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Title: The early evolution of sea turtles
Author: Evers, Serjoscha W.
ISNI:       0000 0004 7654 3458
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2018
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Turtles are a major group of reptiles with moderate species richness (335 extant species) but high ecological diversity, inhabiting terrestrial, freshwater, and marine habitats. Stem-turtles evolved in the Late Permian or Early Triassic, and the crown-group appeared during the Jurassic. Fossils suggest that even the earliest turtles were ecologically diverse. As an ecologically diverse group with an excellent fossil record, turtles provide information on evolutionary dynamics of ecological transitions in vertebrates. For instance, marine ecologies evolved several times within turtles, although only chelonioid sea turtles survive to the present. Uncertainties regarding the relationships of extinct clades render the precise number of marine transitions uncertain. The unresolved phylogeny further impedes on understanding of trait evolution during early chelonioid evolution. I address these knowledge gaps by detailed anatomical study of relevant turtle groups. I present new phylogenetic hypotheses of turtles using different inference methods (parsimony, Bayesian). A taxonomically comprehensive dataset (every extant and most extinct clades sampled) of morphological characters was derived from high-resolution computer-tomography data and vetted to reflect homology. Results suggest that protostegids are crown- or stem-group chelonioid sea turtles, implying a single origin of a pelagic lifestyle in turtles. A general flipper bauplan was established early during chelonioid evolution, and modified subsequently in different subgroups. Time-calibration suggests an Early Cretaceous divergence time for chelonioids. This implies that some main clades of extant cryptodires (Chelonioidea, Chelydroidea, Testudinoidea) diversified earlier that commonly thought, during the Early Cretaceous and simultaneously with pleurodires and trionychians. A range of Jurassic-Cretaceous 'stem- cryptodires' (e.g. sinemydids) are found as stem-turtles. Non-pleurodiran marine turtles from the Jurassic-Paleogene (sandownids, thalassochelydians) form a previously unrecognized clade of stem-turtles, Hyphalochelydia. Marine ecologies evolved five-six times in turtles. All three Late Cretaceous marine turtle clades (hyphalochelydians, chelonioids, bothremydid pleurodires) survived the K/Pg mass extinction despite high levels of extinction in other marine reptiles.
Supervisor: Benson, Roger B. J. ; Barrett, Paul M. Sponsor: Natural Environmental Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Paleontology ; Systematics ; Palaeoecology ; Paleoecology ; Palaeontology ; Evolutionary Biology ; Herpetology