Saadanius and the split between old world monkeys and apes
By William J. Sanders, PhD – Published March 27, 2020
Modern anthropoid primates in the Catarrhini include Old World (OW) monkeys, apes, and humans, or “crown” catarrhines. The oldest known basal, or “stem” catarrhines derive from latest Eocene to early Oligocene sites in Afro-Arabia, dated to 34-30 million years (myr) old. Although ancient apes and OW monkeys are present in the early Miocene (23-16 myr), the timing of their divergence from a common ancestor and the anatomical condition of that ancestor have been the subjects of serious debate. For example, geneticists predict the divergence to have occurred as early as 34.5-29.2 myr ago, while paleontologists estimate it occurred sometime in the late Oligocene between 25 to 23 myr ago. Using comparative studies of living catarrhines, morphologists have reconstructed the crown catarrhine common ancestor to have had a short, non-projecting face, short, broad nasal bones, and a globular braincase, whereas paleontologists have used comparison of early apes, the first stem catarrhines, and early OW monkeys to alternatively reconstruct the ancestral crown catarrhine with a long, projecting face, elongate, narrow nasal bones, and a long, relatively narrow braincase with a mid-line sagittal crest in males.
In 2009, a University of Michigan-Saudi Geological Survey team led by U of M graduate Dr. Iyad Zalmout recovered a partial cranium with a face, palate, teeth, and part of the temporal bone, from 29-myr-old sediments in the Shumaysi Formation in western Saudi Arabia (photo credit Iyad Zalmout, UMMP).
The age of the find was dated by radiometric analysis of associated volcanic rocks and by biochronological comparison of accompanying fossil mammals. The find represents a new catarrhine species and was named after the Arabic collective term for apes and monkeys, as well as for the region where it was discovered. It derives from an animal of medium size, approximately 15 kg in body weight. This fossil provides valuable information relevant to the debates about the emergence of crown catarrhines and the cranio-dental anatomy of the crown catarrhine common ancestor. Most of the features of Saadanius are shared with older stem catarrhines, such as broad molars and modest-sized canines, a long rostrum with a strongly projecting midfacial area, long, narrow nasal bones, presence of maxillary sinuses, a narrow anterior braincase with strong sagittal crests, and absence of a frontal sinus. Critically, however, the canal between the outer and inner ear areas (ectotympanic) is formed by a closed tube, a feature shared by all crown catarrhines but not by stem catarrhines, suggesting that Saadanius was close to the ancestry of both OW monkeys and apes. As many of the features in Saadanius continued to be present in Miocene apes and OW monkeys, and none of these features was specialized in a unique way, Saadanius appears to be a good model for the ancestral crown catarrhine, and its age suggests that the paleontological hypothesis for the timing of divergence of apes and OW monkeys is correct. Since the Arabian discovery, early apes and OW monkeys have been recovered from late Oligocene (<27 myr) localities in Tanzania.
More information about Saadanius can be found in the journal article Zalmout, I. S., Sanders, W. J., MacLatchy, L. M., Gunnell, G. F., Al-Mufarreh, Y. A., Ali, M. A., Nasser, A.H., Al-Masari, A.M., Al-Sobhi, S.A., Nadhra, A.O., Matari, A.H., Wilson, J.A., & Gingerich, P.D. (2010). New Oligocene primate from Saudi Arabia and the divergence of apes and Old World monkeys. Nature, 466(7304), 360-364.