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Who is Anomodont A? Archiving and Reconstructing Serially-sectioned Dicynodont Skulls

2012 REU Project:

Anomodonts are an extinct group of ancient mammal-relatives (nonmammalian synapsids), and during the Permian and Triassic periods of Earth history, they were the most diverse and abundant tetrapod herbivores. In 1944, the paleontologist Everett Olson published a long paper describing the braincase morphology of several nonmammalian synapsid specimens, including anomodonts, in the Field Museum's paleontology collections. To carry out this work, Olson serially-sectioned the specimens, essentially cutting them into thin slices and then studying the arrangement of the bones exposed in the successive cross-sections. Unfortunately, Olson did not provide detailed identifications for the specimens he sectioned, making it hard to use his data in further comparative studies. Of particular interest is a specimen he identified only as “Anomodont A,” which may be a rare specimen of a non-dicynodont anomodont. The sections that Olson made are still housed in the Geology Department's collections.

Research methods and techniques: In this project we will first scan the sections that Olson made, and then will make virtual 3D reconstructions of the specimens using software designed for processing CT scan data. Once the reconstructions are available, it should be possible to identify the specimens more precisely, making the specimens much more valuable for research on synapsid braincase evolution.

Curator/Advisor: Dr. Kenneth D. Angielczyk (Geology)


REU Intern: FLORENCE LIN
Anthropology major
University of Illinois at Urbana-Champaign

Symposium Presentation Title: Who is Anomodont A? Archiving and Reconstructing Serially-sectioned Dicynodont Skulls

Symposium Presentation Abstract: Anomodontia is the most successful group of extinct therapsids, surviving the Earth’s largest mass extinction and occurring on every continent from the middle Permian through the late Triassic. In 1944, Everett Olson serially-sectioned several therapsid specimens and then created nitrocellulose peels from the specimens. Among the remaining peels were those from various specimens of anomodontia, cynodontia, and therocephalia. Due to the fragility of the peels, we wanted to digitally archive them to improve accessibility for researchers. Of particular interest was the specimen that Olson designated Anomodont A, which possessed an elongated snout and anteriorly-positioned canines, similar to those seen in basal anomodonts. Because basal anomodonts are rare and poorly known, it would be of great interest if we could confirm such an identity for Anomodont A. We scanned the individual peels and then stacked the slices in Amira to create a 3-D reconstruction. We labeled prominent morphological features such as the canines, external nares, and postcanine teeth, and used these characteristics in comparisons with 23 species of basal anomodonts and dicynodonts from the middle Permian. Our 3-D reconstruction of Anomodont A revealed the presence of a secondary palate and tusk. Furthermore, it possessed a shorter, very dicynodont-like snout and no premaxillary teeth, making it highly unlikely for Anomodont A to have been a basal anomodont. Instead, it shared a considerable number of similarities with basal dicynodonts such as Brachyprosopus, Chelydontops, and Pristerodon. The resemblance between Anomodont A and basal dicynodonts indicates that Olson’s depiction is inaccurate. Correctly identifying and archiving Olson’s specimens is important because it allows researchers to more accurately apply Olson’s data on braincase morphology to modern phylogenetic and evolutionary questions.