Maobrontops paganus, Averianov & Danilov & Chen & Jin, 2018

Averianov, Alexander, Danilov, Igor, Chen, Wen & Jin, Jianhua, 2018, A new brontothere from the Eocene of South China, Acta Palaeontologica Polonica 63 (1), pp. 189-196 : 190-192

Maobrontops paganus sp. nov.

Fig. 2 .

Etymology: From Latin paganus , rustic; an allusion to the collection of many Maoming fossils by locals.

Type material: Holotype: SYSU-M-4, a left maxillary fragment with P4 and M1–2.

Type locality: The oil shale quarry (21°420 N, 110°530 E) located near Maoming City, Maoming Basin, Guangdong Province, China .

Type horizon: Youganwo Formation , upper Eocene ( Li et al. 2016) .

Diagnosis.―Referred to the Brontotheriidae based on W-shaped ectoloph and isolated lingual cusps of upper molars. Referred to the Brontotheriinae because of tall ectoloph with functional emphasis placed on shearing on the labial enamel of the ectoloph (i.e., the labial ectoloph enamel thicker than the lingual ectoloph enamel). Differs from all Brontotheriinae except some members of Embolotheriita ( Nasamplus progressus , Protembolotherium efremovi , Embolotherium andrewsi , and Embolotherium grangeri ) by upper molar anterolingual cingular cusp large. Differs from the above mentioned taxa of Embolotheriita by P4 paracone rib narrow and P4 hypocone lacking.

Description.— The maxilla is fragmentary, poorly preserved, and offers no morphological details concerning the cranial anatomy.

P3: The tooth is mostly destroyed. It is evident that this tooth was slightly narrower transversely than P4.

P4: The tooth is of rectangular outline in occlusal view, transversely wider than long. The ectoloph is almost straight, without a mesostyle and with a rather strong labial ridge at the paracone, weaker ridge at the parastyle, and a slight labial convexity at the metacone. The wear facet of the ectoloph is widest at the paracone (13.4 mm) and the metacone (9.3 mm), and narrowest between the paracone and the metacone and labial to the central fossa (7.6 mm). The exposed enamel is thicker along the labial side (~ 3.8 mm). On the lingual side, the exposed enamel is thicker at the paracone and metacone apices (maximum 2.5 mm) and decreases in thickness outwards. There are strong vertical ridges on the labial side of the paracone and metacone that bracket anteriorly and posteriorly a very deep central fossa.

The lingual heel is slightly shorter anteroposteriorly than the ectoloph. It is dominated by the cone-like protocone, which is as high as the labial cusps and lacks the preprotocrista and postprotocrisa. The protocone apex is slightly worn, with a small circle of exposed dentine that is flush with the surrounding exposed enamel. The protocone is surrounded by the anterior, lingual, and posterior cingula, from which it is separated by a distinct rounded groove. The anterior and posterior cingula are straight. The continuous lingual cingulum is rounded, convex lingually. There are cusp-like eminences at the anterior and posterior cingula.

M1: It has a quadrangular outline in occlusal view, distinctly longer and wider than P4. The W-shaped ectoloph is heavily worn, with the wear facet widest at the paracone (24 mm) and metacone (18 mm). There are strong labial ridges at the parastyle and mesostyle. The labial sides of the paracone and metacone between the labial ridges are slightly convex. The exposed enamel is thick on the labial side (~ 3.3 mm), and about twice as thin lingually around the paracone and metacone (~ 1.5 mm). There is a large central fossa of crescentic shape, which is bordered lingually by a prominent and significantly worn ridge. This ridge extends between the anterolingual cingular cusp and vertical ridge on the lingual side of the metacone.

The lingual heel is only a little shorter anteroposteriorly than the ectoloph. The protocone and hypocone are distinctly lower than the labial cusps. Both cusps have circular wear facets at the apices with exposed dentine and enamel. On the protocone wear facet the enamel rim is raised above the adjacent dentine surface, while on the hypocone wear facet the small exposed area of dentine is flush with the enamel. This suggests that the wear on the protocone started earlier than on the hypocone. The protocone is cone like, but with a flattened to slightly concave lingual side. The hypocone is more ridge-like and resembles the enlarged posterior cingulum of P4. The anterior cingulum is similar to that of P4 but larger. It has a large cusp-like eminence that is mostly worn down (anterolingual cingular cusp). There is no lingual cingulum.

M2: It is distinctly larger than M1 and is of trapezium outline in occlusal view. The anterior side is about 1.5 times wider than the posterior side, which results in an oblique position of the labial side relative to the lingual side. The W-shaped ectoloph is about twice less worn than that of the M1. The exposed enamel is ~3.0 mm thick along the labial side and ~ 2.5 mm thick around the paracone and metacone apices on the lingual side. There are strong labial ridges at the parastyle and mesostyle and a much weaker ridge at the metastyle. The labial side of the paracone and metacone is slightly convex between the ridges. There is a large central fossa of triangular shape that is bordered posteriorly by a sharp vertical ridge along the metacone lingual side and lingually by a distinct ridge.

The lingual cusps are much lower compared to the labial cusps and correspond in shape to those of the M1. With a flattened lingual side, the cone-like protocone is relatively larger than that of P4. Because of this, the size difference between the protocone and hypocone is more pronounced on M2 compared to M1. The ridge-like hypocone is obliquely extended along the postero-lingual corner of the tooth. There is only slight wear on the protocone and hypocone apices. There is a large cusp-like eminence along the anterior cingulum (anterolingual cingular cusp). This eminence is connected with the base of the hypocone by a ridge extending between the central fossa and the vertical ridge on the lingual side of the metacone. This ridge is unworn and bears several prominent irregular denticles. There is no lingual cingulum.

The enamel is mostly smooth, but there are areas of pitted or wrinkled enamel on the labial side of the P4 and molars, and on the lingual side of P4.

For dental measurements see Table 1.

Stratigraphic and geographic range.—Eocene of South China.

Figures

Fig. 2. Embolotheriit brontothere Maobrontops paganus gen. et sp. nov., SYSU-M-4 (holotype), from Maoming Basin, Guangdong Province, China; Youganwo Formation, upper Eocene. Left maxillary fragment with P4 and M1–2, in labial (A), occlusal (B), and lingual (C) views. Photographs (A1–C1) and interpretive drawings of tooth crowns (A2–C2).