Late Eocene Sea Cows (Mammalia, Sirenia) from Wadi Al Hitan in the Fayum Basin, Egypt.

Abstract

Protosiren, Eotheroides, and Eosiren are all known from the Fayum Basin, and all are represented by exceptionally complete skulls and axial skeletons with pectoral and pelvic girdles. The skeletal remain of Protosiren and Eotheroides of the Birket Qarun Formation in Wadi Al Hitan are of special interest because they represent an intermediate stage of evolution and an intermediate stage of secondary adaptation to life in water. Eotheroides (Dugongidae, Halitheriinae) is described for the first time from the Priabonian of the Wadi Al Hitan. The two Wadi Al Hitan species Eotheroides clavigerum sp. nov. and Eotheroides sandersi sp. nov. are most similar to Eotheroides aegyptiacum (Owen, 1875) from the Lutetian nummulitic limestone beds of Cairo. They share the following derived characteristics: prominent falx cerebri and bony tentorium in the roof of the braincase; nasals long and in contact along the midline; palate broad with its posterior border posterior to the toothrow; and anterior ribs pachyosteosclerotic. Wadi Al Hitan Eotheroides were medium to large dugongs, ranging in length from 1.5 to 2.5 m; the skull is robust and heavy; the rostrum is deflected and bears medium to diminutive tusks; the trunk is widest between the ninth and eleventh thoracics; the end of the tail was fluked; the pelvis is greatly reduced with a shallow acetabulum, but retains an expanded club-like ilium; and the femur is short and slender. There cannot have been any substantial lower leg or foot. Protosiren, Eotheroides, and Eosiren all exhibit secondary sexual dimorphism of the bony pelvis. As in modern dugongs, males of the Eocene species consistently have the distal thickness of the ischium about twice as that of the ramus, where females ischia and rami have approximately equal thickness. The coexistence of Protosiren smithae, Eotheroides clavigerum, and Eotheroides sandersi in the same biotope reflects the diversity of this group in early Priabonian Tethys. The observed morphological diversity reflects dietary and environmental specialization and niche partitioning. Seagrass preserved as leaf impressions in Priabonian marine mammal beds is a direct indicator of the shallowness of Tethyan waters in the Fayum Basin.