Morphology, ecology, and affinity of soft-bodied 'molluscs'

269 Pages·2012·32.61 MB·English

by  Martin R. Smith

Morphology, ecology, and affinity of soft-bodied ‘molluscs’
from Cambrian Burgess Shale-type deposits
by
Martin Ross Smith
A thesis submitted in conformity with the requirements
for the degree of Doctor of Philosophy
Department of Ecology and Evolutionary Biology
University of Toronto
© Copyright Martin Smith 2012
Morphology, ecology, and affinity of soft-bodied ‘molluscs’
from Cambrian Burgess Shale-type deposits
Martin Ross Smith
Doctor of Philosophy
Department of Ecology and Evolutionary Biology
University of Toronto
2012
Abstract
The soft-bodied fossils preserved in Burgess Shale-type deposits provide a unique window on the
origin and early evolution of complex animals some 500 million years ago, but their relationships
to modern phyla can be difficult to determine. Using new fossils collected by the Royal Ontario
Museum, I reconstruct the morphology, ecology and affinity of three contentious fossil
organisms: Wiwaxia corrugata, Odontogriphus omalus, and Nectocaris pteryx.
Nectocaris pteryx is reconstructed as a dorso-ventrally flattened organism with flexible tentacles,
lateral fins, internal gills, camera-type eyes, and an axial cavity and funnel that were adapted for
efficient jet propulsion. Detailed analysis of the slug-like animals Wiwaxia and Odontogriphus
revises some anatomical interpretations, in particular showing that their mouthparts represent
serial unipartite rows that comprise multiple teeth. Despite the proposed similarities between
Wiwaxia sclerites and annelid chaetae, my new observations are not consistent with an annelidan
interpretation.
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The relationships of these taxa are evaluated using Maximum Parsimony and Bayesian
approaches, based on 59 taxa coded for 804 ‘characters’. Both methods strongly support the
molluscan affinity of Odontogriphus and Wiwaxia, and indicate that Nectocaris is related to the
cephalopods. Nectocaris would place the origin of the cephalopods much earlier than the fossil
record suggests, whereas the basal position of Odontogriphus and Wiwaxia implies that the
molluscs had a non-mineralized early history, evolving from ancestor that had a Wiwaxia-like
scleritome.

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“No triumph of science has appeared more marvellous to the intelligent mind
than the reconstruction of a form a life that has passed away long ages ago”
— Richard Owen
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Acknowledgments

I am grateful for the advice, oversight and enthusiasm of my supervisor, J.-B. Caron, and my
advisory committee, B. Chang, E. Larsen and C. Darling.
S. Conway Morris, H. López-Fernández, D. Fuchs, A. Lindgren, A. Scheltema, C. Schander,
M. Vecchione, B. Runnegar, N. Butterfield and four anonymous reviewers provided critical
comments on components of this thesis. R. Gaines provided comments on SEM.
J.-Y. Chen, M.-Y. Zhu, J. Paterson and T. Kawamura provided images. Parks Canada delivered
research and collection permits to Royal Ontario Museum (ROM) teams led by D. Collins. M.-
Y. Zhu and F.-C. Zhao (Chinese Academy of Sciences – Nanjing Institute of Geology and
Palaeontology) and D. Erwin (Smithsonian Institution – National Museum of Natural History,
NMNH), facilitated access to specimens. P. Fenton (ROM), G. Hunt and M. Florence (NMNH)
assisted with collections; G. Kretschmann (U. Toronto), S. Whittaker (NMNH) and S. Lackie
(U. Windsor–GLIER) helped with SEM imaging and EDS.
Funding was provided by Natural Sciences and Engineering Research Council of Canada
Discovery Grant 341944 to J-B. Caron, a University of Toronto fellowship, a Geological Society
of America research grant, and the Paleontological Society H. B. Whittington award.
My parents and soon-to-be wife supported me patiently from great distance. Thank you. It’s
been worth it.
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Table of Contents
Acknowledgments ........................................................................................................................... v
Table of Contents ........................................................................................................................... vi
List of Tables ............................................................................................................................... viii
List of Figures ................................................................................................................................ ix
List of Appendices ........................................................................................................................ xii
Preface ............................................................................................................................................. 1
The Cambrian: a pivotal period in animal evolution ................................................................. 1
The Burgess Shale ...................................................................................................................... 2
Molluscan origins ....................................................................................................................... 6
Material studied and techniques ................................................................................................. 7
Objectives ................................................................................................................................... 7
Chapter 1 The ecology and affinity of Nectocaris pteryx, a stem-group cephalopod from the
Cambrian .................................................................................................................................... 9
1.1 A redescription of Nectocaris pteryx, with notes on its affinity ......................................... 9
1.2 Affinity of the nectocaridids ............................................................................................. 18
1.3 Nectocaridid ecology and diversity .................................................................................. 23
Chapter 2 Odontogriphus and Wiwaxia: some new details, and the case for a molluscan
affinity ...................................................................................................................................... 86
2.1 Introduction ....................................................................................................................... 86
2.2 Materials and Methods ...................................................................................................... 88
2.3 Wiwaxia Scleritome: Morphology and Ontogeny ............................................................. 89
2.4 Wiwaxia and Odontogriphus: Comparative Anatomy of the Mouthparts ........................ 94
2.5 Wiwaxia and Odontogriphus: Other Features ................................................................... 97
2.6 Discussion ......................................................................................................................... 98
Chapter 3 Phylogenetic perspectives .......................................................................................... 119
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3.1 Introduction ..................................................................................................................... 119
3.2 Methods ........................................................................................................................... 125
3.3 Results ............................................................................................................................. 134
3.4 Implications for molluscan evolution ............................................................................. 135
Conclusion .................................................................................................................................. 153
References ................................................................................................................................... 156
Appendix I Description of transformation series ........................................................................ 185
Sources ................................................................................................................................... 185
Transformation series ............................................................................................................. 188
Appendix II Data matrix for phylogenetic analysis .................................................................... 242
Copyright acknowledgements ..................................................................................................... 257

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List of Tables
Table 1-1. Provenance of Nectocaris specimens from Fossil Ridge in Royal Ontario Museum
collections ............................................................................................................. 47
Table 1-2. Measurements of Nectocaris specimens. ................................................................... 49
Table 1-3. Detailed comparison of potential homologies between Nectocaris, cephalopods and
anomalocaridids. ................................................................................................... 50
Table 3-1. Selection of priors. ..................................................................................................... 139
Table 3-2. Monophyly of the halwaxiids. ................................................................................... 140
Table 3-3. Affinity of Odontogriphus and Wiwaxia. .................................................................. 141
Table 3-4. Affinity of Nectocaris. ............................................................................................... 142
Table 3-5. Evaluation of hypotheses of cephalopod ancestry. .................................................... 143
Table 3-6. Characters supporting the relationship of proposed cephalopod ancestors with the
cephalopod crown-group. ................................................................................... 144

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List of Figures
Figure 1-1. Vetustovermis and Petalilium. ................................................................................... 51
Figure 1-2. Nectocaris pteryx from the Middle Cambrian Burgess Shale. .................................. 53
Figure 1-3. Comparison with Nectocaris pteryx holotype. .......................................................... 55
Figure 1-4. Outline of Nectocaris. ............................................................................................... 56
Figure 1-5. Lateral view of the head region of Nectocaris. ......................................................... 57
Figure 1-6. Tentacle rotation and putative mouthparts in Nectocaris. ........................................ 58
Figure 1-7. Tentacle projections in Nectocaris. .......................................................................... 59
Figure 1-8. Interpretative sketch of ROM 59660. ........................................................................ 60
Figure 1-9. Mouthparts and preservation of the eye in Nectocaris. ............................................. 61
Figure 1-10. Sediment infill of the axial canal of Nectocaris. ..................................................... 62
Figure 1-11. Flexibility of Nectocaris fins. ................................................................................. 63
Figure 1-12. Ventral schematic of Nectocaris. ............................................................................ 64
Figure 1-13. Reconstruction of Nectocaris pteryx. ...................................................................... 65
Figure 1-14. Connective tissue in the fins of Ommastrephes (Coleoidea: teuthida), comparable to
bars and lineations in Nectocaris. ......................................................................... 66
Figure 1-15. Funnel morphology in extant cephalopods. ............................................................ 67
Figure 1-16. Phylogenetic position of the nectocaridids. ............................................................ 68
Figure 1-17. Funnel morphology in Nectocaris pteryx. ............................................................... 69
Figure 1-18. Taphonomic spectrum in Burgess Shale specimens of Nectocaris pteryx. ............. 70
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Figure 1-19. Distribution of elements in the body of Nectocaris pteryx (ROM 60079) from the
Burgess Shale. ....................................................................................................... 71
Figure 1-20. Distribution of elements in the head of Nectocaris pteryx (ROM 60146) from the
Burgess Shale. ....................................................................................................... 72
Figure 1-21. Fins in Nectocaris pteryx from the Burgess Shale. ................................................. 73
Figure 1-22. Dark axial stain in Nectocaris pteryx from the Burgess Shale. ............................... 74
Figure 1-23. Large morph of Nectocaris pteryx? from Chengjiang. ........................................... 75
Figure 1-24. Bioturbation in Nectocaris pteryx from the Burgess Shale (ROM 60117). ............ 76
Figure 1-25. Tentacles in Nectocaris pteryx from the Burgess Shale. ......................................... 77
Figure 1-26. Mouthparts in Nectocaris pteryx from the Burgess Shale. ..................................... 78
Figure 1-27. Funnel in Nectocaris pteryx from the Burgess Shale. ............................................. 79
Figure 1-28. Axial cavity and gills in Nectocaris pteryx from the Burgess Shale. ...................... 80
Figure 1-29. Bars (‘iterated elements’) Nectocaris pteryx from the Burgess Shale. ................... 81
Figure 1-30. ‘Petalilium latus’ from Chengjiang. ....................................................................... 82
Figure 1-31. Large morph of Nectocaris pteryx? from the Burgess Shale, ROM 61496. ........... 83
Figure 1-32. Axial length of Nectocaris specimens. .................................................................... 84
Figure 1-33. Nectocaris sp. from the Emu Bay Shale. ................................................................ 85
Figure 2-1. Representative Wiwaxia specimens. ........................................................................ 104
Figure 2-2. Acicular crystals within sclerites of Wiwaxia, UNSM 198674. ............................... 106
Figure 2-3. Wiwaxia corrugata Matthew 1899. .......................................................................... 108
Figure 2-4. Sclerite length (○) and width (+) in Wiwaxia. .......................................................... 109
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