and closeto those of the Middle Cambrian Peytoia (or Lagganiaas it might be more properly be called according theCode of Nomenclature). The mouth parts of anomalo-carids, as it is well known, are so radically different

C3 C4

Fig. 3 - Appendages of Cambrian arthropods with unsclerotisedand unsegmented or irregularly sclerotised coxal region (US = un-sclerotised coxal region) and with exopod partly or entirely unit-ed with the endopod. A, Retifacies abnormalis (modified afterHou & Bergström, 1997). B, Naraoia compacta (modifiesd afterDelle Cave & Simonetta, 1998). C, Branchiocarispreziosa: Cl, in-terpretation by Briggs, 1976; C2, interpretation by Delle Cave &Simonetta, 1998; C3-C4, interpretation by Hou & Bergstrom, 1999.D, Cucumericrus decoratus (modified after Simonetta et al., 2000).

C1

Fig. 4 - A, reconstruction of the morphology of the dinocarid (=anomalocarid) Parapeytoia yunnanensis, ventral view, and theBurgess Shale dinocarids modified from Simonetta et al. (2000); B,Anomalocaris canadensis-, Cj-C3, Peytoia (= Laggania) nathorsti,ventral view, and their masticatory apparatus (modified fromDelle Cave & Simonetta, 1998).

from those of any arthropod that, even supposing thatthe flap-like appendages of the other anomalocarids arehighly modified derivatives of more arthropod-likestructures, anomalocarids must belong to a branch dis-tinct from that of typical arthropods.

Indeed, though there are still some doubts as to thedetails of the genera Anomalocaris and Peytoia fromthe Burgess Shale, their basic features are clearly estab-lished (Fig. 4) and it is plain that, prima facie, apartfrom the great raptorial cephalic appendages, they havelittle in common with arthropods: their dorsal segmen-tation may have been incomplete and their mouthpartsare absolutely unique: these last are made of an exter-nal rosette of plates provided with apical teeth, whichenabled them to tear off pieces of their preys, that hadbeen captured and held by their raptorial appendages,while inside their mouth there was a set of three serrat-ed blades, clearly used for mashing their food.

However, while the post-oral appendages of the Mid-dle Cambrian anomalocarids were flap-like and hardly

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

04:

38 0

6 O

ctob

er 2

014

252 A. M. SIMONETTA

comparable with arthropod legs, in the older Parapey-toia the post-oral appendages were closely similar tothose of many Cambrian typical arthropods (Fig. 4) andthis genus was also provided with strong segmentalsternites, a quite advanced arthropod character.

This last feature is, again, significant in showing howproblematic is the assessment of the phylogenetic sig-nificance of functionally important features. Indeed,sternites are not known in typical trilobites and in al-most all other better known Cambrian arthropods, and,while they do not occur in some species of the genusNaraoia, they are strongly developed in others.

Parapeytoia and Cucumericrus, anyway, leave uswith two alternatives: either these animals had acquiredto some extent some critical features of arthropods en-tirely independently from typical arthropods, or boththey and arthropods had inherited from some commonancestor both a basically, albeit incompletely segment-ed, rather trilobite-like kind of appendages, and theleverage mechanisms related with strong sternal plates,developed independently several times as, for instance,they did not in trilobites. Thus, whichever alternative ispreferred, this implies that there is no special reason forholding that a complete segmentation of the innerbranch of a basically trilobite-like appendage must havebeen attained just once.

If, as a working hypothesis, we consider that, given abasic structure like that of Cucumericrus, complete seg-mentation was achieved more than once, different num-bers of basic leg segments may well have been devel-oped in different lineages, and this, in turn, may wellhave had a significant impact on the development ofthe main joints

Indeed, whether the already mentioned Cucumericrus(Fig. 3) from the lower Cambrian Chenjiang is an anoma-locarid or whether it belongs to the true arthropod lin-eage will remain doubtful so long as it is known onlyfrom isolated appendages, but the significant fact re-mains that such appendages are distinctly segmentedonly in their distal portion, while their general patternconforms with a basic "trilobitic" structure.

Thus, while Cucumericrus corroborates the assump-tion that a basic incompletely segmented appendage of'trilobitic pattern' is indeed the pattern from which allthe varied appendages of arthropods must be derived,Cucumericrus and the genera with unsclerified basalportion of the appendages prove also: A) that a classic-al, completely segmented arthropod appendage was agradual achievement, and B) that the possibility shouldbe considered that this may have been attained morethan once, as there is no doubt that a number of otherArthropods had already achieved a complete segmenta-tion of their legs millions of years previously, when Cu-cumericrus and the Cambrian genera with incomplete-ly segmented appendages were alive.

The number of leg segments of the endopodite inCambrian arthropods varies considerably from onegenus to another, most genera having short ones. We

consider that most Cambrian arthropods had legs madeup of an endopodite of many short articles and an exo-podite basically lamellar, which could be variously artic-ulated or fringed (Incidentally, to attribute a relevantsignificance to the type of fringe of the outer branch ofPalaeozoic arthropods is a gratuitous assumption, as wecan not know whether the lamellae that fringe the outerbranch, for instance in Canadaspis, are modified setaeor not. The considerable variety of morphologies of theouter branch known in Trilobites and the fact that theappendages of the majority of Palaeozoic taxa is usuallyincompletely known should invite caution in all inter-pretation). Only a few, clearly unrelated genera (theMolaria-Habelia-Emeraldella group, Marrella, Waptiaand a few others) have endopods made up of thin, elon-gate segments.

On the other hand, a definite flex point in the en-dopodite appears only in one Middle Cambrian genus,Sidneyia, itself a genus of obscure affinities (Fig. 5). Itfollows that the acquisition of the various types of flexpoints may be considered as a significant character onlywith qualifications. For instance, it is usually consideredthat a locomotory appendage with a double flex point,separated by a "patella", is a diagnostic character ofChelicerata. However, when we consider the fossilrecord, this type of leg was not yet developed either inthe Devonian pycnogonids, or in the palaeophonidaquatic scorpions and, apparently, it was incompletelydeveloped in the Antracomarti. Now we have fossil ter-restrial arachnids in the Devonian, while scorpions be-came terrestrial only in the late Carboniferous and both

Fig. 5 - Sidneyia inexpectans, a species with advanced tagmosisand appendages and an exceedingly primitive segmentation ofthe head (modified after Delle Cave & Simonetta, 1998).

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

04:

38 0

6 O

ctob

er 2

014

MORPHOLOGY, PALAENTOLOGY AND ARTHROPOD SYSTEMATICS 253

pycnogonid fossil genera described to date are LowerDevonian. It is thus apparent that the "typical" chelice-rate arrangement of the flex point of the walking legshas been independently acquired at least three timesand, therefore, neither can it be considered as diagnos-tic of arachnids in a phylogenetic classification, nor canit be coded as being a single character in a cladistic ma-trix. On the other hand, should other conclusive evi-dence sufficiently support the hypothesis of indepen-dent phylogenies for pycnogonids, scorpions and otherarachnids, we should investigate which functional con-straints dictated such homoplasies.

Thus we cannot escape the conclusion that the typicalstructure of arthropod legs and its different specialisa-tions may have been attained gradually and repeatedly.

From this it follows that, if any of these structures de-veloped independently more than once, as indepen-dently acquired characters cannot be considered strictlyhomologous, before coding any characters of the ap-pendages in a cladistic matrix, it is necessary to assesswhether they are truly homologous with the classicalmethods of comparative anatomy, including full consid-eration for the palaeontological evidence.

The previous points also prove that, when discussingthe significance of the joints in the arthropod legs, weshould consider the mechanical requirements of anyadaptive step in their evolution, as these dictate a limit-ed range of possibilities in the evolution of these struc-tures, which, in turn, implies an increased probability ofhomoplasy. Descriptive and functional morphologymust, indeed, always be carefully considered together.For instance, both an animal walking on dry land,where there will be no hydrostatic buoyancy, or a heav-ily protected or swift moving animal in water will be atan advantage if their legs allow for a greater range ofmovement and better leverage than those offered by atypical trilobite leg.

Now, as we said, given a leg with a certain number ofarticles (and we know that Cambrian Arthropods haddifferent numbers of articles) and a basic architecture,engineering will dictate that the range of possibilities toimprove its efficiency for each different purpose isstrictly limited. It is indeed unfortunate that either whenassuming homologies or when coding characters for acladistic analysis most scholars do not consider howfunctional requirements necessarily canalise adaptationswithin an often restricted range of pathways, so that theevolving structures of the whole organism may properlywork as an integrated, efficient system.

Trilobita and related taxa

Typical trilobites with calcified terga are known fromthe beginning of the Cambrian, while the late Pre-Cam-brian has provided as yet unpublished trails of the"Cruziana" type (M. A. Fedonkin, personal communica-tion), in addition, we know that, in later times, somesuch trails were left by trilobites, though many may well

have been left by a number of other Palaeozoic arthro-pods contemporary with the rare trilobites that havebeen found in association with these trails. Indeed boththe later Lower Cambrian and the Middle Cambrianhave left us the fossils of a number of species morpho-logically more or less close to typical trilobites, but de-void of a calcified skeleton, which must have left trailsindistinguishable from those of trilobites (Fig. 6). More-over, as shown by the Ordovician Tariccoia, at leastsome of these unmineralized trilobite-like animals lastedwell into later times. Such species with unmineralizedterga have a varied tagmosis and some, as far as known,have some slightly but clearly functionally specialisedcephalic appendages which are more advanced thanthose of typical trilobites.

While it is obvious that the mineralization of the exo-skeleton must have been achieved independently in dif-ferent lineages of arthropods (some, as Aglaspis and theentirely unrelated Phosphatocopina even acquired aphosphatic exoskeleton, while all the others have a cal-

Fig. 6 - Various unmineralised Cambrian arthropods related totrilobites: A, Phytophilapsis pergamena (modified after Ivantsov,1999); B, Tegopelte gigas (modified after Delle Cave & Simonetta,1998); C, Naraoia compacta (after Delle Cave & Simonetta, 1998);D, Kuamaia muricata (modified after Hou & Bergström, 1997); E,Sapeiron glumaceum (modified after Hou & Bergström, 1997); F,Liwia (modified after Delle Cave & Simonetta, 1998).

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

04:

38 0

6 O

ctob

er 2

014

254 A. M. SIMONETTA

citic one!) as, for instance, in various lineages of "crus-taceans" and of "myriapods", the fossils clearly showthat during the late Pre-Cambrian or, at the very latest,by the earliest Cambrian there must have been a radia-tion of the stock leading to typical trilobites. Only oneor perhaps two of its branches acquired a mineralisedexoskeleton, the typical Trilobites (some Authors, e.g.Ramsköld & Edgecombe (1991) have suggested that thetrilobites may be at least diphyletic), while an unknownnumber of other branches of the same stock followeddifferent pathways (Fig. 6).

Hou & Bergström (1997) thus suggested a new andcomplex taxonomy within a superclass Lamellipedia,subdivided into classes and subclasses, etc, where typi-cal trilobites are ranged as a subclass (Fig. 7). This isclearly an advance in principle, though obviously onlyfurther discoveries will show how far the new arrange-ment proposed mirrors the true relationships of the dif-ferent animals. As is apparent from our Figure 13, Ifavour a somewhat different phylogeny, as I disagreewith Hou and Bergström on the morphologic signifi-cance of some characters coded by them.

— Fuxianhuiida— Canadaspidlda— Leanchiiliida— Fortiforcipida— Yohoiida— Sanctacaris— Malaria

~| Pseudocruslacea

Crustacea

Proschizoramia

Crustaceomorpha

MarrelJoraorpha

• Orientellidae

- Liwiidae• Naraoiidae• Retifaciidae

Helmctiidae

SkioldiidacSaperiidae

TegopeltidaeXandarellidae• AlmeniidaeSinoburiidae

Nectopleura

Conciliterga

Petalopleura

Trilobita

Piymnotagma

r§Cheloniellidae

StrabopidaeSidncyiidae

Emcraldellidae

Xenopoda

- Aglaspidida

Arachnida

—J Merostomata' Scorpionida

Chelicerata

B

Fig. 7 - The complex phylogenetic classification proposed by Houand Bergström (1997).

Fig. 8 - The three related genera Forttforceps (A, modified afterHou & Bergström, 1997), Janfengia (B, modified after Hou &Bergström, 1987), and Yohoia (C, modified after Delle Cave & Si-monetta, 1998), showing different degrees in the evolution of tag-mosis and in the morphology of the appendages; note eyes andantennae in Fortiforceps, these last apparently lost in the othergenera.

Our previous considerations could be further extendedinto more general arguments on the relativity of the as-sessments of homology, as quite often structures may bedeemed to be homologous if considered from one stand-point but merely analogous when viewed from another.

Anyway, as there is much evidence that the first truesegment of both annelids and arthropods is morpholog-ically post-oral and that the only pre-oral structures ofboth phyla are the acron or epistomium of the annelids,the epistomium of trilobites and the labrum (and strictlyrelated structures) of other arthropods. However, asthere is no acron-like structure in many annelids andthe mouth is terminal in the protonychophorans, thereis no special reason to consider it as being a primitivefeature and, again, the morphological interpretation ofthe arthropod labrum-epistomium may need to be re-assessed. In which case then, topologically the first an-tennae may well be serially homologous with the otherpaired appendages, and only their developmental re-quirements have displaced them forwards.

This could be an argument, albeit a questionable one,in favour of the homology of the raptorial appendagesof anomalocarids (dinocarids), of leanchoiliids etc. withthe antennae of trilobites and of most Palaeozoicarthropods etc.

Also the strange "proboscis" of Opabinia, albeit un-segmented, could have the same origin, as it is knownthat, at least in the males of some species of anostra-cans, the first antennae may fuse to make a clasper, andthat modified antennae in different arthropods have lostall traces of segmentation. Thus it could be argued that

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

04:

38 0

6 O

ctob

er 2

014

MORPHOLOGY, PALAENTOLOGY AND ARTHROPOD SYSTEMATICS 255

the "proboscis" of Opabinia might possibly be derivedfrom fused appendages homologous with the raptorialfirst appendages of anomalocarids and with the first an-tennae of arthropods, their frontal position being a sec-ondary displacement.

It might be noted, incidentally, that in all Cambrianarthropods where the position of the mouth is definitelyknown, this is both ventral and directed backwards,contrary to its position in annelids and in Cambrianlobopods, where it is consistently terminal. On the oth-er hand, in living onychophorans, the mouth is, again,rotated ventrally and one pair of legs has been incorpo-rated into it as jaws. It appears that the ventral rotationof the mouth is correlated with the recruitment ofpaired appendages in the exploration of the environ-ments and in the manipulation of food. It is also proba-ble that the development of the cephalic sensory appa-ratuses and of the correlated ganglia must have playedan important part.

The previous discussion implies that the position ofother genera, such as Leanchoilia, Actaeus, Sanctacarisetc., might need re-assessment, as their first pair of ap-pendages is both a raptorial and a sensory structure, sothat they may well be a totally independent achievementin the evolution of the first pair of appendages and be-ing serially homologous with the antennae of Trilobites.

The case of the species related with Yohoia

The late Lower Cambrian of China has left us withtwo genera clearly related to the Middle CambrianYohoia. These have been described with the names For-tiforceps and Jianfengia (Fig. 8); the three genera ap-pear {pace Budd, 2002) to be a natural group. WhileFortiforceps and Jianfengia are approximately contem-porary, these three genera show in all their features atransition from one to another: in Fortiforceps we stillhave small antennae, while all the appendages, apartfrom the first and the last pair, are built on a uniformpattern and, as in other genera from the same locality,the exopodite was incompletely separated from the en-dopodite. Yohoia, on the other hand, is mechanicallymuch more similar to a small shrimp: there is a sharpdistinction among the tagmata and the abdomen hascompletely lost its appendages, including the last pair,which, in Fortiforceps, complete the tail fan. The thora-cocephalic appendages of Yohoia are much moreevolved, with both articles and exopodite strongly differ-entiated while the antennae have vanished. These are allquite significant differences, but they are clearly basicallybridged by the corresponding morphologies of Jainfen-gia. Moreover, each of these changes may be found tooccur in different arthropod taxa, both living and extinct.

The evolution within this small group of genera clear-ly shows how considerable independent evolution oftagmosis has been possible and thus it casts seriousdoubts on the general reliability of tagmosis and on thesignificance of the number of segments included in each

B

Fig. 9 - Marrella splendens (A1-A2) and the two related species ofFurca (B, Furca bohemica; C, Furcapilosd). The reconstruction ofMarrella after Delle Cave & Simonetta (1998), has been modifiedto conform with the amended oral description by Garcia-Bellido &Collins (2001, Abstract in 3 rd int. conf. on trilobites and their rela-tives, Oxford Univ., p. 13); Furca modified after Chlupâc (1999).

tagma itself as a significant feature for purposes of phy-logenetic reconstruction. Moreover, it shows how theend members of an otherwise plain series of topologicaltransformations may appear to be extremely different.

The case of the marrellomorphs

Until recently Marrella, possibly the commonestBurgess Shale arthropod, appeared to be an entirelyisolated genus. At most some connection had been sug-gested, as a tentative hypothesis, with the Lower De-vonian Mimetaster. Actually I consider as a typical mar-rellomorph the genus Furca, which was described byFritsch in 1908 (on material which had been knownsince 1847!) (quoted after Chlupâc, 1999), thus antedat-ing the discovery of Marrella, while another specieswas identified by Chlupâc in 1999 (Fig. 9). However,apart from Perner (1919; quoted after Chlupâc, 1999),these Ordovician materials either have been completelyoverlooked or its marrellid affinities were not recog-nised. The objection that only the cephalon of the Bo-hemian animals is known and that, therefore one can-not be sure of their affinities is not valid. Until the con-trary may be proved the chances that animals consider-ably different from Marrella had such closely similarcephalic structures are minimal and, anyway if only rea-sonably complete animals should be considered as be-longing into any given taxon, probably 99-99% fossiltaxa should be considered as incertae sedis.

Almost all the phylogenetic reconstruction proposedso far place Marrella in an entirely isolated position andsometimes it has been considered as the ideal "out-

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

04:

38 0

6 O

ctob

er 2

014

256 A. M. SIMONETTA

Fig. 10 - A-D, schemes of different types of cephalisation and de-velopment of carapaces found in different crustaceans (modifiedafter Delle Cave & Simonetta, 1998); E, Chengjangocaris longi-formis; F, Fuxianhuia protensa two very primitive unmineralisedgenera from the Lower Cambrian of China (modified after DelleCave & Simonetta, 1998).

group" for the cladistic analysis of the other Cambrianarthropods. A very recent study by Garda-Bellido &Collins (in preparation), based on some hundreds ofspecimens chosen from among over 6,000, producedtwo interesting pieces of evidence: on one side it hasconfirmed that the second pair of cephalic appendagesis not biramous (Fig. 10) and showed that there was apair of small eyes directed downwards and placed at thebase of the first pair of great spines of the cephalon. Thediscovery of the eyes dispels what was held to be a cu-rious anomaly in an obviously active animal living in arather shallow marine environment. On the other handthe fact that the second pair of appendages was unirami-an is strong evidence against considering Matrella as avery primitive animal and, indeed, agrees with the gen-eral morphology of the legs, thin and made of slender,very elongated articles, which, when considered jointlywith the almost unique morphology of the outer branchof the legs, points to a morphology well advanced be-yond that of the really primitive Cambrian arthropods,such as Chengjangocaris and Fuxianhuia (Fig. 10). Ifthese considerations are granted, it follows that allcladistic analyses which have taken Marrella as the "out-group" to the other taxa should be considered as objec-tionable as this group appears, albeit fairly isolated, avery specialised (or in cladistic terms derived) group.

TAGMOSIS

The morphological series discussed above introducesto the problem of the evaluation of tagmosis and of thenumber of segments in the body of the different lin-eages. There is no doubt that the development ofArthropods is primitively anamorphic or, rather, hemi-anamorphic, as usually animals do add new segmentsup to less than a centimetre long at most. By then theyhave, but for a few exceptions, a given, species-specific,number of segments and further moults increase theirsize or change their morphology, but do not add to the

number of their segments. It appears that even compar-atively primitive Palaeozoic arthropods had a fixednumber of segments and that this was attained well be-fore the animals ceased to grow.

Tagmosis is, strictly speaking, the differentiation ofthe body into distinct tagmata or regions, each onefunctionally specialised and morphologically distinct.Fusion of segments belonging to a single tagma, com-plete or incomplete fusion of different tagmata, devel-opment of a carapace, this having itself quite differentmorphologies, have clearly evolved independently in anumber of lineages (Figs 10-11).

Apparently the most primitive arthropod known, fromthe standpoint of tagmatisation, is Chengjiangocaris,where we find only two tagmata: a head region of fourto five segments (Fig. 11), clearly different from the fol-lowing ones, but still with entirely separated segments,and covered by a "carapace", or, rather, by a shield ap-parently attached to the foremost cephalic segment. Thesmall segments of the head are followed by a series ofsimilar segments, the size of which gradually decreasescaudally. The pygidial region (or telson) was apparentlya gently tapering triangular plate, though we cannot ex-clude that one or more segments were fused with thetelson. Chengjiangocaris apparently differs from the

A1

D2

Fig. 11 - Some Cambrian arthropods showing an assortment ofdifferent tagmoses and carapaces growing from different seg-ments. A1-A2, Branchiocaris preziosa; B1-B2, Odaraia alata; C,Waptia fieldensis; D1-D3, Canadaspis obliqua (modified afterDelle Cave & Simonetta, 1998).

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

04:

38 0

6 O

ctob

er 2

014

MORPHOLOGY, PALAENTOLOGY AND ARTHROPOD SYSTEMATICS 257

most primitive olenellids in two features only: olenellidshave well calcified terga, typical of true trilobites, andtheir cephalic segments are completely fused.

Fuxianhuia is also, apparently, close to Chengjiango-caris, but it is even more similar to a primitive olenellid,so that the only significant difference between Fuxian-huia and the most primitive trilobites known appears tobe the calcification of the terga and the consequenttrilobation (I do not discuss here Kleptothule rass-musseni, which has been compared with olenellids, butwhich has no calcified exoskeleton, I consider it ratheras belonging to a complex and varied group of more orless trilobite-like, rather primitive, arthropods; that ap-pear, as far as the species known are concerned, to be-long to a radiation of several branches, one or, perhaps,two having evolved into true Ttilobites).

However, the head region of Chengjiangocaris, as itincludes at least four true segments (the tiny first "seg-ment" may be either a true segment or an epistomiallobe) is, as such, more developed than, for instance,that of Sidneyia (with an entirely different and muchmore advanced tagmosis with clearly recognisablehead, thorax and abdomen), which includes only twofree post-ocular segments (Fig. 5), while several otherearly arthropods (such as Marrelld) have, again, lessthan four fused cephalic segments.

It is equally obvious that to house the most importantsensory organs, fusion of some cephalic segments isuseful and that it may be selectively advantageous toextend it caudally, according to the requirements of thedifferent adaptive pathways and especially in connec-tion with the development of specialised mouth-parts.The very fact that the number of cephalic segments, thedegree of their fusion and the different degree of spe-cialisation of their appendages in Palaeozoic arthropodsranged in a number of assortments, clearly shows thatcephalisation and more generally tagmosis developedindependently in different lineages, that similarity intagmosis may or may not be significant in differentgroups, and that its morphological significance must beassessed in each case by comparison with other, inde-pendent lines of evidence. Thus if the Cambrian arthro-pods prove that tagmosis was quite variously and inde-pendently developed in a number of lineages, whatabout the number of segments making up each tagma?The naraoids provide a clear-cut answer: all of them,and other related genera, are made up of only two tag-mata: a cephalon and an "abdomen", but the latter in-cludes a different number of segments according to thespecies. A simple perusal of the morphology of theCambrian arthropods proves that though the evolutionof tagmosis and consideration of the number of seg-ments included in each tagma in the various groupsmay provide critical evidence as to their relationships,yet on the one hand they offer such a variety that plain-ly shows how complex was the overall pattern and howcharacters commonly deemed to be diagnostic of latermajor conventional taxa could be independentlyachieved more than once.

Such a variety in the specialisation of different groupsof body segments is exemplified by the differentiationof the post-cephalic segments. Both in fossils as old asthe Cambrian and in living arthropods, we find extremeinstances both of lack of differentiation of the post-cephalic segments, as in the Cambrian Protocaris orOdaraia (different as they are) and we have a similarcondition in living conchostracans or, vice versa, ofcomplete differentiation as, again, in the CambrianCanadaspis or Waptia.

It would obviously be tedious to go into the multifari-ous ways by which the postcephalic segments becamedifferentiated or how the different types of carapace de-veloped (Figs 10-11). The important thing is that wemay easily see how these were obtained by the imple-mentation of comparatively few mechanisms, which,while being always the same, not only combined into anumber of different ways, but were implemented in thevarious groups at different times. Such are, for instance,the few patterns in the modification of the appendages(loss, reduction, simplification, development of chelaeor subchelae, etc.) and, being often correlated withthem, changes in the morphology of thoracic and ab-dominal segments (fusions, etc).

These are familiar to anyone, but the question mustbe posed: as they are always functionally significant, isit not probable that they occurred independently anumber of times?

THE TRACHEAL SYSTEMS

Apart from some small mites, the tiny Palpigrada,many collembolans and proturans, which have no res-piratory structures and apparently rely for respiration ondiffusion through their thin cuticle, all other true terres-trial arthropods rely for respiration either on lung-books, on tracheae or on the specialised posterior sur-face of their pleopods.

Although the validity as a natural taxon of the tradi-tional "myriapods" has been increasingly doubted sinceRipper (193D, yet they continue to creep up as such indiscussions on the phylogeny of arthropods. Consensuson the validity of the taxon Insecta or Hexapoda hasbeen traditional, yet it has been challenged more thanonce on very sound evidence (Dallai, 1991) and it hasbeen argued that the taxon Insecta should be restrictedto the Archaeognatha + (Zygaentoma + Pterygota).

Tracheae, especially, in spite of the remarks by Ripper(193D, have often been held to be a significant featureboth for characterising a supposed group of "Antenno--tracheata" (insects + myriapods sensu lato, though theselatter are now being commonly considered to be a para-phyletic assemblage), and they have been considered asa significant apomorphy within the arachnida, taken intheir traditional meaning (e.g., by Shultz, 1990).

As it happens, tracheae occur in different groups ofwoodlice (Isopoda) and there they are variously devel-oped (Fig. 12) even within a single family. We have

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

04:

38 0

6 O

ctob

er 2

014

258 A. M. SIMONETTA

chosen our examples from the family Eubelidae (Ferraraet al., 1990), but we must stress that a similar range ofconditions occurs in other families of terrestrial isopods.As may be seen, in this family there is a complete rangeof morphologies, from species provided with a simplecorrugated area of the back surface of the pleopods,there being underneath it a network of haemolymphaticlacunae, to ingrowth into a thickened shelf of the pleo-pod, to development of multiple stigmata in a spe-cialised surface of the appendages, each stigma leadinginto a more o less complex system of tracheae, and fi-nally to a single stigma leading into an atrium, fromwhich the tracheae branch well into the body of the an-imal (Fig. 12).

The degree of development of the system, in thespecies illustrated, is strictly correlated with habitats:Atracheodillo (Fig. 12A) being a genus typical of thetropical rainforest, while the others are to be found inincreasingly arid habitats up to Periscyphis arabicuswhich is a species typical of sub-desert habitats (Fig.12F). As we said similar ranges of morphologies arefound in other families of woodlice.

The ultrastructure of the walls of the fully developedtracheae of these isopods is indistinguishable from thatof insects, arachnids, etc. We may note here that the

Fig. 12 - Diagrammatic representation of the respiratory structuresin different members of the family Eubelidae (Crustacea, Isopoda)(modified after Ferrara et al, 1990): A, Atracheodillo marmorivir-gatum; B, Eubelum instrenum; C, Gerutha pila; D, Eubelum lu-bricum; E, Somaloniscus taramassoi; F, Periscyphis arabicus.

structure of the taenidia which support the tracheae isthe same in all tracheate arthropods investigated, apartfrom some diplopods that have some very large tra-cheae supported by completely different taenidia in ad-dition to the typical ones (Simonetta et al., 2000).

The varied morphologies of these Isopods may, onthe one hand, suggest by which pathways other tra-chéal systems may have evolved and especially thatthey may not necessarily derive from structures similarto book-lungs. Moreover, their multiple appearance inunrelated families of the same order prove that theymay have independently evolved a number of times.

This is further proved when we consider the situationin "arachnids". Table I, indeed, summarises the situationin the various taxa of this supposed class. It is immedi-ately apparent that, their structural similarity notwith-standing, it is most unlikely that tracheae are homolo-gous throughout this group.

Apart from the fact that they may anastomise, as inSolifugae, or not, as in Ricinulei etc. in some instances,as is proved by some Araneae, tracheae may have orig-inated from book-lungs by a process similar to thatwhich we have seen in isopods. This, however, is hard-ly probable for thoracic tracheae and also the varieddistribution of tracheae in different segments suggestsan entirely independent evolution of this character inthe different orders.

Moreover, as far as terrestrial "Chelicerata" are con-cerned both the fact that they are variously distributedin different segments in the various orders and that theymust have evolved in scorpions much later than in theother "arachnid" orders (it is well known that, whileseveral "arachnid" orders go back to the Late Devonian,terrestrial scorpions appear only during the Upper Car-boniferous, and the aquatic ones, provided with regularranges of gill lamellae, range from the Devonian to thevery end of the Triassic) suggest that also the book-lungs occurring in the various orders may well be ho-mologous only in the broad sense that they are struc-turally similar structures derived from the outer branchof a trilobite-like appendage.

Much the same may be argued concerning the other"classes" of tracheate arthropods: among progoneatediplopods there are considerable variations, and the or-ganisation of the trachéal system and that of glomeridsis hardly comparable with that of the other diplopods(Simonetta et al, 2000).

As for the "Chilopoda", again, the structure and topol-ogy of the trachéal system of Scutigeromorpha clearlypoints to an origin of these structures independent fromthat of the other Chilopoda.

Among insects there are no tracheae in most collem-bolans and in some proturans (assuming that they areinsects, that is quite doubtful; cf. Dallai, 199D- As withthe mites, it could be argued that this depends on thetiny size and delicate teguments of these animals. Thisis quite possible, but it must be noted that the peculiarposition of the stigmata and tracheae both in the tra-cheate Collembola and Protura may well point to an ori-

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

04:

38 0

6 O

ctob

er 2

014

MORPHOLOGY, PALAENTOLOGY AND ARTHROPOD SYSTEMATICS 259

TABLE I - Distribution of the respiratory apparatus in the various classes of terrestrial Chelicerata.

Book lungs (BL) Tracheae (TR) Stigmata localization

Prosomasegments

Opistomasegments

Notes

ScorpionesUropigiSchizomidiAmblypygiPalpigradiRicinuleiPseudoscorpiones

Solifugae

Opiliones WAraneae (4BL)Araneae (2BL+TR)

Araneae (TR)Acari

4 pairs2 pairs1 pair2 pairs

---

-

-2 pairs1 pair

-tracheae of various t\

----

1 pair2 pairs1 pair2 pairs

only one stigma1 pair

-

1 pair2 pairs

roes and number.

III-VI VentrallyII-III Ventrally

II VentrallyII-III Ventrally

VI - Laterally, behind the 4th coxaeIII-IV Ventrally

IV Laterally, behind the 4th coxaeIII-IV Ventrally

V VentrallyII Ventrally

II-III VentrallyII VentrallyIII Ventrally

II-III Ventrally

W Phalangidae present also tibial stigmata and pedal tracheae.

gin for these structures in these animals independentfrom that of ectognathous insects. There is thus over-whelming evidence that structurally almost identical tra-cheae evolved independently not only in the differenttraditional classes of arthropods, but within the same"class" and that, as a whole, this must have happenedseveral times. This implies that in any discussion of theaffinities of arthropods, be it a comparison within a sup-posed major taxon or among different major taxa, thetracheae cannot be listed as homologous a priori, butonly if this is proved by a balanced assessment of theirown possible evolution and of independent evidence.

This appear to be just one good example of the diffi-culty of assuming, a priori, that any two similar struc-tures are strictly homologous in a phylogenetic sense asproblems comparable with those discussed above ob-tain with almost any important structure of arthropods.Obviously, when preparing the matrix for a cladisticanalysis it is essential that only strictly homologouscharacters may be listed as being the same, while thehomoplasic ones must be listed as separate apomor-phies. But then we are again faced with the fact thatthis implies the subjective, albeit well informed judge-ment of the scholar, so that the resulting analysis cannotclaim to any greater objectivity then that attained bymore traditional methods.

BRIEF CONSIDERATIONS ON THE PROPOSED SUPER-PHYLUM "ECDYSOZOA"

Adequate consideration of this controversial issue can-not be given here and it is planned for a future paper.

However, one or two considerations may not be out ofplace as they allow us to mention methodological prob-lems, which impinge also on the underlying methods ofthis, very paper. As for the fact that even molecular evi-dences is contradictory, I may refer to B. K. Penney(2002, http://www.biology.ualberta.ca/courses.hp/biol606/OldLecs/Lecture2K.09.Penney.htmt), or to Gar-cïa-Machado et al. (1999), but I shall here consider thepaper by Giribet & Wheeler (1999), again as one exam-ple of what I deem an inadequate treatment of the evi-dence. These authors note how their evidence, apartfrom supporting the supposed Ecdysozoa, while sup-porting the traditional Deuterostomata (Chordata, En-teropneusta, Echinodermata) does not support mono-phyly for molluscs, brachiopods, phoronids, annelidsand nemerteans. Now it should be obvious that thesephyla are established on such strong morphological andpalaeontological evidence that any scholar should haveasked himself the following questions: assuming that allthe evidence available is correct, and this is undoubted,when contradictory results seem to obtain by the studyof different kinds of evidence, there are but three possi-bilities: A) the methods by which some of the evidenceis analysed are unreliable, B) a given type of evidencehas different significance in the various phyla, C) by anunfortunate chance a given set of evidence was ob-tained from a somehow inadequate sample, that is fromorganisms which, for the particular type of evidence in-vestigated, are not representative of the average condi-tions in the taxon.

Obviously all these alternatives should be properlytested for all the lines of evidence which produce the

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

04:

38 0

6 O

ctob

er 2

014

260 A. M. SIMONETTA

contradictory results. Just as I have pointed to the diffi-culty of having any standard simple rule to assess thesignificance of morphological evidence, I think thatwhen we meet with apparently contradictory results,the whole matter must be re-examined with an openmind, without assuming a priori that any given kind ofevidence is always the significant one.

As I have criticised elsewhere some principles ofcladistics and the use of parsimony (Simonetta, 1992,1993, 1999a), my first suspicion is that when we arefaced by such blatant contradictions between the resultsof comparative morphology and of molecular methods,the fault is not in the evidence, but in some limitationsin the methods employed to analyse it. For instance,just to return to the above-quoted paper, the Authorsfind evidence pointing to an affinity of some bivalveswith some bryozoans, while other bryozoans are foundwithin a bunch of assorted annelids, and the other bi-valves group with gastropods (cephalopods not havingbeen studied). Indeed, as we have a fair understandingof the evolution of molluscs, it is perfectly obvious thatthe separation between molluscs and bryozoans, be-sides their affinities being rather incredible, if it did oc-cur at all, must have occurred well before the molluscsthemselves began their radiation (which must be datedat the very end of the Vendian or even slightly later), itshould have been concluded that, as the monophyly ofmolluscs (with the possible exception of the Caudo-foveata, which may prove to be a "sister-taxon" to the"true molluscs") is supported by an overwhelmingamount of combined evidence, an analysis which pointsto a relation of some of them with some bryozoans,must necessarily be at fault on this point at least.

Just the same may be said of another conclusion inthe same paper, as the annelids appear scattered hereand there in the cladogram, some close to some ne-merteans, some to Sipuncula etc., and as, again, a simi-lar argument may be advanced against the supposedparaphyly of brachiopods, phoronids and nemerteans,such a number of surprising assemblages should havestrongly suggested that it was more than reasonable todoubt the general validity of the methods employed toreach such conclusions.

CONCLUSIONS

In recent years, in spite of a wealth of papers based onmorphological evidence, the general comparative anato-my of arthropods has seldom been considered in anysystematic way. Indeed a variety of morphological char-acters have been used, especially in cladistic analyses,but in such papers the actual significance of each charac-ter has not been discussed for each individual case.

My main purpose in this paper has been to point toproblems of interpreting the evidence. I may here em-phasise anew what I said in the previous section: whenthe interpretation of different sets of evidence results inconflicting conclusions, it is entirely wrong to accept

this or that solution until we have been able to under-stand the reasons underlying such conflicting results.

As I said in the opening lines, this paper does notprovide any new evidence, but it rather advocates aflexible interpretation of such evidence by avoiding theconsideration of separate characters. Closing this briefessay, I wish to stress that my attitude is not a nihilisticdenial that either true homologies may eventually beidentified, nor that the quest for the actual relationshipsof the different groups of arthropods is a hopeless en-deavour. My arguments are rather a warning that homo-plasies are probably much more common than is usual-ly assumed and that they are the more common themore functionally relevant is a set of characters, so thatDarwin's caveat that the best tracers of phylogenetic re-lationships are usually the least functionally significantcharacters is still largely valid. Both functional morphol-ogy and the requirement for pre-adaptation should al-ways be considered in the assessment of possible phy-logenetic links.

I think that, even if we wish to implement a cladisticanalysis of arthropods in general or of any subgroup ofthem, it is unavoidable to face the two horns of adilemma: either we do not try to make a preliminary as-sessment of the morphological significance of eachcharacter considered in each one of the taxa studied,and thence we are bound to enter in our matrixes anunknown number of apparently identical characterswhich in fact are pseudo-apomorph (homoplastic), asthey are simply the result of convergent evolution de-pending on adaptation, or we start from the considera-tion of the comparative morphology of each structureand, therefore from the assessment of its morphologicalsignificance. If we start with the first alternative a biasedmatrix will necessarily lead to biased results. With thesecond approach, as this preliminary assessment of thesignificance of each bit of evidence unavoidably impliesthe tentative reconstruction of the pathways by whichtheir evolution developed, our decisions will necessarilybe to some extent subjective.

This conclusion would entail the consideration of sev-eral basic problems in the philosophy of science whichlie beyond the scope of this paper and that we shall notdiscuss here (but see Simonetta, 1992, 1993, 1999a, b;Wägele, 1996a, b).

In this paper I have considered only the morphologi-cal evidence, as this is the only kind available for bothfossil and living animals. Thus the growing and certain-ly significant amount of information concerning the ge-netics of some of the structures considered, and moregenerally molecular evidence, has been mentioned onlyincidentally. Again: limits of space have allowed us toconsider just a few characters, but examples like thesemay be easily multiplied for almost all characters.

SOME CONSIDERATIONS ON SYSTEMATICS

Basically our previous examples suggest that there isno sufficient evidence to hold that any of the traditional

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

04:

38 0

6 O

ctob

er 2

014

MORPHOLOGY, PALAENTOLOGY AND ARTHROPOD SYSTEMATICS 261

major taxa: Crustacea, Chelicerata, Insecta is a natural,id est, monophyletic taxon.

In order to better clarify this hypothesis I must stressthat, in order to have a clear argument it is necessary todefine the terms we are using. There is conclusive evi-dence to assume that considerable radiation of the truearthropods must have already occurred by the end ofthe Pre-Cambrian, as the range of morphologies that oc-cur in the earliest Lagerstätten and the fact that typicaltrilobites occur in the earliest Cambrian strata cannot beexplained otherwise. This clearly does not rule out thepossibility of considerable differentiation of arthropodtaxa during the Lower Cambrian and later and it is es-pecially noteworthy how, both in the Lower CambrianChinese localities and, to some extent, in the later Mid-dle Cambrian localities, we find, side by side, extremelyprimitive morphologies and fairly advanced and spe-cialised ones and, even more commonly, species show-ing a mix of both advanced and very primitive features(e.g. Sidneyia, showing both an advanced tagmosis andstill separated head segments (Fig. 5). If, as it would bedesirable in principle, we attempt to sort out the affini-ties of the Cambrian arthropods without reference to lat-er taxa, this is extremely difficult, but it would unavoid-ably identify a number of lineages vastly exceeding thepresent major ones (see also Hou & Bergström, 1997).

We must also consider that the radiation of thePalaeozoic arthropod stocks and their acquisition of thevarious characters that we commonly deem to be typi-cal of arthropods must have been gradual, and that itmust have begun some time before the separation ofthe anomalocarids (Dinocarida), with their uniquemouthparts, from the true arthropods.

Moreover there is hardly any doubt that, since the be-ginning of arthropod evolution, functional requirementsconsequent to the emerging of similar ecological nichesmust have often required many instances of parallel andconvergent evolution or, if you like, of pseudo-apomor-phy (homoplasy). This must, obviously, have mainly af-fected functionally significant features, so that it shouldbe assumed that, when considering such structures, wemight expect the development of similar and even iden-tical structures in unrelated lineages. This should becarefully considered in any attempt to reconstruct thephylogeny of arthropods, be it by cladistic or by noncladistic, more traditional, methods.

As it appears, and several Authors have been movingalong these tracks, it is high time to tackle the phyloge-ny of arthropods by first throwing into the waste basketall, and I emphasise all, the traditional classifications or,at least, by considering all of them, in principle, as be-ing questionable, we may then expect to reach quitenew results. Indeed, scholars must beware of analysingtheir evidence with the explicit or implicit assumptionthat any of the living traditional classes is a mono-phyletic assemblage, as this is precisely what has to bedemonstrated and as any such assumption will neces-sarily mislead the argument into circularity.

My collaborators and I (Delle Cave & Simonetta, 1975,1981, 1991, 1998; Simonetta 1975; Simonetta & DelleCave, 1977, Abstract in XIV Conv. U.Z.I., no. 81; Simo-netta & Delle Cave, 1981, Abstract in Boll. Zool. 48(suppl.): 103) have repeatedly proposed diagrams ofthe possible phylogenetic relationships of the differentarthropods groups, modifying them from time to time inthe light of the evidence accruing. The contradictory re-sults that scholars have reached in the last years, a con-tradiction which largely stems from the different selec-tion of characters used by each Author and, to some ex-tent, from the different technicalities used in the elabo-ration of the dendrograms have recently prompted dif-ferent Authors to advocate a conservative evaluation ofthe available evidence. So, for instance Brusca (2000)and Schräm & Koenemann (2001), have envisaged theneed for a re-appreciation of the traditional classifica-tions of recent classes of arthropods. From their attitudethere is but a little step to take in order to doubt thatChelicerata, Insecta, Crustacea and "myriapods" are ac-tually natural, monophyletic assemblages.

During the last years, several new taxinomie arrange-ments have been proposed for arthropods and for therelated taxa, and these are often at odds with one an-other. In fact these differences stem as much from theevidence considered as from the methods by which thishas been analysed. Most of the recent taxonomies havebeen proposed by the implementation of cladistic meth-ods and the differences stem mainly from the choice ofthe characters listed in the matrixes, where, quite often,characters have been entered as homologous whichwere either certainly not such, or, at least, quite ques-tionably so (Simonetta et al., 2000). Moreover, severalsuch new attempts have aimed to give strict definitionsof the taxa, which is necessarily a doomed attempt.

In a previous paper (Simonetta et al., 2000) we ar-gued that the traditional "Chelicerata" is a paraphyleticassemblage made up of at least three and possibly fourlineages which independently acquired the traditional"diagnostic" characters of the Chelicerata. Dallai (199Dand others have questioned the monophyletic origin ofthe entognathous and ectognathous "insects". That"myriapods" are a natural taxon has always been a de-batable issue, but one may add that there is evidence(Simonetta et al., 2000) that even the diplopods maywell be an artificial assemblage, as glomerids may wellhave been separated since their origin from the remain-ing diplopods.

Finally, when we consider the ensemble of the Cam-brian arthropods showing this or that character later"typical" of crustaceans (cf. Walossek, 1999), we are im-mediately aware that none of them may be directly re-lated to all of the later "crustaceans", while, if we sup-pose that the living "crustaceans" acquired indepen-dently some of their supposed "diagnostic" characters,then the root of some crustacean groups may, perhaps,be found in different Cambrian crustacean-like animals.Thus, for instance, the Conchostraca may be related to

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

04:

38 0

6 O

ctob

er 2

014

262 A. M. SIMONETTA

Branchiocaris, provided that we assume that the Con-chostraca evolved their specialised mouthparts indepen-dently from other "crustaceans". It may be noted, in ad-dition, that most of, if not all, the characters whichWalossek (1999) listed as the only ones really commonto all crustaceans are actually unknown (and some willclearly never be known) for the Lower and MiddleCambrian arthropods.

Therefore, I herewith submit a sketch showing thepossible interrelationships between fossil and livingarthropods, as they may be envisaged on the morpho-logical evidence available for the fossil taxa (Fig. 13).

It is my contention, as already stressed, that method-ologically the correct procedure, when trying to assessthe phylogenetic relationships of any of the major taxa,is first to assess, on the available evidence, the mutualrelations of the early fossils known, then to see bywhich gradual topological transformations every singlelater morphology may have evolved from the known

^Tardigrada/\Protonïchophora Gnychophora

Opabinia ? SanctacarisI .-Dinocarida (Anomatocaris)' ^\.Leanchoitia Tegopefte (etc.)

Rhpmb•Retifades

Kuamaia•Uwia

•Mollinsonia "Urokodia Geophylomorpha

.^--Scolopendromorpha^ LIthobiomorpha

Emeraldella^Cambropodus?

—Lfmulava

^Fuxianhiua~~Chengjiangocaris

•"-—Other Arachi

intomorphaPentazonia ^^Scorpiones

Pauropoda ^^—^-EuTypXenda^aleomen^"^ ^^-Borkrevinkium

Triopus ̂ ^ — StrabopidaeW"—Bunaia•SynaphosuraOCyphosura

VCftasmatesp/sDiploaspis

"Alalcomçnaeus-Chelôniellida

MarreflidaMimetasterida

•Tesnusocarida Nectiopoda?LipostracaAnostraca

Acerçostraca ^EumalacostracaBurgessiaNotostraca

yWaptida Douglasjcaris— ^ - — Clrripedia

KazacharthraLeptostraca

raca • ci—*—C

Fig. 13 - Our view of a simplified diagram of the possible phylo-genetic relationships of most Palaeozoic and recent taxa; severaltaxa, especially of those belonging to the so-called crustacean-stem group have been omitted. Living taxa in bold type. This typeof presentation of my ideas has been chosen as I think that, onthe available evidence, phylogenetic relationships cannot be indi-cated by the type of dendrogram usual in cladograms, where thesupposed relationships of the various branches are more preciselyindicated.

earlier fossil ones or from morphologies that may rea-sonably be supposed to have existed on the evidenceof those of the early taxa known. Finally, such phyloge-nies as appear plausible on morphological evidencemust be compared with all other kinds of evidenceavailable: genetic, molecular, etc. At this last stage, ifthere is mutual agreement in the results of such differ-ent analyses, we may take the results as temporarily re-liable and tailor on it a practical taxonomy, while, if theresults of the different analyses are in conflict, we musttry first to find an explanation for the conflict itself.

The scheme proposed here has been elaborated onthe only evidence of comparative morphology and issubmitted merely to be used as a working hypothesis.Practically all the suggested relationships in this schemehave already been proposed separately by some Au-thors, however they are combined here for the first time.

As it appears, there may well be some sort of relation-ship linking some of the higher taxa currently includedin the same traditional taxinomie unit, but to groupthem into a single inclusive taxon will necessitate botha drastic revision of the list of diagnostic characters (forinstance, a single taxon may well group scorpions, eu-rypterids and xiphosurans, provided that it is made in-clusive of Emeraldella, Molaria, Habelia, and assumingthat one of their primitive diagnostic features, later lostin most fossil groups and in all living ones, were theanal valves, and by assuming also that their commonancestor was provided with typical antennae and had atmost sub-chelate spiny appendages). Likewise, the in-clusion of any of the Lower or Middle Cambrian arthro-pods into the "Crustacea" requires the elimination fromthe definition of crustaceans of the characters "two pairsof antennae and two or three pairs of mouth ap-pendages" etc.

Thus, to conclude, we suggest here that taxa wouldbe better defined in terms of evolutionary trends or,even better, of morphologies compatible with gradualtopologic changes rather than in terms of a list of defi-nite characters. Evidently clear definitions in the tradi-tional taxinomie framework and in terms of classicaldefinitory logics are possible, provided that we assumea strictly terministic attitude, that is that we recognisethat the classes thus defined are conventional assem-blages pragmatically arranged so that they may approx-imately portray such phylogenetic relationships be-tween known animals as appear justified in the light ofavailable evidence. Finally, as is clearly apparent, in ourpresent views none of the traditional classes of livingArthropods survives as a monophyletic taxon.

Not only do none of the traditional "classes" of arthro-pods appear to be unquestionably monophyletic, butmany fossil arthropods may well prove to belong toseveral more lineages, at least half of them being extinctwithout descendants.

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

04:

38 0

6 O

ctob

er 2

014

MORPHOLOGY, PALAENTOLOGY AND ARTHROPOD SYSTEMATICS 263

REFERENCES

Abele L. G., Kim W., Felgenhauer B. E., 1989 - Molecular evi-dence for inclusion of the phylum Pentastomida in the Cru-stacea. Mol. Biol. Evol, 6: 685-691.

Adoutte A., Balavoire G., Lartillot N., De Rosa R., 1999 - Animalevolution: the end of the in-termediate taxa? Trends Genet., 15:104-108.

Aguinaldo A. M. A., Turbeville J. M., Linford L. S., Rivera M. C.,GareyJ. R., Raff R. A., Lake J. A., 1997 - Evidence for a clade ofnematodes arthropods and other moulting animals. Nature, 387:489-493.

Arthur W., 1997 - The origin of animal body plan: a study in evo-lutionary developmental biology. Cambridge University Press,Cambridge.

Averof S. M., Akam S. M., 1995 - Insect-crustacean relationshipsinsights from comparative development and molecular studies.Phil. Trans. R. Soc. Lond. B, 347: 293-303.

Ayala F. J., Rzhetsky A., Ayala F. J., 1998 - Origin of the metazoanphyla: molecular clocks confirm paleontological estimates. Proc.Nad. Acad. Sci. U.S.A., 95: 606-611.

Bergström J., 1992 - The oldest arthropods and the origin of theCrustacea. Acta Zool., 73: 287-291.

Bergström J., 1994 - A treat of Cambrian arthropods. Lethaia, 27: 208.Briggs D. E. G., 1983 - Affinities and early evolution of the Cru-

stacea: the evidence of the Cambrian fossils. In: F. R. Scham(ed.), Crustacean phylogeny. Balkema, Rotterdam, pp. 1-22.

Briggs D. E. G., Fortey R. A., 1989 - The early radiation and rela-tionships of the major arthropod groups. Science, 246: 241-243.

Brusca R. C., 2000 - Unraveling the history of arthropod biodiver-sification. Ann. Mo. Bot. Gard., 87: 13-25.

Budd G. E., 1996 - Progress and problems in arthropod phyloge-ny. Trends Ecol. & Evol., 11: 356-358.

Budd G. E., 2002 - A palaeontological solution to the arthropodhead problem. Nature, 417: 271-275.

Chlupâc I., 1999 - Unusual arthropods from the Bohemian Or-dovician - a review. Acta Univ. Carol. Geol., 43: 393-396.

Conway Morris S., 1997 - Molecular clocks: defusing the Cambrian'explosion? Curr. Biol., 7: R71-R74.

Dallai R., 1991 - Are Protura really insects? In: A. M. Simonetta &S. Conway Morris (eds), The early evolution of Metazoa and thesignificance of problematic taxa. Cambridge University Press,Cambridge, pp. 263-269

Delle Cave L., Insom E., Simonetta A. M., 1998 - Advances, diver-sions, possible relapses and additional problems in understand-ing the early evolution of the Articulata. Ital. J. Zool., 65: 19-38.

Delle Cave L., Simonetta A., 1975 - The Cambrian non-trilobitearthropods from the Burgess Shale of British Columbia. A studyof their comparative morphology taxinomy and evolutionarysignificance. Palaeontogr. Ital., 69: 1-37.

Delle Cave L., Simonetta A., 1981 - An essay on the comparativeanatomy and evolutionary morphology of Palaeozoic arthro-pods. In: Atti Conv. Lincei, no. 49. Accademia Nazionale deiLincei, Roma, pp. 389-439.

Delle Cave L., Simonetta A. M., 1991 - Early Paleozoic arthropodsand problems of arthropod phylogeny, with notes on taxa ofdoubtful affinities. In: A. M. Simonetta & S. Conway Morris (eds),The early evolution of Metazoa and the significance of problem-atic taxa. Cambridge University Press, Cambridge, pp. 189-244.

De Robertis E. M., 1997 - The ancestry of segmentation. Nature,387: 25-26.

Eernisse D. J., Albert J. S., Anderson F. E., 1992 - Anellida andArthropoda are not a sister taxa: a phylogenetic analysis of spi-ralian metazoan morphology. Syst. Biol., 41: 305-330.

Emerson M. J., Schräm F. R., 1991 - Arthropod pattern theory: anew approach to arthropod phylogeny. Mem. Queensl. Mus.,31: 1-18.

Fedonkin M. A., Waggoner B. M., 1997 - The late Precambrianfossil Kimberella is a mollusc-like bilaterian organism. Nature,388: 868-871.

Fedonkin M. A., 2001 - Megascopic trace fossils and body fossilsof the oldest metazoans. Am. Zool., 41: 1441-1441.

Ferrara F., Paoli P., Taiti S., 1990 - Morphology of the pleopodallungs in the Eubelidae (Crustacea, Oniscidea). In. P. Juchault &J. P. Mocquard (eds), The biology of terrestrial isopods III. Uni-versité of Poitiers, Poitiers, pp. 9-16.

Fortey R. A., Briggs D. E. G., Wills M. A., 1996 - The Cambrianevolutionary 'explosion': decoupling cladogenesis from mor-phological disparity. Biol. J. Linn. Soc, 57: 13-33.

Fryer G., 1996 - Reflections on arthropod evolution. Biol. J. Linn.Soc., 58: 1-55.

Garcia-Machado E., Malgorzata P., Dennebouy N., Oliva-SuarezM., Mounolou J.-C., Monnerot M., 1999 - Mitochondrial genescollectively suggest the paraphyly of Crustacea with respect toInsecta. J. Mol. Evol., 49: 142-149.

GareyJ. R., 2002 - Ecdysozoa: the relationship between Cycloneu-ralia and Panarthropoda. Zool. Anz., 240: 321-330.

Giribet G., Wheeler C. W., 1999 - The position of arthropods inthe animal kingdom: Ecdysozoa, trees, and the "parsimonyratchet". Mol. Phylogenet. Evol., 13: 619-623

Gould S. J., 1990 - Wonderful life. The Burgess Shale and the na-ture of history. Hutchinson Radius, London.

Hou X., Bergström J., 1997 - Arthropods of the Lower CambrianChengjang fauna, Southwest China. Fossils Strata, 45: 1-116.

Ivantsov A. Y., 1999 - Trilobite-like arthropod from the Lower Cam-brian of the Siberian Platform. Acta Palaentol. Pol., 44: 455-466.

Kraus O., Kraus M., 1994 - Phylogenetic system of the Tracheata(Mandibulata): on "Myriapoda"- Insecta interrelationship, phylo-genetic age, and primary ecological niches. Verh. Naturwiss Ver.Hambg., 34: 5-31.

Kraus O., Kraus M., 1996 - On myriapod-insect interrelationships.Mem. Mus. Nat. Hist. Nat. A, 169: 283-290.

Kukalova Peck J., 1992 - The Uniramia do not exist: the ground planof the Pterygota is revealed by Permian Diphanopterodea fromRussia [Insecta Paleodictyopteroidea]. Can. J. Zool., 70: 236-255 .

Martin M. W., Grazhdankin D. V., Bowring S. A., 2000 - Age ofneoproterozoic bilaterian body and trace fossils, White Sea, Rus-sia: Implications for metazoan evolution. Science, 288: 841-845.

Minelli A., 1998 - Segmented animals: origins, relationships andfunctions. Ital. J. Zool., 65: 1-4.

Moura G., Christoffersen M. L., 1996 - The system of the Mandibu-lata arthropods: Tracheata and Remipedia as sister groups;"Crustacea" non-monophyletic. J. Comp. Biol., 4: 95-113.

Nielsen C., 1997 - The phylogenetic position of the Arthropoda.In: R. A. Fortey & R. H. Thomas (eds), Arthropod relationships.Systematics association special volume, series 55. Chapman &Hull, London, pp. 11-22.

Panganiban G., Sebring A., Nagy L., Carrol S., 1995 - The devel-opment of crustacean limbs and the evolution of Arthropods.Science, 270: 1363-1366.

Ramsköld L., Edgecombe G. D., 1991 - Trilobite monophyly revis-ited. Hist. Biol., 4: 267-283.

Regier J. C., Shultz J. W., 1997 - Molecular phylogeny of the majorarthropod groups indicates polyphyly of crustaceans and a newhypothesis for the origin of hexapods. Mol. Biol. Evol., 14: 902-913.

Ripper W., 1931 - Versuch einer Kritik der Homologiefrage derArthropodentracheen. Z. Wiss. Zool., 138: 303-369.

Salvini Plaven L. von, 1981 - On the origin and evolution of theMollusca. In: Atti Conv. Lincei, no. 49. Accademia Nazionale deiLincei, Roma, pp. 247-293.

Salvini Plaven L. von, 1985 - Early evolution and the primitivegroups In: E. R. Trueman & M. R.Clarke (eds), The mollusca. Vol.10: Evolution. Academic Press, London, New York, pp. 59-150.

Schmidt-Rhaesa A., Ehlers U., Bartolomaeus T., Lemburg C.,Garey J. R., 1998 - The phylogenetic position of the Arthropoda.J. Morphol., 238 :263-285.

Schräm F. R., 1978 - Arthropods: a convergent phenomenon.Field. Geol., 39: 61-108.

Schräm F. R., Koenemann S., 2001- Developmental genetics andarthropod evolution: Part 1, on legs. Evol. Devel., 3: 343-354.

Shultz J. W., 1990 - Evolutionary morphology and phylogeny ofArachnida. Cladistics, 6: 1-38.

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

04:

38 0

6 O

ctob

er 2

014

264 A. M. SIMONETTA

Shear W. A., 1992 - End of the Uniramia taxon. Nature, 359: 477-478.Simonetta A., 1975 - Remarks on the origin of the Arthropoda. At-

ti Soc. Toscana. Sci. Nat. P. V. Mem. Ser. B, 82: 112-134.Simonetta A., 1992 - Problems of systematics: part 1. A critical

evaluation of the "species problem" and its significance in evo-lutionary biology. Boll. Zool., 59: 447-464.

Simonetta A., 1993 - Problems of systematics: part 2. Theory andpractice in phylogenetic studies and in systematics. Boll. Zool.,60: 324-334.

Simonetta A., 1999a - Is parsimony a justified principle in the as-sessment of possible phylogenies? Ital. J. Zool., 66: 159-169.

Simonetta A., 1999b - Una critica al pensiero dicotomico in si-stematica. Systema Naturae, 2: 55-65.

Simonetta A. M., Insom E., Pucci A., 2000 - Are the Chelicerata anatural group? Some problems of comparative anatomy. Mem.Soc. Entomol. Ital., 78: 233-250.

St0rmer L., 1944 - On the relationships and phylogeny of fossiland recent Arachnomorpha: a comparative study on Arachnida,Xiphosura. Eurypterida, Trilobita and other fossil Arthropoda.Nor. Vidensk. Akad. Oslo Arbok, 1: 1-158.

Wägele J. W., 1993 - Rejection of the Uniramia hypothesis and im-plications of the Mandibulata concept. Zool. Jahrb. Abt. Syst.Oekol. Geogr. Tiere, 120: 253-288.

Wägele J. W., 1996a - First principles of phylogenetic systematics,a basis for numerical methods used for morphological and mo-lecular characters. Vie Milieau, 46: 125-138.

Wägele J. W., 1996b - The theory and methodology of phyloge-netic systematics is still evolving: a reply to Wilson. Vie Milieau,46: 183-184.

Walossek D., 1999 - On the Cambrian diversity of Crustacea. In: F.R. Schram & K. von Vaupel Klein (eds), Crustaceans and thebiodiversity crisis. Brill, Leiden, pp. 3-27.

Walossek D., Müller K. J., 1990 - Stem-lineage crustaceans fromthe Upper Cambrian of Sweden and their bearing upon themonophyletic origin of Crustacea and the position of Agnostus.Lethaia, 23: 409-427.

Walossek D., Müller K. J., 1994 - Pentastomid parasites from the Low-er Palaeozoic of Sweden. Trans. R. Soc. Edimb. Earth Sci., 85: 1-37.

Walossek D., Müller K. J., 1998 - Early arthropod phylogeny inlight of the Cambrian "Orsten" fossils. In: G. Edgecombe (ed.),Arthropod fossils and phylogeny. Columbia University Press,New York, pp. 185-231.

Walossek D., Repeteski J. E., Müller K. J., 1994 - An exceptionallypreserved parasitic arthropod, Hiemonsicambria taylori n. sp.(Arthropoda incertae sedis: Pentastomida) from Cambrian-Or-dovician boundary beds of Newfoundland, Canada. Can. J.Earth Sci., 31: 1664-1671.

Wheeler W. C., Hiyashi C. Y., 1998 - The phylogeny of the extantchelicerate orders. Cladistics, 14: 173-192.

Williams J. A., Carroll S. B., 1993 - The origin, patterning and evo-lution of insect appendages. Bioessays, 15: 567-577.

Wills M. A., Briggs D. E. G., Fortey R. A., 1994 - Disparity as anevolutionary index: a comparison of Cambrian and recentarthropods. Palaeobiology, 20: 93-130.

Wills M. A., Briggs D. E. G., Fortey R. A., 1998 - Evolutionary cor-relates of arthropod tagmosis: scrambled legs. Syst. Assoc. Spec.Vol., 55: 57-65.

Wills M. A., Briggs D. E. G., Fortey R. A., Wilkinson M., 1995 -The significance of fossil in understanding arthropod evolution.Verh. Dtsch. Zool. Ges., 88: 203-215.

Wingstrand K. G., 1972 - Comparative spermatology of a pentas-tomid, Raillietiella hemidactyli, and a bachiuran crustacean, Ar-gulus foliaceus, with a discussion of pentastomid relationships.K. Dan. Vidensk. Selsk. Biol. Skr., 19: 1-72.

Yochelson E. L., 1996 - Discovery, collection, and description ofthe Middle Cambrian Burgess Shale biota by Charles DoolittleWalcott. Proc. Am. Philos. Soc., 140: 469-545.

Zrzavy J., Stys P., 1995 - Evolution of metamerism in Arthtropoda:developmental and morphological perspectives. Q. Rev. Biol.,70: 279-295.

Dow

nloa

ded

by [

Ast

on U

nive

rsity

] at

04:

38 0

6 O

ctob

er 2

014