before the Phanerozoic and became abundant during theEarly Cambrian (Kennard and James 1986).7 Stromatolites are rare after the Early Ordovician,except in marine supratidal and lacustrine settings.8 The preservation of microscopic microbial fabricsshowing traces of cell morphology in carbonate stro-matolites is rare in the Archaean and Proterozoic butcommon in the Phanerozoic; this seems to be yet anotherexample of the appearance of calcification near the baseof the Cambrian.

Within this framework many finer patterns have been recognized, and are used in biostratigraphy andpalaeoenvironmental interpretations. Stromatolite bio-stratigraphy has been a controversial field, but it hasbeen used now for more than 50 years, mostly in the Pro-terozoic, and the evidence that it works is compelling.

References

Banfield, J.F. and Nealson, K.H. (1997) Geomicrobiology: interac-tions between microbes and minerals. Reviews in Mineralogy,no. 35. Mineralogical Society of America, Washington DC.

Freytet, P. and Verrecchia, E.P. (1998) Freshwater organismsthat build stromatolites: a synopsis of biocrystallization byprokaryotic and eukaryotic algae. Sedimentology 45, 535–563.

Grotzinger, J.P. (1989) Facies and evolution of the Precambriancarbonate depositional systems: emergence of the modernplatform archetype. Special Publication of the Society of Eco-nomic Paleontologists and Mineralogists 44, 79–106.

Grotzinger, J.P. and Knoll, A.H. (1999) Stromatolites in Precam-brian carbonates: evolutionary mileposts or environmentaldipsticks? Annual Review of Earth and Planetary Sciences 27,313–358.

Hill, A.C., Al Arouri, K., Gorjan, P. and Walter, M.R. (in press)Sedimentology, paleobiology and paleoenvironments of acratonic carbonate platform: the Neoproterozoic BitterSprings Formation, Amadeus Basin, central Australia. SpecialPublication of the Society of Economic Paleontologists and Mineralogists.

Hofmann, H.J., Grey, K., Hickman, A.H. and Thorpe, R.I. (1999)Origin of 3.45 Ga coniform stromatolites in WarrawoonaGroup, Western Australia. Bulletin of the Geological Society ofAmerica 111, 1256–1262.

Kennard, J.M. and James, N.P. (1986) Thrombolites and stro-matolites: two distinct types of microbial structures. Palaios 1,492–503.

Walter, M.R., ed. (1976) Stromatolites. Developments in Sedi-mentology, no. 20. Elsevier, Amsterdam.

Walter, M.R. (1994) Stromatolites: the main source of informa-tion on the evolution of the early benthos. In: S. Bengtson, ed.Early life on Earth, pp. 270–286. Nobel Symposium no. 84.Columbia University Press, New York.

4.1.3 Plant Growth Forms and Biomechanics

T. SPECK and N.P. ROWE

Introduction

Palaeobotanical research aims to reconstruct fossilplants, their form and size, life histories, and ecosystems.In order to begin to understand the specific ecologicalniche occupied by a fossil plant species, its many struc-tural, physiological, and biochemical properties must beconsidered, in addition to biotic and abiotic environmen-tal factors. It is difficult to analyse fossil plants because ofinadequate preservation and the often disarticulatednature of the material. These problems make it particu-larly difficult to infer growth forms and the attitude offossil plants in their original habitat. Size, shape, andposture are important aspects of a plant’s growth formand can be used to infer a range of ecological preferencesof long-extinct plants. Based on experiments with livingplants, growth forms and maximum heights of fossilplants can be quantitatively investigated by calculatingthe biomechanical properties of stem segments from dif-ferent ontogenetic stages (Speck 1994; Speck and Rowe1999a).

Methods

In order to assess the bending mechanical properties ofstems, three fundamental parameters and their variationduring ontogeny must be considered: (1) the flexuralstiffness (EI) quantifies the ability of a stem to resistbending forces, and can be determined on the basis ofbending tests on living plants; (2) structural Young’smodulus (E) describes the stiffness of a stem in bending(the prefix ‘structural’ is used to emphasize that plantstems are inhomogeneous composite materials); and (3)the axial second moment of area (I) is a geometrical para-meter that quantifies the cross-sectional size and shapeof stems and tissues in relation to the direction ofbending. In plants with secondary growth I can be usedto approximate the ontogenetic sequence of stem devel-opment (Speck and Rowe 1999b).

Three main growth forms can be distinguished inliving woody plants by quantitative analyses of bio-mechanical properties and structural changes duringontogeny (Speck and Rowe 1999b): (1) ‘self-supporting’shrubs and trees show a significant increase in structuralYoung’s modulus during ontogeny, caused by anincrease in the amount of strengthening tissues (wood);(2) ‘semi-self-supporting’ (leaning) plants are character-

4.1 Fossils as Living Organisms 379

Palaeobiology IIEdited by Derek E.G. Briggs, Peter R. Crowther

Copyright © 2001, 2003 by Blackwell Publishing Ltd

ized by a structural Young’s modulus and stem structurethat do not change significantly during ontogeny; and(3) ‘non-self-supporting’ lianas show a significant dropof structural Young’s modulus during ontogeny, causedby a drastic reduction in the amount of strengtheningtissues. Detailed analysis of the variation in mechanicalproperties and stem structure of a wider range of plantsshows that this ontogenetic approach can reveal patternsof biomechanical behaviour characterizing a diversity ofgrowth forms (Speck and Rowe 1999a,b).

The method can be extended to fossil material: the relevant parameters are calculated from the composi-tion and arrangement of fossilized stem tissues (Fig.4.1.3.1) (Speck 1994; Speck and Rowe 1999a). A range of ontogenetic stages (diameters) of the fossil plant stem are required; anatomy must be well enough pre-served to allow the geometrical arrangement of thetissues and the structural features of cells comprisingeach stem tissue to be analysed quantitatively. By usingexperimentally measured data (Young’s moduli) fromsimilar extant tissues, the mechanical properties of dif-ferent ontogenetic stages of the fossil stem can be calcu-lated. Finally the growth form of the fossil plant can beinferred by comparing the patterns of variation inmechanical properties during ontogeny with thoseobserved among tested living plants with knowngrowth forms.

Results

The growth form of a variety of woody Palaeozoicspecies with secondary growth has been analysed,including Diaphorodendron vasculare, Pitus dayi,Lyginopteris oldhamia, and Calamopitys sp. (Speck 1994;Speck and Rowe 1994; Rowe and Speck 1998). In the firstthree plants the analysis was based on disconnected per-mineralized fragments from which a hypothesized onto-genetic sequence was constructed. With this type of data(termed ‘general data’ by Rowe and Speck 1998) it isimpossible to distinguish between young growth phasesof a species (i.e. seedlings or saplings) and young distalor regenerative shoots of a mature plant. However, thisdoes not affect the validity of the analysis, as studiesfrom a range of extant plants have shown (Speck andRowe 1999b). Diaphorodendron vasculare, a Carboniferousrhizomorphic lycopsid, has been used to test themethod, based on a reliable interpretation of its probablegrowth form and height. The calculated mechanical dataare consistent with experimental data from extant self-supporting trees in which the structural Young’smodulus increases significantly during ontogeny. In D.vasculare structural Young’s modulus increases by afactor of about 3.5 (Figs 4.1.3.2a and 4.1.3.3a).

The biomechanical approach also allows maximum

height to be calculated (Speck 1994). The calculatedmaximum height of D. vasculare of 12–20m (Speck 1994)is consistent with that of about 15m inferred from fossilmaterial. Pitus dayi comprises exceptionally well-preserved young branch segments of a Lower Carbonif-erous seed plant. Calculations showed unequivocallythat it was self-supporting (Figs 4.1.3.2b and 4.1.3.3b),suggesting that the isolated segments belong to a large-bodied arborescent plant. In P. dayi, some of the tissuescould not be matched with living tissue types andknown Young’s moduli. To overcome this problemmodels were constructed with varying Young’s modulusvalues representing different tissue types characteristicof the cortex/periderm. Despite the range of modelstested, all the results clearly showed that the plant wasself-supporting (Speck and Rowe 1994). Furthermore,large decorticated stumps and branches common in theLower Carboniferous of Scotland appear to represent thestems and basal branches of P. dayi, allowing estimates to

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Determination of the ontogenetic sequence from transverse sections(if only isolated segments are available)

Construction of models of transverse sections of the stemsquantitative analysis of size and shape of the different stem tissues

Measurement of structural features of the cells of each stem tissue:cell lumina, cell-wall thickness, cell length, structure of thewall thickenings

Correlation of each fossil tissue type with the most similar extanttissue type

recalculation of the mechanical parameters for the fossil tissues

By considering fossil plant stems as compound materials themechanical properties for the entire stem and for each stemtissue can be calculated

Calculations for stems of different ontogenetic stagesvariations of the mechanical properties during ontogenyreliable inferences concerning the growth habit of fossil plantsquantitative estimates of upper and lower mechanical limits ofthe maximum height of fossil plants

Fig. 4.1.3.1 Flow chart of the methodology used for analysingmechanical properties, functional anatomy, growth habit, andmaximum height of fossil plants.

be made of variations in the mechanical properties of theentire ‘Pitus-plant’ (Rowe and Speck 1998).

The growth forms of at least two other Carboniferouspteridosperms have been controversial (Lyginopteris oldhamia) or could be inferred only from stem size andanatomy (Calamopitys sp.). Lyginopteris oldhamia shows aslight decrease in structural Young’s modulus duringontogeny (Fig. 4.1.3.2c), a pattern that suggests that thisplant was not self-supporting (Speck 1994). The struc-tural Young’s moduli calculated for L. oldhamia are rela-tively high compared with those of extant lianas, anddecrease only to about 60% of their initial value, suggest-ing that L. oldhamia was a semi-self-supporting plant thatmay have become lianescent in old ontogenetic stages(Fig. 4.1.3.3c) (Speck and Rowe 1999b). The stem ofCalamopitys is consistent with a semi-self-supporting

growth form, as the structural Young’s modulus remainsnearly constant over the entire ontogenetic range tested(Figs 4.1.3.2d and 4.1.3.3d) (Rowe and Speck 1998). Datafor the stem of Calamopitys are consistent with that of asemi-self-supporting stem, although all the points arederived from a single slender axis. The trend towards thebase of the segment is not consistent with that of a self-supporting branch and appears to result from a con-straint on the size of the primary body during ontogeny.The specimen tested was a permineralized plant stem,nearly 75cm long, which provided an opportunity tostudy sections at known intervals along its length. Such‘positional data’ supply information on the finer detailsof adaptive growth, such as structural and mechanicalresponses of the plant to the local environment. Posi-tional data are particularly important for analysing the

4.1 Fossils as Living Organisms 381

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Fig. 4.1.3.2 Structural Young’s modulus (E) calculated forselected fossil plants plotted against the axial second momentof area (I) (double logarithmic plots). In plants with secondarygrowth, the axial second moment of area can be correlated withthe ontogenetic stage of the stems. (a) Diaphorodendronvasculare. (b) Pitus dayi. (c) Lyginopteris oldhamia. (d) Calamopityssp. Symbols characterize different ontogenetic stages: �,young ontogenetic stage; , middle-aged; �, old. The self-supporting growth form is characterized by a distinct increaseof structural Young’s modulus with ontogenetic stage, a data

pattern found for D. vasculare and P. dayi. The semi-self-supporting growth form is characterized by a (nearly) constantstructural Young’s modulus during ontogeny, as found inCalamopitys sp. The data for L. oldhamia show only a slightdecrease of structural Young’s modulus during ontogeny; thispattern is more compatible with extant semi-self-supportingplants than angiospermous lianas, which show dramaticdecreases in structural Young’s modulus of up to 95%. (a,c,From Speck 1994, with permission from Elsevier Science; b,from Speck and Rowe 1994; d, from Speck and Rowe 1999b.)

growth form of small-bodied herbaceous or pseudo-herbaceous fossil plants that lack significant secondarygrowth. Analyses of extant small-bodied plants withoutsecondary growth have shown that the axial diametersdo not necessarily reflect the age of a section or positionon the plant (Rowe and Speck 1998; Speck and Rowe1999b). Therefore, growth form cannot be inferred reli-ably from general data but depends on the availability ofpositional information.

In addition to analysing overall growth forms, the bio-mechanical method also permits calculations of the con-tributions made by different stem tissues to flexuralstiffness. This has been done for the plants mentionedabove as well as for a number of small-bodied Early andMiddle Devonian land plants. Such analyses have beenused to infer the mechanical importance of parenchyma(turgor-stabilized basic tissue of plants), vascularsystems (conducting tissue comprising xylem andphloem), and hypodermal steromes (strengtheningtissue, one to several cell layers thick beneath the epi-dermis) for support among early vascular land plants(Speck and Vogellehner 1994; Bateman et al. 1998). Thedata suggest that many of the earliest land plants withcylindrical axes relied on turgescent parenchyma tomaintain an upright posture, and that size and branch-ing were therefore limited. Plants with turgor-stabilized

axes, such as Aglaophyton major, Rhynia gwynne-vaughanii, Horneophyton lignieri, Zosterophyllumrhenanum, Asteroxylon mackiei, and Drepanophycus spinae-formis (Fig. 4.1.3.4a–f), were confined to habitats withcontinual and sufficient water supply and high humid-ity. With the colonization of water-limited habitats,hypodermal steromes became the most important stabil-izing tissues, as in Psilophyton dawsonii, Gosslingia breco-nensis, and Zosterophyllum llanoveranum (Fig. 4.1.3.4g,h).In practically all of these early plants, the stele was of noimportance for direct mechanical support, neither in the earlier turgor systems nor in later forms with hypodermal sterome. The only exception was theMiddle Devonian protolepidodendralean lycopsidLeclercqia complexa (Fig. 4.1.3.4i) in which the stele andhypodermal sterome both contributed significantly tothe mechanical stability of the stem. Studies of the typeof support, maximum height and branching frequency,and mechanical significance of individual tissues haveprovided important insights into the ecology of earlyland plants and constraints on their evolution (Batemanet al. 1998).

382 4 Palaeoecology

(a) (b)

20mm

(c) (d)

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Fig. 4.1.3.3 Reconstruction of the growth form of somebiomechanically analysed woody Palaeozoic plants. (a)Reconstruction of Diaphorodendron vasculare, a self-supportingtree with a maximum height of 12–20 m. (b) Diagram withschematic cross-sections of a self-supporting young branch of

Pitus dayi, probably belonging to a tree-like ‘Pitus-plant’. (c)Reconstruction of Lyginopteris oldhamia, a semi-self-supportingplant. (d) Diagram with schematic cross-sections of a semi-self-supporting stem segment of Calamopitys sp. (a, Redrawn fromDiMichele 1981, Palaeontogr. B175, 85–125.)

4.1 Fossils as Living Organisms 383

(a) (b) (c) (d)

(e) (f) (g) (h) (i)

Fig. 4.1.3.4 Reconstruction of the growth form of somebiomechanically analysed Early and Middle Devonian landplants: (a–f) plants with turgor-stabilized axes; (g,h) plantswith axes predominantly stabilized by hypodermal steromes.(a) Aglaophyton major. (b) Rhynia gwynne-vaughanii. (c)Horneophyton lignieri. (d) Zosterophyllum rhenanum. (e)Asteroxylon mackiei. (f) Drepanophycus spinaeformis. (g)Psilophyton dawsonii. (h) Gosslingia breconensis. (i) Leclercqiacomplexa, a plant where both the stele and the hypodermalsterome contributed a comparable amount to the mechanical

stability of the upright stems. (a, From Edwards 1986, Bot. J.Linn. Soc. 93, 173–204; b, from Edwards 1980, Rev. Palaeobot.Palynol. 29, 177–188, with permission from Elsevier Science; c, from Eggert 1974, Am. J. Bot. 61, 405–413; d,f, modified fromSchweitzer 1990, Pflanzen erobern das Land; e, from Stewart1983, Palaeobotany and the evolution of plants; g, from Banks et al.1975, Palaeontogr. Am. 8, 77–127; h, from Edwards 1970, Phil.Trans. R. Soc. Lond. B 258, 225–243; i, from Bonamo et al. 1988,Bot. Gaz. 149, 222–239, with permission from The University ofChicago Press.)

References

Bateman, R.M., Crane, P.R., DiMichele, W.A., Kenrick, P.R.,Rowe, N.P., Speck, T. and Stein, W.E. (1998) Early evolution of land plants: phylogeny, physiology, and ecology of theprimary terrestrial radiation. Annual Review of Ecology andSystematics 29, 263–292.

Rowe, N.P. and Speck, T. (1998) Biomechanics of plant growthforms: the trouble with fossil plants. Review of Palaeobotanyand Palynology 102, 43–62.

Speck, T. (1994) A biomechanical method to distinguishbetween self-supporting and non self-supporting fossilplants. Review of Palaeobotany and Palynology 81, 65–82.

Speck, T. and Rowe, N.P. (1994) Biomechanical analysis of

Pitus dayi: early seed plant vegetative morphology and itsimplications on growth habit. Journal of Plant Research 107,443–460.

Speck, T. and Rowe, N.P. (1999a) Biomechanical analysis. In:T.P. Jones and N.P. Rowe, eds. Fossil plants and spores: moderntechniques, pp. 105–109. Geological Society of London.

Speck, T. and Rowe, N.P. (1999b) A quantitative approach toanalytically defining size, form and habit in living and fossilplants. In: M. Kurmann and A.R. Hemsley, eds. The evolutionof plant architecture, pp. 447–479. Royal Botanic Gardens, Kew.

Speck, T. and Vogellehner, D. (1994) Devonische Landpflanzenmit und ohne hypodermales Sterom — Eine BiomechanischeAnalyse mit Überlegungen zur Frühevolution des Leit- und Festigungssystems. Palaeontographica B 233, 157–227.

4.1.4 Sessile Invertebrates

W.I. AUSICH and D.J . BOTTJER

Introduction

The ocean floor offers a myriad of benthic habitats differ-entiated by numerous parameters — physical, chemical,and biological. Some processes and attributes of theenvironment only establish basic conditions, whereasothers are limiting factors. Some have remained relat-ively constant through history, some have variedthrough time and space (i.e. penetration of sunlight andmineralogical composition of substrata), and othershave undergone secular change through geologicalhistory (i.e. predation pressure and tiering). Metazoansand other larger life forms evolved principally in benthichabitats during the Proterozoic, and the late Pro-terozoic–early Palaeozoic metazoan radiation was intimately linked to the resources, conditions, and limitations at and near the sediment–water interface.

Sessile invertebrates are treated here as those in whichadult organisms are permanently affixed (e.g. cementedoysters and encrusting bryozoans), passively affixedthrough a lack of mobility (e.g. free-living strophomenidbrachiopods and inoceramid bivalves), or effectivelysessile despite an ability to move episodically (e.g. pec-tinate bivalves and isocrinid crinoids). In addition, many organisms that are generally considered vagile,such as suspension-feeding ophiuroids and somedeposit-feeding bivalves, are effectively sessile wherefeeding is concerned.

History of tiering

Tiering is the vertical subdivision of space by organisms

within a community, and the processes responsible forthis organization include space, resources, and construc-tional constraints (Ausich and Bottjer 1982). ThePhanerozoic history developed here is for suspension-feeding communities on soft substrata, non-reef, shallowsubtidal, and epicontinental environments. The tieringhistory represents the maximum characteristic tieringheights and depths within tier subdivisions. Clearly, notall communities display the maxima, and taller organ-isms and deeper burrows did exist.

Later studies refined the history and defined the occu-pants of tiers (e.g. Bottjer and Ausich 1986). A distinctionwas made between primary and secondary tierers,where primary tierers are organisms whose bodies are incontact with the sediment–water interface and sec-ondary tierers are organisms that live either attached toprimary epifaunal tierers or in the burrows of infaunalprimary tierers (Bottjer and Ausich 1986). Tiering is nowone of the primary ways in which palaeoecologistsunderstand ancient epifaunal and infaunal commun-ities. This contribution presents a revised history ofprimary tiering among suspension feeders on soft sub-strata (Fig. 4.1.4.1).

Expansion of larger organisms during the Proterozoic

The first, larger multicellular benthic organisms were theEdiacaran biota that evolved and radiated during thelate Proterozoic. Regardless of the affinities of theseorganisms, the Ediacaran biota represents the initialestablishment of soft-substrata benthos into verticallydifferentiated epifaunal tiers. Three epifaunal tier sub-divisions are recognized: 0 to +5cm, +5 to +20cm, and+20 to +75cm. The characteristic maximum height of lateProterozoic tiers was +75cm, but as more data becomeavailable a higher level may be required (Jenkins 1992).Examples of occupants of these tiers from Ediacaranassemblages of South Australia include: 0 to +5cm tier,Arkarua; +5 to +20cm tier, Glaessnerina; and +20 to +75cmtier, Charniodiscus (see Jenkins 1992).

Unfortunately, this well-developed ecological struc-ture does not help to differentiate between competinghypotheses for the affinities of this biota. If erect forms,such as Charniodiscus, were cnidarians, then tieringwould have enabled differentiation of suspension-feeding levels, as was common throughout the Phanero-zoic. Alternatively, tiering within this biota could havebeen a function of competition for space or positioningfor photoreception.

Duality of Cambrian communities and Cambrian tiering

Analysis of Ediacaran organisms extends tiering into theProterozoic and, depending on their affinities, indicateseither a continuity or discontinuity between tiering in

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