Agriarctos nikolovi, Jiangzuo & Spassov, 2022

Jiangzuo, Qigao & Spassov, Nikolai, 2022, A late Turolian giant panda from Bulgaria and the early evolution and dispersal of the panda lineage, Journal of Vertebrate Paleontology (e 2054718) 42 (1), pp. 1-12 : 2-4

? AGRIARCTOS NIKOLOVI sp. nov.

( Fig. 2 )

Etymology —Named in honor of Dr. Ivan Nikolov, who has preserved the fossil teeth in the collections of the NMNHS and for his contributions to the study of the fossil mammal fauna of Bulgaria.

Holotype —Upper right carnassial tooth ( NMNHS FM3546-a ) and upper right canine ( NMNHS FM3546-b ; both numbers from the collection of the NMNHS), belonging to the same individual .

Age —Estimated late Messinian (i.e., latest Turolian) age (MN13).

Diagnosis —Derived Ailuropodini of large size. Canines proportionally very large. P4 with distinct parastyle, but not enlarged to metacone size. Inner lobe expanded with well-developed protocone and especially hypocone. Protocone and hypocone subdivided. Paracone and metacone under very slight angle. Buccal wall straight, without concavity in the middle.

Differential Diagnosis —Differs from Kretzoiarctos and Miomaci in having much stronger P4 parastyle and more complicated P4 inner lobe; differs from Ag. depereti in having larger P4 inner lobe and more developed small cusps in the inner lobe; differs from Ailurarctos in having smaller P4 parastyle.

Description —The canine is large and robust. A wear facet from the contact with the lower canine is seen on the base of the mesial surface of the tooth. The mesial and distal ridges are well marked: A well expressed sharp ridge is present on the distal surface of the C1. A similar, but slightly less pronounced ridge is present on the mesial surface, displaced slightly lingually. The labial surface of the tooth is the largest one. It is slightly convex. The lingual one is relatively flat. A visible rim separates the lingual surface from a third, short, flat to slightly concave postero-lingual surface, reaching distally the distal ridge.

The P4 is unworn, with a triangular shape. There is no distinct concavity in both antero- or posterior border of the inner lobe. The buccal wall is straight, not concave in the middle. Three large buccal cusps are present. The paracone is the dominant cusp, followed by metacone and then parastyle. The parastyle practically reaches the metacone in height, but is significantly shorter mediodistally than the metacone, which is rather elongated. The cutting edge of the three cusps is well developed. The paracone and metacone are oriented in a very slight angle. The medial faces of these three cusps are rather flat, whereas the lateral faces of parastyle and paracone are convex. The inner lobe is greatly extended to most of the medial border. There are two large cusps, representing protocone on the anterior side and hypocone on the posterior side. They are relatively mesially located on the inner lobe. The hypocone is clearly larger than the protocone. It is more distally placed (close to the paracone/metacone boundary) and presents several, weakly expressed corrugations. Both the protocone and hypocone are subdivided around their middle part.

COMPARISON

The P4 of? Agriarctos nikolovi sp. nov. shows clear Ailuropodinae affinity, and in particular Ailuropodini affinity. The large and ridge-like parastyle, and greatly enlarged inner lobe with multiple cusps point to an affinity to either Agriarctos , Ailurarctos , or Ailuropoda . It differs from Indarctos and Agriotherium by its much larger and blade-like parastyle and more expanded and complex inner lobe.

Comparison with Kretzoiarctos and Miomaci

The tooth of? Ag. nikolovi sp. nov. differs significantly from Kretzoiarctos : The P4 of Kretzoiarctos is smaller ( Table 1 ). The parastyle is distinct in Kretzoiarctos ( Abella et al., 2011) but it is relatively small, much smaller (shorter and lower) than that of the Bulgarian one. The inner lobe in Kretzoiarctos is smaller, with the only hypocone developed. Similar differences can be applied to Miomaci , which has a smaller parastyle and simpler inner lobe in a similar way ( de Bonis et al., 2017).

Comparison with cf. Agriarctos depereti

Two P4s of cf. Agriarctos depereti (see below Discussion point 1–3 about the taxonomic status and the scope of this taxon) are known from Soblay ( Viret, 1949). The Bulgarian P4 is similar to the two P4 of cf. Ag. depereti in having a similarly enlarged blade-like parastyle ( Fig. 3 ). But the Bulgarian P4 has a parastyle nearly in line with the paracone, whereas the parastyle in the two P4s from Soblay turns slightly buccally. The paracone and metacone in Bulgarian P4 are nearly in line. The situation in cf. Ag. depereti is variable. There is no buccal concavity in the Bulgarian P4, which is present in both teeth of cf. Ag. depereti . The buccal cingulum is nearly absent in the Bulgarian P4, but well developed in cf. Ag. depereti .

The inner lobe of the Bulgarian P4 is clearly more derived than that of cf. Ag. depereti . The Bulgarian P4 has a distinctly more expanded inner lobe, with clearly developed protocone and hypocone, both subdivided. In the P4 of cf. Ag. depereti , the inner lobe only expends around half of the tooth length, with the protocone located more posteriorly, and both protocone and hypocone are not subdivided. The Bulgarian tooth therefore is more derived than cf. Ag. depereti and also larger.

Comparison with Ailurarctos

Two species of Ailurarctos are known. The Bulgarian tooth is similar to the P4s of both species in having well-developed inner lobe cusps, with both protocone and hypocone subdivided. The parastyle and metacone also form a very small angle or in line in all three species, and the buccal concavity is indistinct or absent. The buccal wall of? Agriarctos nikolovi is straight, not concave, in which it is close to the more derived Ailurarctos lufengensis .

However, the Bulgarian tooth is different from the two species of Ailurarctos in several aspects. The parastyle is distinctly smaller than the metacone in length in the Bulgarian tooth, whereas in both species of Ailurarctos , the parastyle is close to the metacone in length ( Fig. 4 ). The Bulgarian tooth has no cingulum, which is present in both species of Ailurarctos . A small cusp is present between protocone and hypocone in both species of Ailurarctos , which is absent in the Bulgarian tooth, though the latter trait could be variable, as is seen in the situation of Ailuropoda .

Comparison with Ailuropoda

The P4 of? Agriarctos nikolovi is slightly larger than in Ailuropoda microta and smaller than other species of Ailuropoda ( Pei, 1987) , whereas its canine size is comparable to that of the largest individuals of Ailuropoda melanoleuca and larger than that of other species. The P4 is similar to that of Ailuropoda in the absence of the cingulum. The differences between the Bulgarian tooth and Ailuropoda can largely be referred to the differences between the Bulgarian tooth and Ailurarctos . The Bulgarian tooth further differs from Ailuropoda by a less enlarged hypocone.

The canine of? A. nikolovi is very robust, comparable in size (in spite of the smaller dimensions of P4) to those of Ailuropoda melanoleuca .

Figures

FIGURE 2. Teeth of?Ag. nikolovi: A, NMNHS FM3546-b, upper canine in labial and lingual views; B, NMNHS FM3546-a, P4 in buccal, occlusal, and lingual views.

FIGURE 3. Upper dentition (PA) of?Agriarctos nikolovi in comparison with other representatives of Ailuropodini. A, cf. Ag. depereti, uncatalogued cast of MHNL AA52bis in the collection of the University of Vienna; B,?Ag. nikolovi NMNHS FM3546-a (reversed for comparison); C, Ai. yuanmouensis, YICRA YV2509.1 (reversed); D, Ai. lufengensis, IVPP V6892.12; E, A. microta, IVPP V14564; F1-3, A. melanoleuca, IOZ32756, IOZ26449, IOZ32755. Not to scale.

FIGURE 4. Dental ratio of P4 of Ailuropodini. Abbreviations: L, length; W, width, in mm.