Brevimalictis chikasha, 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 : 241-243

Brevimalictis chikasha sp. nov.

Fig. 6 .

Etymology: Named in honor of the unconquerable Chickasaw (Chikasha is the traditional spelling) people, who are known for their highly progressive, education-focused values, and preservation of their native culture.

Holotype: UCMP 113319 , left dentary with p3–m1.

Type locality: UCMP V70140 (OMNH V968), also known as Nevaxel V, near the town of Eastgate, Nevada, USA.

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

Diagnosis.—A small mustelid about the size of an extant female or small male long-tailed weasel, Mustela frenata Lichtenstein, 1831 . Brevimalictis gen. nov. differs from Mustelinae in having the m1 talonid basined, not semitrenchant. It differs from Lutrini in having m1 with a long rather than shortened trigonid and with a reduced rather than strong metaconid. Brevimalictis gen. nov. differs from the Galictinae in having a shallow rather than deep jaw.

Measurements (in mm).—p3: AP, 2.6; T, 1.5; p4: AP, 3.3; T, 1.7; m1: AP, 6.0; AP of trigonid, 4.0; AP of talonid, 2.0; TA, 2.5; TP, 2.2; p2–m2 alveolar length, 14.3; c–m2 alveolar length, 16.1; dentary depth beneath m1 protoconid, 4.0; and dentary total length (from below incisor alveoli to damaged mandibular condyle), 25.7.

Description.— Dentary: The holotype and only known specimen UCMP 113319 is a short, slender left dentary with broken coronoid and condyloid processes but intact angular process. The dentary contains p3–m1 and empty alveoli for c, p2, and m2. The mandibular condyle is transversely oriented. In lateral view, the ventral margin of the horizontal ramus is nearly straight in the region beneath the p3 to m1. The angular process projects ventrally at about 45 degrees and slightly laterally from the long axis of the mandible, and has a troughlike medial surface. Laterally the dentary bears a deep, concave masseteric fossa with a rounded anterior margin that terminates below the m2 alveolus, and has prominent superior and inferior ridges. Anteriorly the dentary has at least four mental foramina. The anteriormost mental foramen occurs below the lower incisors. The second (and largest) occurs below the posterior alveolus of p2, the third occurs below the posteriormost part of the p3, and the fourth occurs below the posterior root of p3 and anterior root of p4. On the lingual aspect of the ramus the bone is weathered but the pterygoideus muscle scar can be discerned at the medial base of the coronoid process just posteromedial to the m2 alveolus. The mandibular foramen is located about halfway between the condyle and m2 alveolus and ventral to the pterygoideus scar. The mandibular symphysis extends posteriad to a level beneath the p3. Incisive alveoli are poorly preserved such that the number of lower incisors cannot be determined with certainty, but at least two appear to have been present. Two alveoli are present for the missing p2, posterior to the single canine root. The posterior of the two alveoli for the p2 is larger than the anterior one; the two are aligned obliquely relative to the long axis of the toothrow and horizontal ramus. The p2 alveoli indicate a relatively large tooth set at an oblique angle to the other premolars. The p3, p4, and m1 each have two roots. Posteriorly a single alveolus is present for the missing m2 on the rising anterior base of the coronoid process.

Lower premolars: The premolars increase in size from front to back and have a single high cusp each. Crowding of the lower premolars and absence of p1 indicates that the species had a short muzzle as in many other mustelids. The p3 and p4 lack posterior accessory cusps; each premolar has an anterior and posterior ridge extending from the apex of the main cusp down to the anterior and posterior cingulid, respectively, where a tiny cingular cuspule is formed at each end of these premolars. Cingulids are absent medially and laterally on each premolar.

Lower molars: The m1 trigonid is transversely narrow (but wider than the talonid), elongate and open lingually. The paraconid is more worn (or the cutting edge is damaged) than the other cusps; this cusp is relatively high but lower than and anterior to the protoconid. The protoconid is the highest cusp and forms a carnassial blade (paracristid) with the paraconid; the carnassial blade is slightly oblique but nearly in line with the long axis of the toothrow. The metaconid is small, distinct, and much shorter than the paraconid. The metaconid is placed lingual and slightly posterior to the protoconid, to which it is connected by a short, rounded ridge (protocristid) lacking a sharp notch— the ridge circumscribes the margin of the talonid to join the hypoconid. The m1 talonid is lower than the trigonid and is basined. The hypoconid is situated on the labial margin of the talonid and shows a shearing facet on its external (labial) slope. A small, vertical, ridgelike accessory cuspule (protostylid) occurs at the posterior base of the protoconid, and a distinct notch is formed between the protostylid and the hypoconid. There is a flattened swelling extending basinward from the lingual base of the hypoconid, filling part of the talonid basin. The entoconid and hypoconulid are absent, and there is a very low rim on the lingual aspect of the basin. The talonid is shallowly basined, with a continuous cingular rim extending from the hypoconid to the posterior base of the metaconid. The lingual rim of the talonid is much lower than the labial rim with hypoconid. Cingulids are absent on the m1 except for a slight swelling beneath the anterior end of the paraconid.

Although the m2 is absent, the dentary has a single alveolus indicating a single root for this tooth locus. The alveolus is tilted forward as in many mustelids. Based on the size of the alveolus, the m2 was small in this specimen, which is typical for mustelids ( Baskin 1998).

Remarks.— Brevimalictis chikasha gen. et sp. nov. (UCMP 113319)is smaller than most known Neogene North American Mustelidae . Its m1 AP of 6.0 mm is roughly similar to that of several genera of Miocene mustelids ( Promartes Riggs, 1942 : AP 8.3–10.4 mm; Miomustela Hall, 1930 : AP 5.1 mm; Pliogale Hall, 1930 : AP 5.7–8.4 mm; Pliotaxidea Hall, 1944 : AP 8.2–10.9 mm; and Plionictis Matthew, 1924 : AP 7.6–9.5 mm; Martes Pinel, 1792 : AP 7.0–13.0 mm; measurements from Baskin 1998). The taxidiine badger Chamitataxus Owen, 2006 is also relatively small but is known only by a cranium and upper teeth ( Owen 2006), which are not yet known for Brevimalictis gen. nov. Brevimalictis gen. nov. differs in various qualitative features of its dental morphology from all of these genera. In particular it differs from members of the mustelid taxa Oligobuninae (e.g., Promartes ), Taxidiinae (e.g., Pliotaxidea ), and ischyrictine Mustelinae (e.g., Plionictis ) in various dental characters of those taxa provided by Baskin (1998). Brevimalictis gen. nov. differs from the Oligobuninae in that the m1 is relatively long and its talonid is not labially indented, and the lower jaw length is reduced. It differs from Taxidiinae in that the p4 lacks a posterolateral accessory cusp, the m1 lacks an entoconid and accessory cusp lateral to the hypoconid, the m1 paraconid and metaconid differ in size and are not approximated, and the m2 has a single small root. Brevimalictis gen. nov. differs from Ischyrictini in having a basined rather than semitrenchant or trenchant talonid on m1. Brevimalictis gen. nov. differs from members of the family Mephitidae (formerly considered a subfamily of Mustelidae ) in lacking accessory roots on m1.

Brevimalictis gen. nov. differs from Miomustela (see Hall 1930: pl. 8) in having its largest lateral mental foramen farther anterior (below the posterior root of p2 instead of below the posterior root of p3), crowded lower premolars, p4 without a posterior accessory cusp, and m1 with a protostylid and hypoconid. As for Pliogale Hall, 1930 , the m1 of Brevimalictis gen. nov. differs in lacking an entoconid and a cuspidate rim on the talonid, a narrow notch separates a protostylid from the hypoconid; and m2 has a single root, which is fused-double in Pliogale . In Pliotaxidea , the m1 is much longer (AP) and has a relatively wider talonid (TAW) than the m1 of Brevimalictis gen. nov. Brevimalictis gen. nov. differs from Plionictis ( Matthew 1901, 1924) in having lower premolars that lack weak posterior accessory cusps, m1 metaconid is smaller and not as well separated from the protoconid, m1 protostylid is present, m1 hypoconid is separated from the protostylid by a notch, and m1 metaconid and hypoconulid are absent. Tseng et al. (2009) described a new genus of mustelid, Legionarictis , from the Temblor Formation in California, but the mandible and lower teeth for this taxon are not known so no direct comparison can be made. Based on the length of the P4 (AP 11.5 mm) of Legionarictis , it is a mustelid that is much larger than Brevimalictis chikasha gen. et sp. nov.

Brevimalictis gen. nov. agrees in some features with those used by Baskin (1998) to characterize the Ischyrictini , the Galictini and the Lutrini , but also shows differences from each of these tribes. The characters of Brevimalictis gen. nov. agree most closely with those of the Galictini as diagnosed by Baskin (2011), in that it has crowded premolars, p1 absent, m1 with an open bladelike trigonid, m1 with a lingually expanded and basined talonid, m1 talonid with a posterolingual cingulum extending from the base of a reduced metaconid to the hypoconid, m1 hypoconid separated from the protoconid by a small notch, and in its small size; it differs from galictins in having a shallow rather than deep jaw.

Brevimalictis gen. nov. shares some characters in common with Lutrini ( Baskin 1998) , specifically in having m1 with an accessory cuspule on the posterior margin of the protoconid and having the metaconid connecting to a broad, basined talonid; it differs from lutrins in having m1 with an elongated trigonid and reduced metaconid. Brevimalictis chikasha gen. et sp. nov. further differs from the lutrin Mionictis Matthew, 1924 in being much smaller than known species of that genus ( Mionictis species range in m1 length 10.6–16.4 mm; measurements from Baskin 1998). It differs from Limnonyx Crusafont-Pairó, 1950 in having m1 trigonid wider than talonid, and in having depth of dentary less than length of m1 ( Limnonyx has an m1 length of 12.1 mm; measurement from Baskin 1998). Brevimalictis differs from Satherium Gazin, 1934 in having p4 without a posterior accessory cusp, m1 with a more anteroposteriorly directed carnassial blade, no metastylid, and much smaller size ( Satherium range in m1 length 16.5–18.3 mm; measurements from Baskin 1998). It differs from Enhydritherium Berta and Morgan, 1985 in having m1 narrow and without inflated cusps, metaconid smaller than protoconid, protostylid (accessory cuspule) present, and much smaller size ( Enhydritherium range in m1 length 15.8–17.0 mm; measurements from Baskin 1998).

Brevimalictis gen. nov. differs from Baskin’s (1998) characterization of Mustelinae , including the Ischyrictini , in having the m1 talonid basined, not semitrenchant to trenchant; it agrees with Mustelinae in having the m1 metaconid reduced and m2 single rooted.

Brevimalictis gen. nov. differs from Negodiaetictis rugatrulleum gen. et sp. nov. from Eastgate (described below) in having a p4 without posterior accessory cusp, m1 with simple basined talonid that is narrower than the trigonid, in lacking crenulated enamel on the teeth, and in much smaller size.

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

Figures

Fig. 6. Mustelid carnivoran Brevimalictis chikasha gen. et sp. nov. from the Eastgate LF, basal-most unit of the Monarch Mill Formation, Barstovian (Miocene), Churchill County, Nevada, USA. UCMP 113319, holotype right dentary with p3–m1 in occlusal (A, stereopair), labial (B), and lingual (C) views.