Remingtonocetus harudiensis, new combination, a Middle Eocene archaeocete (Mammalia, Cetacea) from Western Kutch, India

Abstract

A new genus of archaeocete cetaceans, Remingtonocetus, is based on revision of previously described and recently collected well preserved skull material from the Middle Eocene (Lutetian) Chocolate Limestone, Babia Stage, Harudi, Kutch. Remingtonocetus harudiensis, new combination, was previously described as a species of Protocetus (Sahni and Mishra, 1975). Present studies suggest that R. harudiensis is related to Andrewsiphius kutchensis Sahni and Mishra, 1975, which is known from a slightly younger horizon, about 17 km northeast of Harudi. A. kutchensis was previously classified as an agorophiid (Odontoceti), but is here grouped with R. harudiensis in a new family Remingtonocetidae (Archaeoceti). A slightly smaller species of Remingtonocetus, R. sloani, new combination, is present in the same horizon; it was also originally referred to Protocetus. The cetacean beds are of shallow water marine origin and yield abundant foraminifers and mollusks. Catfishes also occur associated with cetacean elements. The foraminiferal biostratigraphy of these beds indicates a Lutetian age (Tewari, 1957; Biswas and Deshpande, 1970; Tandon, 1971, 1976). The remingtonocetids are characterized by their remarkably long and narrow skulls with long beaklike rostra and mandibles with unusually long symphyses that may extend as far back as the last molar. In Remingtonocetus, the dentition was of the primitive eutherian type with all incisors, canines, premolars and molars present. The Remingtonocetidae were a primitive rapidly evolving group of archaeocetes that had acquired certain derived characters, e.g., large accessory air sinuses, partially isolated ear bones and teeth with crenulated cutting edges. In these features, they were more derived than the protocetids. The remingtonocetids have closer resemblance to the Odontoceti than have the basilosaurids. Their restricted geographical distribution in the Indo-Pakistan region suggests the possibility of their being endemic to the south Asiatic Tethys. The reasons for their limited distribution, and for the dissimilarity in the archaeocete taxa of the eastern and western Tethys, are the loss of connection between the two seaways during the Middle Eocene and the possible origin of archaeocetes in the south Asiatic Tethys. A restudy of all presently known archaeocete material from Kutch indicates that the genus Protocetus Fraas, 1904 was absent in India. The distinctness of Protocetus and Pappocetus Andrews (1920) is justified on the basis of differences in the dentition. The assignment of the genus Pakicetus Gingerich and Russell, 1981, to the family Protocetidae is opposed because Pakicetus was not a truly aquatic animal, as is evident from the structure of its tympano-periotic bone and its relationship with other cranial bones. The presence of odontocetes in Kutch as reported by Satsangi and Mukhopadhyay (1975) is doubtful.