The origins of marine turtles

Turtles have invaded the marine realm many times. The origins of the most diverse and specialized group of sea turtles (the Chelonioidea) are poorly known. Many pre-cladistic investigations into this problem hypothesized that Chelonioidea represents a grade of turtle specialization. Alternatively, all cladistic studies accept the monophyly of a marine Chelonioidea a priori. Given the conservative morphology of turtles, the possibility that proposed chelonioid synapomorphies are actually parallelisms has not been rigorously investigated. For example, the most relevant fossils (sea turtle-like “macrobaenids” and extinct sea turtles) are exlcuded from most cladistic analyses of cryptodire phylogeny.; My dissertation is the first rigorous test of chelonioid monophyly. In addition to including new data on “macrobaenids” from Asia, I also include new data on two Cretaceous “macrobaenids” from North America, Judithemys and Osteopygis. Judithemys was included in many cladistic analyses over the past seven years, but is described here for the first time. Osteopygis has been known for over 130 years, but for the last 50 years, paleontologists have assumed that well-preserved sea turtle skulls from the same formation could be attributed to it. Consequently, the “macrobaenid” affinities of Osteopygis have been obscured. I integrate historical collection information, taphonomy, osteological characters, and functional interpretations to correct this longstanding mistake. These new data and interpretations set the stage for a detailed phylogenetic study of neocryptodiran turtles including a broad sampling of Cretaceous fossil taxa and mitochondrial characters.; The result of my phylogenetic analysis is less support for the basal node of Chelonioidea than reported by previous studies. Reassessments of the proposed morphological synapomorphies for Chelonioidea show that some of them are correlated with one another, a marine habit, or with a single functional complex. A reanalysis of the available data shows that, with minor changes to initial assumptions of correlation vs. independence, parsimony analyses of combined morphological and molecular data recover a paraphyletic “traditional” Chelonioidea. Therefore, it would be premature to claim that chelonioids have a single marine origin. The parallel evolution of marine chelonioids is a plausible competing hypothesis.