Oreopithecus

SARMIENTO, ESTEBAN E. & MARCUS, LESLIE F., 2000, The Os Navicular of Humans, Great Apes, OH 8, Hadar, and Oreopithecus: Function, Phylogeny, and Multivariate Analyses, American Museum Novitates 3288, pp. 1-1 : 1-

Oreopithecus

The large sagittal taloectocuneiform and entoectocuneiform angles and the relatively small transverse mesoectocuneiform angle (table 1 and 2) indicate the Oreopithecus foot had markedly divergent rays. The low navicular torsion with the strong medial inclination of the talar neck specifically indicate a markedly divergent hallux. Among the hominoids such a marked divergence is unique to Oreopithecus and is corroborated when the associated tarsal and metatarsal bones are articulated (Kohler and Moya­Sola, 1997). A shallow talar facet with small and approximately equal radii of curvature (table 4) corresponds to a loose talonavicular joint with rotatory ability. Such joint laxity is also reflected in the calcaneo­cuboid joint (Sarmiento, 1987; Szalay and Langdon, 1987) and in the single continuous navicular facet on the talar head. A talar facet which is large relative to the cuneiform facets (fig. 8) suggests an emphasis on mobility at the talonavicular relative to the naviculocuneiform joints. The absence of weight­bearing tubercles on the navicular and entocuneiform (Sarmiento, 1987) suggest that the foot lacked a commitment to terrestrial behaviors and/or was not often used for walking along large diameter horizontal supports. The large cuboid facet (fig. 6 and table 6) is in accord with powerful hallucal opposability and a foot loaded in supinated postures.

The Oreopithecus navicular best corresponds to the foot of an arboreal vertical climber. A relatively mobile foot with a wide opposable grasp, this foot could be apposed against vertical trunks when climbing, or used along horizontal supports of diameters permitting hallucal grasps.

PHYLOGENY IMPLICIT IN NAVICULAR MEASUREMENTS

It is striking how close the phenetic tree based on the Mahalanobis distances between samples of human and great ape navicular measurements (fig. 13) coincide with hominoid phylogenies as hypothesized by early anatomists (Keith, 1916, 1934, 1940; Morton 1927; Schultz, 1936; Le Gros Clark, 1971). Because navicular measurements were chosen to reflect largely functional concerns, co­ inciding phylogenies and phenetics may be unexpected, especially when epigenetic and inherited characters are given equal weight. Early anatomists and systematists, however, did not usually test the inferred homologies used to construct phylogenies for the likelihood of parallelisms (Sarmiento, 1998). Thus, these phylogenies are in essence phenetic trees, i.e., they equate degree of overall morphological similarity with the degree of relationship. Without the morphological complexity necessary to test for homologies, quantification of overall similarities in fragmentary fossils may be the only analysis possible. Results from such an analysis, however, do not accurately reflect phylogenetic affinities, since they conflate shared derived characters (synapomorphies) and parallelisms (plesiomorphies) and do not distinguish inherited characters from those originating ontogenetically with use.

Through cladistic analysis and knowledge as to what navicular characters are primitive vs. derived, testing for homologies may be possible. For instance, a weight­bearing plantar tubercle with a variable sustentacular articulation is a shared derived character of the African ape navicular absent in other catarrhines and the earliest known fossil hominoids (i.e. Oreopithecus and Sivapithecus ; Sarmiento, 1994, this study). The presence of a large weight­bearing plantar tubercle and a sustentacular facet on the navicular of Hadar forms may be used to argue that many of the angles and metric characters shared by Hadar and African apes are also homologous. As such, the Hadar naviculars would seem to share a special relationship with African apes exclusive of humans and orangutans. As with all cladistic analyses this argument assumes orthoselection with minimal reversals. In this case, the assumptions are that the large plantar tubercle and sustentacular facet was not (1) independently acquired by chimpanzees, gorillas and/or Hadar fossils, (2) a shared trait of humans and African apes which was later lost in humans, or (3) a shared hominoid trait that was independently lost in humans, orangutans, and Oreopithecus . With relatively few comparative taxa as outgroups and a limited number of shared derived characters for comparison, such an analysis may prove equivocal. In most cases more than a bit of localized morphology is necessary for phylogenetic resolution (Sarmiento, 1987).

Regardless of its inaccuracy, navicular phenetics and the cladistic analysis that can be done on the limited number of navicular characters does reflect phylogeny to some degree. In this regard, there must be a limit as to how much the naviculars of closely related forms may differ from each other and/ or how similar the navicular of more distally related forms may be. Similarities in inherited anatomy predispose the types of problems an organism encounters in its environment and the behaviors used to solve these problems. Differences in inherited anatomy, on the other hand, reduce the likelihood of encountering similar problems in the environment or arriving at similar solutions. Even if the same problems are encountered and similar solutions arrived at, inherited differences are more apt to result in greater differences in navicular morphology in more distally related taxa than in closely related ones.

Notably navicular phenetics also approximates the currently accepted hominoid phylogeny (Sarmiento, 1998). The only disagreement exists at the point of human divergence, and humans have a decidedly specialized foot committed to terrestrial bipedality (Weidenreich, 1922; Morton, 1924; Schultz 1963; Sarmiento, 1994, 1998). The close similarities in the navicular of OH 8 and humans, and of Hadar australopithecines and African apes, and the unique navicular of Oreopithecus must reflect to some degree phylogenetic affinities. This applies whether or not the measured characters also reflect functional concerns. As regards the navicular, the AL 333/333w remains are more likely to represent ancestral African apes than ancestral humans. The measured differences between OH 8, Hadar, and Oreopithecus naviculars are consistent with generic differences among humans and living great apes.