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Old World monkeys and apes diverged later than thought, new fossil shows

A remarkable new discovery redates the evolutionary split between the Old World monkeys and the ape-human lineage.

By Hans-Dieter Sues

The higher primates of the Old World (Catarrhini) are divided into two major lineages, one comprising the living monkeys of Africa, Asia, and Europe and their fossil relatives (Cercopithecoidea), and the other, humans, great apes, and their extinct relatives (Hominoidea).

Based on fossils and other lines of evidence, experts long assumed that the two catarrhine lineages diverged from a common ancestor at some point during the Oligocene epoch. An analysis of DNA from present-day apes and Old World monkeys published in 2004 more specifically inferred that this divergence took place between 34.5 and 29.2 million years ago, during the early Oligocene.

The discovery of a remarkable new primate fossil, an incomplete skull from the middle unit of the Shumaysi Formation of western Saudi Arabia, by a team from the Saudi Geological Survey and the University of Michigan has now challenged this assessment.

SAADANIUS_HIJAZENSIS.jpg

Facial skull fragment of Saadanius hijazensis in front and left side view. The original fossil is housed in the Paleontology Unit of the Saudi Geological Survey in Jeddah. The white arrow indicates a bite mark on the forehead. Click image to enlarge the photo.

Photograph courtesy of Wikipedia.

The sedimentary rocks of the Shumaysi Formation were laid down before the Red Sea rift opened between Africa and the Arabian Peninsula. Radiometric dates of associated igneous rocks and evidence from elephant fossils found associated with the primate remains indicate an age between 28 and 29 million years before present.

The new primate has been named Saadanius hijazensis, which is derived from the Arabic word “saadan,” which is used for both monkeys and apes, and the Al Hijaz region of western Saudi Arabia where the specimen was discovered.

Based on the size of new skull (belonging to a male), the researchers estimated the weight of Saadanius at between 15 and 20 kg (33 to 44 pounds).

Saadanius retained a projecting snout, broad molars with thin enamel, and other features present in earlier, more primitive primates. It still lacks characteristic features of the later apes such as frontal sinuses and large canine teeth. However, Saadanius already shares with more advanced catarrhines a fully formed, tube-like ear canal, which its predecessors lacked.

The researchers found a deep bite mark on the forehead of the only known specimen of Saadanius. Another puncture hole is present on the right side of the brain cavity. It is not clear whether this damage was caused by predation or scavenging.

The skull of Saadanius strikingly resembles that of the hypothetical last common ancestor of Old World monkeys and apes reconstructed by two American primate researchers in 1993. Based on this new discovery, it would appear now that the divergence between Old World monkeys and apes took place later than previously assumed–somewhere between 29 and 24 million years before present.

This would still leave a gap of several million years in the fossil record between Saadanius and the oldest known undisputed fossils of apes and Old World monkeys, which date from the early Miocene epoch, 23 million years and later. A paleontologist’s job is never done!

Hans-Dieter-Sues.jpgHans-Dieter (Hans) Sues is a vertebrate paleontologist based at the National Museum of Natural History in Washington, D.C. He is interested in the evolutionary history and paleobiology of vertebrates, especially dinosaurs and their relatives, and the history of ecosystems through time.

A former member of the National Geographic Committee for Research and Exploration, Hans has traveled widely in his quest for fossils and loves to share his passion for ancient life through lectures, writings, and blogging.

Blog entries by Hans-Dieter Sues >>

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