Carr, R. K., 1995a. - Biological Sciences

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evolving chondrichthyans and actinopterygians. The Frasnian-Famennian extinction episode, by
reducing placoderm diversity, may have provided a "window of opportunity" for early radiation
of chondrichthyans and actinopterygians. Placoderm evolution had centered on diverse, but
limited modifications of primitive patterns of locomotion and suspensorium. The evolution of
specialized actinopterygian subcarangiform and carangiform locomotion provided cost effective
improvements over primitive anguilliform or modified anguilliform patterns (WEBB, 1982). Actinopterygians
and chondrichthyans also demonstrated a greater plasticity in development of structural
modifications for feeding. The Early Mississippian extinction of placoderms is consistent
either with competitive displacement or with opportunistic replacement following the global
Famennian-Tournaisian extinction event. It is possible, even likely, that both factors may play
a role. Distinguishing between these two models requires a more complete understanding of
early osteichthyan diversity. Due to the high reported levels of endemism among placoderms,
the interrelationships among gnathostomes should be verified regionally as well.
Finally, additional study is needed to evaluate these and other hypotheses of placoderm
evolution. An integrative approach considering environmental, geographic, and biological interactions
with a renewed emphasis on field work will shed new light on placoderm, as well as
overall gnathostome, evolution.
Acknowledgments
I would like to thank Dr Herve LELIEVRE and the members of my dissertation committee (Drs Daniel
FISHER, Carl GANS, Daniel GOUlET, Philip GINGERICH, and Gerald R. SMITH) for their reviews of this
manuscript and for Daniel GOUlET'S assistance in tracking down a number of difficulties to find references.
I am greatly indebted to Shen MAl for her assistance in translating stratigraphic data from numerous Chinese
references. Also, I would like to thank Rob Cox for our many discussions; Dr Michael FOOTE for discussions
on diversity; and Dr Carl GANS for discussions on muscle function and architecture. I want to thank Dr
Michael WILLIAMS for his time in showing me the material used in his 1990 paper and the placoderm
pectoral fin material with preserved ceratotrichia. Finally, the life reconstructions of Dunkleosteus terrelli
and Cladoselache sp. were drawn by Joseph C. WINANS, 1992. His work has helped to bring to life an
interpretation of placoderms as active predators in the Devonian seas. Field work, referred to in this paper,
was supported in part by grants from the Geological Society of America and Scott TURNER Awards in
Earth Science, The University of Michigan. This report was submitted in partial fulfillment of the requirements
for a Doctor of Philosophy in Geological Sciences in the Horace H. Rackham School of Graduate
Studies at The University of Michigan.
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