Aegyptocetus tarfa, n. gen. et sp. (Mammalia, Cetacea), from the middle Eocene of Egypt: Clinorhynchy, olfaction, and hearing in a protocetid whale

摘要

A new protocetid archaeocete, Aegyptocetus tarfa, is represented by a nearly complete cranium and an associated partial skeleton. The specimen was recovered when marbleized limestone was imported commercially to Italy and cut into decorative facing stone. It came from middle Eocene Tethyan marine strata of the Gebel Hof Formation of Wadi Tarfa in the Eastern Desert of Egypt. Exceptional preservation and preparation enables study of some internal features of the skull as well as its external morphology. The skull of Aegyptocetus is unusual in having the rostrum and frontal portions of the cranium deflected more ventrally relative to the braincase than is typical for archaeocetes. This ventral deflection, clinorhynchy, is a rare specialization related to feeding or hearing that is widely distributed across mammals. Aegyptocetus has well-developed ethmoidal turbinal bones, indicating retention of a functional sense of smell. It also has cranial asymmetry, thinning of the lateral walls of the dentaries, enlarged mandibular canals, and thinning of the anterolateral walls of the tympanic bullae, indicating enhanced ability to hear in water. Neural spines are long on thoracic vertebrae T1 through T8, suggesting that Aegyptocetus was able to support its weight on land like other protocetids. This combination of terrestrial and aquatic characteristics is consistent with interpretation of protocetids as semiaquatic. The pattern of tooth marks preserved on the ribs of Aegyptocetus indicates that the individual studied here was attacked by a large shark, but it is not certain whether this was the cause of death.