Thrinacodus dziki, Ginter & Duffin & Dean & Korn, 2015

Ginter, Michał, Duffin, Christopher J., Dean, Mark T. & Korn, Dieter, 2015, Late Viséan pelagic chondrichthyans from northern Europe, Acta Palaeontologica Polonica 60 (4), pp. 899-922 : 905-907

publication ID	https://doi.org/ 10.4202/app.00084.2014
persistent identifier	https://treatment.plazi.org/id/251B879F-FFD3-FFAC-FF39-32F60EC5FB14
treatment provided by	Felipe
scientific name	Thrinacodus dziki
status	sp. nov.

Treatment

Thrinacodus dziki sp. nov.

Figs. 6 View Fig , 9B, C View Fig , 11 View Fig .

1993 Thrinacodus ferox (Turner, 1982) ; Duffin 1993: 2, pl. 1: 2.

2010 “Thrinacodont teeth”; Ginter and Turner 2010: fig. 4A.

Etymology: In honour of Professor Jerzy Dzik (Institute of Palaeobiology, Polish Academy of Sciences, Warsaw), one of the most famous Polish palaeontologists.

Type material: Holotype, tooth ( ZPAL P.IV/212).

Type locality: Sample TG-D of Todowa Grząba , north of Ostrówka Quarry , Gałęzice Region, Holy Cross Mountains, Poland .

Type horizon: Carboniferous , upper Viséan , upper Asbian or lower Brigantian, Gnathodus bilineatus Conodont Zone .

Material.— Seventeen teeth: six from upper Viséan of Cawdor Quarry, Matlock , Derbyshire, England, UK (five from sample Cawdor 5, one from sample Cawdor 11); two from upper Viséan , Ticknall , Derbyshire, England, UK; nine from upper Viséan of Todowa Grząba, north of Ostrówka Quarry , Gałęzice Region, Holy Cross Mountains, Poland (sample TG-D); NHMUK PV P73271–P73275, NHMUK PV P73279, NHMUK PV P73289, MWGUW /Ps/11/8–11, ZPAL P.IV/212 .

Diagnosis.—Species of Thrinacodus whose teeth display the following combination of features: there are three cusps in the tooth-crown, either equal to each other or the distal cusp may be slightly larger than the other two. The median cusp is offset from its normal position and displaced lingually. The basal/ labial part of the crown below the median cusp forms a bulge. In this bulge, there occurs a distinct basal canal opening. The angle between the mesial and median cusps is slightly smaller than the angle between the latter and the distal cusp.

Description.—Most of the specimens of T. dziki sp. nov. from Cawdor Quarry are in a rather poor state of preservation, with broken cusps and bases, so this description is based mostly on the collection from Todowa Grząba and two specimens from Ticknall. The labio-lingual size of the base ranges from 0.6 to more than 1.5 mm. The lingual part of the base is usually long and thin. It seems that in larger specimens it is relatively shorter and thicker, but this may be only an illusion, as the lingual tips of the bases in such specimens are usually broken off (except for Fig. 11B View Fig ). The lingual one-third of the base is flattened and the sharp lingual tip is upturned ( Figs. 9B View Fig 2 View Fig , 11B, E View Fig ). The part more proximal to the crown is elevated and on the lingual side of this elevation there opens a large foramen. This foramen most probably corresponds to the basal/labial opening of the overlying tooth in a tooth family. In two specimens ( Fig. 11A View Fig 1 View Fig , D 2 View Fig , D 3 View Fig ) it seems that there is more than one foramen in the orolingual part of the base, but this is, perhaps, due to the partial abrasion of the surface, uncovering the internal osteodentine canals.

The most characteristic features of this species are the lingually displaced median cusp of the crown and the bulge, sometimes quite distinct ( Figs. 6B View Fig 1 View Fig , D 2 View Fig , 9C View Fig ), formed at its base on the labial side. The distal cusp, which usually seems to be slightly larger than the other two, is gently sigmoidal ( Fig. 9B View Fig 1 View Fig ); the mesial cusp is sigmoidal, too ( Fig. 11B View Fig ); the full shape of the median cusp is unknown, because in all the specimens studied so far the apical tip is broken. The greater distance (angle) between the median and distal cusps (see especially Fig. 11C View Fig 2, E 2 View Fig ) indicates that the latter had a somewhat different function to that of the other two cusps.

Remarks.—Hitherto, six species of Thrinacodus have been recognised (see Ginter and Turner 2010). In stratigraphic order they are: T. tranquillus Ginter, 2000 (middle–late Famennian), T. ferox (Turner, 1982) (latest Famennian–Tournaisian), T. nanus St. John and Worthen, 1875 (early Tournaisian), T. bicuspidatus Ginter and Sun, 2007 (middle Tournaisian), T. incurvus ( Newberry and Worthen, 1866) (early Viséan), and T. gracia ( Grogan and Lund, 2008) (Serpukhovian) . Of these, T. tranquillus , T. ferox , and T. bicuspidatus were described based on reasonably large collections of teeth and recorded from more than one region in the world; T. gracia was described from Bear Gulch (Montana, USA) and based on complete and articulated specimens containing well preserved dentitions, and similar teeth have been found in Russia and New Mexico ( Ivanov and Lucas 2011); T. nanus , the genotype, is known from only two specimens, of which one is atypical and the other incomplete. Based upon our knowledge of other species ( Ginter and Turner 2010: fig. 1) it is possible that T. ferox is conspecific with T. nanus , but at the moment this cannot be verified. The type specimen of T. incurvus was apparently lost and although we do have a good drawing of it and several teeth from the same locality, any reliable comparison with this species is impossible.

This being the case, we can meaningfully compare our new species only to T. tranquillus , T. ferox , T. bicuspidatus , and T. gracia . The latter two species can be excluded at once:

typical specimens of T. bicuspidatus have only two cusps and all the specimens of T. dziki sp. nov. have three; the cusps of T. gracia are thick and coarsely ornamented with numerous cristae, whereas the cusps of T. dziki are slender and the cristae on them are subtle. Moreover, the base of T. gracia is shorter and thicker.

Teeth of T. dziki differ from typical teeth of T. tranquillus in the lingual displacement of the median cusp, a character unknown in Famennian thrinacodont species. However, it has been observed in several Tournaisian specimens of T. ferox , particularly from the Irish collection presented by Duncan 2003: fig. 5B, 6A, B). T. ferox has a wide range of tooth morphotypes in a single jaw (monognathic heterodonty), varying from specialised subsymmetrical symphyseal and parasymphyseal teeth, through moderately asymmetrical anterolaterals, to the extremely asymmetrical (with an enormous distal cusp) in the lateral and posterolateral parts of the jaw. Teeth of T. dziki are very similar to those of the second, anterolateral morphotype, but are rather more symmetrical than that, i.e., the twist of the crown and the enlargement of the distal cusp are greater in T. ferox (second morphotype) than in T. dziki .

Thus, it is clear that the material described here as Thrinacodus dziki sp. nov. represents a new species, probably closely related to T. ferox , but with a more homodont dentition and a lesser degree of asymmetry in lateral teeth. It is not clear from the material available whether T. dziki has specialised, subsymmetrical (parasymphyseal?) teeth, as in T. ferox and T. nanus (see, e.g., Ginter and Turner 2010: fig. 3A–F). Such teeth are very rare (their frequency in large collections never exceeds 1: 10 in relation to the other morphotypes), so their absence from the studied material may be incidental.Morphologically, the teeth of T. dziki which we do have in our samples, fall between morphologies of T. tranquillus and T. ferox .

There occurs a tiny collection of thrinacodont teeth from the lower Tournaisian of Armenia, designated as Thrinacodus aff. tranquillus ( Ginter et al. 2011: fig. 12H–J), which appear to be intermediate forms between T. tranquillus and T. dziki . The crowns in the Armenian specimens are almost symmetrical and the median cusp is only slightly displaced lingually. The basal/labial bulge, if it exists, is only rudimentary. One might consider that such thrinacodonts gave rise to both T. ferox , by the increase of asymmetry and differentiation of the dentition, and T. dziki , by the further change of the position of the median cusp, while the asymmetry did not increase much.

Stratigraphic and geographic range.—Currently known from the upper Viséan of Derbyshire, England, UK ( Duffin 1993; this paper) and the Holy Cross Mountains, Poland ( Ginter and Turner 2010; this paper). The identity of similar teeth from the upper Viséan of Fife, Scotland, UK ( Ginter and Turner 2010: fig. 4C) is uncertain.