Evolutionary rates and patterns of artiodactyl limb reduction

Mammalian morphological response to Cenozoic environmental change has long been studied. Traditionally, the appearance of hypsodonty in equids is considered an adaptation to the introduction of abrasive material to the diet (e.g. grass), and has been used as a means of signifying the opening of environments. However, the timing of the radiation of hypsodont equids conflicts with the paleobotanical appearance of grasses and, in fact, hyspodont dentition may not necessarily indicate the addition of grasses to the diet. Despite the conflict arising between dentition and timing of environmental opening, other skeletal characters can be examined for morphological adaptation to environmental opening (e.g. cursoriality). In an attempt to further explore ungulate adaptation to environment, I am evaluating the Order Artiodactyla for rate of cursorial evolution. Artiodactyla was selected because of the availability of a large number of extant and extinct specimens and the fact that these ungulates emerged and radiated during the time of environmental opening. This study represents novel research by exploring fusions and reductions in the number of distal limb elements across the entire order. Reductions and fusions are important to cursoriality because they reduce the limb’s distal weight and render the distal limb elements more stable, allowing for faster and more efficient locomotion across open terrain. The spread of open environments (e.g., savannas), as a result of Cenozoic environmental change, might have provided an impetus for these types of cursorial adaptations. Cenozoic environmental opening is believed to have occurred gradually, beginning as small patches of low-biomass vegetation in the Eocene, leading to short grasslands in the Oligocene, followed by the tall, modern grasslands of the Late Miocene. With this in mind, my working hypothesis that the opening of environment drives cursorial adaptation, and therefore, rates of cursoriality should be highest from the Eocene to Late Miocene. To test this, 306 artiodactyls were analyzed for 47 discrete characters in limb morphology that were indicative of cursorial adaptations and recorded in Mesquite 2.72 (build 514). I calculated the rate of limb evolution across the entire order and found that there was a very high peak throughout the Eocene, which has also been found in ungulates by a study examining metatarsal/femur ratios- a method used to determine cursoriality. In addition, the earliest artiodactyls (about 55Mya) were already showing signs of cursoriality. The evidence suggested by my results, metatarsal/femoral ratios and the morphology of the earliest artiodactyls indicate that artiodactyls were evolving cursorial adaptations early on in their evolutionary history. In addition to the high rate of the Eocene, I found a slight increase of rate during the Late Oligocene. This increase in rate coincides with a dramatic increase in open habitat phytolith assemblages from the Great Plains of North America. This may indicate that artiodactyl cursoriality rate was increasing simultaneous to North American environmental opening. My hypothesis that the opening of environments drives cursorial adaptation is supported by the second, smaller peak of the Late Oligocene, but more research is needed to explore this drastically high rate of cursorial adaptation during the Eocene, which was considered to be a time of closed, widespread forests with few pockets of low-biomass vegetation