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3. The origin of conodonts and of vertebrate mineralized skeletons

Author

Murdock, Duncan J.E., Dong, Xi-Ping, Repetski, John E., Marone, Federica, Stampanoni, Marco, and Donoghue, Philip C.J.

Subjects
Skeleton -- Origin, Conodonts -- Origin -- Physiological aspects, Environmental issues, Science and technology, Zoology and wildlife conservation
Abstract

Conodonts are an extinct group of jawless vertebrates whose tooth-like elements are the earliest instance of a mineralized skeleton in the vertebrate lineage (1,2), inspiring the 'inside-out' hypothesis that teeth evolved independently of the vertebrate dermal skeleton and before the origin of jaws (3-6). However, these propositions have been based on evidence from derived euconodonts. Here we test hypotheses of a paraconodont ancestry of euconodonts (7-11) using synchrotron radiation X-ray tomographic microscopy to characterize and compare the microstructure of morphologically similar euconodont and paraconodont elements. Paraconodonts exhibit a range of grades of structural differentiation, including tissues and a pattern of growth common to euconodont basal bodies. The different grades of structural differentiation exhibited by paraconodonts demonstrate the stepwise acquisition of euconodont characters, resolving debate over the relationship between these two groups. By implication, the putative homology of euconodont crown tissue and vertebrate enamel must be rejected as these tissues have evolved independently and convergently. Thus, the precise ontogenetic, structural and topological similarities between conodont elements and vertebrate odontodes appear to be a remarkable instance of convergence. The last common ancestor of conodonts and jawed vertebrates probably lacked mineralized skelet al tissues. The hypothesis that teeth evolved before jaws and the inside-out hypothesis of dental evolution must be rejected; teeth seem to have evolved through the extension of odontogenic competence from the external dermis to internal epithelium soon after the origin of jaws., The soft tissue anatomy of euconodonts substantiates their vertebrate affinity (12,13), but homology of euconodont and vertebrate skelet al tissues (1,14,15) remains the subject of controversy (16,17). The mineralized skeleton [...]

Published
2013
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4. Evaluating scenarios for the evolutionary assembly of the brachiopod body plan

Author

Murdock, Duncan J.E., Bengtson, Stefan, Marone, Federica, Greenwood, Jenny M., Donoghue, Philip C.J., University of Zurich, and Murdock, Duncan J E

Subjects
Tommotiida, Evolutionary Biology, Histology, Brachiopoda, Geology, 610 Medicine & health, Zoologi, Evolutionsbiologi, 170 Ethics, 1309 Developmental Biology, 1105 Ecology, Evolution, Behavior and Systematics, Cambrian, Geologi, 10237 Institute of Biomedical Engineering, Zoology, Phylogeny
Abstract

The fossil faunas of the Cambrian provide the only direct insight into the assembly of animal body plans. However, for many animal groups, their early fossil record is linked to disarticulated remains, interpretation of which is problematic since they possess few characters from which their affinity to phyla can be established and, indeed, few characters at all. One such group is the tommotiids, which has been interpreted, on the basis of skeletal anatomy, as a paraphyletic assemblage uniting brachiopods and phoronids, through the acquisition and subsequent modification, or loss, of an imbricated set of dorsal phosphatic sclerites. Here we present a reexamination of the fossil evidence uniting the tommotiids and brachiopods, supplemented with new anatomical data from synchrotron radiation X-ray tomographic microscopy of key tommotiid taxa. The characters used to support the complex hypothesis of character evolution in the brachiopod stem lineage relies on scleritome reconstructions and inferred mode of life which themselves rely on brachiopods being chosen as the interpretative model. We advocate a more conservative interpretation of the affinity of these fossils, based a priori on their intrinsic properties, rather than the modern analogue in whose light they have been interpreted.

Published
2014

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