Adcrocuta eximia

Adcrocuta eximia (Roth and Wagner, 1854)

Figures 3–5

Synonymy. Refer to Werdelin and Solounias (1991) for list and discussion of synonymy

Type species. Hyaena eximia (Roth and Wagner, 1854, 396-398)

Occurrence. Bahean-Daodean; Vallesian-Turolian; MN Zones 10-13; Late Miocene

Localities. Bulgaria, China, France, Greece, Hungary, Iran, Libya, Pakistan, Romania, Spain, Turkey, Russia, and Yugoslavia (Werdelin and Solounias, 1991); UO-4605 Ortok.

Referred specimen. UOMNH F-70508

Description

Preserved material consists of a lower right partial dentary, isolated left p3 and P4 (Figures 3 and 4). The angular process and condyloid process are intact, but the coronoid process is damaged and incomplete (Figure 3A). The mandibular corpus is elongated, high, and thick (13.3 mm), and the inferior border is convex. At the horizontal ramus below the m1, it curves deeper and rises at the ventral border at a 12˚ angle. There are two mental foramina (Figure 3); the anterior foramen mentale is located 11 mm below the distal root of the p2, the smaller posterior foramen mentale is 16.5 mm below the mesial root of p3. The foramina are both positioned in the upper half of the ramus. The mandibular depth is 46.8 mm behind the m1 (lingual) and 36.4 mm between the p2 and p3 (lingual). Behind the m1, the mandible is generally deeper, but due to the broken mandibular surface, we could not take reliable measurements. A portion of the fossa masseterica is preserved (Figure 3B); it is oval, relatively deep, and the anterior portion ends 28 mm from the distal end of the m1. This measurement could be impacted by preservation (Figure 3). The angular process is quite well-preserved and is in line with the alveolar end of the m1.

Upper dentition. The upper carnassial P4 is elongated and slender with a strongly reduced protocone (Figure 4). The protocone is a distinct conical cusp that projects anterolingually between the parastyle and paracone (Figure 4A). As characterizes Adcrocuta, the reduced protocone is situated posterior to the anterior margin of the parastyle (Werdelin and Solounias, 1991; Ghaffar et al., 2011; Koufos, 2016). The protocone is low, the parastyle is lower than the paracone, and the metastyle blade is long with a buccally angled distal end (Figure 4A).

Lower dentition. The dentition is well-preserved and a small rounded alveolus for the p1 is present (Figure 5A). The other premolars, p2 to p4, increase in size posteriorly (Figure 5C). The p2 has two roots, with a large ovoid protoconid that is unworn. The mesial cingulum thickens, but there is no anterior accessory cuspid. There is a posterior accessory cuspid that is closely appressed to the main cuspid (protoconid). The p3 has the same features as the p2, although the thickening of the cingulum is slightly greater, and it is larger and more robust in general. The posterior accessory cuspid is slightly larger than in the p3 but is still closely appressed to the main cuspid. There is a shallow lingually situated basin between the posterior accessory cuspid and the main cuspid. The position of the p3 is slightly lower than the p 2 in the mandible. The p3 is near equal in size to the p 4 in width and length. The p4 is the largest and most robust premolar, with a large anterior accessory cuspid separated from the main cuspid, and a smaller posterior accessory cuspid. The posterior accessory cuspid is situated on the buccal side of the distal projection of cingulum. There is a narrow open basin between the smaller posterior cuspid and the main cuspid apparent on the lingual side, similar to an m1 talonid. The lower carnassial m1 is elongated and relatively narrow. There is no distinct metaconid, and there is a small and low bicuspid talonid (Figure 5E). The talonid is separated from the trigonid by a small groove and there is a very low, distinct hypoconid, and second reduced labial cuspid, separated by a clear but shallow groove (Figure 5E). The other small cuspid has been identified as the entoconid. This reduced cuspid was also slightly worn with dentine exposure (Figure 5B). The reduction of the talonid is associated with hypercarnivory, but the continued presence of even a reduced talonid suggests its continued functional use in crushing (Solé and Ladevèze, 2017). There is a shallow talonid basin on the buccal side of the distal projection of the carnassial. The buccal cingulum thickens around the entire carnassial creating a small projection on the trigonid. There is a degree of wear on both the protoconid and paraconid, exposing the dentine with <1mm of enamel around the anterolingual edge, which is minimal wear compared to many specimens of Adcrocuta (based on museum comparatives). There is no trace of an m2 or any alveoli, however, A. eximia has a wide variation in presence of m2, as well as the presence of p1, and the presence of a metaconid, so this does not confound our identification (Werdelin and Solounias, 1990; Kovachev, 2012).

Comparisons

The large size (length and width) of the teeth of UOMNH F-70508, the present alveolus of the first premolar (p1), the short m1 talonid, and the absent m2 are diagnostic of the Hyaenidae. There are three current subfamilies recognized in Hyaenidae: Ictitheriinae, Hyaeninae, and Percrocutinae (McKenna and Bell, 1997). The morphology of the specimen excludes subfamily Ictitheriinae as UOMNH F-70508 does not have a second molar, the premolars are relatively short compared to the carnassial, there is no metaconid, and there is a relatively short talonid (F: AM 144905, 144906; Semenov, 2008).

During the Miocene, there are several possible hyaenids present in Eurasia, including the Percrocutinae hyaenids (Werdelin and Solounias, 1990). The features of UOMNH F-70508 distinguish it from a Percrocuta grandis as our p4 and m1 are larger and longer (relative) to the p2 and p3 (Howell and Petter, 1985; Ghaffar et al., 2011). Dinocrocuta gigantea was eliminated based on premolar morphology as they possess anterior cusps on p2 and p3, with no p1, unlike the premolars in our specimen UOMNH F-70508 (Zhang, 2005). Generally, the size for Percrocuta mandibular teeth are also substantially larger (± 10 mm on average, length, and width) than UOMNH F-70508 (Howell and Petter, 1985). Given the disqualification of UOMNH F-70508 from Percrocutinae, the remaining characteristics of the dentition were able to denote membership within the subfamily Hyaeninae, and subsequently the species Adcrocuta eximia. Adcrocuta does share certain derived characteristics with the percrocutoid hyaenids (see Howell and Petter, 1980, 1985; Werdelin and Solounias, 1990). While there is variation in the characteristics of A. eximia, diagnostic features such as two mental foramina, present lower first permanent premolar, the lower carnassial being longer relative to the fourth premolar, and a less reduced talonid, distinguish Adcrocuta from potential percrocutoid hyaenids (Howell and Petter, 1980, 1985; Werdelin and Solounias, 1990; Kovachev, 2012). The specimen UOMNH F-70508 possesses many of these features, including two mental foramina, present p1, short p2 and p3 relative to the taller p4 and m1, absent anterior accessory cuspids in the p2 and p3, absent metaconid and small talonid in the m1, and lower carnassial longer relative to p4. Therefore, UOMNH F-70508 bears the most similarity to A. eximia.

Remarks

UOMNH F-70508 possesses largely unworn dentition, particularly compared to other adult specimens in museum collections, however, it is not newly erupted either (Figure 5B). The maximum length and width of the m1 is 2-4 mm longer and wider than the compared museum specimens (e.g., AMNH 22880, F: AM 144903). This could suggest endemic variation in our specimen compared to other Eurasian Adcrocuta. Ratio of maximum length m1:P4 is within 1.0 mm of the museum specimens, as is length of paracone:metastyle blade. As mentioned above, A. eximia has been considered highly variable within the species, but Werdelin and Solounias (1990, 1991) found that there was no statistical significance to this variation, and A. eximia are no more variable than any other hyaenids from the same region.