Mode de vie et affinités de Paschatherium (Condylarthra, Hyopsopontidae) d'après ses os du tarse

Marc Godinot, Thierry Smith and Richard Smith

Cite this article: Godinot M., Smith T., Smith R., 1996. Mode de vie et affinités de Paschatherium (Condylarthra, Hyopsopontidae) d'après ses os du tarse. Palaeovertebrata 25 (2-4): 225-242.

Abstract

Tarsals that can be confidently attributed to Paschatheríum dolloi from Dormaal (Belgium) are described. The astragalus is short; its broad neck is set at an angle of 30 degrees to the trochlea. The trochlea is pulley-shaped and proximally extended. There is no astragalar foramen. The medial tibial facet extends distally in a cup deeply excavated in the neck and buttressed. The sustentacular facet extends toward the navicular facet but is not fully confluent with it. The calcaneum bears a proximo-distally extended proximal facet for the astragalus. A relatively large peroneal tubercle projects from the body, and is situated distally. Functionally, the trochlea indicates extensive flexion-extension movements of the foot. Calcaneo-astragalar facets indicate sliding and rotation between the two tarsals. The inclination of the navicular facet suggests frequent foot inversion. The peroneal tubercle reflects good muscular capacities for foot rotation. Overall morphology is interpreted as a scansorial adaptation similar to that of sciurids.Comparisons are made with the tarsals of Macrocranion vandebroeki from Dormaal and Hyopsodus paulus from the Bridgerian of Wyoming. Some similarities between the astragali of Paschatherium and Macrocraníon (trochlea) are interpreted as convergences for rapid locomotion. However absence of mobility at the lower ankle and midtarsal joints in M. vandebroekzi suggests that this species was cursorial, as is known for M. tenerum from Messel. Similarities in the calcanea of Paschatherium and Hyopsodus are probably the result of close phylogenetic relations, and confirm the placement of Paschatherium in the hyopsodontids. The differences in the calcanea of Paschatherium and Macrocranion underline that Paschatherium is distinct from erinaceomorph insectivores. The differences in astragalar morphology between Paschatherium and Hyopsodus show that a marked adaptive divergence separates the two genera. We speculate about the common occurrence of a deep tibial cup (“cotylar fossa”) and a pulley-shaped trochlea in Paschatherium and hyracoids, suggesting that an adaptive scenario similar to that having led to Paschatherium (scansoriality) might explain the acquisition of the peculiar hyracoid tarsal characters; such a scenario contradicts the concept of Pantomesaxonia. Other peculiar characters of Hyopsodus suggest that hyopsodontids might be given more consideration in the search for hyracoid (and tethythere) origins.