The majority of living mammals are nocturnal, but when did this daily activity pattern evolve in the 300 million year history or mammals and their closest relatives, and did it evolve more than once? Answering this question requires data from both fossil and living mammals and their relatives. Of particular importance is determining how well the dimensions of the eye socket correlate with daily activity patterns in living mammals: if the correlation is good, we can use the same measurements to make predictions about the daily activity patterns of fossil mammals. In this project, we will examine the correlation between daily activity patterns and eye socket dimensions in living and fossil members of Sciuridae, the large group of rodents that includes such familiar animals as squirrels, chipmunks, groundhogs, and prairie dogs.
Research methods and techniques: This project will involve measuring eye socket dimensions on sciurid skulls using digital calipers, recording information on body mass and/or measuring skeletal proxies for body mass, and compiling information on daily activity patterns from literature sources. We will analyze the data using various statistical methods, including a discriminate function analysis developed by a collaborator (Dr. Lars Schmitz, UC Davis).
Curator/Advisors: Dr. Kenneth Angielczyk, Geology
REU Intern: STEPHANIE M. SMITH
Johns Hopkins University
Symposium Presentation Title: Do orbit dimensions predict daily activity patterns in rodents?
Symposium Presentation Abstract: Inability to directly observe the activities of extinct species means that all information must come from specimens and geologic context. However, this information is limited; for example, how does one determine patterns of daily activity based only on skeletal remains? Successful distinction among nocturnal, diurnal, and crepuscular amniotes has been achieved in those organisms having bony structures that provide close eyeball size correlates, including the scleral ring and postorbital bar; these structures can indicate, according to optical principles, whether an organism is adapted to a high- or low-light environment. But some taxa do not possess these features, and those that do often have them poorly preserved. Other eye size correlates are available, but their reliability as predictors of diel activity pattern has not been established in mammals or basal non-mammalian synapsids. Here we use skeletal data from 51 species of extant sciurids and one fossil specimen to determine the possibility for such predictions. Using a variety of statistical analyses, we show that the use of orbital dimensions alone is sufficient to distinguish nocturnal species from non-nocturnal species, but only in cases of morphological extremes. However, differences in structure between extant sciurids and basal non-mammalian synapsids indicate that it might be possible to obtain a cleaner distinction in the latter group.