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The Origin and Evolution of Aquatic Adaptations in Cretaceous Squamates Open Access


Other title
aquatic adaptation
Type of item
Degree grantor
University of Alberta
Author or creator
Mekarski, Michelle M
Supervisor and department
Caldwell, Michael (Biological Sciences)
Examining committee member and department
Holmes, Robert (Biological Sciences)
Murray, Alison (Biological Sciences)
Acorn, John (Agricultural, Life and Environmental Sciences)
Laarson, Hans (Biology)
Department of Biological Sciences
Systematics and Evolution
Date accepted
Graduation date
2017-11:Fall 2017
Doctor of Philosophy
Degree level
Three species of Cretaceous marine squamates are described or redescribed. The first, Pontosaurus ribaguster is described from a relatively complete specimen discovered on Hvar Island, Croatia. Preservation of identifiable nektonic teleosts within the gastric cavity (the first identifiable gastric contents described from a dolichosaur) provide strong evidence of a primarily piscivorous diet. The second described species is a new genus and species of plesiopedal mosasauroid, Portunatasaurus krambergi, from the Cenomanian-Turonian (U. Cretaceous) of Dugi Otok, Croatia. The specimen preserves an exquisite forelimb showing a unique anatomy that appears to be transitional between Aigialosaurus and Mosasaurus. The new and unique limb anatomy contributes to a revised scenario of mosasauroid paddle evolution, whereby the abbreviation of the forelimb and the hydrofoil shape of the paddle evolves either earlier in the mosasaur lineage than previously thought, or more times than previously considered. The third description is a reassessment of the lizard Aphanizocnemus libanensis. Re-examination suggests that characters cited as supporting varanoid-dolichosaur affinities are misinterpreted, are common to many squamates, or are homoplastic and tightly linked to aquatic adaptation. Available data support the conclusion that Aphanizocnemus is not a dolichosaur, a varanoid, nor in fact an anguimorph, but may represent a new form of aquatic scincomorph, a group not previously recognized as having evolved aquatic adaptations. The three descriptions highlight morphological data that has been erroneously used, or were unavailable for, previous studies. A systematic analysis of the Pythonomorpha (inclusive of Pontosaurus ribaguster and Pontosaurus krambergi¸ and exclusive of A. libanensis) shows strong evidence for a monophyletic Aigialosauridae from which the hydropedal mosasauroid condition evolved at least twice. The results also support dolichosaurs as a non-monophyletic assemblage that form successive sister taxa to the derived ophidians. The relationship between mosasauroids (Aigialosauridae) and ophidiomorphs is less conclusive, recovering the ophidiomorph lineage as arising from within the Mosasauroidea, most often as a sister group to the Mosasaurinae. To provide context for their evolutionary history, non-ophidian ophidiomorphs are further investigated through paleobiogeography. Fragmentary discoveries suggest that dolichosaurs originated in the Valanginian (Early Cretaceous) or even the latest Jurassic. Diversity and density peaked in the Cenomanian as a result of a large radiation in the Tethys and Western Interior Seaway. This radiation was likely interrupted by the Cenomanian-Turonian Boundary Event, an extinction event which caused a considerable drop in diversity. Non-ophidian ophidiomorphs persist until the Maastrichtian, while achieving their largest geographical distribution: spanning Europe, North America and South America. Their fossil record indicated that this geographically widespread group inhabited nearshore and offshore marine environments, and made several independent radiations into freshwater environments. Their radiative success was driven by features that were predisposed to functionality in the marine environment. Such ‘preadaptive’ features could explain the propensity of the pythonomorph lineage to invade the water, as evidenced by multiple lineages independently radiating into the marine environment. Fast evolution of aquatically adapted features would subsequently allowed them to colonize the aquatic environment worldwide.
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