Protaspides of uppermost Cambrian trilobite Missisquoia , with implications for suprafamilial level classification of the genus

Can. J. Earth Sci. 44: 493–506 (2007) doi:10.1139/E06-113 © 2007 NRC Canada

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Protaspides of uppermost Cambrian trilobiteMissisquoia, with implications for suprafamiliallevel classification of the genus

Dong-Chan Lee and Brian D.E. Chatterton

Abstract: The protaspides of Missisquoia depressa (Missisquoiidae, Trilobita), from the uppermost Cambrian part ofthe Rabbitkettle Formation, Mackenzie Mountains, northwestern Canada, are described. Three metaprotaspid stages arerecognized, using size as well as features of the anterior border of the cranidium and the shape of the glabella. Themorphology of the protaspides is not typical of the Order Corynexochida, suggesting that Missisquoia is not a memberof the order to which the genus has been previously assigned. This further indicates that its affinity to thestratigraphically younger styginids and illaenids is questionable.

Résumé : Une description des protaspides de Missisquoia depressa (Missisquoiidae, Trilobita), du Cambrien supérieurde la Formation de Rabbitkettle, des monts Mackenzie (Nord-Ouest canadien), est présentée. Les dimensions ainsi queles caractéristiques de la bordure antérieure du cranidium et la forme de la glabelle permettent de distinguer trois sta-des de métaprotaspides. La morphologie des protaspides n’est pas typique de l’ordre des Corynexochida, ce qui laissecroire que Missisquoia n’appartient pas à l’ordre auquel ce genre avait antérieurement été affecté. Cela jette en retourun doute sur son affinité avec les styginides et les illaenides stratigraphiquement plus jeunes.

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Introduction

Ludvigsen (1982) reported the occurrence of twoMissisquoia species, M. depressa and M. mackenziensis, inthe Rabbitkettle Formation, Mackenzie Mountains, northwest-ern Canada. Missisquoia is a biostratigraphically importanttrilobite taxon, used to correlate strata within Laurentia(Winston and Nicholls 1967; Stitt 1971, 1977; Ross 1982;Loch et al. 1993; Ross et al. 1997) and between Laurentianstrata and those of Gondwana (Shergold 1988; Geyer andShergold 2000). However, a variety of contrasting opinionshave been expressed on the taxonomy of Missisquoia at thegeneric level (Fortey 1983; Westrop 1986; Jell in Jell andAdrain 2003) and at higher taxonomic levels (Shergold 1975;Ludvigsen 1982; Lane and Thomas 1983), indicating theproblems of classifying this genus.

Protaspides of Missisquoia depressa from the RabbitkettleFormation, northwestern Canada are figured and documented.Previous taxonomic opinions on Missisquoia are re-evaluatedin light of the protaspid morphology described herein, focus-ing on its suprafamilial classification. All the specimens arestored in University of Alberta Paleontology Collection (UAnumbers: Department of Earth and Atmospheric Sciences,University of Alberta, Edmonton, AB T6G 2E3, Canada).

The specimens were collected by the junior author andprocessed by the senior author; the limestone samples werecollected from horizon “KK-116” belonging to theMissisquoia depressa subzone of the Parabolinella Zone inLudvigsen (1982). The Missisquoia depressa subzone is re-garded as the uppermost part of the Upper Cambrian, sinceCooper et al. (2001) placed the base of the Ordovician at thebase of the Iapetagnathus fluctivagus Zone which is locatedabove the base of Missisquoia depressa subzone. Detailedgeographical information on the sampling locality is foundin Ludvigsen (1982, fig.1). Some protaspid specimens ofMissisquoia, described by Hu (1971), and some others fromthe same locality, were borrowed from the Cincinnati Mu-seum Center, Cincinnati, Ohio (CMC-P numbers), and com-pared with specimens from the Rabbitkettle Formation toinvestigate whether they are correctly associated.

Tectonic distortion and association ofprotaspides

Silicified specimens from the Rabbitkettle Formation aredistorted in varying amounts. Some specimens seem to havebeen compressed or extended along a single plane, but othersare distorted in more complicated ways. Tectonic force

Received 1 May 2006. Accepted 5 October 2006. Published on the NRC Research Press Web site at http://cjes.nrc.ca on25 May 2007.

Paper handled by J. Jin.

D-C. Lee. Department of Museum, Daejeon Health Sciences College, 77-3, Gayang2-Dong, Dong-Gu, Daejeon, 300-711, SouthKorea.B.D.E. Chatterton.1 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada.

1Corresponding author (e-mail: [email protected]).

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(pressure or stress) caused the distortion of the specimensafter burial. Such distortion causes problems in reconstruct-ing original morphology correctly, and in assigning speci-mens correctly to a species. To examine the effects oftectonic forces on some of the Rabbitkettle trilobites, a spec-imen was selected that is distorted solely by unidirectionalcompression (Fig. 1.1). Its image was then distorted in vari-ous angles and directions using the “Distort” tool in AdobePhotoshop version 7.0 (Figs. 1.2–1.4). It was found possibleby this means to produce distorted images that are very sim-ilar to undistorted images of other specimens recovered from

the same limestone block (compare Fig. 1.2 with Fig. 4.18,1.3 with 4.23, and 1.4 with 5.16). Two pygidia and acranidium were distorted in the same way (Figs. 1.5–1.10).The distorted images are again similar to actual specimens.This allows us to be confident in assigning all of these spec-imens to the same species.

Not all specimens were deposited parallel to the planealong which the maximum distortion took place; in otherwords, specimens were deposited at various angles to thedirection of maximum pressure or stress. Such a randomthree-dimensional orientation of specimens results in com-plicated morphologies. Furthermore, if pressure or stress onthe specimens were applied during multiple tectonic events,specimens would be distorted in more complicated ways. Forexample, some specimens show a very distinct ocular ridge(Fig. 5.25) compared with others. This “ocular ridge” islikely to have been caused by tectonic buckling of the ante-rior portion of the specimen, rather than be the remnant ofan original morphological characteristic. It is concluded thatmorphological differences such as variation in length versuswidth ratios observed in many specimens are more due to tec-tonic distortion than original variation. The reconstructions ofprotaspides and meraspid degree 1 of Missisquoia depressa(Figs. 2.1–2.5) are based on the specimens that are consid-ered to be least deformed, and only deformed along a singleplane parallel to the original horizontal plane on which thespecimens were deposited (perpendicular to the direction ofmaximum pressure). This type of distortion is very commonin trilobites deposited in outer detrital facies due to compres-sion as the result of dewatering of shale and the weight ofsucceeding layers of sediments during lithification, even inregions that have not undergone significant tectonism.

Division of protaspid and meraspidontogeny into stages

Due to the effects of tectonic distortion, conventional lengthversus width plots that are normally used to identify or dis-criminate ontogenetic instars would be unreliable for thispurpose. Morphologic features such as distinctive glabellar

Fig. 1. Distortions of the specimens from the Rabbitkettle Formation.(fig. 1.1) Original metaprotaspid specimen that is considered tohave been deformed only laterally in a single planar direction,UA8137, × 50. (fig. 1.2) With Adobe Photoshop, the specimenimage in fig 1.1 is compressed along the axis. The compressedimage is similar to the specimen illustrated in fig. 4.18. (fig. 1.3)With Adobe Photoshop, the specimen image is axially compressedand then laterally sheared off. The sheared image is similar tothe specimen illustrated in fig. 4.23. (fig. 1.4) With Adobe Photoshop,the specimen image is sheared along the axis. The sheared imageis similar to the specimen illustrated in fig. 4.16. (fig. 1.5) Originalpygidial specimen, UA8138, × 15. (fig. 1.6) Original pygidialspecimen, UA8139, × 14. (fig.1.7) With Adobe Photoshop, thespecimen image in fig. 1.5 (UA8138) is compressed along theaxis, which is similar to fig. 1.6. (fig. 1.8) With Adobe Photoshop,the specimen image in fig. 1.6 (UA8139) is extended along the axis,which is similar to fig. 1.5. (fig. 1.9) Original cranidial specimen,UA8140, × 14. (fig. 1.10). With Adobe Photoshop, the specimenimage from fig. 1.9 (UA8140) is laterally sheared.

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Fig. 2. Reconstruction of protaspides and meraspid degree 1 of Missisquoia depressa. The reconstruction is based on the specimens that are considered to be only distortedalong a single plane and by unidirectional force. All drawings are × 60. (2.1) Early metaprotaspid stage, dorsal view. (2.2) Intermediate metaprotaspid stage, dorsal view. (2.3)Late metaprotaspid stage, dorsal view. (2.4) Meraspid degree 1 stage, dorsal view. (2.5) Meraspid degree 1 stage, ventral view.

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shapes and differentiation of an anterior cranidial border,among others, are used herein to divide the protaspid ontogenyinto three stages: early, intermediate, and late. Since eventhe smallest protaspid specimens (Figs. 3.1–3.5) securedhave a differentiated, but not released, protopygidium, allprotaspides available are considered metaprotaspides. Theseparation between the late protaspid stage and degree 0meraspides is based on observation of ventral surfaces, tosee whether the pygidial region is clearly separated from thecranidial region (see “Systematic paleontology” for detailedobservation).

Systematic paleontology

?Order Corynexochida Kobayashi, 1935?Suborder Leiostegiina Bradley, 1925Family Missisquoiidae Hupé, 1953Genus Missisquoia Shaw, 1951

REMARKS: The generic concept of Missisquoia has been dis-cussed by Ludvigsen (1982), Fortey (1983), Zhou and Zhang(1984), and Westrop (1986). Many genera have been assignedto the family Missisquoiidae and many of them have beensynonymized into Missisquoia. Ludvigsen (1982) synonymizedthe following genera with Missisquoia: Lunacrania Kobayashi,1955, Macroculites Kobayashi, 1955, Rhamphopyge Kobayashi,1955, Tangshanaspis Zhou and Zhang, 1978, and ParanumiaHu, 1973. Fortey (1983) synonymized Missisquoia withParakoldinioidia Endo in Endo and Resser, 1937. Zhou andZhang (1984) synonymized Pseudokoldinioidia Endo, 1944with Missisquoia. Westrop (1986) considered Missisquoia asa valid separate genus from Parakoldinioidia. It is apparentthat the concept of Missisquoia has been confused and needsto be rigorously investigated, which is beyond the scope of

this study. The generic concept of Missisquoia in the scopeof the family Missisquoiidae will be reviewed elsewhere.

Missisquoia depressa Stitt, 1971(Figs. 1, 2, 3.1–3.5, 3.11–3.12, 4–7)

SYNONYMY: Missisquoia depressa Stitt, 1971, p. 25, pl. 8,figs. 5–8

Missisquoia depressa, Ludvigsen, 1982, p. 121, figs. 42,43, 65, 66A–66G

Missisquoia depressa, Westrop, 1986, p. 67, figs. 30–34

HOLOTYPE: An incomplete cranidium from the Signal Moun-tain Limestone, Joins Ranch Section, Arbuckle Mountains,Oklahoma, illustrated by Stitt (1971, pl. 8, fig. 5).

STRATIGRAPHIC AND PALEOGEOGRAPHIC DISTRIBUTION: Alberta,Canada (Westrop 1986); Mackenzie Mountains, Canada(Ludvigsen 1982); Oklahoma, USA (Stitt 1971).

REMARKS: Ludvigsen (1982) synonymized Tangshanaspiszhaogezhuangensis from China into this species. The Chi-nese cranidia figured by Zhou and Zhang (1978, 1984) aresimilar to those of Missisquoia depressa. The Chinesepygidia (Zhou and Zhang 1978, pl. 16, fig. 23; Zhou andZhang 1984, pl. 20, fig. 14; Duan et al. 1986, pl. 3, fig. 26)have long pleural spines, which are absent in M. depressa(see Figs. 7.4, 7.15). This difference indicates thatT. zhaogezhuangensis is separate from M. depressa.

DESCRIPTION OF ONTOGENy:

Early metaprotaspid stage (Figs. 2.1, 3.1–3.5, 3.11, 3.12):Exoskeleton sub-oval in outline. Axis spindle-shaped. Glabellareaches anterior exoskeletal margin; anterior cranidial bordernot differentiated. Occipital ring moderately distinct. Poste-rior cephalic marginal furrows very faintly impressed. Proto-

Fig. 3. Metaprotaspid specimens figs. 3.1–3.5, 3.11, and 3.12 are from KK-116 sampling horizon of the Rabbitkettle Formation, Mac-kenzie Mountains, northwestern Canada. The rest of the specimens are from the Deadwood Formation exposed in Wyoming, USA.(figs. 3.1–3.3) Early metaprotaspid stage of Missisquoia depressa, UA8141, × 100, fig. 3.1, dorsal view; fig 3.2, lateral view; fig. 3.3,ventral view. (figs. 4, 5) Early metaprotaspid stage of Missisquoia depressa, UA8142, × 100, fig. 4.4, dorsal view; fig. 5, ventral view.(fig. 3.6–3.9) Metaprotaspid specimen of ?Missisquoia cyclochila, CMC-P 38749e, × 100, fig. 3.6, dorsal view; fig. 3.7, lateral view;fig. 3.8, posterior view; fig. 3.9, anterior view. (fig. 3.10) Metaprotaspid specimen of ?Missisquoia cyclochila, CMC-P 38740d, dorsalview, × 100. (figs. 3.11, 3.12) Early protaspid stage of Missisquoia depressa, UA8143, × 100, fig. 3.11, ventral view; fig. 3.12, dorsalview. (fig. 3.13) Small cranidium of Missisquoia typicalis, CMC-P 38749o, dorsal view, × 50. (fig. 3.14) Small cranidium of Missisquoiatypicalis, CMC-P 38749p, dorsal view, × 50. (fig. 3.15) Small cranidium of Missisquoia typicalis, CMC-P 38749r, dorsal view, × 25.(fig. 3.16) Metaprotaspid specimen of ?Missisquoia depressa, CMC-P 38740a, dorsal view, × 100. (fig. 3.17–3.20). Metaprotaspid specimenof ?Missisquoia depressa, CMC-P 38740b, × 100, fig. 3.17, dorsal view; fig. 3.18, lateral view; fig. 3.19, posterior view; fig. 3.20,anterior view. (fig. 3.21) Small cranidium of Missisquoia cyclochila, CMC-P 38749h, dorsal view, × 50. (fig. 3.22) Small cranidium ofMissisquoia cyclochila, CMC-P 38749j, dorsal view, × 50. (fig. 3.23) Metaprotaspid specimen of ?Missisquoia cyclochila, CMC-P 38740e,dorsal view, × 100. (fig. 3.24) Metaprotaspid specimen of Ptychoparioidea sp. A, CMC-P 38749g, dorsal view, × 100. (fig. 3.25)Metaprotaspid specimen of Ptychoparioidea sp. A, CMC-P 38749d, dorsal view, × 100. (fig. 3.26) Metaprotaspid specimen ofPtychoparioidea sp. B, CMC-P 38749f, dorsal view, × 100. (fig. 3.27) Large pygidium assigned to Missisquoia cyclochila, CMC-P 38749y,dorsal view, × 25. (fig. 3.28) Large pygidium assigned to Missisquoia cyclochila, CMC-P 38749w, dorsal view, × 25. (fig. 3.29)Metaprotaspid specimen of Ptychoparioidea sp. A, CMC-P 41556k, dorsal view, × 100. (fig. 3.30). Metaprotaspid specimen of?Missisquoia cyclochila, CMC-P 41556l, dorsal view, × 100. (fig. 3.31) Large cranidium assigned to Missisquoia cyclochila, CMC-P 38749,dorsal view, × 25. (fig. 3.32) Anaprotaspid specimen of Ptychoparioidea sp. A, CMC-P 38749a, dorsal view, × 100. (fig. 3.33)Anaprotaspid specimen of Ptychoparioidea sp. A, CMC-P 38749b, dorsal view, × 100. (fig. 3.34) Small pygidium assigned toMissisquoia cyclochila, CMC-P 38749v, dorsal view, × 25. (fig. 3.35) Metaprotaspid specimen of Ptychoparioidea sp. C, CMC-P 38740c,dorsal view, × 100. (fig. 3.36) Metaprotaspid specimen of Ptychoparioidea sp. D, CMC-P 41556m, dorsal view, × 100. (fig. 3.37)Metaprotaspid specimen of Ptychoparioidea sp. E, CMC-P 38749c, dorsal view, × 100.

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pygidial margin widely and shallowly indented sagittally; noaxial rings recognized.

Intermediate metaprotaspid stage (Figs. 2.2, 4.1–4.17):Exoskeleton sub-oval in outline. Glabella sub-rectangular inoutline; three pairs of glabellar furrows faintly impressed.

Anterior cranidial border narrow sagittally and exsagittally.Anterior pits weakly impressed. Posterior cranidial borderfurrows shallow and widen distally. Two to three axial rings,including terminal piece in protopygidium.

Late metaprotaspid stage (Figs. 2.3, 4.18–4.29, 5.1–5.15,

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5.18, 5.20, 5.23, 5.25, 5.28): Exoskeleton elongated oval inoutline. Anterior cranidial border furrow wide sagittally andexsagittally. Glabella subcylindrical, but slightly expandingforward. Palpebral lobe slightly convex outwards. Posteriorcranidial border furrows widen distally. Librigenae blade-shaped, with short genal spine. Protopygidium with three tofour axial rings, including terminal piece. Posterior protopy-gidial margin indented. Posterior cranidial edge inturned insome specimens.

Meraspid degree 0 and 1 (Figs. 2.4, 2.5, 4.30, 4.31, 5.16,5.17, 5.19, 5.21, 5.22, 5.24, 5.26, 5.27, 6.1–6.6, 6.9, 6.10):Exoskeleton elongated oval in outline. Glabella subcylin-drical in shape and expanding forward; glabellar front in-dented sagittally. Anterior cranidial border wide (sagitally)and flat, without distinctively incised border furrow. Palpebrallobe large and proximally defined by shallow and widepalpebral furrow. Ocular ridge faintly developed. Three pairsof glabellar furrows weakly impressed. Anterior pits wide.Posterior cranidial border wide exsagittally. Hypostomeelongated, waisted and shield shaped; pair of short spinespresent at posterior lateral corner. Rostral plate transverselyelongated, inverted subtrapezoid in outline, and as trans-versely wide as hypostome. Free cheek with short genalspine with blunt tip. Transitory pygidium with five to sixdistinct axial rings and terminal piece; posterior marginindented medially. Transitory pygidial border flat and wideand defined by slope change representing border furrow;pleural furrows wider than interpleural furrows; transitorypygidial lateral margin weakly saw-toothed in region ofprotothoracic segments.

Later meraspid and holaspid degrees (Figs. 6.7, 6.8, 6.11–6.26, 7): Glabellar furrows deeper. Fourth pair of glabellarfurrows impressed. Genal spine becomes longer. Anteriorcranidial border furrows separated from anterior palpebrallobe margin. Posterior pygidial margin entire.

Articulation between trunk and cephalonduring early ontogeny of Missisquoiadepressa

The protopygidium of metaprotaspides is demarcated fromthe cranidium by a posterior cranidial marginal furrow.Meraspid degree 0 stage commences with the protopygidiumbeing released from the cephalon to become a transitory

pygidium. At this stage, the transitory pygidium and cranidiumare considered to have been connected by a flexible integu-ment and articulated with each other when the individualwas alive.

The topographical configuration of the articulatory boundarybetween adjacent free segments of most trilobites is that theposterior margin of the more anterior segment is ventrallyinturned as doublure and the anterior margin of the moreposterior segment lies underneath the doublure as anarticulatory half ring in the axial region, and as a flange inthe pleural region (see Fig. 8.3 for cross-sectional view). Anidentical configuration between the protopygidium and thecephalon is found in the meraspid degree 0 stage ofMissisquoia depressa (Fig. 5.22 for lateral view and 5.26 forventral view). The metaprotaspid ontogeny of M. depressadisplays how the topographical configuration would haveproceeded. Intermediate metaprotaspid specimens have asimple posterior cephalic marginal furrow that is ventrallyprojected as a transverse ridge (see Figs. 4.5, 4.14, 4.16).Some late metaprotaspid specimens display the same furrowconfiguration (see Figs. 5.2, 5.6; see also Fig. 8.1). Othermetaprotaspid specimens, however, display a different con-figuration, where the furrow ventrally deepens, pointing for-ward; this condition is clearly shown in lateral extremity ofposterior cephalic region (see Figs. 4.24, 4.27, 5.11; see alsoFig. 8.2). This difference may be due to distortion. As a mat-ter of fact, specimens that are considered compressed alongthe anterior–posterior axis (for example, Figs. 4.18–4.29)appear to have deeper furrows and more highly raised bor-ders and pleural ribs. However, some less-distorted speci-mens also show a ventrally deepening and forward-directedposterior cephalic marginal furrow (for example, Figs. 5.11,5.28). Thus, the two different types of configuration mayrepresent the ontogenetic pathway through which the articu-lation between the protopygidium and cephalon proceeds.The deep invagination of the exoskeleton (Fig. 8.2) indicatesthat articulation between the protopygidium and cephalonmay be underway even during the protaspid period. The ar-ticulation proceeds from a simple furrow, pre-articulatoryinvagination, finally to true articulation with the two partsseparated (Figs. 8.1–8.3).

Other Missisquoiid protaspides

Hu (1971) described protaspides of Missisquoia cyclochilafrom the Lower Ordovician Deadwood Formation exposed

Fig. 4. Metaprotaspides and meraspid degree 0 of Missisquoia depressa. All specimens are from KK-116 sampling horizon of RabbitkettleFormation, Mackenzie Mountains, northwestern Canada. All specimens are × 50. (figs. 4.1, 4.2) Intermediate metaprotaspid stage,UA8144, fig.4.1, dorsal view; fig. 4.2, ventral view. (figs. 4.3–4.6) Intermediate metaprotaspid stage, UA8145, fig. 4.3, dorsal view;fig. 4, lateral view; fig. 4.5, ventral view; fig. 4.6, anterior view. (figs. 4.7, 4.8, 4.14). Intermediate metaprotaspid stage, UA8146,fig. 4.7, lateral view; fig. 4.8, dorsal view; fig. 4.14, ventral view. (figs. 4.9, 4.10) Intermediate metaprotaspid stage, UA8147, fig. 9,dorsal view; fig. 10, ventral view. (figs. 4.11–4.13) Intermediate metaprotaspid stage, UA8148, fig. 4.11, lateral view; fig. 4.12, dorsalview; fig. 4.13, ventral view. (figs. 4.15–4.17) Intermediate metaprotaspid stage, UA8149, fig. 4.15, dorsal view; fig. 4.16, ventralview; fig. 4.17, lateral view. (figs. 4.18, 4.19) Late metaprotaspid stage, UA8150, fig. 418, dorsal view; fig. 4.19, ventral view.(figs. 4.20, 4.25) Late metaprotaspid stage, UA8151, fig. 4.20, ventral view; fig. 4.25, dorsal view. (figs. 4.21, 4.22) Late metaprotaspidstage, UA8152, fig. 4.21, dorsal view; fig. 4.22, ventral view. (figs. 4.23, 4.24) Late metaprotaspid stage, UA8153, fig. 4.23, dorsalview; fig. 4.24, ventral view. (figs. 4.26–4.29) Late metaprotaspid stage, UA8154, fig. 4.26, dorsal view; fig. 4.27, ventral view;fig. 4.28, lateral view; fig. 4.29, anterior view. (figs. 4.30, 4.31) Meraspid degree 0, UA8155, fig. 4.30, dorsal view; fig. 4.31, ventralview.

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in northeastern Wyoming, and he figured seven protaspidspecimens. Westrop (1986) regarded this species as a juniorsynonym of Missisquoia typicalis, implying that the smallerWyoming specimens are earlier growth stages of M. typicalis.The relatively large Wyoming cranidium (e.g., Fig. 3.31) issimilar to an Albertan cranidium (Westrop 1986, pl. 1, fig. 37)and cranidia of M. typicalis from other localities (e.g., Stitt1971, pl. 8, fig. 1). However, the subcircular pygidia lackingmarginal spines (Figs. 3.27, 3.28) differ from the subtri-angular pygidia of M. typicalis that possess short, distinctmarginal spines (e.g., Westrop 1986, pl. 1, fig. 35). The on-togeny of Missisquoia depressa demonstrates that thepygidium develops a maximum of two pairs of marginalspines in the holaspid period, while the holaspid pygidiumof M. typicalis develops a maximum of six pairs of spines,indicating different ontogenetic pathways. In addition, thespines of M. depressa and M. cyclochila appear to be a modi-fication of the marginal border, whereas those of M. typicalisare an extension of pygidial pleurae. M. cyclochila is re-garded as a separate species from M. typicalis.

Seven protaspid specimens figured by Hu (1971; Figs. 3.6–3.9, 3.24–3.26, 3.32, 3.33, 3.37) display morphological dis-parity that does not support their association within a singlespecies. Specimen CMC-P 38749e (Figs. 3.6–3.9), a meta-protaspis, is distinguished from other specimens in having aparallel-sided axis. It shows the most gradual morphologicaltransformation from the smallest cranidium (Fig. 3.21) bysharing a parallel-sided axis and glabella. The larger cranidia(Figs. 3.13, 3.14, 3.22, 3.15) also show a gradual morpho-logical transformation in size to the largest cranidium(Fig. 3.31).

Specimens CMC-P 38749a (Fig. 3.32) and CMC-P 38974b(Fig. 3.33), although poorly preserved, clearly show longitu-dinal subdivision (bilobation) of two lobes in the axis thatare likely to be L2 and L3. Specimens CMC-P 38749d(Fig. 3.25) and CMC-P 38749g (Fig. 3.24) are characterizedby a spindle-shaped axis with two bilobed axial lobes. Itseems possible that these four specimens belong to a singleontogenetic sequence, since they all have a bilobed, spindle-shaped axis. Specimen CMC-P 38749c (Fig. 3.37) is charac-terized by having a strongly annulated axial lobe. SpecimenCMC-P 38749f (Fig. 3.26) has a subquadrate exoskeletaloutline and relatively narrow transverse (tr.) and forward-tapering axis. Each of these two specimens clearly belongsto a different ontogenetic series.

Hu (1971, 1973) described protaspides of Highgatellafacila and Paranumia triangularia from what appears to be

the same locality and figured five and three protaspidspecimens for these species, respectively. Of the protaspidesassigned to H. facila, specimens CMC-P 38740d (Fig. 3.10)and CMC-P 38740e (Fig. 3.23) are similar to one anotherand, in turn, are similar to specimen CMC-P 38749e(Figs. 3.6–3.9). Specimen CMC-P 38740b (Figs. 3.17–3.20)is distinguished from the other protaspides by having themost elongated exoskeleton and a well developed posteriorcranidial border furrow. Specimen CMC-P 38740a (Fig. 3.16)appears to be an earlier stage of metaprotaspid CMC-P 38740b.These two specimens are similar to the early metaprotaspidspecimens of Missisquoia depressa (Figs. 3.1–3.5, 3.11, 3.12)in having an elongated exoskeleton. Specimen CMC-P 38740c(Fig. 3.35) is characterized by having a transversely narroweraxis and distinct transglabellar furrows. Of the protaspidesassigned to P. triangularia, specimen CMC-P 41556k(Fig. 3.29) is most similar to specimens CMC-P 38749d andCMC-P 38749g (Figs. 3.24, 3.25). Specimen CMC-P 41556l(Fig. 3.30) is similar to CMC-P 38749e. Specimen CMC-P41556m (Fig. 3.36) is characterized by having a strongly ta-pering, spindle-shaped, bilobed axis and at least two pairs ofshort fixigenal spines.

Highgatella facila was synonymized into Apoplanias rejectusby Ludvigsen (1982). Most olenids, including Apoplanias,have a strongly annulated glabella during the protaspid andearly meraspid periods (for example, Olenus gibbosus andAcerocare ecorne; see Hu 1971, pl. 18, figs. 6–10, pl. 19,figs. 5–8). None of the Wyoming protaspid specimens hassuch an annulated glabella or axis. The figured smallestcranidium (Hu 1971, pl. 21, fig. 6) of A. rejectus is typicalof the olenid. Paranumia was regarded as a subjective syn-onym of Missisquoia by Ludvigsen (1982). All the cranidiaof Paranumia triangularia figured by Hu (1973) have a for-ward-tapering glabella.

The four Wyoming protaspid specimens, CMC-P 38749e,CMC-P 38740d, CMC-P 38740e, and CMC-P 41556l mayrepresent protaspides of Missisquoia cyclochila; they arequestionably assigned to this species. They are considerablysmaller than early protaspides of Missisquoia depressa(Figs. 3.1–3.5, 3.11, 3.12), and differ in having a parallel-sided axis. Specimens CMC-P 38740a and CMC-P 38740bseem to display morphologies that transform into the earlyprotaspides of M. depressa in the most acceptable range;they are questionably assigned to this species. Better con-trolled sampling is required to assess these possible associations.Other protaspides, showing a generalized ptychoparioidprotaspid morphology (Chatterton and Speyer in Whittington

Fig. 5. Metaprotaspides and meraspids degree 0 of Missisquoia depressa. All specimens are from KK-116 sampling horizon ofRabbitkettle Formation, Mackenzie Mountains, northwestern Canada. All specimens are × 50, unless otherwise noted.(figs. 5.1, 5.2)Late metaprotaspid stage, UA8156, fig. 5.1, dorsal view; fig. 5.2, ventral view. (figs. 5.3, 5.4) Late metaprotaspid stage, UA8157, fig.5.3, dorsal view; fig. 5.4, lateral view. (figs. 5.5, 5.6, 5.12) Late metaprotaspid stage, UA8158, fig. 5.5, dorsal view; fig. 5.6, ventralview; fig. 5.12, posterior view. (figs. 5.7, 5.8, 5.14, 5.15) Late metaprotaspid stage, UA8159, fig. 5.7, lateral view; fig. 5.8, dorsalview, fig. 5.14, anterior view; fig. 5.15, ventral view. (figs. 5.9–5.11, 5.13) Late metaprotaspid stage, UA8160, fig. 5.9, dorsal view;fig. 5.10, lateral view; fig. 5.11, ventral view; fig. 5.13, anterior view. (figs. 5.16, 5.17) Meraspid degree 0, UA8161, fig. 5.16, dorsalview; fig. 5.17, ventral view. (figs. 5.18, 5.23, 5.28) Late metaprotaspid stage, UA8137, fig. 5.18, anterior view; fig. 5.23, dorsal view;fig. 5.28, ventral view. (figs. 5.19, 5.21, 5.24) Meraspid degree 0, UA8162, fig. 5.19, ventral view; fig. 5.21, lateral view; fig. 5.24,ventral view of hypostome, × 200. (figs. 5.20, 5.25) Late metaprotaspid stage, UA8163, fig. 5.20, ventral view; fig. 5.25, dorsal view.(figs. 5.22, 5.26, 5.27) Meraspid degree 0, UA8164, fig.5.22, lateral view; fig.5.26, ventral view; fig. 5.27, dorsal view.

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et al. 1997) are tentatively assigned to the Ptychoparioidea:CMC-P 38749a, CMC-P 38749b, CMC-P 38749d, CMC-P38749g, and CMC-P 41556k are assigned to Ptychoparioideasp. A; CMC-P 38749f to Ptychoparioidea sp. B; CMC-P38740c to Ptychoparioidea sp. C; CMC-P 41556m to Ptycho-parioidea sp. D; CMC-P 38749c to Ptychoparioidea sp. E.

Comparison of Missisquoia protaspides andsuprafamilial taxonomy

Various opinions have been proposed for the suprafamilialtaxonomy of Missisquoia. Shergold (1975, p. 195) notedcranidial similarities to the Leiostegiidae and assigned theMissisquoiidae to the superfamily Leiostegiacea (see alsoShergold 1988). Ludvigsen (1982, p. 119) suggested that thefamily was the ancestor to the Styginidae, which in turn ap-pears to be ancestral to the Illaenidae and Scutelluidae; theScutelluidae is now regarded as a junior synonym of theStyginidae (Fortey in Whittington et al. 1997) or as a sub-family of the Styginidae (Whittington 2000). He presentedthe similarities between Missisquoia depressa and Peri-schoclonus capitalis (Whittington 1963, pl. 22, figs. 4, 13)as evidence. Later, Lane and Thomas (1983, p. 155) contra-dicted Ludvigsen’s view and claimed that the cephalicmorphologies of the Missisquoiidae are much more similarto Cambrian Corynexochida than post-Cambrian Scutelluina(= Illaenina of Fortey in Whittington et al. 1997). In particu-lar, they claimed that the morphologies of the rostral plateand pygidium are readily distinguished from those of thestyginids. The characteristic features shared by Corynexo-chida and post-Cambrian Illaenina are a forward-expandingglabella and a large, wide rostral plate (Lane and Thomas1983, p. 154). However, most missisquoiids have a subrect-angular or forward-tapering glabella and a small, triangularrostral plate. The lack of a preglabellar field appears to bethe only feature shared by Corynexochida and Illaenina. Jelland Stait (1985, p. 43) suspected the corynexochid affinityand argued that the morphological similarities could have re-sulted from a similar feeding habit. Fortey (in Whittington etal. 1997), in the most recent and comprehensive review oftrilobite classification, assigned Leiostegiina to the Corynexo-chida along with Illaenina and Corynexochina, implicitlysupporting the view that the Missisquoiidae belong to theCorynexochida, but the Missisquoiidae are not listed in thatwork under the Leiostegiina nor under any other taxon. TheMissisquoiidae clearly share several cranidial features withOrdovician Leiostegiidae (excluding the Pagodiidae, see Forteyin Whittington et al. 1997). Both taxa possess a trapezoidal

cranidial outline, a subrectangular or slightly forward-taperingglabella, an inflated palpebral area of the fixigenae, andmost importantly, the lack of a preglabellar field (see Leeand Chatterton 2003, pl. 2, figs. 10, 11). In contrast, theirpygidia show some differences, including the lack of a broadmarginal border in the missisquoiids; compare Figs. 7.4, 7.6,7.7, 7.10, 7.15 with Lee and Chatterton 2003, pl. 2, fig. 12.Nonetheless, holaspid morphologies are obviously indicativeof the inclusion of the Leiostegiidae and Missisquoiidae withinthe same higher taxon.

Lee and Chatterton (2003) figured and compared protaspidesof Ptarmigania (a corynexochid), Leiostegium (a leiostegiid),Failleana (an illaenid), Kosovopeltis (a styginid), andBumastoides (an illaenid). Protaspides of Missisquoia differfrom these protaspides in having sawtooth-shaped protopygi-dial margins (contrasting with distinct, long protopygidialmarginal spines in the other protaspides), an ellipticalexoskeleton (contrasting with a subhexagonal exoskeleton),and a subcylindrical to slightly forward-expanding glabella(contrasting with strongly forward-expanding or “pestle-shaped”glabella). Of several distinctive features of protaspides ofsuborder Corynexochina listed by Chatterton and Speyer (inWhittington et al. 1997), a widely (tr.) anteriorly expandedglabellar frontal lobe, and very shallow to indistinct trans-glabellar furrows are shared by the protaspides of Missisquoia.Of the illaenine protaspid features listed by Chatterton andSpeyer (in Whittington et al. 1997), the forward-expandingglabella (although Missisquoia protaspides have a much lessstrongly expanding glabella), large palpebral lobes, and theopisthoparian facial suture are found in the protaspides ofMissisquoia. A small, nonadult-like bulbous “anaprotaspid”stage is not discovered for Missisquoia depressa. Such stageshave been found for styginid and illaenid trilobites (Hu 1976;Chatterton 1980; Chatterton and Speyer in Whittington et al.1997).

Given that protaspid morphology is indicative of member-ship of more inclusive groups, the protaspid morphology ofMissisquoia depressa leads us to suspect that Missisquoiashould not be assigned to the Corynexochida; and we con-sider that its affinity to post-Cambrian illaenines is suspect.The protaspides of Missisquoia also display similarities withtype C proetide protaspides (Chatterton et al. 1999, figs.1.22–1.25), except for lacking a preglabellar field and pos-sessing large distinct palpebral lobes and a truncatedglabellar front. Previously published taxonomic opinions,based primarily on holaspid morphologies, are not supportedby the protaspid morphologies of Missisquoia. Further workon other Upper Cambrian and Lower Ordovician trilobite

Fig. 6. Meraspides of Missisquoia depressa. All specimens are from KK-116 sampling horizon of Rabbitkettle Formation, MackenzieMountains, northwestern Canada. (figs. 6.1–6.6) Meraspid degree 1, UA8165, × 50, fig. 6.1. dorsal view, fig. 6.2. lateral view, fig. 6.3.ventral view, fig. 6.4. hypostome in ventral view, fig. 6.5. anterior view, fig. 6.6. posterior view. (fig. 6.7) Meraspid cranidium,UA8166, dorsal view, × 50. (fig. 6.8) Meraspid cranidium, UA8167, dorsal view, × 50. (figs. 6.9, 6.10) Meraspid degree 0, UA8168, × 50,fig. 6.9. dorsal view, fig. 6.10. anterior view. (fig. 6.11) Transitory pygidium, UA8169, dorsal view, × 50. (fig. 6.12) Transitory pygidium,UA8170, dorsal view, × 50. (fig. 6.13) Meraspid cranidium, UA8171, dorsal view, × 25. (figs. 6.14, 6.15, 6.24) Hypostome, UA8172, ×25, fig. 6.14. ventral view, fig. 6.15. dorsal view, fig. 6.24. lateral view. (fig. 6.16) Meraspid cranidium, UA8173, dorsal view, × 25.(fig. 6.17) Meraspid cranidium, UA8174, dorsal view, × 25. (fig. 6.18) Meraspid cranidium, UA8175, dorsal view, × 25. (figs. 6.19,6.20) Transitory pygidium, UA8176, × 25, fig. 6.19. dorsal view, fig. 6.20. ventral view. (figs. 6.21, 6.23) Free cheek, UA8177, × 25,fig. 6.21. ventral view, fig. 6.23. dorsal view. (fig. 6.22) Meraspid cranidium, UA8178, dorsal view, × 25. (fig. 6.25) Transitorypygidium, UA8179, dorsal view, × 25. (fig. 6.26) Meraspid cranidium, UA8180, dorsal view, × 25.

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Fig. 7. Meraspides and holaspides of Missisquoia depressa. All specimens are from KK-116 sampling horizon of Rabbitkettle Formation,Mackenzie Mountains, northwestern Canada. All specimens are × 25, unless otherwise noted. (figs. 7.1, 7.3, 7.5, 7.8) Holaspidcranidium, UA8140, fig. 7.1, dorsal view; fig. 7.3, anterior view; fig. 7.5, ventral view; fig. 8, lateral view. (fig. 7.2) Holaspid freecheek, UA8181, dorsal view. (figs. 7.4, 7.6, 7.7, 7.10) Holaspid pygidium, UA8139, fig. 7.4, dorsal view; fig. 7.6, lateral view; fig. 7.7,posterior view; fig. 7.10, ventral view. (figs. 7.9, 7.11, 7.12, 7.16) Holaspid cranidium with right free cheek, hypostome, and rostralplate, UA8182, fig. 7.9, ventral view; fig. 7.11, dorsal view; fig. 7.12, lateral view; fig. 7.16, ventral view of hypostome, × 100.(fig. 7.13) Meraspid cranidium, UA8183, dorsal view. (fig. 7.14) Meraspid cranidium, UA8184, dorsal view. (fig. 7.15) Holaspidpygidium, UA8138, dorsal view.

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ontogenies should illuminate the relationships of this enig-matic genus.

Acknowledgements

G.D. Edgecombe and S.R. Westrop and an anonymous re-viewer provided constructive comments that greatly improvedthe paper. This project was supported by Korea Science andEngineering Foundation grant R01-2004-000-10167-0 toD.-C. Lee; and by a Natural Sciences and EngineeringResearch Council of Canada Discovery Grant to B.D.E.Chatterton.

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Protaspides of uppermost Cambrian trilobite Missisquoia , with implications for suprafamilial level classification of the genus