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Journal of Paleontology 92(2):289–304


large jaw-adducting muscles, high bite forces, and the ability to bite into hard objects (Goswami et al., 2011). Multiple lines of evidence for strong jaw-closing and jaw-opening muscles (i.e., pterygoideus, temporalis, digastricus, and masseter) further sup- port a high bite force suggestive of crushing in L. oregonensis. Based on comparisons with extant mustelids, the presence of a sagittal crest, strong-lipped and narrow glenoid fossae of the squamosals, small orbits, and dorsally curved zygomatic arches in L. oregonensis supports a carnivorous diet for this mustelid (Dumont et al., 2016). The short snout of L. oregonensis (based on LACM CIT 206) is consistent with the interpretation of L. oregonensis as a carnivorous species with a strong bite force (Goswami et al., 2011; Dumont et al., 2016). Our estimate of the bite force relative to body mass of L. oregonensis indicates that although not durophagous, it had a stronger bite force than the extant European badger (Meles meles). This high bite force is consistent with an animal-dominated diet. Wroe et al. (2005) showed that carnivorous species tend to have higher BFQs than omnivorous species.All of the omnivores and animalswith plant- dominated diets included our analysis have lower BFQs than L. oregonensis (Table 4). The bite force of L. oregonensis is superior to that of the golden jackal, Canis aureus, a taxon that feeds on small mammals, large ones (gazelles), other vertebrates, insects, and fruits (Nowak, 2005). The bite force of L. oregonensis is most similar to that of Lynx rufus, a carnivorous taxon that feeds mostly on small mammals and birds, but is capable of hunting deer, amuch larger prey than itself (Nowak, 2005). Thus, it seems that L. oregonensis would have been able to subdue small verte- brates as well as vertebrate prey larger than itself, and had a powerful bite force conferring it considerable crushing ability. The tooth morphology of L. oregonensis also supports the


may reflect the ecologically intermediate status of L. oregonensis in the evolution of leptarctines. The oldest leptarctine (19–22.5 Ma; Wang et al., 2004), Kinometaxia from Asia, displays a long P4 blade (Wang et al., 2004, fig. 3A). The oldest North American leptarctines, Craterogale and Schultzogale, also display long P4 blades and reduced or altogether missing hypocones on P4 andM1


interpretation of this fossil mustelid as a crushing animal. The protocone of P4 is robust, a feature associated with crushing behavior (Valenciano et al., 2015). Although L. oregonensis lacks a strong lingual cingulum on M1 (unlike L. martini, for example), another feature associated with durophagy (Valenciano et al., 2015), it does bear a strong hypocone on that tooth. The presence of a strong hypocone on M1 is indicative of crushing in carnivorans (Hunter and Jernvall, 1995; Popowics, 2003).Despite being often associated with herbivory, a hypocone is also found in a number of generalist and even a fewfaunivorous carnivorans (Hunter and Jernvall, 1995). The presence of a hypocone in L. oregonensis therefore cannot be interpreted as evidence for herbivory. Nevertheless, L. oregonensis possesses large crushing surfaces on both M1 and P4 that suggest the ability to process invertebrate cuticles and plant material. This crushing toothmorphology is associatedwith a long P4 blade that usually is interpreted as evidence for carnivory (Friscia et al., 2006). Thus, the skull morphology and some aspects of the tooth morphology of L. oregonensis are consistent with a carnivorous diet, while other aspects of dental morphology suggest the consumption of invertebrates and possibly plant material. This mosaic of dental characters in Leptarctus oregonensis


(Gazin, 1936; Lim and Martin, 2000), which are characteristics associated with carnivorous diets. The oldest leptarctines, L. ancipidens and L. oregonensis, from the Hemingfordian and early Barstovian respectively, also display long P4 blades (Fig. 6.1, 6.2). However, the younger (late Barstovian) L. primus shows a reduced P4 blade (Korth and Baskin, 2009). Other mid to late Miocene species (e.g., L. mummorum and L. martini) show more robust hypocones and less blade-like P4s than L. oregonensis, indicating increased crushing and possibly more


omnivorous diets relative to L. oregonensis (Lim andMiao, 2000, fig. 1.1; Korth and Baskin, 2009, fig. 4; Fig. 6.3). Even younger species, from the Clarendonian through Hemphillian (e.g., L. kansaensis and L. webbi), essentially lack a functional P4 blade (Lim and Martin, 2001, fig. 1A; Baskin, 2005, fig. 3A) and may have had more herbivorous diets than older Leptarctus species. Thus, L. oregonensis may represent the initial stage of an ecomorphological transition from an ancestrally carnivorous diet to a more derived herbivorous diet in leptarctines. Future investiga- tions of the phylogenetic relationships of leptarctines will be critical in exploring this issue. In light of the conflicting dietary signals of comparative


anatomy, we undertook a quantitative comparison of the propor- tions of the P4 and M1 of Leptarctus oregonensis with modern carnivores. Our results suggest an animal-dominated omnivorous diet rich in invertebrates similar to that of the similarly sized Mephitis mephitis and Mydaus javanensis or the smaller Spilogale putorius. The morphology of the teeth of L. oregonensis is very similar to that of the teeth of Spilogale. They both share a long P4 blade as well as a large and posteriorly expanded metacone (van Gelder, 1959; Kinlaw, 1995, fig. 2c). This long P4 blade could provide the sharp edges required to tear and shear invertebrates (Popowics, 2003) in addition to shearing vertebrate prey. The M1 of Spilogale,likethatof L. oregonensis, is very broad and would


offer a large surface area for grinding vegetation (Kinlaw, 1995, fig. 2c) or invertebrate cuticles (Popowics, 2003). Although they are very similar in dental morphology, the skull of L. oregonensis is much more robust than the skulls of S. putorius, Me. mephitis,and My. javanensis. The cranial morphology of L. oregonensis does not however prevent it from having an invertebrate-rich diet. Indeed, the proportions of the skull of L. oregonensis are similar to those of Meles meles (European badger), which includes a high proportion of earthworms in its diet in addition to some plant material and small vertebrates (Nowak, 2005; Myslajek et al., 2013). Like L. oregonensis, Meles meles displays a broad skull with a short snout, a high sagittal crest, prominent mastoid processes, and robust zygomatic arches (Hidaka et al., 1998; Yom-Tov et al., 2003). Our interpretation of an invertebrate-rich diet for


L. oregonensis may explain the conflicting results of our ana- lyses of its craniodental morphology. Indeed, the study of extant small carnivorans shows that the morphological overlap among invertivorous carnivorans and both carnivorous and omnivorous species can sometimes prevent the assignment of a taxon to one of these three dietary categories (Friscia et al., 2006). Our interpretation is consistent with data on body mass and energetic constraints on carnivore diets. Our estimate of maximum prey size for L. oregonensis suggests that it would have been able to consume many of the small mammals known from the Mascall Formation, including the leporid Hypolagus, many rodents (e.g., Protospermophilus and Nototamias), and the shrew


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