Aeretes melanopterus (Milne-Edwards, 1867)

Pang, Li-bo, Chen, Shao-kun, Hu, Xin, Wu, Yan & Wei, Guang-biao, 2024, Fossil flying squirrels (Petauristinae, Sciuridae, Rodentia) from the Yumidong Cave in Wushan County, Chongqing, China, Fossil Record 27 (1), pp. 209-219 : 209-219

Aeretes melanopterus (Milne-Edwards, 1867)

Fig. 4 ; Suppl. material 1: table S 5

Materials.

As in Suppl. material 1: table S 1, there are one isolated tooth from the layer ②-2 and three maxillary bones and two mandibular bones from the layer ④.

Description.

The mandible is robust and the diastemal portion is short. The tip of the incisor is slightly higher than the worn surface of the cheek teeth. Its posterior end lies under the posterior root of m 3. The diastema is short and a medium-sized mental foramen is located at the middle part of the buccal side of the diastema. The inferior margin of the horizontal ramus is smoothly curved. A vascular notch is obvious and under the masseter muscle fossa. The masseter muscle fossa is relatively shallow with a weak ridge. Its anterior angle is at the level of the anterior root of m 1. The pterygoid muscle fossa at the lingual side is very wide and deep. Its anterior end is at the level of the posterior part of m 3. The mandibular foramen is large, oval-shaped and positioned more ventrally than the worn surface of the cheek teeth. The angular process is broken, but the preserved part shows it should be almost as wide as the ascending ramus. The upper part of the ascending ramus is badly preserved, but it seems not very high. The condylar process is a short transverse axis and its neck is long. The coronoid process is thin and higher than the condylar process.

DP 4 is smaller than P 4 and M 1. Its occlusal outline is triangular. The parastyle is fused with the anteroloph, forming a high and isolated anterolingual corner. Besides the parastyle, the paracone, the metacone and the protocone are all well developed on the crown surface. The protostyle is invisible. The hypocone is very weak, close behind the protocone. The protoloph is short and straight. No obvious protoconule. The metaloph bends back at the metaconule. The enlarged metaconule is connected with the posteroloph by a short ridge, forming a small hypoconule on the posteroloph and dividing the posterior valley into two. The posteroloph is low. The posterolingual flexus is narrow and closed after being moderately worn. The anterior valley and the central valley are short and the anterior one is wider than the central one. There are one large root at the lingual side and two small ones at the buccal side.

P 3 is not preserved. On the basis of the alveolar, it should be single-rooted, not particularly small and visible from the buccal side.

P 4 is molariformed. Its lingual side is much shorter than the buccal side, making its occlusal outline triangular. The lingual wall is wrinkled and higher than the buccal wall. The protocone is small and separated from the developed hypocone by a vertical groove (the anterolingual flexus) which extends to the base of the tooth crown. The anterolingual flexus is shallow, visible after being moderately worn. The posterolingual flexus is quite deep and narrow. There are four transverse ridges on the crown surface (the anteroloph, the protoloph, the metaloph and the posteroloph). The anteroloph is well developed and the parastyle is fused with it. The anteroloph is connected with the protocone at the slightly worn stage, but separated after being moderately worn by the anterolingual flexus. The protoloph is relatively straight. There are two ridges extending from the protoconule, the smaller one backwards and the larger one forwards. The metaconule is marked and the metaloph bends back at the metaconule. There are three small ridges extending backwards from the metaloph and two of them are connected with the posteroloph. The posteroloph is continuous and is connected with the hypocone only after being very deeply worn. Three roots, one larger at the lingual side and two smaller at the buccal side.

The occlusal outline of M 1 is close to a square. The occlusal structure is mainly composed of four cusps (the paracone, the metacone, the protocone and the hypocone) and four transverse ridges (the anteroloph, the protoloph, the metaloph and the posteroloph). The anteroloph and the posteroloph are lower than the protoloph and the metaloph. The protocone is not completely separated from the hypocone and both of them are ridge-shaped. The anteroloph and the protoloph converge with the protocone. The metaloph is connected with the hypocone. The paraconule and the metaconule are small. The main structure of the posterior lobe is similar to that of DP 4.

The occlusal outline of p 4 is a trapezoid with a narrow anterior lobe and a wide posterior lobe. The anteroconid is small, located at the anterobuccal side of the metaconid. The metaconid is the highest amongst all cusps of the tooth. The metastylid is barely visible and is separated from the well-developed mesostylid by a deep groove. The protoconid is weaker than the metaconid, but larger than other cusps. The anterobuccal sinusid is shallow and V-shaped. The buccal valley is wide. The mesoconid is obvious. The mesolophid divides the talonid basin into two. The protolophulid is tiny and the extra posterolophid is short. The posterolophid is curved. Two roots.

The lower molars have similar occlusal structure with p 4. The anterolophid is developed and connected with the anteroconid and the metaconid. The anterobuccal sinusid is much deeper than that of p 4. The mesoconid is connected with the protoconid. The metalophid is more developed than that of p 4. The posterior lobe of m 3 tapers obviously with a weak entoconid. Four roots.

Comparison.

Based on the dental dimensions (in Suppl. material 1: table S 5), the described fossils should be a large species of flying squirrel, which is most likely to belong to either Aeretes or Petaurista .

In Petaurista , M 1 / 2 lacks the hypocone and the lower cheek teeth have marked anterobuccal sinusid, which can be used for generic diagnosis and are obviously different from the features of the described fossils.

The diagnosis of the genus Aeretes includes (summarised by Li CK et al. (2019 a)): P 3 small, P 4 larger than molars, the hypocone of P 4 - M 2 weak and located close behind the protocone, the protoconule and the metaconule weak, the hypoconule well developed, the posterolingual flexus very deep, the entoconid of lower molars small, the mesostylid developed and separated from the metastylid and entoconid by deep grooves and the mesoconid small. The described fossils resemble Aeretes .

There are three species in the genus Aeretes , A. premelanopterus , A. grandidens and A. melanopterus . The primitive A. premelanopterus has weak protoconule, but strong metaconule; A. grandidens has stronger protoconule and metaconule than A. premelanopterus . The protoconule and metaconule of A. melanopterus from different regions varies in morphology: specimens from Chongqing and Guizhou have a very weak protoconule and developed metaconule ( Zheng 1993), while specimens from Beijing have a marked protoconule and metaconule ( Tong 2007). The Yumidong specimens are most similar to A. melanopterus from Beijng. Furthermore, comparing the Yumidong specimens with A. melanopterus from the Baotansi Cave in Chongqing, there is another obvious dental morphological difference: the lingual vertical groove of P 4 is clear on the Yumidong specimens, but unclear on the Baotansi specimens.

Compared with the Aeretes melanopterus from the Tianyuan Cave and extant specimens (based on Tong (2007)), the morphological structure of the described specimens falls within their variable range. Based on dimensions, the described specimens are much smaller than the Tianyuan Cave specimens and fall within the variable range of the living species. It is reasonable to attribute these fossils to A. melanopterus . Tong (2007) mentioned the diagnosis of this species, including: cheek teeth subhypsodont, enamel slightly rugose, the lingual wall of upper cheek teeth higher than the buccal wall, the posterolingual flexus very narrow and deep.

Figures

Figure 4. Aeretes melanopterus from the Yumidong Cave. A. 12 YMDT 4 ④ 60.54, right DP 4 - M 1, occlusal view; B. 12 YMDT 4 ② 35.81, right P 4, occlusal view; C. 12 YMDT 4 ④ 60.1, left mandible, labial view; D. 12 YMDT 4 ④ 60.2, right p 4 - m 3, occlusal view. Scale bar: 5 mm (A, B, D); 10 mm (C).