Minutolestes submersus, Martin & Averianov & Schultz & Schellhorn & Schwermann, 2022

Martin, Thomas, Averianov, Alexander O., Schultz, Julia A., Schellhorn, Rico & Schwermann, Achim H., 2022, First spalacotheriid and dryolestid mammals from the Cretaceous of Germany, Acta Palaeontologica Polonica 67 (1), pp. 155-175 : 162-166

publication ID	https://doi.org/ 10.4202/app.00914.2021
persistent identifier	https://treatment.plazi.org/id/EB57E55D-9157-6551-FCA5-F8F8FD41F633
treatment provided by	Felipe
scientific name	Minutolestes submersus
status	sp. nov.

Treatment

Minutolestes submersus sp. nov.

Figs. 6–8 View Fig View Fig View Fig . ZoobankLSID: urn:lsid:zoobank.org:act:7DD409B2-0C58-4AAA-875C-7F3F881AAD64

Etymology: From Latin submersus , sunken, referring to the origin of the fossils from a sinkhole.

Holotype: WMNM P82306 , left upper molar.

Type locality: Fissure filling within the Busche quarry near Balve, Sauerland, North Rhine-Westphalia, Germany.

Type horizon: Barremian–Aptian (Lower Cretaceous).

Material.— Holotype and left upper ( WMNM P82304 ) and lower ( WMNM P82307 ) molars from the type locality and horizon.

Diagnosis.—Tiny dryolestid with small stylocone and flat primary trigon basin. Referred to Dryolestidae based on the following derived characters: upper molars much wider than long, stylocone large, median cusp present, metaconid more than half of protoconid height, and mesial root much larger than distal root on lower molars.

Differs from Dryolestes Marsh 1878 , Laolestes Simpson 1927 , Krebsotherium Martin 1999 , and Portopinheirodon Martin 1999 , by smaller size and flat primary trigon basin without median ridge between stylocone and paracone ( Laolestes , Portopinheirodon , Lakotalestes Cifelli,Davis,and Sames 2014 , Groebertherium Bonaparte, 1986 ) or bulge between metacone and stylocone ( Dryolestes , Krebsotherium ) on upper molars. Differs from Amblotherium Owen, 1871 by less prominent (not hook-like) parastyle on the upper molars and blunt metaconid. Differs from Anthracolestes Averianov, Martin, and Lopatin, 2014 , by a blunt metaconid on the lower molars. Differs from Hercynodon Martin, Averianov, Schultz, Schwermann, and Wings, 2021 , by presence of a metacone. Differs from Achyrodon Owen, 1871 , by the presence of cups along the metacrista on the upper molars. Similar to Crusafontia Henkel and Krebs, 1969 by the following characters: small size, shallow ectoflexus on upper molars present, metacone reduced (derived), labial cingulid on lower molars (derived). Differs from Crusafontia by a weaker ectoflexus and a stronger metacone on upper molars and a blunt metaconid on the lower molars.

Measurements (in mm).—WMNM P82305, upper molar (holotype): length: 0.75; width: 0.86. WMNM P82304, upper molar: length: 0.65; width: 0.97. WMNM P82307, lower molar: length: 0.60 (without talonid); width: 0.61.

Description.— Upper molars: WMNM P82306 (holotype) and attributed specimen WMNM P82304 ( Figs. 6 View Fig , 7 View Fig ) are unilaterally hypsodont with the crown being higher lingually than labially. In occlusal view, the crown forms an asymmetrical triangle with the distal border longer than the mesial border in the holotype and vice versa in WMNM P82304 Figs. 6A View Fig 1 View Fig , 7A View Fig 1 View Fig ). In the holotype, the labial border is slightly shorter than the distal border, and in WMNM P82304 it has only half the length of the distal border. It is almost straight in the holotype and exhibits a slight labial bulging in the stylocone area in WMNM P82304. The tooth crown is compressed mesio-distally and the angulation of the principal cusps is 43° in the holotype specimen and only 25° in WMNM P82304. Judging by the degree of compression, the holotype specimen derives from an anterior position and WMNM P82304 from a middle position in the tooth row.

Of the three primary trigon main cusps, the paracone is the highest. Its outline is triangular in cross section with flat mesial and distal flanks, a rounded lingual angle, and a slightly concave labial flank. The paracone bears an oval apical wear facet with exposed dentine that extends along the metacrista ( Fig. 6A View Fig 1 View Fig , A 4 View Fig ). The stylocone is broken on the holotype but was likely the second largest cusp. In WMNM P82304, the stylocone is present and small ( Fig. 7A View Fig 1 View Fig , A 3 – A View Fig 6 View Fig ). It is worn (likely by corrosion) apically and on the mesial and labial sides; it is the second-highest cusp of the primary trigon. The metacone, third-largest cusp, is a prominent swelling on the metacrista that bulges towards the primary trigon basin ( Figs. 6A View Fig 1 View Fig , A 4 View Fig , 7A View Fig 1 View Fig ) and is separated from the paracone by a sharply indented notch. The metacone bears a large linguo-labially extended wear facet ( Figs. 6A View Fig 1 View Fig , A 6 View Fig , 7A View Fig 1 View Fig ). Labially of the metacone, cusp C is represented as a slight elongate swelling on the metacrista (present in both upper molars) ( Figs. 6A View Fig 1 View Fig , A 3 View Fig , A 4 View Fig , A 6 View Fig , 7A View Fig 1 View Fig ). The parastyle is triangular in shape, bulging mesio-labially, and clearly separated from the primary trigon by a marked lingual notch ( Fig. 6A View Fig 1 View Fig , A 3 View Fig , A 4 View Fig , A 5 View Fig ). It bears an oval apical wear facet in the holotype specimen and is corroded in WMNM P82304. The metastyle ( Fig. 6A View Fig 1 View Fig , A 3 View Fig , A 4 View Fig , A 6 View Fig ) is much smaller and bears a large oval wear facet in the holotype and is strongly corroded in WMNM P82304. In the middle of the labial rim there is a tiny median cusp ( Fig. 6A View Fig 1 View Fig ) with an apical wear facet; from the median cusp extends a small worn crest in distal direction in the holotype. In WMNM P82304 the median cusp and the area distally from it including the metastyle are corroded. Paracrista (so far preserved) and metacrista are worn with exposure of dentin. The mesial and distal flanks of the crown bear fine striations that are oriented about 45° in labio-cervical direction. The primary trigon basin is flat without any crest or bulge.

There are three roots, of which two are broken off in the holotype. According to the broken base with opened pulp cavity, largest root was the one supporting the paracone, whereas the roots supporting the para- and metastyle were much smaller ( Fig. 6A View Fig 2 View Fig ). The root supporting the parastyle is largely intact and only its tip is missing ( Fig. 6A View Fig 2 –A View Fig 6 View Fig ). The root is slightly bent disto-labially and has an oval cross section with a slight mesio-distal compression. In WMNM P82304 all three roots are partially preserved. The larger root supporting the paracone is almost complete. It is curved mesio-labially ( Fig. 7A View Fig 3 –A View Fig 6 View Fig ) and has a sub-quadrangular outline in cross section ( Fig. 7A View Fig 2 View Fig ). Of both smaller roots supporting the para- and metastyle only the proximal confluent portion is preserved ( Fig. 7A View Fig 2 –A View Fig 6 View Fig ); the distal parts are broken off. The incompletely separated roots have oval cross sections with mesio-distal compression. A slight curvature of the remaining portion of the roots indicates a former labial bending.

Left lower molar: The left lower molar WMNM P82307 ( Fig. 8 View Fig ) is unilaterally hypsodont with the labial part higher than the lingual. The part of the tooth where a talonid would be expected is broken off. On the crown, the three main cusps, proto-, para-, and metaconid are preserved. The protoconid ( Fig. 8A View Fig 1 View Fig , A 3 –A View Fig 6 View Fig ) is the highest cusp and has a triangular cross section; it has a flat mesiolingual and slightly concave (in the lower part) distal side and a slightly convex mesiolabial side. The distal side of the protoconid shows faint striations that are oriented at about 45° in labio-cervical direction in relation to the occlusal plane ( Fig. 8A View Fig 6 View Fig , inset). The protoconid is separated from the para- and metaconid by sharp notches. Second largest cusp is the metaconid ( Fig. 8A View Fig 1 View Fig , A 3 –A View Fig 6 View Fig ), with a cross section that is quadrangular in outline; the lingual and distal sides are flat, the labial side is slightly concave, and the anterior side slightly convex. At the distal side of the metaconid, a small portion of the distal metacristid is preserved ( Fig. 8A View Fig 6 View Fig ). The upper third of the metaconid is broken. The base of the metaconid sits close to the base of the protoconid; both cusps are separated by a sharp notch in the metacristid. The paraconid Fig. 8A View Fig 1 View Fig , A 3 –A View Fig 5 View Fig ) is crest-like, compressed mesio-distally and tapering labially. In lingual view, the paraconid appears pointed and inclined mesially; it attains about half the height of the protoconid. The trigonid is moderately compressed mesio-distally and has an angle of 38°. Due to the close approximation of protoconid and metaconid, the basin is largely confined to the mesial portion of the trigonid. The talonid is broken off. On the mesial side of the paraconid, close to its base, there is a short mesial cingulid ( Fig. 8A View Fig 3 View Fig , A 4 View Fig ) which is inclined cervically and exhibits slight wear in its middle part. The cingulid becomes weaker in labial direction, but continues along the preserved part of the labial side; the distal side is broken. The paracristid is moderately worn along the paraconid, and the lingual portion of the metacristid is broken off including the tip of the metaconid. The protoconid bears a small oval wear facet that continues along the preserved part of the metacristid. Of the mesial, much larger root, the anterior portion is preserved ( Fig. 8A View Fig 2 View Fig , A 4 View Fig , A 6 View Fig ) whereas its distal part and the posterior small root are broken off together with the talonid and the disto-cervical part of the protoconid, exposing the large pulp cavity Fig. 8A View Fig 2 View Fig , A 3 View Fig , A 6 View Fig ).