Research

Functional traits of mammals

To better understand the ecology and history of mammals, it is critical to examine their ecologies (e.g., diets) and functional traits linked to ecologies (e.g., tooth shapes). By examining these functional traits across diverse groups of mammals (e.g., bats), we can link evolutionary changes factors such as ecological diversification and underlying genetic changes.

And examining these functional traits in modern mammals allows us to make inferences about the behaviors of extinct mammals. For example, paleontologists can use clues from the shapes of fossil teeth and jaws to help infer diets. Due to similar functional demands, jaws of mammals with particular diets tend to have similar shapes. Thus, I have focused on using jaw morphologies of modern mammals with known diets as analogs for inferring the unknown diets of Mesozoic mammals (250-66 million years ago). For quantitative comparisons of jaw shapes, I've applied regression models and a method called geometric morphometrics. I've also used results from these analyses to look broadly at macroevolutionary patterns in Mesozoic mammals. See Grossnickle & Polly (2013).

Mammalian jaw mechanics

Mammals underwent considerable evolutionary changes during the Mesozoic. For over a century, paleontologists have been particularly interested in the evolutionary changes to molars and middle ears. However, evolutionary transitions of the functional anatomy of the jaw have attracted less attention. I believe jaw morphologies may provide important clues concerning the biology and evolution of early mammals. Thus, I have examined macroevolutionary patterns of jaw morphologies, and I have used digital models of the jaws and attached muscles to examine functional changes in early mammal groups. In a recent paper (Grossnickle 2017), I argue that evolutionary changes to the jaws, molars, chewing muscles, and ears in the ancestors of most modern mammals allowed for greater 'yaw' rotation (right). This type of jaw movement may have been critical in expanding the dietary range of these mammals by allowing a grinding function of the molars.

Inferring locomotor modes

Paleontologists are detectives. They take limited fossil evidence and extract as much information as possible to help inform our understanding of ancient life. Fossil jaws and teeth often offer clues about diet (as mentioned above), and limb bones can offer considerable information about the the ways in which mammals moved around (i.e. locomotor modes). For example, long finger bones are most likely associated with tree climbers, while short, stout fingers are often indicative of borrowers (left; from Meng et al. 2015). I have used these basic functional concepts to help infer the locomotor modes of early mammals from the Jurassic (200-145 million years ago). These projects are in collaboration with Zhe-Xi Luo.