Protocetid (Cetacea, Artiodactyla) bullae and petrosals from the middle Eocene locality of Kpogamé, Togo: new insights into the early history of cetacean hearing
Résumé
Extant cetaceans are fully aquatic mammals with deep modifications of their sensory organs, notably of the sound perception pathway. Early diverging cetaceans, known as archaeocetes, show a diversity of morphologies of the petrotympanic complex and middle ear ossicles, documenting a variety of sound transmission mechanisms from a mostly terrestrial configuration to a fully aquatic layout. Protocetids are a paraphyletic assemblage of semi-aquatic archaeocetes known from the Eocene. The auditory region of these ‘transitional’ forms is only partly known. The middle Eocene locality of Kpogamé, Togo (46–43 Ma) has yielded abundant material of protocetid whales documenting the auditory region, including isolated bullae, one petrosal and a skull fragment preserving the petrotympanic complex. Detailed study of this material leads us to reassess the original taxonomic attribution of this material, first attributed to Togocetus traversei, and to recognize three different protocetid taxa on the basis of bullar and petrosal remains: ?Carolinacetus sp., Togocetus traversei and Protocetidae indeterminate (morphotype γ). Associating isolated petrosals or bullae with dental remains solely based on size criteria can be misleading and we show here that there is no correlation between the size of tympanic bulla and size of dental remains in protocetids. CT-scan investigation of the in situ petrotympanic complex reveals that Protocetidae retained a complete tympanic ring similar to that of terrestrial artiodactyls and that the involucrum could probably articulate with the medial side of the ventral surface of the petrosal. This bulla/petrosal articulation is absent in fully aquatic cetaceans. Finally, the close phylogenetic relationships between protocetids from Kpogamé and North American protocetids suggest successive dispersals of these non-fully aquatic archaeocetes from African coasts to North America.