Investigator:

Tyler Lyson

Advisor: Jacques Gauthier

Description:

CT data allows gives paleontologists unprecedented access to morphological and phylogenetic data. Unfortunately, these data require computationally intensive programs which are lacking in the Geology Department. Along with Brian Andres and Rachel Racicot, I would like to use the Center's facilities to run the Avizo program to manipulate my CT data.

Specifically, I plan to address the puzzling relationship of the shoulder girdle inside the ribcage in turtles. This apparent relationship of the shoulder girdle inside the ribcage of turtles has puzzled neontologists, embryologists, and paleontologists for centuries. Unlike other extant amniotes where the scapula lies dorsal to several thoracic ribs, the turtle scapula appears to lie inside the thoracic ribs and thus inside the ribcage. However, while the shoulder girdle clearly lies inside the shell, its precise relationship to only those portions of the shell that are considered homologous with other amniotes ribs and vertebrae remains untested. The neural and costal bones, which make up the majority of the turtle shell, undergo both endochondral and intramembranous ossification and the whole structures have been hypothesized by some to be homologous to the endochondrally ossified ribs and vertebrae of other amniotes. However, fossil, histological, and some embryological data indicate that only the endochondrally ossified portion of the costal and neural is homologous to the ribs and vertebrae.

Computed Tomography images manipulated to show only the endochondral portion of the costals and neurals of a sample that covers the full extant tree space of turtles clearly indicate that the turtle scapula does not lie inside the ribcage, but rather frames the neck and lies anterior to the thoracic ribs. This relationship is homologous to the condition found in basal amniotes (e.g. Limnoscelis paludis). In addition, the position of the coracoid is similar to other amniotes in that it lies underneath the ribcage posteriodorsally to the clavicle and interclavicle. These observations indicate that turtles essentially retained the shoulder girdle of basal amniotes and that the position of this girdle within the ribcage is an illusion created by secondary dermal ossifications.