The fossil horse sequence is a popular example of a phylogenetic pattern resulting from the evolutionary process. However, the seemingly transitional stages are actually derived from a scattered sampling of horse fossils within the multi-branched horse evolutionary tree. The current hypothesis is that the reduction in the number of digits was necessary for the cursorial lifestyle of today's horses. Lengthening of the distal segments and toe-tip running increased the stride length and thus the speed with which the animal can move. However, to avoid the increase in cost of swinging the heavier limbs, the digital number was reduced. Reinforcement of the middle toe might have also led to an improvement of the stability. The drawback to the reduction lies in loss in versatility and a decreased ability to run on compliant substrates.
This project proposes to perform detailed comparative research on horses and their extant relatives. By studying donkeys, zebras, horses and the closely related rhinoceros and tapir, we will detect evolutionary patterns within this group. By studying how the animals move combined with a detailed study of the limb anatomy of the same species, we aim at providing insights in the mechanisms behind the reduction in digital number.
The limb movements of the animals will be studied using traditional gait analyses techniques. Video material will be obtained synchronized with ground reaction forces and pressure data under the hooves. This information can be combined with inertial information in a calculation technique called inverse dynamics that will yield the net joint moments and power profiles over time. These profiles can be regarded as indications for motor control patterns. This will allow us to compare motor control patterns between the different species. These gait analysis experiments will be done in collaboration with European zoos.
Detailed anatomical descriptions of the joint surfaces and segment proportions will be used to obtain measures for range of motion and joint center locations. These will be compared between species and will create a basis for assessing range of motion in extinct species. These descriptions will be based on 3D scans of osteological samples obtained from museum material.
Models of trait evolution will be used to discern how limb skeletal morphology and motor control patterns have evolved in perissodactyls. This work will then form the basis for future work on extinct species of the Perissodactyli.