Oreopithecus

Swamp Ape; Temporal range: Miocene
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Primates
Family:	Hominidae
Genus:	Oreopithecus
Species:	O. bambolii
Binomial name
	Oreopithecus bambolii; Gervais, 1872

The Swamp Ape (Oreopithecus bambolii) is a prehistoric primate species from the Miocene epoch whose fossils have been found in Italy (Tuscany and Sardinia) and in East Africa. To date, over 50 individuals have been discovered from the Tuscan mines of Monte Bamboli, Baccinello, Montemassi, Casteani, and Ribolla, making Oreopithecus one of the best-represented fossil apes.

Oreopithecus bambolii is estimated to have weighed 30-35 kg. It possessed a relatively short snout, elevated nasal bones, small and globular neurocranium, vertical orbital plane, and gracile facial bones. The shearing crests on its molars suggest a diet specializing in plant leaves. The very robust lower face, with a large attachment surface for the masseter muscle and a sagittal crest for attachment of the temporal muscle, indicates a heavy masticatory apparatus. Its teeth were small relative to body size. The lack of a diastema (gap) between the second incisor and first premolar of the mandible indicates that Oreopithecus had canines of size comparable to the rest of its dentition. In many primates, small canines correlate with reduced inter-male competition for access to mates and less sexual dimorphism.

Its habitat appears to have been swampy, and not savanna or forest. The postcranial anatomy of Oreopithecus features adaptations to both suspensory arborealism and bipedalism. Functional traits related to suspensory locomotion include its broad thorax, short trunk, high intermembral index, long and slender digits, and extensive mobility in virtually all joints. At the same time, it also features adaptations to upright walking such as the presence of a lumbar curve, in distinction to otherwise similar species known from the same period. Since the fossils have been dated to about 8 million years ago, this represents an unusually early appearance of upright posture, lending itself to the aquatic ape theory of human bipedalism, especially in conjunction with other evidence of a possible earlier date for the evolution of hominids, such as the six to seven million year old, very human-looking Toumai skull. How adapted for bipedal walking it was is not known, but its fingers and arms also seems to show adapapations for climbing and swinging.

Another piece of evidence lies in the semicircular canals of the inner ear. The semicircular canal serves as a sense organ for balance and controls the reflex for gaze stabilization. The inner ear has three canals on each side of the head, and each of the six canals encloses a membranous duct that forms an endolymph-filled circuit. Hair cells in the duct’s ampulla pick up endolymph disturbances caused by movement, which register as rotatory head movement. They respond to body sway of frequencies greater than 0.1 Hz and trigger the vestibulocollic (neck) reflex and vestibuloocular (eye) reflex to recover balance and gaze stability. The bony semicircular canals allow estimates of duct arc length and orientation with respect to the sagittal plane.

During upright walking, human head rotation frequencies range between 2-8 Hz along the sagittal and coronal planes and between 1-3 Hz along the horizontal plane. Across species, the semicircular canals of agile animals have larger arcs than those of slower ones. For example, the rapid leaper Tarsius bancamus has semicircular canals much bigger than the slow-climbing Nycticebus coucang. The semicircular canals of brachiating gibbons are bigger than those of arboreal and terrestrial quadrupedal great apes. As a rule of thumb, arc size of the ducts decreases with body mass and consequently slower angular head motions. Arc size increases with greater agility and thus more rapid head motions. Modern humans have bigger arcs on their anterior and posterior canals, which reflect greater angular motion along the sagittal plane. The lateral canal has a smaller arc size, corresponding to reduced head movement from side to side.

Allometric measurements on the bony labyrinth of BAC-208, a fragmentary cranium that preserves a complete, undeformed petrosal bone suggest that Oreopithecus moved with agility comparable to extant great apes. Its anterior and lateral semicircular canal sizes fall within the range for great apes. Its relatively large posterior arc implies that Oreopithecus was more proficient at stabilizing angular head motion along the sagittal plane.

Some have suggested the unique locomotory behavior of Oreopithecus requires a revision of the current consensus on the timing of bipedality in human developmental history, but there is little agreement on this point among paleontologists. Among other things, its foot was birdlike and had an anatomy different from the early bipedal human ancestors. Some researchers have related Oreopithecus to the early Oligocene Apidium, a small arboreal proto-ape that lived nearly 34 million years ago in Egypt. Others claim it is an extinct great ape without descendants and a sister taxon of the European Dryopithecus, a Miocene genus that resembles modern great apes. Still others have suggested that Oreopithecus is an arboreal hoofed mammal. For the moment, this species is a considerable anomaly, and may represent an independent development of bipedality other than that which led to humans, and which came to a dead end some time later. More fossil discoveries may help settle this and other questions. Likewise, the taxonomic placement of Oreopithecus is unsettled. Some scientists place it as a very early catarrhine, shortly after the split from the New World monkeys. Other scientists place it in Hominidae just before the split of the orangutans from the rest of the great apes.

It evolved in isolation from other animals for at least two million years on an island in the Mediterranean where Tuscany in Italy is found today. A cooling phase around 9 million years ago transformed a tropical island into a temperate one characteristic of middle European biomes at the time. There were no large predators on the island and the apes didn't have any natural enemies. Later, probably during the ice age when the sea level dropped all over the world, a land bridge emerged and connected the island with the mainland. New species, among them large predators, were then free to invade this isolated environment where animals like the Swamp Ape were easy prey. Soon this strange primate, as well as other creatures on the island, was gone forever; a parallel to what happened when the land bridge between North America and South America joined the two continents.

The scientific name Oreopithecus quickly inspired the commonly used though unofficial name for the species among paleontologists, Cookie Monster. The true etymology, however, is rather more mundane: it comes from the Greek "oros" and "pithekos" meaning "hill-ape".

External links

Bibliography

Alba, David M. et al. “Canine reduction in the Miocene hominoid Oreopithecus bambolii: behavioural and evolutionary implications.” Journal of Human Evolution, vol. 40 (2001): pp. 1-16.
Carnieri, E. and F. Mallegni. “A new specimen and dental microwear in Oreopithecus bambolii.” Homo, vol. 54 (2003): pp. 29-35.
Rook, Lorenzo et al. “The bony labyrinth of Oreopithecus bambolii.” Journal of Human Evolution, vol. 46 (2004): pp. 347-354.
Spoor, Fred. “The semicircular canal system and locomotor behavior, with special reference to hominin evolution” in Walking Upright: Results of the 13th International Senckenberg Conference at the Werner Reimers Foundation, eds. Jens Lorenz Franken et al., E. Schweitzerbart’sche Verlagsbuchhandlung, 2003.