298


Journal of Paleontology 92(2):289–304


paroccipital process (Lim andMartin, 2001a). The ventral end of the tympanic projections is smooth and superficially resembles a condyle. The tympanic projections bear a number of round to oval holes, although those seem to be taphonomic in origin, as interpreted from their location and structure (Fig. 4). The medial edge of the bulla is not preserved and cannot be described. The lobate shape of the tympanic projections ventral to the


paroccipital process, combined with the nearby, prominent mastoid process, may have provided L. oregonensis additional surface attachment for jaw muscles involved in crushing and chewing. Olsen (1958) suggested that they may have served as surface attachment for the digastric muscles. In some felids, the auditory bulla is an area of insertion for the digastric muscle (Antón et al., 2004). Korth and Baskin (2009) pointed out that a raphe from the masseter attaches just lateral to the auditory bulla in Canis, and proposed that the tympanic projections of Leptarctus provided surface area for the attachment of the masseter. Whether the masseter, or the digastricus, or both attached onto the tympanic projections of Leptarctus,itis apparent that these projections provided critical surface area for the attachment of muscles important in jaw action. The teeth are unworn with most cusps fully intact (Fig. 5).


The dental morphology most closely resembles that of L. ancipidens, but is smaller in size (Table 1). The M1 is longer than in many mustelids of similar skull size (Table 1, Fig. 6). None of the teeth anterior to P3 is preserved. Only the left P3 is present. Only partial roots of the right P3 are preserved. There are two roots with the posterior larger than the anterior of the right P3. It has a single, conical cusp, almost as high as the paracone of P4 (Fig. 6). The cusp is surrounded by a cingulum that is most prominent on the anterior and posterior ends of the tooth. The overall shape of the P4 is a rounded square with a metacone blade that projects posterobuccally (Fig. 6). The dental morphology of UOMNH F-35458 is similar overall to


that of the type specimen, LACM(CIT) 206, both in shape and size (Table 1). There are differences in the proportions of the teeth between the specimens from the Olcott Formation and those from the Mascall Formation. The P4 of AMNH 18241 from Nebraska is shorter and wider than the P4s of the specimens from Oregon (Table 1). The M1 of AMNH 18241 is longer and wider than that of LACM (CIT) 206. The P4 of UOMNH F-35458 resembles closely the P4 figured by Downs (1956), UCMP 39102. It is longer than it is wide, unlike in L. progressus (Table 1; Lim and Martin, 2001a). All cusps, except the hypocone of the left P4 and the metacone of the right P4, are preserved (Fig. 6). Similarly to L. mummorum, P4 has a prominent parastyle at the anterior margin of the tooth that is subequal in height to the protocone. This parastyle, as noted by Downs (1956) and Olsen (1957) is not as strong as in L. ancipidens. Two strong cingula extend from this parastyle to the anterior margin of the protocone and the anterobuccal margin of the paracone, respectively. The paracone is the largest and most robust cusp of the P4 and, indeed, of the whole tooth row. The paracone is smaller and more acute than in L. desuii and L. martini (Lim and Martin, 2001b; Fig. 6). A paraloph (a crest attaching the paracone to the protocone) closes off a valley between the paracone, parastyle, and protocone (Fig. 6). The protocone is located slightly anterior to the paracone. It is higher than the hypocone. The hypocone of L. oregonensis is small compared to those of L. primus (as noted by Korth and Baskin, 2009) and L. ancipidens (see Downs, 1956; Olsen, 1957). The posterior end of the protocone is connected to the


Figure 5. Occlusal view of the dentition of Leptarctus oregonensis, UOMNH F-35458: (1) right partial P4 and partial M1; (2) left P3, partial P4, and partial M1 (reflected). Scale bar is 2mm.


Figure 6. Occlusal view of the dentition (P3–M1) of Leptarctus: (1) L. oregonensis (UOMNH F-35458, composite of left and right dentitions); (2) L. ancipidens (UF 5706; redrawn from Lim et al., 2001); (3) L. martini (UNSM 20843). Scale bar is 5mm. The dentition of AMNH 18241 is figured in Matthew (1924).


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