Evidence of a false thumb in a fossil carnivore clarifies the evolution of pandas

Abstract

The "false thumb" of pandas is a carpal bone, the radial sesamoid, which has been enlarged and functions as an opposable thumb. If the giant panda (Ailuropoda melanoleuca) and the red panda (Ailurus fulgens) are not closely related, their sharing of this adaptation implies a remarkable convergence. The discovery of previously unknown postcranial remains of a Miocene red panda relative, Simocyon batalleri, from the Spanish site of Batallones-1 (Madrid), now shows that this animal had a false thumb. The radial sesamoid of S. batalleri shows similarities with that of the red panda, which supports a sister-group relationship and indicates independent evolution in both pandas. The fossils from Batallones-1 reveal S. batalleri as a puma-sized, semiarboreal carnivore with a moderately hypercarnivore diet. These data suggest that the false thumbs of S. batalleri and Ailurus fulgens were probably inherited from a primitive member of the red panda family (Ailuridae), which lacked the red panda's specializations for herbivory but shared its arboreal adaptations. Thus, it seems that, whereas the false thumb of the giant panda probably evolved for manipulating bamboo, the false thumbs of the red panda and of S. batalleri more likely evolved as an aid for arboreal locomotion, with the red panda secondarily developing its ability for item manipulation and thus producing one of the most dramatic cases of convergence among vertebrates.

Fig. 1.

Skeletal anatomy of S. batalleri.(A) Skeletal reconstruction of S. batalleri. The pelvis, femora, tibiae, fibulae, sacrum, and caudal vertebrae are not known and have been reconstructed on the basis of related taxa. (B) Articulated right carpus and metacarpus in palmar view, showing the position of the radial sesamoid (rs) in Ailuropoda melanoleuca (Left; 1) and S. batalleri (Right; 2) (not at scale). Art by M. Antón; manus of giant panda modified from ref. .

Fig. 2.

Comparisons of left radial sesamoids of the three species of carnivores with false thumbs, represented at the same size. (Left) Internal face. (Right) External face. Am, Ailuropoda melanoleuca; Sb, Simocyon batalleri; Af, Ailurus fulgens; 1, abductor pollicis brevis muscle; 2, abductor pollicis longus muscle; 3, opponens pollicis muscle; a, articulation facet with scapholunar.

Fig. 3.

Schematic tree summarizing the phylogenetic relationships and temporal ranges of the members of Arctoidea referred to in this work, highlighting the independent appearance of the false thumb (gray circle). Data on sister-group relationships and divergence dates are from refs. -, , , and .