Calede et al.—Ecomorphology of Leptarctus oregonensis
posteriorly as in other species of Leptarctus (Olsen, 1958; Lim et al., 2001; Korth and Baskin, 2009), but also display a smooth articular surface medially. The ventral surface of the skull, particularly the basicra-
nium, is better preserved in the new skull from the Mascall Formation of Oregon (UOMNH F-35458) than in the specimen from the Olcott Formation of Nebraska (AMNH 18241). The following description is based on the specimen from Oregon except when specifically referencing the specimen from Nebraska. In ventral view (Fig. 1.2), both the maxilla and the palatine are slightly concave in UOMNH F-35458. The maxilla of AMNH 18241 also displays a concavity. The anterior border of the suture between the maxilla and palatine is at the mesial end of P4. The lateral branches of the maxilla and palatine suture extend just medial to M1. The posterior palatine foramen cannot be distinguished. The palatine is much narrower posterior to the
tooth row and ends with the pterygoid flanges. The pterygoid flanges are curved and extend posteriorly; they are tall (Table 1) and thickened anteriorly (Fig. 1.2). There is a ridge of bone lateral to this thickening, parallel to the pterygoid flange. The size and thickening of the pterygoid and the presence of this bony ridge may indicate an enlarged pterygoideus muscle, a jaw-closing muscle (Hall, 1926; Davis, 1964; Endo et al., 2003). A ridge also extends medially to the pterygoid flanges,
starting at the posterior end of the palatine and ending at the posterior end of the basioccipital. It is tall and medio-laterally compressed anteriorly but is low and broadened posteriorly. The suture on the right flange of the pterygoid indicates that both the palatine and the basisphenoid contribute to the pterygoid flange. The foramen ovale of the basisphenoid is within the size range of that of L mummorum (Korth and Baskin, 2009). The bone ridge lateral to the pterygoid flange extends lateral to the foramen ovale and contacts the auditory bulla at the posterior end of the foramen. The foramen is teardrop shaped with the broader end located posteriorly; the opposite end extends between the pterygoid flange and the ridge of bone. Laterally to the foramen ovale and medially to the glenoid fossa of the squamosal is a narrow concave bone surface (Fig. 1.2). The squamosal wraps around the fossa farther anteriorly and ventrally in UOMNH F-35458 than in AMNH 18241, which is evidence for a tighter dentary-squamosal articulation in the Mascall specimen. The eustachian canal is located anteriorly to the bulla and posterior to the foramen ovale. It is small, but slightly larger in UOMNH F-35458 than in AMNH 18241. As mentioned by Korth and Baskin (2009) for other specimens, there is no evidence for a foramen anterior to the paroccipital process and posterior to the auditory bulla in either one of the adult skulls (UOMNH F-35458 or AMNH 18241). This foramen would be the entrance of the auricular branch of the vagus nerve, purportedly lost during ontogeny in Leptarctus (Korth and Baskin, 2009). The paroccipital process of L. oregonensis, visible in UOMNH F-35458 (Fig. 1.2), is likely the site of the origin of a powerful digastric muscle, as in other members of Mustelidae, Mephitis, the giant panda, and some felids (Hall, 1926; Davis, 1964; Riley, 1985; Antón et al., 2004). The strong mastoid process of UOMNH F-35458 would have provided additional surface for the origin of a powerful digastricus muscle as it does in Mephitis, Spilogale, and Martes (Hall, 1926). The digastricus muscle depresses the mandible,
297
opening the mouth (Antón et al., 2004). In Enhydra, the sea otter, the mastoid also provides surface attachment for the temporalis and the horizontal portion of the masseter muscles (Riley, 1985), giving the animal a greater bite force at the canines and/or cheek teeth than in relatives without this expanded muscle attachment area (e.g., Lontra), although body mass may also play a role in this increased bite force (Riley, 1985).
Posterior to the glenoid fossa of the squamosal is an oval,
anterolaterally to medio-laterally oriented post-glenoid foramen (Fig. 1.2). It is larger and located more laterally than in L. mummorum (Korth and Baskin, 2009). The auditory bulla is fully fused to the base of the skull, as in Leptarctus mummorum. The auditory bulla is narrow medio-laterally. It bears a peculiar process extending anteroventrally (Figs. 1.2, 2.1, 4). This is the bullar process of Korth and Baskin (2009), also called tympanic projection by Lim and Martin (2001a). This is a common feature of the genus Leptarctus, with the exception of L. webbi, which lacks processes (Baskin, 2005). Overall, the tympanic projec- tion of L. oregonensis is most similar to that of L. ancipidens (see Olsen, 1958). It is very tall and its ventral edge is the ventral-most feature of the skull (Fig. 2.1). The posterior side of the bulla bears two parallel ridges extending dorso-laterally to ventro-medially. These two ridges are thicker on their dorsal- most end and thinner towards their attachment to the bulla. The anterior process of the bulla (Fig. 4) consists of a series (unlike the simple processes of L. mummorum and L. supremus; Lim et al., 2001; Korth and Baskin, 2009) of non-overlapping (unlike L. primus, but as in L. ancipidens; Olsen, 1958; Lim and Martin, 2001a) anteroventral projections, the two anterior-most of which are separated by a canal, as in L. ancipidens (see Olsen, 1958) and L. primus (Lim and Martin, 2001a). The tympanic projection is broken posterior to a third damaged projection. Additional projections can be distinguished at the dorsal end of the posterior surface of the tympanic projection, forming lobes ventral to the paroccipital process (Fig. 4; jugular process of Lim and Martin, 2001a). These lobes and the third projection (posterior to the ones than form the canal) are not present in other species of Leptarctus. The contact between the two projections and the canal are aligned with the external auditory meatus, located dorso-posteriorly to the tympanic projection, as in L. ancipidens. This state is unlike L. primus, where the contact and canal are aligned with the postglenoid foramen, located anterodorsally to the tympanic projection farther from the mastoid process than the external auditory meatus. As in L. primus, but unlike L. kansaensis, the tympanic projections of L. oregonensis do not extend posteriorly to the edge of the
Figure 4. Tympanic projections of the auditory bullae of Leptarctus oregonensis (UOMNH F-35458): (1) medial view; (2) lateral view. White * denote the taphonomic damage mentioned in the text. Scale bar is 5mm.
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