Thrinacodus St. John and Worthen, 1875

Duncan, Mags, 2003, Early Carboniferous chondrichthyan Thrinacodus from Ireland, and a reconstruction of jaw apparatus, Acta Palaeontologica Polonica 48 (1), pp. 113-122 : 114-117

Genus Thrinacodus St. John and Worthen, 1875 Type species: Diplodus incurvus Newberry and Worthen, 1866 . Thrinacodus St. John and Worthen, 1875: 289 , pl. V: 1, 2.

Harpago Turner 1982: 113 , figs. 2–4.

Harpagodens Turner 1983: 38 .

Harpagodens Wang and Turner 1985: 226 , pl. 2: 11, 12. Harpagodens Derycke 1988 : pl. 1: 1, 2.

Harpagodens Wang 1989: 105 , pl. 28: 6, 7; pl. 29: 2a, b; pl. 30: 1, 2. Thrinacodus (Harpagodens) Turner in Long 1990: fig. 5L, M. Thrinacodus Ginter 1990: 76 , pl. 3: 2–4.

Thrinacodus Ginter 1991: 12 , pl. A: 6–8.

Thrinacodus Kietzke and Lucas 1992: 18 , fig. 2D–H.

Thrinacodus (Harpagodens) Turner 1991 : fig. 6, pl. 2G.

Sub− system Series (this Ireland paper) Britain Belgium USA SILESIAN (part) in NAMURIAN in part) (PENDLEIAN PENDLEIAN PENDLEIAN part () in BRIGANTIAN BRIGANTIAN CHESTERIAN ASBIAN ASBIAN WARNANTIAN VISEAN HOLKERIAN HOLKERIAN VISEAN LIVIAN MERAMECIAN (in part) MISSISSIPPIAN DINANTIAN ARUNDIAN ARUNDIAN " MOLINIACIAN Lower " Visean DIAN Late "FREYRIAN" CHA "FREYRIAN" TOURNAISIAN HAST− IVORIAN COURCEYAN Early TOURNAISIAN HAST− IVORIAN KINDER− OSAGEAN ARIAN ARIAN HOOKIAN

Thrinacodus Derycke 1992: 30 , pl. 1: 3.

Thrinacodus Turner 1993 : fig. 8.7F.

Thrinacodus Duffin 1993: 1 , pl. 1, 2.

Thrinacodus Derycke et al. 1995: 472 , pls. IIIB, IVA.

Thrinacodus Ginter 1995: 27 , 28, figs. 4G, 5A, B.

Thrinacodus Ivanov 1996: 418 , fig. 4a–e.

Thrinacodus Ginter and Ivanov 1996: 267 , figs. 1, 2a–d, 7. Thrinacodus Ginter 1999: 34 , pl. 3: 1–7.

Thrinacodus Ivanov 1999: 273 , pl. 4: 2, 4.

Thrinacodus Ginter 2000: 377 , fig. 2D–F.

The genus Thrinacodus was proposed by St. John and Worthen (1875), based on their new species Thrinacodus nanus from the upper fish bed of the Mississippian Kinderhook Division, Burlington, Iowa, USA. It is known only from isolated teeth. The taxonomy of Thrinacodus is complicated because of possible synonymies, involving at the generic level, Diplodus , and at the specific level Diplodus incurvus , Thrinacodus nanus , and Thrinacodus ferox .

St. John and Worthen (1875: 289) described the genus as follows: “Teeth of small size. Base posteriorly produced in a long, sometimes twisted, vertically flattened, or laterally compressed, clavate plate, longer than wide, anterior face narrow, and abruptly beveled from the basal line of the crown; posterior extremity more or less obtusely rounded; inferior surface narrow, plane and faintly excavated; superior surface gently convex, concave antero−posteriorly, or corresponding to the curvature of the inferior surface. From the antero−superior extremity of the base spring three more or less relatively stout, nearly equal, trenchant, acutely pointed recurved cusps, the exterior pair divergent, the central one more or less vertical, slightly sigmoidally curved, transverse section sublenticular, compressed in front, rounded behind, with simple cutting edges, and more or less strongly costate in either face.

The generic peculiarities of the teeth here referred to, as contrasted with Diplodus, Agassiz , with which they are most intimately allied, consists in the relative more slender base, which lacks both the antero−inferior protuberance and the postero−superior pad−like prominence characteristic of the above genus. The coronal cusps are also nearly equal in size, the cutting edges are destitute of crenulations, and their anterior and posterior faces more or less strongly ridged vertically. The form of the trident−like cusps are very like that of Cladodus , being slightly constricted basally in front and faintly defined from the base behind.

As here defined, the genus includes the forms originally described by Messrs. Newberry and Worthen, under the name Diplodus incurvus and D. duplicatus , of the Keokuk limestone, which seem to be indistinguishable from a form not uncommon in the upper Burlington limestone.”

The species Diplodus incurvus Newberry and Worthen is assigned here to Thrinacodus . Thrinacodus nanus described by St. John and Worthen and Irish material described below are morphologically identical to the specimens on which Newberry and Worthen (1866) based the species Diplodus incurvus except that they are an order of magnitude smaller. This raises a general question in relation to microscopic fish teeth: are teeth that are morphologically identical except for their size conspecific? The tentative conclusion is that the difference in size does not rule out two teeth being conspecific. Therefore Thrinacodus [ Diplodus ] incurvus Newberry and Worthen, 1866 is regarded here as a senior synonym of Thrinacodus nanus St. John and Worthen, 1875 , and is thus the type species of Thrinacodus .

Turner (1982) proposed the genus Harpago with the type species Harpago ferox Turner, 1982 . The name Harpago was a b c d

found to be pre−occupied and was replaced by Harpagodens ( Turner 1983) .

At the specific level, the taxonomy is complicated by the question of whether symmetrical forms of Thrinacodus ( Thrinacodus incurvus = Thrinacodus nanus ) occurred in the same fish as asymmetrical forms ( Thrinacodus ferox ). While this question cannot at present be answered conclusively, the balance of the evidence suggests that in Carboniferous examples, at least, left− and right−handed asymmetrical forms and symmetrical forms did co−occur in the same fish (reconstructions by Turner 1982, refigured in Fig. 3E herein, illustrated this). Therefore the practice followed here is to recognise the asymmetrical and symmetrical forms of Thrinacodus in Carboniferous rocks as belonging to the same taxon, Thrinacodus incurvus . In order to distinguish the symmetrical and asymmetrical teeth, they are referred to here as the nanus (symmetrical) and ferox (asymmetrical) morphotypes of Thrinacodus incurvus .

As noted above, the type of Thrinacodus [ Diplodus ] nanus was described by St. John and Worthen as having “Teeth minute ... with coronal cusps nearly equal in size ...” andabase“...moderatelyproducedposteriorly,orapparently sub elliptical in outline ...”. However, they qualified the description suggesting that it applied to “specimens which show the crown entire” and noting “that within the collections are several imperfect teeth: ... apparently referable to the same species, which show the base considerably produced posteriorly, flattened and slightly twisted precisely in the same manner as in the allied forms occurring in the succeeding Burlington and Keokuk divisions. In case of this identity, the laterally elliptical outline of the base, as described above, is probably due to mechanical abrasion...”. (St. John and Worthen 1875: 289–290).

The specimens described here as Thrinacodus incurvus nanus morphotype, are those with a symmetric to slightly asymmetric elliptical outline; those with more flattened and twisted base are referred to as Thrinacodus incurvus ferox morphotype as described below.

In a discussion on the relationships of Phoebodus, Ginter and Ivanov (1996) suggested that two “varieties” of Carboniferous Thrinacodus teeth could be distinguished: asymmetrical and symmetrical. This is consistent with both the findings of St. John and Worthen (1875) and of Turner (1982). However, St. John and Worthen implied in their description of Thrinacodus nanus that the “principal teeth” are the symmetrical ones with the asymmetrical teeth as the variations, whereas Turner, in her description of Thrinacodus ferox ( Harpago ferox ), suggested the opposite with the asymmetricalteethasthe“principalteeth”,sincetheynumericallydominated her samples. In this study the ratio of asymmetrical to symmetrical teeth is 10:2, a ratio similar to that reported by Turner (1982). A similar ratio has been observed in samples from Eastern Europe (M. Ginter personal communication 1997). In the Irish material, seven asymmetrical teeth with the principal cusp to the right when viewed from the labiolateral side with the crown to the bottom (referred to here as right handed) and three asymmetrical teeth with their principal cusp to the left (left handed) were recovered from the Kilbride Limestone Formation. A further two left−handed asymmetrical teeth were recovered from the Urlingford borehole core, and a single right−handed asymmetrical tooth was recovered from Hook Head; neither of the two latter localities yielded symmetrical or near symmetrical teeth.

It seems most probable that the left− and right−handed teeth belong to the same species, as it is unlikely that a jaw contained either all left−handed or all right−handed teeth ( Turner 1982). The more likely reconstruction is that the left−handed teeth were located on one side of the jaw and the right−handed on the other ( Turner 1982: fig. 5A, B). It is suggested here that the Thrinacodus incurvus nanus morphotype teeth were housed in the same jaw as the T. incurvus ferox morphotype. However, Ginter and Ivanov (1996) found that symmetrical teeth with a “... typical phoebodont−type crown seems the older one. Only this variety has been found thus far in the Late Famennian deposits of the Palmatolepis trachytera to P. expansa (?Early Siphonodella praesulcata ) zones, in the samples constrained by conodonts ( Long 1990; Ginter 1990)”. They suggested that the asymmetrical teeth of Thrinacodus appeared for the first time in the S. praesulcata Zone and dominated the Early Carboniferous faunas ( Wang and Turner 1985); rare occurrences of the symmetrical teeth in the Carboniferous “... may be because they were retained only in a special part of a jaw (symphyseal, as suggested by Turner 1982).” Subsequently Ginter (2000) erected a new species Thrinacodus tranquillus (with a current range of mid−late Famennian) characterised by an almost symmetrical crown and long slender recurved cusps with a long (labiolingually), narrow base. This species, too, appears to have two morphotypes; one with lingually flattened bases, asymmetrically situated in relation to the crown, and secondly, a morphotype represented by very small teeth, which are almost entirely symmetrical.

Taking into account the varying opinions, the most reasonable conclusion is that during the Mississippian, at least, the two Thrinacodus morphotypes discussed here were housed within a single jaw and thus the specific name applied to them should be the same. Thrinacodus incurvus has priority.

Thrinacodus teeth have been described from the late Famennian of Ostrowka, Holy Cross Mountains, Poland (tranquillus morphotype, Ginter 1990; nanus and ferox morphotypes, Ginter and Ivanov 1996); late Famennian of Tafilalet, Morocco ( Derycke 1988; 1992); late Famennian of Broken River, Queensland, Australia (includes type material of Thrinacodus ferox ( Harpago ferox )butthedatingisnotprecise,feroxmorphotypesdominatebutnanusmorphotypesalso occur, Turner 1982); Late Famennian of Thuringia, Germany (ferox morphotype, Ginter 1999);?Late Devonian of the Sacramento Mountains, south−central New Mexico, USA (ferox morphotype,KietzkeandLucas1992); Devonian/Carboniferous boundary of Daihua and Wangyou Formations of Changshu County, Guizhou Province, China (ferox morphotype, Wang 1993); Devonian/Carboniferous boundary at La Serre, MontagneNoire,Hérault, France (feroxmorphotype,Derycke etal.1995);earlyMississippian(earlyTournaisian)ofNovaya Zemlya, North Island (dominated by ferox morphotype with one/twonanusmorphotype, Ivanov1999);earlyMississippian (early Tournaisian) South Urals (ferox morphotype, Ivanov 1996); Mississippian (late Tournaisian to early Viséan) Bingleburra Formation, New South Wales, Australia (morphotype non specified, Turner 1982); Mississippian (Viséan) of Derbyshire, England (ferox morphotype, Duffin 1993); late Mississippian (Serpukhovian) of the Moscow Region (ferox morphotype, Ginter and Ivanov 1996).

Figures

Fig. 3. Possible reconstructions of front part of a Thrinacodus mouth. A, B. Using Thrinacodus incurvus ferox and nanus morphotype teeth. C, D. Using Thrinacodus incurvus ferox morphotype teeth only. E. Reconstruction by Turner (1982) for comparison. Key: a is the robust end member, b the fine end member and c the sharply recurved tooth of T. incurvus ferox morphotype, d is T. incurvus nanus morphotype tooth.