A skeleton ofMesoscalops(Mammalia, Insectivora) from the Miocene Deep River Formation, Montana, and a review of the proscalopid moles: evolutionary, functional, and stratigraphic relationships

A nearly complete and partially articulated skeleton of Mesoscalops, a genus of fossorial insectivore hitherto known only by isolated teeth and some bones of the forelimb, is described and compared with other proscalopids, talpids, and chrysochlorids. The skeleton (UWBM 54708) is more complete than that of any proscalopid known to date, and provides a basis for description of the previously unknown vertebral column, ribs, articulated forelimb including the scapula, clavicle, manubrium, pelvis, femur, fibula, astragalus, and metatarsals. UWBM 54708, from the Hemingfordian or Barstovian Deep River Formation, Meagher County, Montana, represents a new species, M. montanensis. The skull of M. montanensis more closely resembles Whitneyan than late Arikareean proscalopids, suggesting that Mesoscalops was derived from a Whitneyan form of Proscalops. The morphology of proscalopids, as exemplified by Mesoscalops montanensis, reflects a unique digging stroke and differs from that of talpids to a greater degree than has been previously recognized. Functional analyses show that motions of the forelimb differed from those of any modern fossorial animal and that the head of M. montanensis, unlike that of talpids, was probably important in displacing soil. In view of the importance of the head in digging, chrysochlorids provide a closer living analogue than do talpids. The dissimilarities in morphology and digging techniques indicate that talpids probably represent a lineage distinct from that of the Proscalopidae. The specialized morphology that suggests divergence of the talpid and proscalopid lineages extends back to at least Chadronian time. Present samples of proscalopid humeri are morphologically distinct at the generic but not specific level, and the genera Mesoscalops and Arctoryctes are probably synonymous. Successive, progressive changes in the skulls and humeri of proscalopids indicate that fossorially more efficient genera completely replaced earlier, less efficient genera through competition for the same fossorial niche. This evolutionary model is consistent with the restriction of Cryptoryctes kayi to Chadronian time, Oligoscalops galbreathi to the Orellan, Proscalops to the Whitneyan and Arikareean, and Mesoscalops to the Hemingfordian (and possibly Barstovian).