The Early Eocene (~54.5 Ma) Cambay Shale Formation at Vastan lignite mine (Gujarat, India) has yielded remains of both adapoid and omomyoid primates. The collection of primates includes not only jaws and teeth, but numerous exquisitely preserved postcranial elements. We report on the first cranial specimen for a primate from these deposits: an isolated left petrosal that preserves a partial stapes in anatomical position. The petrosal is identified as a primate based on the remnants of a petrosal bulla, and the presence of an ossified tube for the stapedial artery. The specimen documents a posterolateral entry of the internal carotid artery to the middle ear and a lateral course for the promontorial artery across the promontorium, characters most consistent with an attribution to Adapoidea. Of the adapoids published from the Vastan mine, body mass estimates based on the radii of the semicircular canals, calculated from high resolution microCT data, are most in line with previously calculated estimates for Marcgodinotius indicus, so the specimen is provisionally attributed to that species. Preserved anatomy is largely consistent with that described for Cantius. In particular, although the stapedial artery passed through a bony tube, the promontorial artery ran in an open groove from its origin off the internal carotid artery. This contrasts with the condition in omomyoids and most other adapoids, in which the promontorial artery was carried in a bony tube. The identification of an open groove for this artery in Cantius has been somewhat controversial, based on the state of preservation of published specimens. The petrosal from Vastan is extremely well preserved, demonstrating a clear opening in the internal carotid artery bony tube for the exit of the promontorial artery, and a well-demarcated groove on the promontorium for the latter artery that was clearly not enclosed. The absence of a bony tube for the promontorial artery in the oldest known adapoids suggests that the tube arose independently, in parallel, in Omomyoidea and Adapoidea. The promontorial artery is always enclosed in modern haplorhines, but when this artery is retained in living strepsirrhines it is often not fully enclosed by bone. Therefore, the primitive adapoid condition is more similar to that observed in Strepsirrhini. The antiquity and fine quality of preservation of this specimen make it relevant to reconstructing auditory morphology near the base of the primate tree. Grant Information Supported by an NSERC Discovery Grant to MTS and grants from the National Geographic Society and the Leakey Foundation to KDR.
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