Orang-like manual adaptations in the fossil hominoid Hispanopithecus laietanus: first steps towards great ape suspensory behaviours

Abstract

Morphological and biometrical analyses of the partial hand IPS18800 of the fossil great ape Hispanopithecus laietanus (=Dryopithecus laietanus), from the Late Miocene (about 9.5Ma) of Can Llobateres (Catalonia, Spain), reveal many similarities with extant orang-utans (Pongo). These similarities are interpreted as adaptations to below-branch suspensory behaviours, including arm-swinging and clambering/postural feeding on slender arboreal supports, due to an orang-like double-locking mechanism. This is confirmed by the long and highly curved phalanges of Hispanopithecus. The short and stout metacarpals with dorsally constricted heads, together with the dorsally extended articular facets on proximal phalanges, indicate the persistence of significant degrees of palmigrady. A powerful grasping capability is indicated by the great development of basal phalangeal tubercles, the marked insertions for the flexors on phalangeal shafts and the large pits for the collateral ligaments. The morphology of the Hispanopithecus long bones of the hand indicates a unique positional repertoire, combining orthogrady with suspensory behaviours and palmigrade quadrupedalism. The retention of powerful grasping and palmigrady suggests that the last common ancestor of hominids might have been more primitive than what can be inferred on the basis of extant taxa, suggesting that pronograde behaviours are compatible with an orthograde bodyplan suitable for climbing and suspension.

Figure 1

Reconstruction of the H. laietanus partial hand IPS18800 from Can Llobateres 2, (a) in dorsal and (b) palmar view.

Figure 2

Proximal and ulnar views of the fourth proximal phalanx in selected fossil and extant taxa: (a) Papio sp., (b) Sivapithecus, (c) Hispanopithecus, (d) Pongo, (e) Pan and (f) Gorilla. All the depicted specimens come from the right side (the Sivapithecus one being a mirror image of a cast from the left side).

Figure 3

Dorsal, radial and palmar views of the fourth metacarpal in selected fossil and extant taxa: (a) Papio, (b) Hispanopithecus, (c) Pongo, (d) Pan and (e) Gorilla.

Figure 4

Line drawing of selected bones from the H. laietanus partial hand IPS18800 from Can Llobateres, showing the main anatomical features discussed in the text: (a) fourth metacarpal in distal, (b) dorsal, (c) radial and (d) palmar views, (e) fourth proximal phalanx in palmar view, (f) third proximal phalanx in palmar, (g) radial and (h) dorsal views, third intermediate phalanx in (i) radial and (j) palmar views, possible fourth distal phalanx in (k) radial and (l) palmar views. Characters: 1, enlarged pits for the collateral ligaments; 2, marked dorsal constriction of metacarpal heads; 3, smooth palmar surface of metacarpal heads; 4, enlarged ulnar tubercle; 5, protruding radial tubercle; 6, deep groove for channelling the long flexor tendons; 7, well-developed palmar tubercles; 8, dorsal extension of the proximal articular surface; 9, strong and distally positioned flexor sheath ridges; 10, palmarly bent trochlea; 11, well-developed insertions for the superficial flexors; 12, conspicuous muscular insertions at the palmar side only; 13, larger palmar lips.

Figure 5

Results of the three discriminant analyses, displayed by means of UPGMA (Unweighted Pair Group Method with Arithmetic Mean) clusters based on Euclidean distances computed from group centroids (extant genera) and discriminant scores (Hispanopithecus) for the four canonical axes (see electronic supplementary material, table 2): (a) overall manual proportions, (b) metacarpal robusticity and (c) phalangeal robusticity.

Figure 6

Bivariate plot of second versus first canonical axes (discriminant functions) for the three discriminant analyses: (a) overall manual proportions, (b) metacarpal robusticity and (c) phalangeal robusticity. The results for Hispanopithecus by interchanging third and fourth proximal phalanges are displayed within brackets.

Figure 7

Intrinsic manual ray IV proportions displayed as an allometric bivariate plot of phalangeal versus metacarpal length in hominines, orang-utans and baboons. The results for Hispanopithecus by interchanging third and fourth proximal phalanges are displayed within brackets. Abbreviations: MC, metacarpal; PP, proximal phalanx; MP, intermediate phalanx; L, length.

Figure 8

Robusticity of metacarpal and proximal phalanx IV displayed as allometric bivariate plots of articular area versus bone length in extant hominids and baboons: (a) robusticity of metacarpal base, (b) robusticity of metacarpal head, (c) robusticity of proximal phalanx base and (d) robusticity of proximal phalanx trochlea. The results for Hispanopithecus by interchanging third and fourth proximal phalanges are displayed within brackets. Abbreviations: MC, metacarpal; PP, proximal phalanx; B, base; H, head; T, trochlea; L, length; A, area.

Figure 9

Relative metacarpal and phalangeal length in manual ray IV displayed as allometric bivariate plots of bone length versus body mass in extant hominids and baboons: (a) relative metacarpal length and (b) relative proximal phalanx length. The several points corresponding to Hispanopithecus do not represent different individuals, but different size estimates. The results for Hispanopithecus by interchanging third and fourth proximal phalanges are displayed within brackets. Black symbols, males; open symbols, females. Abbreviations: MC, metacarpal; PP, proximal phalanx; BM, body mass.