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On the Origin and Evolution of the Ophidia Open Access


Other title
Fossil snakes
Type of item
Degree grantor
University of Alberta
Author or creator
Palci, Alessandro
Supervisor and department
Caldwell, Michael W. (Biological Sciences)
Examining committee member and department
Murray, Alison (Biological Sciences)
Caldwell, Michael W. (Biological Sciences)
John Acorn (Renewable Resources)
Evans, David C. (Royal Ontario Museum)
Nydam, Randall (Midwestern University)
Department of Biological Sciences
Systematics and Evolution
Date accepted
Graduation date
Doctor of Philosophy
Degree level
With well over 3,400 described species, snakes undoubtedly represent one of the most successful groups of reptiles. Much has been written about their ecology, behavior, anatomy, relationships and evolution. However, despite the debate about the origin of this taxonomic group dating back to the second half of the XIX century, no consensus has been reached, yet. Scenarios that portray the first snakes as evolving from aquatic lacertilian ancestors are countered with others that see the first snakes as the result of long-term adaptations to a burrowing, cryptic lifestyle. The supporters of the first type of scenario found their evidence mostly in osteological comparisons of non-burrowing snakes with extinct aquatic lizards (e.g., mosasaurs, dolichosaurs and adriosaurs), while the supporters of the second type of scenario base their conclusions mostly on anatomical comparisons between modern legless squamates and burrowing snakes. The debate is further complicated by the scarcity of well-preserved fossil remains that may help elucidate the origin of the group, and by the contradictory interpretations that different authors have provided after examination of the same fossil specimens. Therefore, the goal of this work was that of analyzing all the evidence that has been put forward so far in support of the two main origin scenarios, critically evaluate the contradictory evidence provided by different researchers, examine first-hand all the pivotal extant and fossil taxa that have been used in the formulation and support of each scenario, and provide a phylogenetic analysis of snakes within squamate reptiles that is based both on molecular and morphological data. The examination of over 400 specimens of squamates, including both extant and fossil species, lead to the following results: (1) the redescription of several important fossil taxa (Pachyrhachis, Eupodophis, Haasiophis, Najash, and Dinilysia), and the identification, in some of them, of anatomical features never reported before (e.g., chevron bones in Haasiophis, mental foramina and a sacral vertebra in Pachyrhachis); (2) the discovery of new material (a pelvic girdle) attributable to the fossil species Wonambi naracoortensis, a snake whose pelvic anatomy was previously unknown; (3) the retrieval of evidence that supports a reinterpretation of the circumorbital bones of snakes, with particular regard to the “postorbital” and the “supraorbital”, here reinterpreted as primary homologues of the jugal and postfrontal, respectively; (4) a detailed assessment of what constitutes the “crista circumfenestralis” of snakes and how this anatomical feature varies within the Ophidia; (5) new hypotheses regarding the ingroup relationships of snakes, which imply a possible convergent evolution of the macrostomatan skull condition, and the possibility that scolecophidians may represent an aberrant lineage of alethinophidian snakes.
This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for the purpose of private, scholarly or scientific research. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.
Citation for previous publication
Palci, A., M. W. Caldwell, and R. L. Nydam (2013), Reevaluation of the anatomy of the Cenomanian (Upper Cretaceous) hind-limbed marine fossil snakes Pachyrhachis, Haasiophis, and Eupodophis, Journal of Vertebrate Paleontology, vol. 33, issue 6, 1328–1342.Palci, A., M. W. Caldwell, and A. M. Albino (2013), Emended diagnosis and phylogenetic relationships of the Upper Cretaceous fossil snake Najash rionegrina Apesteguía and Zaher, 2006, Journal of Vertebrate Paleontology, vol. 33, issue 1, 131–140.Palci, A., M. W. Caldwell, and J. D. Scanlon (2013), First report of a pelvic girdle in the fossil snake Wonambi naracoortensis Smith, 1976, and a revised diagnosis for the genus, Journal of Vertebrate Paleontology, vol. 34, issue 4, 965–969.Palci, A., and M. W. Caldwell (2013), Primary homologies of the circumorbital bones of snakes. Journal of Morphology, vol. 274, issue 9, 973–986.Palci, A., and M. W. Caldwell (2014), The Upper Cretaceous snake Dinilysia patagonica Smith-Woodward, 1901, and the crista circumfenestralis of snakes. Journal of Morphology, vol. 275, issue 10, 1187–1200.

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