Protaspides of uppermost Cambrian trilobite ...

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Protaspides of uppermost Cambrian trilobite Missisquoia, with implications for suprafamilial level 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 of the Rabbitkettle Formation, Mackenzie Mountains, northwestern Canada, are described. Three metaprotaspid stages are recognized, using size as well as features of the anterior border of the cranidium and the shape of the glabella. The morphology of the protaspides is not typical of the Order Corynexochida, suggesting that Missisquoia is not a member of the order to which the genus has been previously assigned. This further indicates that its affinity to the stratigraphically younger styginids and illaenids is questionable. Résumé : Une description des protaspides de Missisquoia depressa (Missisquoiidae, Trilobita), du Cambrien supérieur de la Formation de Rabbitkettle, des monts Mackenzie (Nord-Ouest canadien), est présentée. Les dimensions ainsi que les caractéristiques de la bordure antérieure du cranidium et la forme de la glabelle permettent de distinguer trois stades de métaprotaspides. La morphologie des protaspides n’est pas typique de l’ordre des Corynexochida, ce qui laisse croire que Missisquoia n’appartient pas à l’ordre auquel ce genre avait antérieurement été affecté. Cela jette en retour un doute sur son affinité avec les styginides et les illaenides stratigraphiquement plus jeunes. [Traduit par la Rédaction]

Lee and Chatterton

Introduction Ludvigsen (1982) reported the occurrence of two Missisquoia species, M. depressa and M. mackenziensis, in the Rabbitkettle Formation, Mackenzie Mountains, northwestern Canada. Missisquoia is a biostratigraphically important trilobite 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 Laurentian strata and those of Gondwana (Shergold 1988; Geyer and Shergold 2000). However, a variety of contrasting opinions have been expressed on the taxonomy of Missisquoia at the generic level (Fortey 1983; Westrop 1986; Jell in Jell and Adrain 2003) and at higher taxonomic levels (Shergold 1975; Ludvigsen 1982; Lane and Thomas 1983), indicating the problems of classifying this genus. Protaspides of Missisquoia depressa from the Rabbitkettle Formation, northwestern Canada are figured and documented. Previous taxonomic opinions on Missisquoia are re-evaluated in light of the protaspid morphology described herein, focusing on its suprafamilial classification. All the specimens are stored in University of Alberta Paleontology Collection (UA numbers: Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada).

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The specimens were collected by the junior author and processed by the senior author; the limestone samples were collected from horizon “KK-116” belonging to the Missisquoia depressa subzone of the Parabolinella Zone in Ludvigsen (1982). The Missisquoia depressa subzone is regarded as the uppermost part of the Upper Cambrian, since Cooper et al. (2001) placed the base of the Ordovician at the base of the Iapetagnathus fluctivagus Zone which is located above the base of Missisquoia depressa subzone. Detailed geographical information on the sampling locality is found in Ludvigsen (1982, fig.1). Some protaspid specimens of Missisquoia, described by Hu (1971), and some others from the same locality, were borrowed from the Cincinnati Museum Center, Cincinnati, Ohio (CMC-P numbers), and compared with specimens from the Rabbitkettle Formation to investigate whether they are correctly associated.

Tectonic distortion and association of protaspides Silicified specimens from the Rabbitkettle Formation are distorted in varying amounts. Some specimens seem to have been compressed or extended along a single plane, but others are 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 on 25 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, South Korea. B.D.E. Chatterton.1 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada. 1

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

Can. J. Earth Sci. 44: 493–506 (2007)

doi:10.1139/E06-113

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Can. J. Earth Sci. Vol. 44, 2007 Fig. 1. Distortions of the specimens from the Rabbitkettle Formation. (fig. 1.1) Original metaprotaspid specimen that is considered to have been deformed only laterally in a single planar direction, UA8137, × 50. (fig. 1.2) With Adobe Photoshop, the specimen image in fig 1.1 is compressed along the axis. The compressed image is similar to the specimen illustrated in fig. 4.18. (fig. 1.3) With Adobe Photoshop, the specimen image is axially compressed and then laterally sheared off. The sheared image is similar to the specimen illustrated in fig. 4.23. (fig. 1.4) With Adobe Photoshop, the specimen image is sheared along the axis. The sheared image is similar to the specimen illustrated in fig. 4.16. (fig. 1.5) Original pygidial specimen, UA8138, × 15. (fig. 1.6) Original pygidial specimen, UA8139, × 14. (fig.1.7) With Adobe Photoshop, the specimen image in fig. 1.5 (UA8138) is compressed along the axis, 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 specimen image from fig. 1.9 (UA8140) is laterally sheared.

(pressure or stress) caused the distortion of the specimens after burial. Such distortion causes problems in reconstructing original morphology correctly, and in assigning specimens correctly to a species. To examine the effects of tectonic forces on some of the Rabbitkettle trilobites, a specimen was selected that is distorted solely by unidirectional compression (Fig. 1.1). Its image was then distorted in various angles and directions using the “Distort” tool in Adobe Photoshop version 7.0 (Figs. 1.2–1.4). It was found possible by this means to produce distorted images that are very similar 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 a cranidium 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 specimens to the same species. Not all specimens were deposited parallel to the plane along which the maximum distortion took place; in other words, specimens were deposited at various angles to the direction of maximum pressure or stress. Such a random three-dimensional orientation of specimens results in complicated morphologies. Furthermore, if pressure or stress on the specimens were applied during multiple tectonic events, specimens would be distorted in more complicated ways. For example, some specimens show a very distinct ocular ridge (Fig. 5.25) compared with others. This “ocular ridge” is likely to have been caused by tectonic buckling of the anterior portion of the specimen, rather than be the remnant of an original morphological characteristic. It is concluded that morphological differences such as variation in length versus width ratios observed in many specimens are more due to tectonic distortion than original variation. The reconstructions of protaspides and meraspid degree 1 of Missisquoia depressa (Figs. 2.1–2.5) are based on the specimens that are considered to be least deformed, and only deformed along a single plane parallel to the original horizontal plane on which the specimens were deposited (perpendicular to the direction of maximum pressure). This type of distortion is very common in trilobites deposited in outer detrital facies due to compression as the result of dewatering of shale and the weight of succeeding layers of sediments during lithification, even in regions that have not undergone significant tectonism.

Division of protaspid and meraspid ontogeny into stages Due to the effects of tectonic distortion, conventional length versus width plots that are normally used to identify or discriminate ontogenetic instars would be unreliable for this purpose. Morphologic features such as distinctive glabellar © 2007 NRC Canada

Lee and Chatterton 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 distorted along 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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Fig. 3. Metaprotaspid specimens figs. 3.1–3.5, 3.11, and 3.12 are from KK-116 sampling horizon of the Rabbitkettle Formation, Mackenzie 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, dorsal view, × 100. (figs. 3.11, 3.12) Early protaspid stage of Missisquoia depressa, UA8143, × 100, fig. 3.11, ventral view; fig. 3.12, dorsal view. (fig. 3.13) Small cranidium of Missisquoia typicalis, CMC-P 38749o, dorsal view, × 50. (fig. 3.14) Small cranidium of Missisquoia typicalis, 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 specimen of ?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 of Missisquoia 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 of Ptychoparioidea 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 to Missisquoia 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.

shapes and differentiation of an anterior cranidial border, among others, are used herein to divide the protaspid ontogeny into three stages: early, intermediate, and late. Since even the smallest protaspid specimens (Figs. 3.1–3.5) secured have a differentiated, but not released, protopygidium, all protaspides available are considered metaprotaspides. The separation between the late protaspid stage and degree 0 meraspides is based on observation of ventral surfaces, to see whether the pygidial region is clearly separated from the cranidial region (see “Systematic paleontology” for detailed observation).

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

The generic concept of Missisquoia has been discussed by Ludvigsen (1982), Fortey (1983), Zhou and Zhang (1984), and Westrop (1986). Many genera have been assigned to the family Missisquoiidae and many of them have been synonymized into Missisquoia. Ludvigsen (1982) synonymized the following genera with Missisquoia: Lunacrania Kobayashi, 1955, Macroculites Kobayashi, 1955, Rhamphopyge Kobayashi, 1955, Tangshanaspis Zhou and Zhang, 1978, and Paranumia Hu, 1973. Fortey (1983) synonymized Missisquoia with Parakoldinioidia Endo in Endo and Resser, 1937. Zhou and Zhang (1984) synonymized Pseudokoldinioidia Endo, 1944 with Missisquoia. Westrop (1986) considered Missisquoia as a valid separate genus from Parakoldinioidia. It is apparent that the concept of Missisquoia has been confused and needs to be rigorously investigated, which is beyond the scope of

this study. The generic concept of Missisquoia in the scope of 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 Mountain 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 Tangshanaspis zhaogezhuangensis from China into this species. The Chinese cranidia figured by Zhou and Zhang (1978, 1984) are similar to those of Missisquoia depressa. The Chinese pygidia (Zhou and Zhang 1978, pl. 16, fig. 23; Zhou and Zhang 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 that T. 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. Glabella reaches anterior exoskeletal margin; anterior cranidial border not differentiated. Occipital ring moderately distinct. Posterior cephalic marginal furrows very faintly impressed. Proto© 2007 NRC Canada

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pygidial margin widely and shallowly indented sagittally; no axial rings recognized. Intermediate metaprotaspid stage (Figs. 2.2, 4.1–4.17): Exoskeleton sub-oval in outline. Glabella sub-rectangular in outline; three pairs of glabellar furrows faintly impressed.


Anterior cranidial border narrow sagittally and exsagittally. Anterior pits weakly impressed. Posterior cranidial border furrows 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, © 2007 NRC Canada

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Fig. 4. Metaprotaspides and meraspid degree 0 of Missisquoia depressa. All specimens are from KK-116 sampling horizon of Rabbitkettle Formation, 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, dorsal view; fig. 4.13, ventral view. (figs. 4.15–4.17) Intermediate metaprotaspid stage, UA8149, fig. 4.15, dorsal view; fig. 4.16, ventral view; 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 metaprotaspid stage, UA8152, fig. 4.21, dorsal view; fig. 4.22, ventral view. (figs. 4.23, 4.24) Late metaprotaspid stage, UA8153, fig. 4.23, dorsal view; 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, ventral view.

5.18, 5.20, 5.23, 5.25, 5.28): Exoskeleton elongated oval in outline. Anterior cranidial border furrow wide sagittally and exsagittally. Glabella subcylindrical, but slightly expanding forward. Palpebral lobe slightly convex outwards. Posterior cranidial border furrows widen distally. Librigenae bladeshaped, with short genal spine. Protopygidium with three to four axial rings, including terminal piece. Posterior protopygidial margin indented. Posterior cranidial edge inturned in some 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 subcylindrical in shape and expanding forward; glabellar front indented sagittally. Anterior cranidial border wide (sagitally) and flat, without distinctively incised border furrow. Palpebral lobe large and proximally defined by shallow and wide palpebral furrow. Ocular ridge faintly developed. Three pairs of glabellar furrows weakly impressed. Anterior pits wide. Posterior cranidial border wide exsagittally. Hypostome elongated, waisted and shield shaped; pair of short spines present at posterior lateral corner. Rostral plate transversely elongated, inverted subtrapezoid in outline, and as transversely wide as hypostome. Free cheek with short genal spine with blunt tip. Transitory pygidium with five to six distinct axial rings and terminal piece; posterior margin indented medially. Transitory pygidial border flat and wide and defined by slope change representing border furrow; pleural furrows wider than interpleural furrows; transitory pygidial lateral margin weakly saw-toothed in region of protothoracic segments. Later meraspid and holaspid degrees (Figs. 6.7, 6.8, 6.11– 6.26, 7): Glabellar furrows deeper. Fourth pair of glabellar furrows impressed. Genal spine becomes longer. Anterior cranidial border furrows separated from anterior palpebral lobe margin. Posterior pygidial margin entire.

Articulation between trunk and cephalon during early ontogeny of Missisquoia depressa The protopygidium of metaprotaspides is demarcated from the cranidium by a posterior cranidial marginal furrow. Meraspid degree 0 stage commences with the protopygidium being released from the cephalon to become a transitory

pygidium. At this stage, the transitory pygidium and cranidium are considered to have been connected by a flexible integument and articulated with each other when the individual was alive. The topographical configuration of the articulatory boundary between adjacent free segments of most trilobites is that the posterior margin of the more anterior segment is ventrally inturned as doublure and the anterior margin of the more posterior segment lies underneath the doublure as an articulatory half ring in the axial region, and as a flange in the pleural region (see Fig. 8.3 for cross-sectional view). An identical configuration between the protopygidium and the cephalon is found in the meraspid degree 0 stage of Missisquoia depressa (Fig. 5.22 for lateral view and 5.26 for ventral view). The metaprotaspid ontogeny of M. depressa displays how the topographical configuration would have proceeded. Intermediate metaprotaspid specimens have a simple posterior cephalic marginal furrow that is ventrally projected as a transverse ridge (see Figs. 4.5, 4.14, 4.16). Some late metaprotaspid specimens display the same furrow configuration (see Figs. 5.2, 5.6; see also Fig. 8.1). Other metaprotaspid specimens, however, display a different configuration, where the furrow ventrally deepens, pointing forward; this condition is clearly shown in lateral extremity of posterior cephalic region (see Figs. 4.24, 4.27, 5.11; see also Fig. 8.2). This difference may be due to distortion. As a matter of fact, specimens that are considered compressed along the anterior–posterior axis (for example, Figs. 4.18–4.29) appear to have deeper furrows and more highly raised borders and pleural ribs. However, some less-distorted specimens also show a ventrally deepening and forward-directed posterior cephalic marginal furrow (for example, Figs. 5.11, 5.28). Thus, the two different types of configuration may represent the ontogenetic pathway through which the articulation between the protopygidium and cephalon proceeds. The deep invagination of the exoskeleton (Fig. 8.2) indicates that articulation between the protopygidium and cephalon may be underway even during the protaspid period. The articulation proceeds from a simple furrow, pre-articulatory invagination, finally to true articulation with the two parts separated (Figs. 8.1–8.3).

Other Missisquoiid protaspides Hu (1971) described protaspides of Missisquoia cyclochila from the Lower Ordovician Deadwood Formation exposed © 2007 NRC Canada

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Fig. 5. Metaprotaspides and meraspids degree 0 of Missisquoia depressa. All specimens are from KK-116 sampling horizon of Rabbitkettle 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, ventral view; 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, dorsal view, 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, dorsal view; 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.

in northeastern Wyoming, and he figured seven protaspid specimens. Westrop (1986) regarded this species as a junior synonym of Missisquoia typicalis, implying that the smaller Wyoming specimens are earlier growth stages of M. typicalis. The relatively large Wyoming cranidium (e.g., Fig. 3.31) is similar to an Albertan cranidium (Westrop 1986, pl. 1, fig. 37) and cranidia of M. typicalis from other localities (e.g., Stitt 1971, pl. 8, fig. 1). However, the subcircular pygidia lacking marginal spines (Figs. 3.27, 3.28) differ from the subtriangular pygidia of M. typicalis that possess short, distinct marginal spines (e.g., Westrop 1986, pl. 1, fig. 35). The ontogeny of Missisquoia depressa demonstrates that the pygidium develops a maximum of two pairs of marginal spines in the holaspid period, while the holaspid pygidium of M. typicalis develops a maximum of six pairs of spines, indicating different ontogenetic pathways. In addition, the spines of M. depressa and M. cyclochila appear to be a modification of the marginal border, whereas those of M. typicalis are an extension of pygidial pleurae. M. cyclochila is regarded 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 disparity that does not support their association within a single species. Specimen CMC-P 38749e (Figs. 3.6–3.9), a metaprotaspis, is distinguished from other specimens in having a parallel-sided axis. It shows the most gradual morphological transformation from the smallest cranidium (Fig. 3.21) by sharing a parallel-sided axis and glabella. The larger cranidia (Figs. 3.13, 3.14, 3.22, 3.15) also show a gradual morphological 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 longitudinal subdivision (bilobation) of two lobes in the axis that are likely to be L2 and L3. Specimens CMC-P 38749d (Fig. 3.25) and CMC-P 38749g (Fig. 3.24) are characterized by a spindle-shaped axis with two bilobed axial lobes. It seems possible that these four specimens belong to a single ontogenetic sequence, since they all have a bilobed, spindleshaped axis. Specimen CMC-P 38749c (Fig. 3.37) is characterized by having a strongly annulated axial lobe. Specimen CMC-P 38749f (Fig. 3.26) has a subquadrate exoskeletal outline and relatively narrow transverse (tr.) and forwardtapering axis. Each of these two specimens clearly belongs to a different ontogenetic series. Hu (1971, 1973) described protaspides of Highgatella facila and Paranumia triangularia from what appears to be

the same locality and figured five and three protaspid specimens for these species, respectively. Of the protaspides assigned to H. facila, specimens CMC-P 38740d (Fig. 3.10) and CMC-P 38740e (Fig. 3.23) are similar to one another and, 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 the most elongated exoskeleton and a well developed posterior cranidial 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 metaprotaspid specimens 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 narrower axis and distinct transglabellar furrows. Of the protaspides assigned to P. triangularia, specimen CMC-P 41556k (Fig. 3.29) is most similar to specimens CMC-P 38749d and CMC-P 38749g (Figs. 3.24, 3.25). Specimen CMC-P 41556l (Fig. 3.30) is similar to CMC-P 38749e. Specimen CMC-P 41556m (Fig. 3.36) is characterized by having a strongly tapering, spindle-shaped, bilobed axis and at least two pairs of short fixigenal spines. Highgatella facila was synonymized into Apoplanias rejectus by Ludvigsen (1982). Most olenids, including Apoplanias, have a strongly annulated glabella during the protaspid and early meraspid periods (for example, Olenus gibbosus and Acerocare ecorne; see Hu 1971, pl. 18, figs. 6–10, pl. 19, figs. 5–8). None of the Wyoming protaspid specimens has such an annulated glabella or axis. The figured smallest cranidium (Hu 1971, pl. 21, fig. 6) of A. rejectus is typical of the olenid. Paranumia was regarded as a subjective synonym of Missisquoia by Ludvigsen (1982). All the cranidia of Paranumia triangularia figured by Hu (1973) have a forward-tapering glabella. The four Wyoming protaspid specimens, CMC-P 38749e, CMC-P 38740d, CMC-P 38740e, and CMC-P 41556l may represent protaspides of Missisquoia cyclochila; they are questionably assigned to this species. They are considerably smaller than early protaspides of Missisquoia depressa (Figs. 3.1–3.5, 3.11, 3.12), and differ in having a parallelsided axis. Specimens CMC-P 38740a and CMC-P 38740b seem to display morphologies that transform into the early protaspides of M. depressa in the most acceptable range; they are questionably assigned to this species. Better controlled sampling is required to assess these possible associations. Other protaspides, showing a generalized ptychoparioid protaspid morphology (Chatterton and Speyer in Whittington © 2007 NRC Canada

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Fig. 6. Meraspides of Missisquoia depressa. All specimens are from KK-116 sampling horizon of Rabbitkettle Formation, Mackenzie Mountains, 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) Transitory pygidium, UA8179, dorsal view, × 25. (fig. 6.26) Meraspid cranidium, UA8180, dorsal view, × 25.

et al. 1997) are tentatively assigned to the Ptychoparioidea: CMC-P 38749a, CMC-P 38749b, CMC-P 38749d, CMC-P 38749g, and CMC-P 41556k are assigned to Ptychoparioidea sp. A; CMC-P 38749f to Ptychoparioidea sp. B; CMC-P 38740c to Ptychoparioidea sp. C; CMC-P 41556m to Ptychoparioidea sp. D; CMC-P 38749c to Ptychoparioidea sp. E.

Comparison of Missisquoia protaspides and suprafamilial taxonomy Various opinions have been proposed for the suprafamilial taxonomy of Missisquoia. Shergold (1975, p. 195) noted cranidial similarities to the Leiostegiidae and assigned the Missisquoiidae to the superfamily Leiostegiacea (see also Shergold 1988). Ludvigsen (1982, p. 119) suggested that the family was the ancestor to the Styginidae, which in turn appears to be ancestral to the Illaenidae and Scutelluidae; the Scutelluidae is now regarded as a junior synonym of the Styginidae (Fortey in Whittington et al. 1997) or as a subfamily of the Styginidae (Whittington 2000). He presented the similarities between Missisquoia depressa and Perischoclonus capitalis (Whittington 1963, pl. 22, figs. 4, 13) as evidence. Later, Lane and Thomas (1983, p. 155) contradicted Ludvigsen’s view and claimed that the cephalic morphologies of the Missisquoiidae are much more similar to Cambrian Corynexochida than post-Cambrian Scutelluina (= Illaenina of Fortey in Whittington et al. 1997). In particular, they claimed that the morphologies of the rostral plate and pygidium are readily distinguished from those of the styginids. The characteristic features shared by Corynexochida and post-Cambrian Illaenina are a forward-expanding glabella and a large, wide rostral plate (Lane and Thomas 1983, p. 154). However, most missisquoiids have a subrectangular or forward-tapering glabella and a small, triangular rostral plate. The lack of a preglabellar field appears to be the only feature shared by Corynexochida and Illaenina. Jell and Stait (1985, p. 43) suspected the corynexochid affinity and argued that the morphological similarities could have resulted from a similar feeding habit. Fortey (in Whittington et al. 1997), in the most recent and comprehensive review of trilobite classification, assigned Leiostegiina to the Corynexochida along with Illaenina and Corynexochina, implicitly supporting the view that the Missisquoiidae belong to the Corynexochida, but the Missisquoiidae are not listed in that work under the Leiostegiina nor under any other taxon. The Missisquoiidae clearly share several cranidial features with Ordovician Leiostegiidae (excluding the Pagodiidae, see Fortey in Whittington et al. 1997). Both taxa possess a trapezoidal

cranidial outline, a subrectangular or slightly forward-tapering glabella, an inflated palpebral area of the fixigenae, and most importantly, the lack of a preglabellar field (see Lee and Chatterton 2003, pl. 2, figs. 10, 11). In contrast, their pygidia show some differences, including the lack of a broad marginal 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 indicative of the inclusion of the Leiostegiidae and Missisquoiidae within the same higher taxon. Lee and Chatterton (2003) figured and compared protaspides of Ptarmigania (a corynexochid), Leiostegium (a leiostegiid), Failleana (an illaenid), Kosovopeltis (a styginid), and Bumastoides (an illaenid). Protaspides of Missisquoia differ from these protaspides in having sawtooth-shaped protopygidial margins (contrasting with distinct, long protopygidial marginal spines in the other protaspides), an elliptical exoskeleton (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 of suborder Corynexochina listed by Chatterton and Speyer (in Whittington et al. 1997), a widely (tr.) anteriorly expanded glabellar frontal lobe, and very shallow to indistinct transglabellar furrows are shared by the protaspides of Missisquoia. Of the illaenine protaspid features listed by Chatterton and Speyer (in Whittington et al. 1997), the forward-expanding glabella (although Missisquoia protaspides have a much less strongly expanding glabella), large palpebral lobes, and the opisthoparian facial suture are found in the protaspides of Missisquoia. A small, nonadult-like bulbous “anaprotaspid” stage is not discovered for Missisquoia depressa. Such stages have 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 membership of more inclusive groups, the protaspid morphology of Missisquoia depressa leads us to suspect that Missisquoia should not be assigned to the Corynexochida; and we consider that its affinity to post-Cambrian illaenines is suspect. The protaspides of Missisquoia also display similarities with type C proetide protaspides (Chatterton et al. 1999, figs. 1.22–1.25), except for lacking a preglabellar field and possessing large distinct palpebral lobes and a truncated glabellar front. Previously published taxonomic opinions, based primarily on holaspid morphologies, are not supported by the protaspid morphologies of Missisquoia. Further work on other Upper Cambrian and Lower Ordovician trilobite © 2007 NRC Canada

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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) Holaspid cranidium, UA8140, fig. 7.1, dorsal view; fig. 7.3, anterior view; fig. 7.5, ventral view; fig. 8, lateral view. (fig. 7.2) Holaspid free cheek, 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 rostral plate, 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) Holaspid pygidium, UA8138, dorsal view.

© 2007 NRC Canada

Lee and Chatterton Fig. 8. Three different possible skeletal configurations between the cranidium and the protopygidium during the late protaspid and meraspid degree 0 stages of Missisquoia depressa. (8.1) Posterior cranidial marginal furrow: A simple furrow between the two parts. (8.2) An articulation relationship between the two parts without their separation. (8.3) An articulation relationship between the two parts with their being separated. At this stage, the two parts are connected with the flexible articulatory integument (not shown in the figure).

ontogenies should illuminate the relationships of this enigmatic genus.

Acknowledgements G.D. Edgecombe and S.R. Westrop and an anonymous reviewer provided constructive comments that greatly improved the paper. This project was supported by Korea Science and Engineering Foundation grant R01-2004-000-10167-0 to D.-C. Lee; and by a Natural Sciences and Engineering Research Council of Canada Discovery Grant to B.D.E. Chatterton.

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