Guangxicyon sinoamericanus, Zhai & Ciochon & Tong & Savage & Morlo & Holroyd & Gunnell, 2003

Zhai, Renjie, Ciochon, Russell L., Tong, Yongsheng, Savage, Donald E., Morlo, Michael, Holroyd, Patricia A. & Gunnell, Gregg F., 2003, An aberrant amphicyonid mammal from the latest Eocene of the Bose Basin, Guangxi, China, Acta Palaeontologica Polonica 48 (2), pp. 293-300 : 294-299

publication ID	https://doi.org/ 10.5281/zenodo.13345762
publication LSID	lsid:zoobank.org:pub:1FDA1A7A-DBAD-493A-B184-E08060DB7826
persistent identifier	https://treatment.plazi.org/id/03E787CF-B92E-FF92-F80C-F821FAD3FCD1
treatment provided by	Felipe
scientific name	Guangxicyon sinoamericanus
status	sp. nov.

Treatment

Guangxicyon sinoamericanus sp. nov.

Figs. 2–7 View Fig View Fig View Fig View Fig View Fig View Fig .

Guanxicynodon sinocaliforniae nomen nudum; Russell and Zhai, 1987: 200.

Guanxicynodon sinocaliforniae nomen nudum; Tsubamoto et al., 2000: 63.

Holotype: IVPP V11818−1 , a left mandible with c1 and p3–m1 and with alveoli for p1–2 and m2–3; V11818−2 , left humerus; and V11818−3 , a right tibia.

Type locality and stratigraphic position: Holotype specimens were found in a mudstone with associated pebble lens in the Nadu Formation, IVPP locality 73083, latest Eocene, 1 km east of Quelin Village, Tiendong , Guangxi ( Fig. 1 View Fig ; see also Russell and Zhai, 1987: figs. 41 and 42).

Diagnosis.—As for genus.

Measurements — See Table 1 for measurements of holotype and comparative taxa.

Description.—The outstanding feature of the mandible is the short and relatively deep horizontal ramus ( Figs. 3B, C View Fig , 4B View Fig ). It is similar to Brachycyon Filhol from the Oligocene of Europe, but the ramus is deeper than in B. reyi ( Ginsburg 1966) . The mandible is slightly deeper than the dentary of B. palaeolycos , especially in the anterior part, maintaining an almost constant depth along the ramus from the symphysis to the area below m3. The lower border of the ramus is straight and there is no premasseteric fossa on the lateral face as in some ursids. The area of the horizontal ramus beneath the masseteric fossa is considerably deeper than in true canids such as Enhydrocyon , and is comparable in depth to Cynelos . The symphysis is not fused. There are three mental foramina present, a large one beneath the anterior root of p3, another large one beneath the anterior root of p4, and a very small one beneath the root of p1.

The incisor region in this specimen is broken away, so no information on these anterior teeth is available. The canine is large and robust as is usual in amphicyonids. It is oval in cross−section and had no cutting blade.

Unfortunately, p1–2 and m2–3 are missing in this specimen. However, the alveoli of these lost cheek teeth provide a limited knowledge of their size and morphology. Anterior to the preserved p3 two small, well−separated alveoli are present. These two alveoli could be interpreted as having held a double rooted p2 (with p1 lost) or as having held two single rooted premolars. We interpret these alveoli as representing two small single rooted premolars (p1 and p2), because the length from the anterior edge of the first alveolus to the posterior edge of the second alveolus (8.7 mm) is greater than that for the preserved p3 (8.1 mm) and the lack of a bony elevation between the alveoli that is typical if two alveoli belong to the same tooth. If these alveoli were those of a single tooth, it would have possessed a crown length in excess of 9 mm and would have been greater in length than p3. Since no amphicyonid is known to have p2 longer than p3, the presence of two reduced premolars is more likely.

The three preserved cheek teeth of Guangxicyon also show reduction and simplification, being shorter, blunter, and less compressed than in other short−faced forms such as Daphoenocyon (= Brachyrhynchocyon , see Hunt 1998a), Brachycyon and Enhydrocyon . The p3 is small, simple and lacks an accessory cuspule posterior to the protoconid. The labial cingulid is weakly developed and discontinuous over the face of the protoconid, but continuous anteriorly and antero−lingually. The p4 is also simplified, lacking the posterior accessory cusp usually seen in amphicyonids, but it does have a short posterior heel and exhibits slight internal expansion. On both p3 and p4 pre− and postprotocristids are weak, but distinct. In both teeth the roots are close together, and are even fused in their dorsal part. This configuration is consistent with the interpretation of the existence of both single rooted p1 and p2, rather than only a double−rooted p2 with widely separated roots. On p4, two additional short crests extend posterolabially and posterolingually from the protoconid, ending slightly above and disjunct from the cingulid, providing the posterior face of the p4 a triangular outline. The trigonid of m1 is similar to that seen in Brachycyon and Haplocyon with the paraconid−protoconid blade obliquely oriented and the metaconid strong though lower than the protoconid. The talonid is long relative to Brachycyon and is comprised of a large and centrally placed hypoconid, and a relatively high and rounded cristid obliqua. The entoconid is lacking and the posterior rim of the tooth is rounded. This condition contrasts with most amphicyonids where the hypoconid is more laterally positioned and the posterior rim is rectangular (see, e.g., Kuss 1965; Ginsburg 1966; Viranta 1996); the only exception is Pseudocyonopsis , which shows a morphology similar to that observed in Guangxicyon . The former genus, however, possesses much higher talonid cusps (see, e.g., Kuss 1965: fig. 86). Based on a regression of ln m1 area using the all carnivore equation of Legendre and Roth (1988), the estimated body mass of Guangxicyon would have been approximately 10.6 kg.

Molars posterior to m1 are missing, but two alveoli are preserved. The anterior of the two is comparatively large (7.3 mm length × 5.1 mm width) and has a conical root with the deepest point just in the middle of the alveolus ( Fig. 4A View Fig ). The posterior one is very small (4.1 mm length × 4.2 mm est. width). Together, the two alveoli measure 15.7 mm in length. As in the case of the anterior premolar alveoli, two interpretations are possible. These two molar alveoli could have held reduced, single−rooted m2 and m3, or a relatively large, double−rooted m2 with m3 lost. Retention of all three molars and maintaining m1> m2> m3 is the plesiomorphic condition for amphicyonids ( Hunt, 1998a), and a trend toward reducing the size of the posterior molars is seen in several amphicyonid lineages. For example, Brachycyon has fused roots in m2. Loss of m3 without concomitant reduction of m2, only occurs in the most derived amphicyonids. If m3 were lost in Guangxicyon , this would have required that this loss occurred in conjunction with the retention of a relatively large m2 with a much reduced talonid and a conical anterior root typical for a single−rooted tooth. This interpretation seems the less likely, and it is more parsimonious to interpret the posterior molar alveoli to have held reduced, single−rooted m2 and m3. This produces a lower dental formula of i?/c/p1–4/m1–3.

Guangxicyon is distinct among the Amphicyonidae in having both size and root number of anterior premolars and posterior molars reduced, accompanied by a shortening of the face. By contrast, facial shortening in Enhydrocyon is accompanied by loss of p1 and in some specimens m3 and oblique reorientation of the premolars. In the short−faced Brachycyon , the shortening of the jaw is accomplished by having all cheek teeth crowded with regard to the roots of p2 and m2. Reduction of the first premolar in the upper dentition is known from B. gaudryi . In Aktaucyonini, a third amphicyonid group exhibiting facial shortening, P1 is strongly reduced ( Kordikova et al. 2000).

Found in direct association with the dentary, and undoubtedly belonging to the same individual, was a left humerus and a right tibia ( Figs. 2 View Fig , 5–7 View Fig View Fig View Fig ). The humerus lacks the proximal portion, and the distal articular surface is detached from the shaft. The total length of the humerus can be estimated at approximately 205 mm. There is a pronounced deltopectoral crest which extends along about two−thirds the length of the bone ( Fig. 5A View Fig ). The shaft does not exhibit much torsion. There is a strong brachioradialis (supinator) crest on the distal portion of the shaft. The grooves and depressions on the medial side of the distal humerus suggest the possible presence of an entepicondylar foramen ( Fig. 5A View Fig ). The distal articular surface is relatively wide without a strongly projecting medial trochlear ridge or capitulum. The medio−lateral distal articular width is 46 mm.

The tibia ( Figs. 6B View Fig , 7A, B View Fig ) is complete, but is cracked and somewhat distorted in the central portion of the shaft. At its proximal end the medial condylar surface is broken away, and the intercondylar eminence is damaged. There is a pronounced tibial tuberosity which is knob−like. The tibial shaft appears flattened medio−laterally, but part of this could be due to its damaged condition. On the distal articular surface the grooves for articulation with the astragalus can be seen. The medial malleolus is cracked and slightly displaced in a proximal direction. The total length of the tibia is estimated to be 165 mm.

Comparison of the humerus and tibia of Guangxicyon with those of other amphicyonid taxa shows relatively strong similarities to Amphicyon . However, these elements of Guangxicyon are more robust and relatively shorter than those of Amphicyon . Postcranially, Guangxicyon appears quite divergent from more specialized Temnocyon and Enhydrocyon . It differs from Cynelos ( Springhorn, 1977) , as well as Brachycyon ( Filhol, 1873) , in having broader distal condyles on the humerus and tibia, suggesting a less cursorial gait. Therefore, Guangxicyon probably retained the primitive, ancestral condition for the Amphicyonidae .

Hunt (1972) discussed in detail the humerus of a Miocene amphicyonid,? Ysengrinia . He concluded that, in nearly all details of morphology, the? Ysengrinia humerus is very similar to living ursids, especially the Giant Panda, Ailuropoda . The humerus of Guangxicyon exhibits some of the features discussed by Hunt (1972) such as a large deltopectoral crest, a strong brachioradialis crest, a broad distal trochlea, and an elaborate lateral epicondyle. These features suggest an animal capable of extensive lateral rotation of its forearm with a plantigrade posture ( Ginsburg 1961). Guangxicyon was a less cursorially adapted animal than Daphoenodon or Cynelos , both of which had more elongate limbs although not as relatively elongate as in extant canids.