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Evolution of pacing in the family Camelidae

Thompson, Mary Elizabeth

Titre : Evolution of pacing in the family Camelidae

Auteur : Thompson, Mary Elizabeth

Université de soutenance : IDAHO STATE UNIVERSITY

Grade :  : Doctor of Philosophy (PhD) 2005

The family Camelidae is of general interest because of its adaptations to arid habitats and its distinctive mode of locomotion, the pace. The pace is a medium speed symmetrical gait like the trot, both using limbs in alternating pairs. The ipsilateral (same side) limbs are paired in the pace while diagonal limbs are paired in the trot. The family Camelidae has a long evolutionary history that spans some 45 million years, first appearing during a drying trend that produced areas of true savannah in North America. The limb morphology of early primitive camelids suggests adaptation to a cursorial unguligrade existence. During the Miocene, morphological changes occurred in foot skeletal structure indicating a digitigrade stance. These changes, considered together with a fossil camel trackway (i.e. a series of footprints) have been interpreted as evidence of the evolution of a pacing gait in Miocene camelids. Others dispute that a pacing gait can be inferred from a trackway, and point out that analogous morphological changes of the foot skeleton are lacking in horses that pace efficiently. Can pacing behaviour be correlated to morphological changes in the skeleton ? Can a pace be inferred from tracks alone ? Research methods involving extinct and extant camelid skeletal morphology and trackway analysis of extinct camels was used to answer these questions. Skeletal morphological changes in the camelids suggest adaptations to enhance lateral stability. These changes may have developed more in camelids as they progressed towards being “obligate” pacers. Animals that do not possess these adaptations to the same degree can still effectively pace. Trackways analysis of extinct animals constitute a way to describe a brief interval in an animal’s life. However, caution needs to be exercised when using trackways to differentiate between two gaits within a category such as a pace and a trot. If capping is observed, the trackway probably represents a trot not a pace. But if capping is absent and the stride length and straddle indicates a faster medium speed gait, it is difficult to ascertain whether the trackmaker is pacing or trotting. Further work examining the distance of overstride occurring in trotting and pacing trackways may provide a better prospective regarding this problem.

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