Drygantacanthus semirotunda, Voichyshyn & Szaniawski, 2018

Voichyshyn, Victor & Szaniawski, Hubert, 2018, New ischnacanthiform jaw bones from the Lower Devonian of Podolia, Ukraine, Acta Palaeontologica Polonica 63 (2), pp. 327-339 : 330-332

publication ID	https://doi.org/ 10.4202/app.00456.2018
persistent identifier	https://treatment.plazi.org/id/9F0087B5-403E-1A37-FFD2-C0722745BD5C
treatment provided by	Felipe
scientific name	Drygantacanthus semirotunda
status	sp. nov.

Treatment

Drygantacanthus semirotunda sp. nov.

Figs. 3 View Fig , 4 View Fig . Etymology: After the semicircular-like arrangement of the tooth additional dentition.

Holotype: SMNH BP.1581/5, a jaw bone fragment, 1.63 mm long, with two teeth of the lateral tooth row.

Type locality: Right bank of the Dniester River opposite Zalishchyky, Zastavna (Chernivtsi) region, Podolia, Ukraine; sample 2a/11 (after Drygant 2010); Fig. 1C View Fig : point 1.

Type horizon: Upper part of Ivanie Horizon of the Tyver Series, Lower Devonian.

Material.— Except the holotype, two jaw bone fragments with one tooth of the lateral tooth row, paratypes: SMNH BP.1587/3 (1.2 mm long) and SMNH BP.1590/11 (0.65 mm long). Both the specimens are from the same rock sample which was found on the left bank of the Dniester River ,

north-west Zalishchyky outskirts ( Fig. 1C View Fig : point 2), Podolia, Ukraine; Upper Lochkovian, Ivanie Horizon.

Diagnosis.—Same as for the genus.

Description.—The teeth bodies are bulky, especially in their lower half, with one medial, vertically oriented, ridge, curved forward ( Figs. 3A, C View Fig , 4B View Fig 1 View Fig , B 2 View Fig ). The ridge almost reaches the tooth tip and divides the tooth into two parts. The posterior surface is slightly concave linguolabially, increasing the size of the adjoined inter-tooth pit. The teeth parabasal section generally looks like a circular sector with the central angle of about 140° or less. Anterior part of tooth is convex linguolabially and outlined at its base (under the ridge lower end) by a set of 4–6 denticles in a semicircular arrangement ( Figs. 3A, B View Fig , 4A, B View Fig 2 View Fig , B 3 View Fig ). The denticle set is arranged horizontally on the medial tooth side but can rise up on the anterior side. The denticles have a hollow interior ( Fig. 3B View Fig ). A border between crown and base of tooth is visible at its base, beneath the denticle set ( Fig. 3E View Fig ). The teeth are somewhat inclined backwards and have medium-sized anterior and posterior flanges of equal dimension ( Figs. 3C View Fig , 4A View Fig 2 View Fig ). One can see at least five broken inter-tooth denticles in the holotype but, judging by the distance between the teeth, there could be twice as many of them ( Figs. 3A, B, D View Fig , 4A View Fig 1 View Fig , A 3 View Fig ). The inter-tooth pit is clearly defined. Pores of the vascular canal are hardly visible within the pit area of the holotype, possibly because they are encrusted with silt. There are some sporadic pores on the lateral jaw bone side ( Figs. 3D View Fig , 4A 4 View Fig ).

The lingual ridge is hardly distinguishable along the two preserved teeth of the holotype ( Fig. 3C View Fig ), which has at least one remnant of the lingual denticle group ( Fig. 3B, C, E View Fig ). The remnant looks like a hollow base of multiple denticles, being somewhat elongated at a small angle to the longitudinal jaw bone axis. It is surrounded by pores of the vascular canal system. The appearance of the remnant is similar to the lingual denticle groups of Podoliacanthus (cf. Voichyshyn and Szaniawski 2012: figs. 5F, 8C). The lingual ridge of the specimen BP.1587/3 has no denticle group Fig. 4A View Fig 1 View Fig , A 3 View Fig ), most likely because the fragment represents the anterior part of the jaw bone where the denticle groups are usually not developed (cf. Podoliacanthus zychi jaw bone fragments, Voichyshyn and Szaniawski 2012: figs. 4B, C, E, 5A, 6B). The width/height tooth ratio of the species is estimated at 0.8 to 1.0.

The specimen BP.1587/3, in comparison with the others, has some differing features. It does not have the vertical medial ridge on the tooth. Instead, its tooth profile has a slight concavity on both the posterior and anterior sides ( Fig. 4A View Fig 2 View Fig ). The inter-tooth pit part before the tooth in BP.1587/ 3 specimen reveals numerous pores, up to 65 μm in diameter ( Fig. 4A View Fig 3 View Fig ).

Remarks.—The shape of the new species’ teeth, in occlusal and lingual views, is somewhat similar to those of Grenfellacanthus zerinae Long, Burrow, and Ritchie, 2004 Long et al. 2004 : fig. 1A, C) but this similarity has no systematic importance. The main difference is in presence of nine vertical striae in Grenfellacanthus teeth of the lateral tooth row and in number of denticles in lingual tooth row.

Locality and stratigraphic range.—North-western and southern vicinities of Zalishchyky. Upper part of the Ivanie Horizon of Tyver Series (Late Lochkovian or Early Pragian).