Skull and endocranial cast ofEoryctes melanus, a new palaeoryctid (Mammalia: Insectivora) from the early Eocene of western North America

Abstract

Early Eocene (Wasatchian) Eoryctes melanus, gen. et sp. nov., is a relatively large palaeoryctid insectivore (skull length ca. 32 mm, body weight ca. 51 gm) found in the Clarks Fork Basin of northwestern Wyoming. The skull has a short rostrum and prominent lambdoid plates. It appears to have lacked zygomatic arches. The middle ear cavity is large, with an enlarged epitympanic recess. Arteries of the middle ear include an internal carotid artery and a stapedial artery. Both are enclosed in bony tubes for their whole intrabullar course. The neopallium of the brain exhibits a rhinal fissure, a marginal sulcus, and a pseudosylvian sulcus. Venous blood from the cranial cavity drained mainly through the petro—occipital fissure. Endocranial volume is about 1.0 cm³, which is equivalent to a brain weight of 1.0 gm. The Jerison encephalization quotient (EQ) of E. melanus is estimated at 0.60 and the Martin EQ is estimated at 0.85. Proportional development of the brain indicates that olfaction was the dominant sense, although Eoryctes probably had acute hearing and a sensitive rostrum also. Palaeoryctidae appear to be most closely related to extinct Leptictidae and extant Lipotyphla, and are probably best included in the order or superorder Insectivora.