Calede et al.—Ecomorphology of Leptarctus oregonensis
anterior end of the hypocone, although this connection is smaller and less robust than the aforementioned cingula or paraloph. There is a triangular valley between the hypocone, paracone, and protocone (Fig. 6). Another valley posterior to the hypocone is formed by the paracone, the hypocone, and the blade-like metacone. This metacone blade is longer than in L. primus, as noted by Korth and Baskin (2009). A cingulum extends posteriorly to the hypocone around the posterior end of the metacone and terminates on the posterior-buccal side of the paracone. The metacone of P4 is the buccal-most cusp of the tooth row and, at its posterior end, contacts the parastylar crest of M1 (Fig. 6). It is a robust and tall blade, longer and more blade-like than in L. desuii and L. martini (Lim and Martin, 2001b; Fig. 6). The overall shape of the M1 is a rounded square with a projection formed by the paracone and parastylar crest that extends anterobuccally and contacts the projecting meta- cone of the P4 (Fig. 6). The parastyle of M1 is longer and more- blade like in L. oregonensis than in L. martini (Table 1, Fig. 6). Leptarctus oregonensis differs from L. ancipidens in that
the M1 is wider than long in the latter (see Downs, 1956; Olsen, 1957; Table 1). The left M1 is missing two of the lingual cusps (protocone and hypocone). The right M1 preserves all cusps except the hypocone and the anterior surface of the metaconule (see Korth and Baskin, 2009 for a discussion of the metaconule in the genus Leptarctus). The four main cusps of M1 (paracone, metacone, protocone, hypocone) are subequal in heightm as in L. mummorum. There are two main valleys. The largest, deepest valley is located between the paracone, protocone, and metacone. The other, smaller valley is located posteriorly, between the metacone, metaconule, and the posterior edge of the tooth (Fig. 6). There is a strong parastylar crest (anteroexternal cingulum of Korth and Baskin, 2009) subequal in height to the paracone and located directly anterobuccally to it. The paracone connects to the protocone via a robust paraloph. A robust cingulum extends along the buccal margin of the tooth and connects posteriorly to the posterior end of the metacone. The lingual cingulum of the M1 of L. oregonensis is weak compared to those exhibited by L. ancipidens and L. martini. The paracone and metacone are very similar in shape.
Referred specimens.—From the Mascall Formation, Oregon: UCMP 39102, a single right P4 from the Mascall type area (UCMP V4824, Mascall 10) (by Downs, 1956); UOMNH F-35458 (UO 2993, Field Creek Road), mostly complete skull including the right P4 and M1, and the left P3, P4, and M1, but lacking the rostrum, incisors, and canines; from the Olcott Formation, Nebraska: AMNH 18241, almost complete skull of Leptarctus oregonensis lacking incisors and canines, AMNH 18270, partial right lower jaw, both specimens from the Snake Creek Quarries (23miles south of Agate); from the Sand Canyon Beds Formation (Observation Quarry), Nebraska: F:AM 25385, partial cranium lacking the left auditory bulla, the incisors, and canines; fromthe Lower Snake Creek Beds (EchoQuarry): F:AM 25261, partial posterior portion of the cranium.
Remarks.—Since publication of Leptarctus oregonensis by Stock (1930) and Downs (1956), many other species of Lep- tarctus have been recognized (Korth and Baskin, 2009), but no differential diagnosis with L. oregonensis has been provided for
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many of them. The completeness of the new specimen, UOMNH F-35458, also offers the opportunity to consider how many characters (e.g., projections of the auditory bullae) unknown for L. oregonensis before differ from other species of the genus. For these two reasons, we chose to provide an emended diagnosis.
Results of the quantitative analyses
We estimate that Leptarctus oregonensis would have been able to feed on prey as large as 1.85 kg (95% confidence intervals of 0.94–3.60 kg). We calculated a bite force at the carnassials of 136.45 N, and a bite force at the M1s of 177.15N(Table 3). Our calculation of the BFQ at the carnassials of L. oregonensis yields a value of 89. Of the 30 extant mammals included in our analysis, 22 have a higher BFQ than L. oregonensis. None- theless, L. oregonensis has a higher BFQ than the only other mustelid included in our dataset, Meles meles Linnaeus, 1758, the European badger (BFQ of 86). The BFQ of L. oregonensis is intermediate between that of Canis aureus Linnaeus, 1758, the golden jackal (BFQ of 87), and Lynx rufus Schreber, 1777, the bobcat (BFQ of 90) (Table 4). The proportions of the teeth of mustelids are associated to their diets (Popowics, 2003). Our analysis of tooth shape (Fig. 7) supports broad differences in tooth morphology between carnivorous taxa whose diet is dominated by vertebrates and those that are omnivorous or invertivorous, although there are exceptions to this pattern (Popowics, 2003). Thus, omnivorous and invertivorous taxa are generally characterized by large crushing surfaces on both P4 (PRBL/P4W) and M1 (M1LL/M1W), whereas carnivorous taxa are usually typified by high values of carnassial shear (P4LB/ P4W) and carnassial blade length (P4PM/P4W) (Table 5; Fig. 7.1; Popowics, 2003). The proportions of the P4 and M1 of Leptarctus oregonensis are most similar to those of invertivorous and omnivorous taxa (Fig. 7.1). Our canonical variate analysis (Table 5; Fig. 7.2) further supports this conclusion. The propor- tions of the teeth of L. oregonensis most closely resemble those of Mephitis mephitis, the striped skunk, Spilogale putorius Linnaeus, 1758, the eastern spotted skunk, and Mydaus javanensis Desmarest, 1820, the Indonesian stink badger (Fig. 7.1). All three of these taxa eat large amounts of inverte- brates, small amounts of plant materials, and small vertebrates (Wade-Smith and Verts, 1982; Kinlaw, 1995; Hwang and Larivière, 2003; Nowak, 2005).
Table 3. Estimate of bite force in Leptarctus
oregonensis.BFQ is thebiteforce quotient (see text for details).
Variable
Muscle stress (N/cm2)30 Temporal force (N) Masseter force (N) P4 out-lever (cm) M1 out-lever (cm)
Temporal moment (cm) Masseter moment (cm)
Temporal area (cm2) Masseter area (cm2)
Bilateral bite force at P4 Bilateral bite force at M1 BFQ at P4
L. oregonensis 3.05
3.34 1.08 1.03
91.41
100.23 2.96 2.28
136.45 177.15 89
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