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Transformation from plantigrady to digitigrady: Functional morphology of locomotion in Hesperocyon (Canidae: Carnivora)

Authors:
  • Natural History Museum of Los Angeles

Abstract

The most primitive canid Hesperocyon from the late Eocene to early Oligocene (Duchesnean to Whitneyan) of North America possesses a combination of both cursorial and noncursoiial limb characteristics. Compared to 'the archaic miacoid carnivorans, Hesperocyon has a number of advanced features suggestive of running adaptations, such as absence of a large teres major process on the scapula, a relatively reduced deltoid crest on .the humerus, elongated and adjoined metapodials, and laterally oriented fovea capitis femoris. On the other hand, Hesperocyon retains some primitive , arboreal characteristics such as a short, broad scapula with long acromion, a stout humerus with a deep bicipital groove and a thin-bladed bra-chioradialis crest, an anteriorly bowed ulna, a sharply angled pelvic floor, a relatively large muscle scar for flexor digitorum longus and caudal tibialis on the tibia, and a short, deep, and hooked distal phalanx suggestive of a retractile claw. This combination of cursorial and noncursorial characters shows that Hesperocyon represents a transitional stage from its arboreal plantigrade ancestors to its cursorial digitigrade descendants. Examination of a series of miacoids and primitive canids (Vulpavus, Miacis, Hesperocyon, Cormo-cyon, Leptocyon, and Tomarctus) reveals that the canid metatarsus did not become erect until the Arikareean Cormocyon and Leptocyon. Increased extension angle of the tibioastragalar joint was achieved through realignment of the plantar tendon groove to be continuous with the astragalar trochlea and elimination of the astragalar foramen. By combining anatomical characters with results of a morphometric analysis, I have concluded that Hesperocyon was plantigrade in posture and probably scansorial in life.
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... -Plantigrade: the heel (= calcaneus + astragalus) is placed completely flat on the ground during locomotion (Wang 1993; Polly 2010); -Semi-digitigrade: the heel is occasionally in contact with the substrate at rest, but in motion the animal adopts a more digitigrade position (Polly 2010); -Digitigrade: the animal stands on the distal ends of the middle metapodials and phalanges; the heel does not come into contact with the ground during locomotion (Wang 1993;Polly 2010). ...
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