The early evolution of sea turtles

2009 REU Intern William Adams


Junior Biology major at Loyola University

REU Mentors: Dr. Kenneth D. Angielczyk (Assistant Curator of Paleomammalogy, Geology) and Dr. James Parham (Postdoctoral Research Associate, Biodiversity Synthesis Center)

Symposium Presentation Title: A Geometric Morphometric Analysis of Plastron Morphology in Marine Chelonioids and their Relatives (Chordata: Reptilia: Testudines: Chelonioidea)

Symposium Presentation Abstract: Chelonioid turtles include the extant sea turtles and extinct taxa extending back to the Cretaceous. A distinguishing feature of chelonioids is the reduction of their plastron or ventral shell. The shape and proportions of chelonioid plastra have been examined in the past as a source of information for inferences about the process of marine adaptation in sea turtles. Previous research was based on simple sets of linear measurements that only provided a crude picture of plastral shape variation. We examined the plastra of several genera of extinct chelonioids and compared them to those of extant cheloniids, extant chelydrids (snapping turtles), and kinosternids (mud turtles and musk turtles) to determine whether marine turtles are characterized by a distinctive plastron shape, and how that shape differs from those of other turtles. We used landmark-based geometric morphometrics to quantify plastral shape variation, and also conducted an extended eigenshape analysis of the outlines of the extant specimens and the best preserved extinct specimens to determine if the plastral outline preserved additional relevant information. Our results indicate that plastron shape can be used to differentiate between advanced marine forms and their less-derived relatives. Advanced marine turtles have reduced plastra that are longer and narrower than those of non-marine turtles, with large central fontanelles almost completely separating the plastra into two halves. More basal chelonioids have robust plastra with little or no central fontanelle. Plastra of protostegids, advanced marine turtles that may not be chelonioids, resembled the cheloniidae plastra. This similarity emphasizes the link between plastron shape and ecology. Finally, our shape data are highly correlated with the “plastral index” used by previous workers, showing that the latter metric captures similar shape information.

Original Project Title: Sea turtles are one of the most popular and appreciated groups of extant reptiles and are icons for global conservation efforts. They also are fascinating from an evolutionary perspective because of their adaptations to a pelagic marine lifestyle. Nevertheless, the origin and details of this major ecological transition have received little recent attention. We will use phylogenetic and geometric morphometric techniques to investigate the origin and early morphological evolution of sea turtles. The geometric morphometric component of the study will focus on shell shape because it is a strong indicator of ecology, largely reflecting the defensive and locomotory requirements of marine turtles compared to their freshwater ancestors. Furthermore, previous workers, particularly Rainer Zangerl of The Field Museum, developed detailed hypotheses about the iterative evolution of pelagic specializations (e.g., Zangerl 1953, 1980) that relied strongly on shell shape. By combining modern geometric morphometric analyses of shell shape with analyses of sea turtle phylogeny based on morphological and molecular data, we can provide a more detailed, accurate, and holistic picture of the early evolutionary history of sea turtles. The Field Museum is an idea location for this research because the collections of the Department of Geology include key specimens of early sea turtles, most of which were described by Zangerl (1953). "The past five decades have witnessed major advances in phylogenetic and morphometric methods, as well as digital imaging technology, but most of these critical fossils have received little or no attention during this time. Therefore, a reassessment of these specimens in a modern framework will be a very fruitful endeavor.

Research methods and techniques: The intern will be given and introduction to the theories and practice of geometric morphometrics, as well instruction on turtle morphology and phylogenetics, taxonomic principles, the handling and use of paleontological specimens, as well as scientific description, illustration, and publication. They will collect and analyze morphometric and morphological data directly from fossil and extant sea turtle specimens and learn to integrate these data into a phylogenetic framework. The intern will be encouraged to present their results as a published paper and at scientific meetings such as the annual meetings of the Society of Vertebrate Paleontology or the Society for Integrative and Comparative Biology.