“Snagging” Teeth and Premolar Homologies in Paleoparadoxiidae (Mammalia: Desmostylia)

In recent years, controversies about mammalian phylogeny have focused on the relative merits of morphological and molecular characters for phylogenetic analysis (Novacek, 1992; Murphy et al., 2001; Springer et al., 2004; Asher, 2007). Molecular datasets exceed the size of morphological datasets by orders of magnitude, but only morphological characters can be used for fossil taxa. In mammalian phylogeny, molecular analyses have converged on the same phylogenetic hypotheses for relationships among clades of living mammals (Springer et al., 2004). Among these mammalian clades, Afrotheria has some of the strongest molecular support among placental mammals (Springer et al., 1997; Murphy et al., 2001; Yang et al., 2003), although the molecular results have significantly disagreed with morphological data (Asher, 1999) until recently (Tabuce et al., 2007). Within Afrotheria, the subclade Tethytheria (with the Recent members Sirenia and Proboscidea) appears to be a molecular grouping strongly congruent with morphological data. However, several tethythere groups, like Embrithopoda and Desmostylia (Gheerbrant et al., 2005), are extinct; therefore a consideration of their phylogenetic relationships requires morphological data. Paleontologists heavily rely on dental data to estimate relationships among fossil mammals, but the derived dentitions of many fossil lineages challenge attempts to determine homologies. Desmostylians are one such case, with cheek tooth morphologies that are unique among placental mammals. Numerous interpretive problems have beset hypotheses of desmostylian tooth homologies since the group’s discovery in the nineteenth century (reviewed by Inuzuka et al., 1995). Some of these interpretive problems remain unresolved—including the homologies of the anterior lower premolars in the Family Paleoparadoxiidae (the more dentally primitive of the two recognized families). In the well-known genus Paleoparadoxia, the lower incisors and canines are typically separated from the premolars by a long diastema, and a caniniform premolar is isolated from the remaining cheek teeth by a second, shorter diastema (e.g., in P. tabatai; Fig. 1C). This second diastema is followed posteriorly by two more premolars and three molars (Domning et al., 1986:fig. 19). All authors (e.g., Ijiri and Kamei, 1961; Inuzuka, 1986; Clark, 1991; Hasegawa and Taketani, 1994; Inuzuka et al., 1995; Panofsky, 1998; Taru, 2000) have agreed that the last two premolars are p3-4. However, the anteromost, isolated, caniniform premolar—on which Panofsky (1998:122) aptly bestowed the informal designation of ‘snagging tooth’—could represent either p1 or p2. This ambiguity arises because the diastema behind it may have been the former site of another tooth. Functionally, this ‘snagging tooth’, which does not occlude with any upper tooth, seems best interpreted as helping to keep aquatic plants moving backward in the mouth during intraoral food transport, just as Panofsky (1998) suggested. New observations of the type specimen of the more primitive species Paleoparadoxia weltoni (Fig. 1B) indicate that this ‘snagging tooth’ is p1 rather than p2, contrary to the usage of almost all recent authors. Here, we present and evaluate this new interpretation in the context of other known paleoparadoxiid taxa, as well as in the context of previous nomenclature and hypotheses of homology. We conclude this discussion of premolar homologies in P. weltoni with caveats for further interpretations of dental homology among Desmostylia. These revisions highlight the importance of precision in dental nomenclature and homologies, given the emphasis on dental characters in higher-level mammalian phylogeny. Institutional Abbreviations—LACM, Natural History Museum of Los Angeles County, California, USA; UCMP, University of California Museum of Paleontology, Berkeley, California, USA; USNM, Department of Paleobiology, U.S. National Museum of Natural History, Smithsonian Institution, Washington, D.C., USA. Comparisons of Desmostylian Dentitions—Among known desmostylian lineages, interpretation of premolar number and morphology varies depending on the interpretation of diastemata on the mandible. In the most basal paleoparadoxiid, the Oligocene Behemotops, several researchers have interpreted the presence of four lower premolars (Ray et al., 1994; Inuzuka, 2000). However, based on our own observations of specimens from both the United States and Japan, it is not clear whether any diastema was consistently present within the premolar row. The adult specimen USNM 244033 (“B. emlongi” B. proteus), cited by Clark (1991; see below), has a small diastema between p2 and p3, but this gap may or may not have been present in the only other comparable specimen of the species, LACM 124106 (Ray et al., 1994:fig. 1). The slightly larger diastema shown between p1 and p2 in composite reconstructions of B. proteus by Ray et al. (1994:figs. 12, 13; Fig. 1A here) is conjectural because the p1 and p2 were not found in direct association. The sole dentary fragment of B. katsuiei that preserves parts of the roots of c and p1-3 also does not allow firm conclusions about diastemata, and it gives no evidence that any existed (Inuzuka, 2000:fig. 20). By the late early Miocene, Paleoparadoxia tabatai (Fig. 1C) had already lost one lower premolar and developed the diastema within the premolar row, and this condition is present in all geochronologically younger and more derived paleoparadoxiids (including P. repenningi; Domning and Barnes, 2007). To underCorresponding author. Journal of Vertebrate Paleontology 28(3):923–927, September 2008 © 2008 by the Society of Vertebrate Paleontology