Meeting Abstract

18.5  Friday, Jan. 4  Reconstructing the Diel Activity Patterns of Fossil Nonmammalian Synapsids ANGIELCZYK, KD*; SCHMITZ, L; Field Museum of Natural History; Claremont McKenna, Pitzer, and Scripps Colleges kangielczyk@fieldmuseum.org

The majority of extant mammals are nocturnal, and it has been assumed that this trait characterized the earliest mammals. It also has been hypothesized that the shift to nocturnality caused a fundamental reorganization of the circadian system in mammals. However, the diel activity patterns (DAP) of the nonmammalian synapsid ancestors of mammals have never been examined in detail, even though this could provide insight into whether nocturnality is characteristic of mammals or a deeper lineage, and whether nocturnality evolved multiple times among synapsids. Eye dimensions are correlated with light sensitivity, and eye shape can be used to effectively discriminate amniotes of different DAP. Orbit and scleral ring dimensions are reliable skeletal proxies for eye shape, and can be used to extend reconstructions of DAP into the fossil record. Extant mammals lack scleral rings, but they are present, although infrequently preserved, across much of nonmammalian synapsid diversity. We compiled a data set of 40 specimens from 28 synapsid species. We used previously published data on scleral ring and orbit dimensions of extant squamates and avians with known DAP to establish classification rules with a linear discriminant analysis. Using prior probabilities derived from proportions of DAP among extant amniotes, we classified fossil synapsids (species averages) into diurnal, nocturnal, and cathemeral categories. Our results suggest that diurnality was the most common DAP in the analyzed sample. However, nocturnality was present in several clades, including Varanopidae, Sphenacodontidae, Therocephalia, and Cynodontia. Nocturnality likely evolved multiple times within synapsids, with its earliest appearance in the Permo-Carboniferous.