Actiocyon parverratis, Smith & Czaplewski & Cifelli, 2016

Smith, Kent, Czaplewski, Nicholas & Cifelli, Richard L., 2016, Middle Miocene carnivorans from the Monarch Mill Formation, Nevada, Acta Palaeontologica Polonica 61 (1), pp. 231-252 : 236-240

Actiocyon parverratis sp. nov.

Figs. 4 , 5 .

Etymology: From Latin parvus, small; and erratis, wanderer—a small wanderer.

Type material: Holotype: UCMP 141928 , associated right and left dentaries, each with p2–m2 ( Fig. 4 ) . Paratypes: UCMP 141911 , left maxilla fragment with P4 from UCMP V70147 ( OMNH V972 ) ( Fig. 5 ) and OMNH 54988 , right partial dentary with p3–m1 from UCMP V70138 ( OMNH V967 ), both Eastgate , Churchill County, Nevada, USA, Monarch Mill Formation , Barstovian (Miocene) .

Type locality: UCMP V74103 ( OMNH V973 ), Eastgate , Churchill County, Nevada, USA .

Type horizon: Basal-most unit of the Monarch Mill Formation ( Axelrod 1985), Barstovian (Miocene).

Diagnosis.— Actiocyon parverratis sp. nov. differs from Simocyon Wagner, 1858 in its much smaller size, in having p2 and p3 present and/or unreduced, in lacking a protocone and variably lacking a small parastyle on P4, in having m2 with joined trigonid crests forming a small subcircular structure, and in having m2 trigonid narrower than talonid. Actiocyon parverratis sp. nov. differs from Alopecocyon Viret, 1951 in characters of the m2, which has protoconid larger (instead of smaller) than metaconid, joined trigonid crests forming a subcircular instead of a semicircular structure (fide Beaumont 1964), talonid longer and wider than trigonid, and prominent cusps forming a median talonid ridge. Actiocyon parverratis sp. nov. differs from Actiocyon leardi Stock, 1947 in having the maxilla with relatively larger and lower-positioned infraorbital foramen, P4 with less well-developed and undivided small hypocone on internal cingulum, and smaller overall size.

Measurements (in mm).—P4: AP, 8.3; T, 5.1. For measurements of lower jaws and teeth see Table 3.

Description.— Cranium ( Fig. 5 ): The cranial fragment UCMP 141911 includes a small portion of the left maxilla with the proximalmost base of the zygomatic arch (broken at the articulation with the jugal), a very short section of the anteroventral margin of the orbit, a moderately large infraorbital foramen above the anterior root of the P4 ( Fig. 5A ), and a very small portion of the palate adjacent to the P4 Fig. 5B ). The palate shows a deep hollow posterolingual to the P4 that would have accommodated the protoconid of the lower carnassial during occlusion. The maxillary fragment also shows some of the alveoli for adjacent teeth. It preserves half of the posterior alveolus of the P3. There are two alveoli preserved for the M1: an anterolabial alveolus filled with a root that had a triangular occlusal outline, and an incomplete posterior alveolus showing an anterior bifurcation and longitudinal groove. There is also a rugose, pitted area in the palate into which the base of the M1 nested; although the bone is broken, the pitted area extends lingually indicating that there would have been an additional (third) lingual alveolus for this molar.

Upper teeth ( Fig. 5 ): The P4 is in excellent condition with only minor wear along the blade. Three roots are present, two side-by-side anterior roots and one massive, anteroposteriorly elongated posterior root. The crown has a subtriangular outline in occlusal view, is relatively short, and is widest anteriorly. The paracone is the most prominent cusp; from its apex a low but sharp ridge extends anteriorly to the anterior cingulum. The blade of the paracone (postparacrista) joins the blade of the metastyle (metacone; premetacrista), forming an oblique blade. These blades join on the occlusal surface at a deep, narrow carnassial notch. The metastylar blade is relatively short; it is highest near the carnassial notch and low at its posterolabial end. The lingual surface of the carnassial blade shows slightly crenulated enamel where it is unworn, whereas the labial enamel is smooth. A strong anterior cingulum is present but a parastyle is absent. The anterior cingulum is continuous with a broad protoconal shelf lacking a protocone. In occlusal view there is a shallow indentation in the cingulum between the anterior parastylar shelf and the protoconal shelf. The protoconal shelf does not extend quite as far forward as the parastylar shelf. A low but distinct swelling of the wide internal cingulum indicates an incipient hypocone. The internal cingulum narrows just posterior to the hypoconal swelling and widens again posteriorly. The labial cingulum is discontinuous with the anterior cingulum, but narrow sections are present from the midpoint of the tooth posteriorly.

Dentary ( Fig. 4 ): The dentary is moderately robust with dentary depth beneath the protoconid of m1 exceeding the height of m1 ( Fig. 4A ). Both dentaries are broken anteriorly and in the more complete left dentary ( Fig. 4A ) only parts of the roots of two lower incisors can be distinguished among the fragmented bone. The mandibular symphysis is long and low and extends posteriorly to the level of the middle of p2. The broken root of a robust canine is present in its alveolus in the left dentary. Both dentaries show an alveolus for a single-rooted p1. The left dentary preserves most of the ascending ramus. The condyloid process is hemicylindrical, horizontally oriented, transversely widened (broken laterally), and situated slightly above the level of the tips of the main cusps on the lower teeth. The angular process is broken off at its tip. The coronoid process is tall, canted only slightly outward from the plane of the horizontal ramus, laterally has a large and deep masseteric fossa, and is subparallel-sided in lateral profile in its upper half, with a rounded dorsal edge. The dentary possesses at least one mental foramen ventral to the incisors and three or four lateral mental foramina in a series positioned at the same level at mid-depth of the horizontal ramus; in the left dentary the first is small and situated ventral to the small diastema between c1 and p1 alveoli (absent in the right dentary of the holotype), the second is large and situated below the small diastema between p1 and anterior root of p2 alveoli (below the p 1 in the right dentary of the holotype), the third is large and situated below the p 3 in all three available dentaries, and the fourth is small and situated below the anterior root of p4 (possibly absent in the right dentary of the holotype). The mandibular foramen is positioned about level with the middle depth of the horizontal ramus and below the middle of the coronoid process. There is a prominent tubercle for insertion of the temporalis muscle at the anterolingual base of the coronoid process. A long, prominent, teardrop-shaped insertion scar for the internal pterygoid muscle internus occurs on the lingual side of the angular process. Low on the lateral side of the ascending ramus below the masseteric fossa is a long, low horizontal ridge that probably separates the insertion scar for the deep portion of the masseter muscle from that for the superficial masseter part of the masseter muscle. A possible tooth puncture mark appears on the right dentary below the front of the p2 of the holotype specimen.

Lower teeth ( Fig. 4 ): The p1 is lost from available specimens but a single alveolus between the lower canine and p2 indicates it was a small, single-rooted tooth ( Fig. 4 ). The p2 and p3 are respectively larger in size, with two roots and a single main cusp (protoconid) each (no posterior accessory cusps are present).

The p4 has a posterior accessory cusp that is strong and separated from the protoconid by a narrow notch. The notch bears a small, narrow, carnassial-like slit. The principal cusp of the p4 is lower than the protoconid of the m1.

The m1 (lower carnassial) shows a moderately open trigonid, with the protoconid-paraconid blade (paracristid) oblique to the long axis of the lower toothrow and bearing a carnassial notch with a deeply incised, narrow slit beneath the notch. The protoconid of the m1 is the tallest cusp on the tooth, followed by the metaconid and paraconid, respectively. In the transverse plane, the metaconid of the m1 occurs lateral but slightly posterior to the protoconid. The protoconid-metaconid ridge (protocristid) also has a deeply incised narrow slit beneath the notch between these two cusps. The talonid of the m1 is slightly narrower than the trigonid; it bears a prominent hypoconid, a small hypoconulid adjacent to the hypoconid, and no entoconid. The m1 hypoconid is separated from the protoconid by a small notch, is slightly lingually to the protoconid, and has a broad, flat base that fills the labial part of the talonid basin. Lingual to this, the talonid basin is open. Posterolingually the talonid basin has a continuous rim. On the m1 there is a short, weak anterolabial cingulid beneath the carnassial notch (longer and stronger in OMNH 54988), and another short, weak labial cingulid below the notch between the protoconid and hypoconid (stronger in OMNH 54988) showing a distinct small wear facet. In OMNH 54988, the enamel is weakly crenulated, whereas the enamel is smooth on both m1s of the holotype.

The m2 is relatively long, its length exceeding half the length of the m1, and its occlusal outline is an elongate oval. The m2 is positioned higher in the dentary than the talonid of m1, rising posteriorly. The m2 talonid is slightly wider and distinctly longer than its trigonid. An anterolabial cingulid is present. The trigonid bears small but distinct protoconid and metaconid cusps, with a very low paraconid. The paraconid is connected by low crests to the metaconid and protoconid; together the crests form a continuous subcircular rim enclosing the trigonid basin. The protoconid-metaconid ridge (protocristid) has a median small notch and corresponding slit. The talonid basin is largely filled with a longitudinal ridge that bears two swellings, possibly representing the lingually-shifted hypoconid and hypoconulid (?) and contacting the posteromedial margin of the talonid cingulid. Lingual to this median longitudinal ridge is a small, shallow, rugose talonid basin. The talonid is rimmed by a nearly continuous cingulid.

Remarks.—Except for its larger size and other differences mentioned above, the second specimen (OMNH 54988) agrees with the holotype in anatomical details. Referral of this species to the genus Actiocyon is based on the similarity of the available P4 (UCMP 141911) to that of the type and only known specimen of Actiocyon leardi, LACM /CIT 2747, and to other teeth of the related Holarctic Simocyon and Eurasian Alopecocyon . Confirmation of this assignment awaits the discovery of associated lower jaws and teeth of A. leardi .

Simocyonines are relatively widespread but rare in Eurasia, and fossils of members of this group are extremely rare in North America. In North America, a single species (and specimen) of Actiocyon , A. leardi is known from a single locality in California. Similarly, the simocyonine Simocyon is known in North America by a single species S. marshi ( Thorpe, 1921) and a single specimen from Oregon, plus referred specimens from Idaho and Nevada ( Baskin 1998; Tedrow et al. 1999). Webb (1969b), Baskin (1998), and Salesa et al. (2011) noted the strong resemblance of Actiocyon to the related Eurasian genus Alopecocyon . Viret (1951) noted the resemblance between Eurasian Alopecocyon and Simocyon . Simocyon includes large-bodied ailurids, whereas Actiocyon and Alopecocyon include small-bodied forms. Actiocyon parverratis sp. nov. shares with these taxa a very long m2 and a tall, nearly parallel-sided coronoid process of the dentary; it differs from Simocyon in its much smaller size ( Simocyon species range in m1 length from 21.0– 25.5 mm; measurements from Peigné et al. 2005; Thorpe 1921; Wang 1997) and in having p2 and p3 present and/or unreduced. Differences between Actiocyon and Alopecocyon , if any, need to be studied in detail with reference to numerous and scattered Eurasian specimens.

The only elements represented in specimens of both Actiocyon parverratis sp. nov. and A. leardi are the P4 and adjacent parts of the maxilla. The infraorbital foramen in A. parverratis sp. nov. is lower and appears to be relatively larger than that in Actiocyon leardi (in which this area is poorly preserved). The infraorbital foramen, anterior base of the zygomatic arch, and observable palatal characters are generally similar to the conditions seen in Alopecocyon gaillardi ( Wegner, 1913) (see Viret 1933: pl. 2: 1, 1a, 2, 2a). The small, relatively robust P4 bearing an expanded protoconal shelf but lacking a protocone resembles the condition seen in the simocyonines Simocyon and Alopecocyon , as described by Beaumont (1964); and in particular for Actiocyon leardi , in which there is “no anterointernal cusp protocone], its place being taken by a very well developed cingulum that continues along the inner side of the tooth as shelf with bordering crenulate edge but is more moderately developed from about the middle of the crown to the rear of the tooth” ( Stock 1947: 85). Actiocyon parverratis sp. nov. differs from Simocyon and resembles A. leardi and published drawings of at least the Eurasian species Alopecocyon gaillardi and A. leptorhynchus ( Filhol, 1883) see Viret 1933; Thenius 1949b) in lacking a protocone and tending to have a small or no parastyle. Other characters of the P4 also generally compare favorably with those of A. leardi . The lack of a parastyle resembles the condition in A. leardi as illustrated by Stock (1947), although Beaumont 1964) indicated that the parastyle is highly variable in Eurasian Alopecocyon and Simocyon . The well-marked internal cingulum is apparently shared with Simocyon and Alopecocyon . The tiny incipient hypocone occurring as rise in the internal cingulum in the Eastgate specimen differs from the condition in A. leardi , wherein the edge of the internal cingulum is crenulated along its widened front half and “divided by a notch into two parts” ( Stock 1947: 87). The metastylar blade of the Eastgate P4—being tallest near the carnassial notch and low at its posterolabial end—is opposite the normal situation in carnivoran P4s, in which the metastyle is highest at the posterolabial end of the blade. We observed this somewhat unusual condition of the Eastgate specimen also in published illustrations of some species of Simocyon such as S. zdanskyi Kretzoi in Kadić and Kretzoi, 1927, and S. batalleri Viret, 1929 (e.g., see illustrations in Spassov and Geraads [2011: figs. 1a, 2a–c] and in Peigné et al. [2005: fig. 3]). However, in other Simocyon species such as S. primigenius ( Roth and Wagner, 1854) there are prominences on the metastylar blade of about equal height near the carnassial notch and at the posterolabial end, separated by a slightly lower intermediate section (e.g., see the illustration in Spassov and Geraads 2011: fig. 1b). In Alopecocyon , the metastylar blade appears similarly variable; Actiocyon leardi has both anterior and posterior ends about equal in height ( Stock 1947: pl. 19: 1a), while Alopecocyon gaillardi and A. goeriachensis ( Toula, 1884) have the anterior end higher than the posterior ( Viret 1933: pl. 2: 1, 2; Toula 1884: pl. 8: 8a).

Unfortunately, no dentaries or lower teeth are known for Actiocyon leardi and the P4 is the only upper tooth known in A. parverratis sp. nov., restricting size comparisons between these taxa to the P4s. The P4 of A. parverratis sp. nov. is smaller than the same tooth in A. leardi (AP, 8.3 mm; T, 5.1 mm compared to AP, 11.6 mm; T, 8.0 mm; latter measurements from Stock 1947). Thus, the P4 of A. parverratis sp. nov. is 28% shorter in anteroposterior length and 36% narrower in transverse width than the same tooth in the larger A. leardi . However, in A. parverratis sp. nov. there is a size difference between the two available specimens, the holotype mandible UCMP 141928 and the second partial dentary OMNH 54988 ( Table 3). In the depth of the dentary and in various dimensions of the lower teeth that can be observed in both specimens, the two specimens of A. parverratis sp. nov. differ by 2.6% to 16.1% of the dimensions of the larger specimen. Depending in part on contemporaneity of the two Eastgate localities from which the two specimens originated (V74103 and V70138, which are only separated by about 12 m stratigraphically), this size difference within the hypodigm specimens might represent simple intraspecific variation, sexual dimorphism, or change through time. We cannot speculate about the significance of this size difference without additional data.

The m2 of Actiocyon parverratis sp. nov. is generally similar in its great length and presence of a median talonid ridge to the m2 of North American Simocyon cf. S. marshi ( Tedrow et al. 1999) and Eurasian Simocyon primigenius and possibly to some Alopecocyon species as described by Beaumont (1964: 834–835, pl. 1: 3, 4). However, it differs from m2 proportions in other related species. For example, the m2 of A. parverratis sp. nov. differs from those of Alopecocyon getti (see Mein 1958: fig. 41) and Alopecocyon goeriachensis (see Viret 1951: pl. 1: 9a, b), in having a much longer and wider talonid, a larger protoconid, a prominent median talonid ridge, and a larger hypoconulid.

The lower teeth of Actiocyon parverratis sp. nov. generally resemble those of some Eurasian Alopecocyon species (e.g., A. leptorhynchus, Thenius 1949b : figs. 9, 10; Beaumont 1964), but differ from others (e.g., A. getti ; Mein 1958: fig. 41). Published drawings of these Eurasian taxa are generally insufficient for detailed comparisons. Like other authors before us, we note the great similarity between and possible synonymy of North American Actiocyon and Eurasian Alopecocyon . However, a revision of these genera is beyond the scope of the present paper.

Stratigraphic and geographic range.—Western Nevada, USA; Monarch Mill Formation, Barstovian (Miocene).

Figures

Fig. 4. Ailurid carnivoran Actiocyon parv- erratis sp. nov. from the Eastgate LF, bas- al-most unit of the Monarch Mill Formation, Barstovian (Miocene), Churchill County, Nevada, USA. UCMP 141928, holotype left (A) and right (B) dentaries in occlusal (A1, B1, stereopairs), lingual (A2, B2), and labial (A3, B3) views.

Fig. 5. Ailurid carnivoran Actiocyon parverratis sp. nov. from the Eastgate LF, basal-most unit of the Monarch Mill Formation, Barstovian (Miocene), Churchill County, Nevada, USA. UCMP 141911, paratype left maxilla fragment with P4 in labial (A), occlusal (B, stereopair), and lingual (C) views.