The Toothcomb of Karanisia clarki: Was this Species an Exudate-feeder?
Toothcombs have evolved independently in various mammalian lineages, including primates, scandentians and dermopterans, but the presence of a six-toothed toothcomb composed of four lower incisors and two canines (I1, I2 and C1, bilaterally) is a distinctive feature of extant strepsirrhine primates (Yamashita, 2017). There is some variation within the group with respect to the nature of the anterior teeth: indriids have a reduced toothcomb as a result of losing the lower canines (Gingerich, 1977), whereas Daubentonia has only one enlarged, ever-growing incisor, and no canines, in each quadrant (Quinn and Wilson, 2004). Nonetheless, it seems clear that a six-toothed toothcomb is primitive for Strepsirrhini (Hill, 1953b; Martin, 1990; Szalay and Delson, 1979). The earliest fossil strepsirrhine that indisputably possesses a toothcomb is Karanisia clarki (Seiffert et al., 2003), therefore the appearance of this trait can be established by as late as 36.9 million years ago (Mya). Molecular dates for the divergence of crown Strepsirrhini are generally much earlier than this (49.8–77.0 Mya; Pozzi et al., 2014b; Yang and Yoder, 2003, and sources cited therein). If the toothcomb was present in the common ancestor of the group, it could therefore be expected to have appeared much earlier than its first record in K. clarki. However, the fossil record for crown strepsirrhines remains very limited, which means that K. clarki provides the first window into the adaptive context in which the toothcomb evolved.
