Trogontherium (Euroxenomys) minutum minutum Meyer, 1938

Apoltsev, D. A. & Rekovets, L. I., 2015, Beavers Of The Genus Trogontherium (Castoridae, Rodentia) From The Late Miocene Of Ukraine, Vestnik Zoologii 49 (6), pp. 419-528 : 521-522

Trogontherium (Euroxenomys) minutum minutum Meyer, 1938

I n v e s t i g a t e d m a t e r i a l. Grytsev: 1 skull fragment with maxillary part and P 4 –M 3 ( fig. 1, 1 ), a fragment of the mandible with P 4 and M 1 ( fig. 1 , 2 ), an incisor fragment, 4 P 4, 1 M 1-2, 4 M 3, 4 P 4, 5 M 1-2, 3 M 3.

G e o l o g i c a l a g e: MN 9, middle Sarmatian (Vallesian).

According to Lychev (1973, 1983), fossil remains of T. (Euroxenomys) minutum from the Late Miocene of Ukraine, including the material from Grytsev, are belonging to Monosaulax savinovi , typical North American and Asian species. Molars from Grytsev ( fig. 1 , 3 ) have relatively smaller crowns and less-developed roots than those in Monosaulax savinovi , which was described from the Miocene of Kazakhstan.

D e s c r i p t i o n a n d c o m p a r i s o n. Skull fragment has full dentition — 2 P 4 and M 1-3. Palate without expressed median crest, its posterior part is broken; the smallest palatal width between the P 4 alveoli — 2.3 mm, and between the M 1 alveoli — 4.5 mm. Alveolar length of the P 4 –M 3 is 12.4 mm, coronary length — 12.2 mm. Teeth without cement have well-developed roots, hypostriae and hypoflexus. Mesofossette is convex, other fossettes are nearly straight. There is a fourth fossette on M 3. P 4, M 1, and incisor are preserved in front of the mandible. Coronoid process and the back part of the mandible are broken down to M 2. Teeth with greatly developed hypostriids and hypoflexids do not have cement; length of diastema is 9.1 mm, the restored height at P4 — 13.0 mm. For comparison, these values on material from Petersbuch are 8.87 and 20.05 mm, respectively ( Stefen, Rummel, 2003).

The fragment of triangular lower incisor has a weak curvature; the enamel slightly covers the lateral surface. Enamel anterior side is convex. Incisor width is 3.4 mm, anteroposterior diameter — 3.9 mm. Average length of the P 4 is 4.1 mm, width — 4.5 mm (the length ratio — 109 %). The tooth is medium-worn. Para- and hypoflexi are straight and contrasted (not shifted) at the middle part of the tooth. Mesoflexus crosses the entire width of the chewing surface and has a slight bend. According to these features, P 4 from Grytsev is inferior in size compared to teeth from Petersbuch, and Trogontherium minutum ozansoyi ( Ünay, 1975) from the Late Miocene of Turkey.

Length of M 1-2 is 2.4 mm, width — 2.9 mm (W/L ratio is 120 %). Parafossette is small, shifted to edge of the chewing surface and opposed to the hypoflexus. Metaflexus is under closure into fossette. The subspecies from Petersbuch has a markedly larger size of these teeth with a W/L ratio of 159.0 %. Perhaps, this is due to different degrees of the tooth wear ( Stefen, Rummel, 2003).

Length of M 3 is 3.1 mm, width — 2.9 mm (W/L ratio — 93 %). The teeth are shaped like an elongated triangle, which distinguishes the genus Trogontherium from others. Paraflexus is in the process of circuit into a fossette, slightly shifted in direction to the hypoflexus. Mesofossette is straight, stretched out over the entire tooth width. Dimensional parameters of the M3 from Grytsev are less than those from Petersbuch, as well as from the Late Miocene of Turkey. At the same time its W/L ratio remains almost in the range of 75–93 % (table 1).

Length of P 4 is 4.0 mm, width — 3.0 mm, W/L ratio — 75.0 %. Anterior part of the tooth is trapezoidal (not rectangular) unlike Trogontherium minutum ozansoyi , and the hypoflexus is not parallel to the labial side of the tooth.

Four M 1-2 with weak roots belong to adult individuals ( fig. 1 , 3 ); maximum crown height — 6.7; 7.0; 7.0; 7.3 mm, hypostriid height — 3.3; 3.4; 3.5; 3.6 mm, mesostriid height — 1.3; 1.4; 1.4; 1.6 mm. There are well-developed meso- and paraflexids, metafossetids on the chewing surfaces. The length exceeds the width (3.4 × 2.7 mm, W/L ratio — 79.4 %; 3.4 × 3.0 mm, W/L — 88.2 %; 3.5 × 2.7 mm, W/L — 77.1 %).

Two M 3 with well-marked four half-fused roots belong to adult individuals. Tooth height is 6.0 and 5.3 mm; hypostriid (2.4 and 2.0 mm) is well expressed, the mesostriid length is 0.9 and 1.7 mm. There are meta- and parafossetids on the crown surface; mesoflexid is weak (narrow and short) unlike the hypoflexid. Tooth length is 3.2 and 2.8 mm, width — 3.2 and 2.8 mm, W/L ratio is 100 %.

R e m a r k s. P 4 from Petersbuch are on average about 5–7 % larger than those from Grytsev. M 1-2, conversely, are larger in Grytsev versus those from Petersbuch. P 4 and M 3 from Petersbuch are larger than those from Grytsev. M 1-2 from Grytsev are longer and less wide than those from Petersbuch. M 3 are not represented in materials from Petersbuch, thus it is not possible to compare these teeth.

Figures

Fig. 1. Chewing surface of upper (1) and restored lower (2) dentition, character of the molar roots development (3) of Trogontherium (Euroxenomys) minutum minutum from the Grytsev locality. Scale bar 5 mm.

Fig. 2. Right mandible of Trogontherium (Euroxenomys) minutum rhenanum. Cherevichnoe 3 locality: 1 — labial view; 2 — view on the point of chewing surface. Scale bar 5 mm.

Fig. 3. Chewing surface of upper (1–4) and lower (5–8) dentition of Trogontherium (Euroxenomys) minutum rhenanum: 1 — Р4, Cherevichnoe 3; 2–3 — М1-2, Andreevka; 4 — М3, Andreevka; 5 — M, Andreevka; 6–7 — 3 М1-2, Cherevichnoe 3; 8 — P4, Cherevichnoe 3. Scale bar 5 mm.