SCANDENTIA, Wagner, 1855

Silcox, Mary T., Bloch, Jonathan I., Boyer, Doug M. & Houde, Peter, 2010, Cranial anatomy of Paleocene and Eocene Labidolemur kayi (Mammalia: Apatotheria), and the relationships of the Apatemyidae to other mammals, Zoological Journal of the Linnean Society 160 (4), pp. 773-825 : 796

SCANDENTIA

Although a close relationship between apatemyids and scandentians, to the exclusion of other forms, has not been suggested as likely by any authors writing on this subject, there are a number of postcranial similarities between L. kayi and modern tupaiids (J.I. Bloch, D.M. Boyer, M.T. Silcox & P. Houde, unpubl. data), which makes cranial comparisons with tree shrews worthwhile. There are a number of lines of evidence that suggest that the most primitive living scandentian is Ptilocercus lowii ( Sargis, 2002, 2004, 2007; Olson, Sargis & Martin, 2005). In light of the fact that L. kayi is more closely comparable with tupaiids than with ptilocercids in the postcranium ( Bloch et al., 2007), if these shared features are to be considered synapomorphies, rather than homoplasies, then the ear region of L. kayi should, at a minimum, exhibit the shared characteristics of the two living families. These similarities include the presence of an entotympanic that forms not only the floor of the tympanic cavity, but also the outermost layer of the tympanic roof, bony tubes for the branches of the internal carotid artery and facial nerves, an ectotympanic that is extremely narrow, and a postorbital bar. None of these features are seen in the cranium of L. kayi .

MacPhee (1981) has argued that scandentians and lemuriforms are unusual for mammals in retaining an ontogenetically early form of the ectotympanic, resulting in this bone being very narrow. Novacek (1986) expressed concern about the difficulty of distinguishing a slightly expanded ectotympanic from a completely unexpanded one. However, we consider the difference between the extremely attenuated ectotympanic of scandentians and lemuriforms, and the somewhat expanded although still narrow form of leptictids, to be sufficiently clear to form a valid contrast. The ectotympanic of L. kayi is not as narrow as in scandentians, and indeed the rostromedial corner of the bone widens notably, so the width of this bone is not as uniform as it is in scandentians and lemuriforms. Based on the position of the ectotympanic in USNM 530211, this bone was closer to horizontal in L. kayi than in scandentians. Unlike scandentians we consider it likely that this bone was athictic or slightly semiphaneric, and not aphaneric. Scandentians also appear to lack the basisphenoid tympanic process seen in apatemyids, and possess a persistent piriform fenestra (occluded by the entotympanic), both of which were lacking in L. kayi . Unlike all scandentians, L. kayi (and all other apatemyids known from the relevant region) lacks a postorbital bar.

The similarities that are present between L. kayi and scandentians are few, and occur in broadly distributed and/or primitive traits. For example, the shared presence of a medially positioned internal carotid artery (ICA) that follows a transpromontorial route is likely to be primitive ( Wible, 1986). Although both apatemyids and scandentians possess alisphenoid canals, it is adjacent to the entopterygoid in L. kayi , whereas it runs through the base of the ectopterygoid plate in scandentians. We agree with Wible & Zeller (1994) that the broad distribution and variable ontogenetic origin of alisphenoid canals make them problematic features for inferring relationships. In all, there are no cranial traits that provide clear evidence of a relationship between scandentians and apatemyids.