296


Journal of Paleontology 92(2):289–304


Figure 3. Oblique view of the skull of Leptarctus oregonensis, UOMNH F-35458. Scale bar is 1 cm.


located dorsally to the infraorbital foramen. The canal extends ventrally from the foramen. The vascular foramen of the lacrimal is located immediately dorso-medially to the infra- orbital foramen. It is as large as the nasolacrimal foramen. This foramen is present in most species of Leptarctus, including L. mummorum (Korth and Baskin, 2009). The minute foramen dorsal to the nasolacrimal foramen described by Korth and Baskin (2009) in L. mummorum and purportedly not present in any other Leptarctus can be observed inUOMNHF-35458. The round sphenopalatine foramen is posterodorsal to M1, medial to the posterolateral edge of the palate. No ethmoid foramen could be distinguished. The optic foramen is small and round, and unlike in L. mummorum, is not directly anterior to the sphenorbital fissure, but rather anterodorsal to it. The sphen- orbital fissure itself is several times broader at its anterior opening than at its posterior one. The foramen rotundum, located posteriorly to the sphenorbital fissure, is broader at its posterior end than at its anterior one. The skull of Leptarctus oregonensis from Oregon,


UOMNH F-35458, bears two prominent parasagittal crests extending anteroposteriorly from just anterior to the postorbital processes of the frontal to the lambdoidal crests (Fig. 1.1). They only extend posterior to the postorbital processes of the frontals


in AMNH 18241 from the Olcott Formation (Lim et al., 2001, fig. 1). The parasagittal crests are parallel to one another in UOMNH F-35458, do not converge (unlike in L. mummorum; Korth and Baskin, 2009), and are taller posteriorly. The postorbital pinching of the frontals is stronger in AMNH


18241 than in UOMNH F-35458. In AMNH 18241, the parasagittal crests diverge posteriorly. The left parietal of UOMNH F-35458 is missing but the


right one can be observed. Both are preserved inAMNH18241. The parietals are laterally inflated, although not as much as in Leptarctus primus (Fig. 1.1). The dorsal surface of the parietals is roughened (Fig. 2.1). This roughening is more accentuated posteriorly, closer to the lambdoidal crests. The bone there is porous and textured alike an irregular lattice. Lim et al. (2001) also described roughening of the parietal and occipital bones in Leptarctus supremus. The intensity of the roughening is similar in both L. oregonensis specimens and between sides of a single specimen, but the pattern of the lattice differs both within (i.e., between left and right sides) and across specimens. This roughening of the parietals is evidence of the attachment of a large jaw-closing temporalis muscle (Riley, 1985; Vizcaíno et al., 1998). The presence of parasagittal and lambdoidal crests increases the surface area for the origin of this temporalis muscle, as in the giant panda (Davis, 1964). In posterior view (Fig. 2.2), the skull is rectangular, much


more so than Leptarctus mummorum. The lambdoidal crests meet dorsally and extend ventrally along the sides of the skull thinning ventrally until the mastoid process connected to the basicranium. With the exception of the occipital condyles, the occipital region is concave. The occipital condyles are enlarged compared to other similarly sized mustelines. The dorsal side of the foramen magnum is surrounded by a bony lip connecting posteroventrally to the skull. The two occipital condyles project


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