Calede et al.—Ecomorphology of Leptarctus oregonensis


UOMNH F-35458 that the morphological similarities between AMNH 18241 and the L. oregonensis material from Oregon support the presence of L. oregonensis in Nebraska. Because mustelids are known for their ecological diversity


and resource partitioning (e.g., Bonesi et al., 2004; Harrington and McDonald, 2008; Dumont et al., 2016; Valenciano et al., 2016), the ecology of the Leptarctinae cannot be easily under- stood through extant phylogenetic bracketing. Inferences from skeletal anatomy are therefore critical to determining the ecology of the leptarctines and informing future studies of community structure in the Miocene of eastern Oregon and more broadly across North America and Asia. Leptarctine ecology and diet have been the topic of much debate and many modern analogs for Leptarctus have been proposed. Starting with the publication of L. primus by Leidy (1856), many simi- larities between Leptarctus and procyonid carnivorans, such as Nasua, Potos, and Procyon, were recognized, leading to debates as to the taxonomic affinities of Leptarctus (see Korth and Baskin, 2009). Although Gazin (1936) established that leptarc- tines belong to Mustelidae, the morphological similarities with procyonids have been interpreted as evidence for similarities in ecology. Olsen (1957) proposed that Leptarctus was similar to the extant mustelid Taxidea taxus Schreber, 1777, the North American badger, a carnivore (Long, 1973), but some have since then disputed this analogy (Lim and Martin, 2001b). Lim and Miao (2000) noted the molariform shape of the upper fourth premolar of Leptarctus and, together with its even wear, inter- preted it as indicative of an omnivorous diet similar to that of the raccoon (Procyon lotor Storr, 1780; Lotze and Anderson, 1979). Lim and Martin (2002) noted similarities in the wear of the canine of the leptarctine Hypsoparia with the coati (Nasua nasua Linnaeus, 1766, an omnivore; Gompper and Decker, 1998), interpreting the subfamily as more herbivorous than most carnivorans. The enlarged zygomatic arches and large tympanic projections ventral to the auditory bullae that are superficially similar to those found in the extant koala (Phascolarctos cinereus Goldfuss, 1817, an arboreal herbivore; Cork et al., 1983) were seen as additional evidence for Leptarctus incor- porating foliage in its diet and possibly being arboreal (Lim and Martin, 2001a). Lim and Martin (2001b) interpreted a suite of craniodental characters of Leptarctus as evidence for “an omnivorous diet, with a strong plant component” (Lim and Martin, 2001b, p. 317). They compared the peculiar morpho- logy of the upper third premolar of L. desuii to that of the fruit- eating bat Pteropus rodricensis Dobson, 1878 (Lim and Martin, 2001b). They also noted several similarities in the morphology of the cheek teeth (including the enlarged molariform morphology of the upper fourth premolar and the relatively long talonid of the lower first molar) and the skull (notably an enlarged surface for the attachment of the masseter muscle) with Procyon lotor or even Ursus americanus Pallas, 1780 (the American black bear) and the koala (Lim and Martin, 2001b). Korth and Baskin (2009) refuted the interpretation of leptarc- tines as herbivorous koala-like animals, pointing out the lack of specialized incisors, the presence of a prominent canine, the absence of a diastema, and the cuspate shape of the teeth as opposed to the shearing crests found in herbivorous taxa (including the koala). The new material of L. oregonensis described herein also allows a functional and paleoecological


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study of the craniodental morphology of one of the oldest species of Leptarctus and thus helps shed light on the dietary habits of the Leptarctinae.


Materials and methods


We describe UOMNH F-35458, a mostly complete skull of Leptarctus oregonensis, including the right P4 and M1, the left third premolar (P3), P4, and M1, and lacking only the rostrum. We compare this new material to UCMP 39102 and LACM (CIT 206), all previously known specimens of L. oregonensis,a cast of AMNH 18241 (UCMP 27295), photos of F:AM25385, the skulls of several extant mustelids, mephitids, and procyonids (Table 1), and published specimens of the subfamily Leptarcti- nae. F-35458 was collected at the early Barstovian locality UO 2993 in 1997 during a road construction project west of Dayville, Oregon, along Highway 26 near the Mascall type area. We use nomenclature following recent publications of other species of Leptarctus (in particular Korth and Baskin, 2009) for skull terminology and cranial foramina. We use the terminology of Olsen (1957) for the description of the dentition. Our measurements (Table 1) are either drawn from the literature or were taken directly on the specimens using Mitutoyo Absolute Digimatic CD-6”Cand Mitutoyo Absolute Digimatic CD-8”CX calipers. In the absence of the rostrum, which prevents the mea-


surement of skull length, we measured the occiput to orbit length of UOMNH F-35458 to estimate the body mass of Leptarctus oregonensis; we used the regression formula for mus- telids of Van Valkenburgh (1990). This new body mass estimate for L. oregonensis allowed us to calculate themaximumbody size of its prey using the approach ofMeers (2002).We used the “all predators” equation of Meers (2002, table 4) and corrected the estimate for logarithmic transformation bias using the ratio estimator correction factor calculated byWilson et al. (2016). We used the dry skull method of Thomason (1991) to


estimate the bite force of Leptarctus oregonensis. This approach has been successfully applied to both mustelids and fossil taxa by Wroe et al. (2005) and Wilson et al. (2016). We used photos of the dorsal, ventral, and lateral views of UOMNH F-35458 (see Figs. 1, 2) to calculate the physiological surface area of the temporal and masseter muscles, the moments of the temporal and of the masseter, and the lengths of the carnassial out-lever and the M1 out-lever. All measurements were made on the better-preserved right side of the specimen. The surface area and position of the centroid of the muscles were estimated in ImageJ 1.51 (Schneider et al., 2012). We used a standard muscle stress value of 300kPa following Wroe et al. (2005) and Wilson et al. (2016) to calculate the force of the masseter and temporal muscles. Because bite force scales with body mass (Meers, 2002; Wroe et al., 2005; Wilson et al., 2016), we estimated a bite force quotient (BFQ) at the carnassial for L. oregonensis and 30 other extant mammals using data and methodology from Wroe et al. (2005). BFQs allow the comparisons of bite force across species of different body masses by using the residuals of a dataset-specific regression of the log of bite forces and log of body masses (Wroe et al., 2005). Our regression (R2= 0.89, p = 2.10−16) led us to estimate BFQ at the carnassial for our dataset using the following formula: BFQ = (bite force at the


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