Ape-like or hominid-like? The positional behavior of Oreopithecus bambolii reconsidered

Abstract

Comparative morphological and functional analyses of the skeletal remains of Oreopithecus bambolii, a hominoid from the Miocene Mediterranean island of Tuscany-Sardinia (Italy), provides evidence that bipedal activities made up a significant part of the positional behavior of this primate. The mosaic pattern of its postcranial morphology is to some degree convergent with that of Australopithecus and functionally intermediate between apes and early hominids. Some unique traits could have been selected only under insular conditions where the absence of predators and the limitation of trophic resources play a crucial role in mammalian evolution.

Figure 1

The lumbar region of Oreopithecus. Dorsal view of the lumbar region. (A1) Ape pattern with a caudally progressive decrease of the interfacet distances (Pan troglodytes, wild shot). (A2) Oreopithecus reconstructed, showing increasing distances from L3 to L5. (A3) Hominid pattern with a caudally progressive increase of the interfacet distances [Australopithecus, after Robinson (33)]. (B) Dorsal view of the Oreopithecus lumbar region with L3-L5 (BA72, + marks the center of the arch). (C1) Sagittal section of L5 of Dryopithecus laietanus. (CLL-18800, left ventral, right dorsal). (C2) Sagittal section of the lumbar region of Oreopithecus (BA72, left ventral, right dorsal). (BA = Naturhistorisches Museum in Basel, Switzerland, which provided A and B.) (Scale = 2 cm.)

Figure 2

Pelvic elements of O. bambolii. (A) Complete pubic symphysis (BA71), dorsal view. ps, pubic symphysis; of, obturator foramen. (B) Dorsal view of the left partial pubis of Oreopithecus, drawn to the same scale and overlapping those of (1) Pan, (2) Pongo, (3) Symphalangus, and (4) A. afarensis (AL 288–1). (C) Partial ischium (BA71) with acetabulum (a) and ischial spine (is). The ischial spine is only weakly developed in living apes and is associated with the diaphragma that joins the sacrospinous ligament, stressing it when supporting the viscera during vertical postures of the trunk (17). In bipedal hominids, however, the ischial spine is very large, due to the increased loading and strength of the sacrospinous ligament, which prevents rotation of the sacrum under the load of the trunk (34). (Scale = 2 cm.)

Figure 3

Foot morphology. (A) Oreopithecus (based on BA79 and BA83, right and left foot of the same individual). (B) Pan. (Upper) Dorsal view. Continuous line, long axis of the foot; interrupted line, axis of Mt3; lower dotted line, tarso-metatarsal joint axis, showing the permanent abduction of the lateral metatarsals; upper dotted line joining the distal ends of the Mt2–5 diaphyses, showing the medially decreasing length of the metatarsals. (Lower) Posterior view of the articulated tarsal elements. 1, Line of gravity; 2, inclination of the tuber calcis. (Scale = 2 cm.)

Figure 4

The foot proportions of Oreopithecus. A calculation of the power arm/load arm ratio (EMA) (21) for a broad spectrum of anthropoids (platyrrhines, cercopithecids, and hominoids). We measured R (load arm) from the head of MT 3 to the center of the trochlear surface of the talus, and r (power arm) from the latter to the distal part of the tuber calcis. Values for Gorilla, Homo, and Pongo are an average of body weight classified by sex; for Oreopithecus, the body weight is that estimated for the male individual IGF 11778 (32 kg) (35), whose size is consistent with that of the foot. Platyrrhines, cercopithecids, and chimpanzees show an allometric relationship with a slope 0.11. r = 0.933. IGF = Institute of Geology of Florence.