Thrinacodus incurvus ( Newberry and Worthen, 1866 )

Duncan, Mags, 2003, Early Carboniferous chondrichthyan Thrinacodus from Ireland, and a reconstruction of jaw apparatus, Acta Palaeontologica Polonica 48 (1), pp. 113-122 : 117-120

publication ID	https://doi.org/ 10.5281/zenodo.13307638
persistent identifier	https://treatment.plazi.org/id/5B6687DA-510C-FFA2-FCCA-F8FC0C127CE7
treatment provided by	Felipe
scientific name	Thrinacodus incurvus ( Newberry and Worthen, 1866 )
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Thrinacodus incurvus ( Newberry and Worthen, 1866)

Figs. 4–6 View Fig View Fig View Fig .

Diplodus incurvus Newberry and Worthen, 1866: 62 , pl. IV: 4, 4a. Thrinacodus nanus St. John and Worthen 1875: 289 , pl. V: 1, 2. Harpago ferox Turner 1982: 113 , figs. 2–4. Harpagodens ferox (Turner) ; Turner 1983: 38. Harpagodens ferox (Turner) ;WangandTurner1985:226,pl.2:11,12. Harpagodens ferox (Turner) ; Wang 1989: 105, pl. 28: 6, 7; pl. 29: 2a, b;

pl. 30; 1, 2. Thrinacodus ferox ( Harpagodens ) (Turner); Long 1990: fig. 5L, M. Thrinacodus ferox (Turner) ; Kietzke and Lucas 1991: 18, fig. 2D–H. Thrinacodus (Harpagodens) ferox (Turner) ; Turner1991:fig.6,pl.2G. Thrinacodus ferox (Turner) ; Derycke 1992: 30, pl. 1: 3. Thrinacodus ferox (Turner) ; Turner 1993: fig. 8.7F. Thrinacodus ferox (Turner) ; Duffin 1993: 1, pl. 1, 2. Thrinacodus ferox (Turner) ; Derycke et al. 1995: 472, pls. IIIB, IVA. Thrinacodus ferox (Turner) ; Ginter 1995: 28, figs. 5A, B. Thrinacodus ferox (Turner) ; Ivanov 1996: 418, fig. 4a, b, d, e. Thrinacodus ferox (Turner) ; Ginter and Ivanov 1996: 267, figs. 1 (in part), 2c, d. Thrinacodus sp. Ginter 1999: 34, pl. 3: 1–3, 5–7. Thrinacodus cf. Thrinacodus ferox (Turner) ; Ginter 1999: 377, fig.

2D–F. Material.—Thirteen illustrated, isolated, asymmetrical, teeth (ferox morphotype), some complete (specimen numbers: TCD.36792–36801 and TCD.36803–36805). Two illustrated, isolated, incomplete, symmetrical teeth (nanus morphotype; specimen numbers: TCD.36802 and 36808). Some broken cusps (TCD.36984) which are not illustrated.

Description.—As discussed above, T. incurvus is considered to include both asymmetrical (ferox morphotype) and symmetrical (nanus morphotype) teeth. These are described separately below.

Ferox morphotype.—Asymmetrical teeth vary from 0.7 mm to1.94mminlengthfromthebaseoftheprincipalcusptothe distal margin of the base and from 0.47 mm to 1.5 mm across the widest extent of the cusps. Only in specimen TCD.36799 are the cusps entire; most have broken tips.

Specimens TCD.36792–36801 are left−handed forms and TCD.36803–36805 are right−handed forms. They are tricuspidate with each cusp curving lingually and in some cases laterolingually. Cusps vary in length and width. The principal cusp initiates at the proximal base growing down slightly and then curving sharply right or left laterolingually; the middle cusp initiates above the level of the principal cusp growing out and then curving lingually; the left or right lateral cusp commonly initiates above the level of the middle cusp (otherwise at the same level) and curves laterolingually. In well preserved specimens, each cusp shows six to eight raised ribs from tip to base; the ribs are more pronounced and numerous on the lingual and lateral areas than on the labial. In many specimensatleastoneribextendsfromtheinsidelateraledge of the outside cusp across and up the lateral edge of the middle cusp ( Fig. 5A, C, E View Fig 1 View Fig , G 2).

The bases are elongate, roughly rectangular and normally at least twice as long as wide, ovoid in cross section close to the cusps but flattening dorsoventrally towards the distal margins as the base twists through ca. 50°. In many cases the distal area of the base opposite the principal cusp is extended lengthways and twisted inwards over the dorsal area of the base, while the other distal margin (in line with the principal cusp) may widen but be shorter and twist away to the ventral side ( Fig. 5B View Fig 2). One or two major basal canal openings are situated laterally, typically at about mid−length ( Fig. 5B View Fig 1 View Fig ) and typically approximately in line with the middle cusp ( Fig. 6A View Fig 2). At the base of the middle cusp along the labioventral surface one or two smaller canal openings may occur ( Fig. 6A View Fig 1 View Fig and A 4 View Fig ) from which a groove to the ventral surface may outline a ventral node ( Fig. 5B View Fig 3 View Fig ). In some specimens there is a small canal opening on the ventral surface of the base close to the base of the principal cusp. Other minor foramina occur on the ventral, dorsal and labioventral surface of the base in some specimens.

Nanus morphotype.—The symmetrical tooth TCD.36802 ( Fig. 4A View Fig ) and slightly asymmetrical tooth TCD.36808 ( Fig. 4B View Fig ) are smaller overall in dimension (0.77mm in length from the base of the median cusp to the distal margin of the base and at least 0.9 mm across the widest extent of the cusps in TCD.36802) than the associated, ferox morphotype teeth. The three cusps are roughly equal in width, curved lingually with up to ten raised ribs from the tip of each cusp, less pronouncedandnumerousontheirlabialsurfaces( Fig.4A View Fig 2,A 3 View Fig ). Cusps are slightly sigmoidal ( Fig. 4B View Fig 2), sub−circular in cross section and a central pulp cavity is present ( Fig. 4A View Fig 2).

The bony base is half as wide as long with the widest extentclosetothebaseofthecusps.Thereisaslighttwistofthe base with a flare to one side ( Fig. 4B View Fig 1 View Fig ); otherwise the base tapers to the distal margin culminating in a rounded tip. Abasal canal opening occurs on the dorsal surface halfway along the length in line with the median cusp ( Fig. 4A View Fig 2). Secondary foramina occur in the same area. A prominent node occurs at the junction of the ventral and labial surfaces to one side of the mid−line, in some specimens associated with a basal canal opening.

Discussion.—The asymmetrical teeth found in this study are variable but conform to the description of Thrinacodus ferox by Turner (1982: 118–119) and to descriptions and illustrationsofotherauthors.Thevariationcanbedescribedinterms of two end members. At one extreme are finer, more delicate specimens (specimens TCD.36794, 36798, 36799, and 36800), which show a principal cusp to the right when viewed from the labiolateral side with the crown to the bottom. At the other extreme are more robust teeth with the principal cusp on the left (TCD.36796, 36801, 36804). The apparent correlation of degree of robustness with left and right handed teeth is probably a statistical artefact because Duffin (1993: pls. 1, 2) illustrated left− and right−handed robust teeth from the Mississippian (late Viséan) of Derbyshire ( England) and Derycke et al. (1995: pl. III) showed an example from the Mississippian (early Tournaisian) with the principal cusp to the right. Turner (1982: figs. 2, 3) illustrated “delicate” teeth with the principal cusp on either side from the late Devonian of Australia.

The remaining Irish specimens of the ferox morphotype exhibitintermediatemorphology.AnexceptionisTCD.36797( Fig. 5H View Fig ) in which, although the principal cusp is broken close to its base, it is still possible to see the typical latero−lingual curvature to the right and the left lateral cusp shows latero−lingual curvature to the left. The middle cusp is straight and bends back sharplyatanacuteangleoverthelengthofthebasewithlittleor no curvature. This specimen is similar in many respects to symmetrical tooth illustrated by Ginter and Ivanov (1996: fig. 2B), but specimen TCD.36797 is clearly asymmetrical.

The two specimens of the nanus morphotype described above fall within the range of variation of Thrinacodus nanus described by St. John and Worthen 1875. In the slightly asymmetrical specimen (TCD.36808), the main cusp is larger and the base tapers, flaring to the distal margin with a low angle twist; canal openings occur on both labial or lingual surfaces together with a foramen or several foramina on the labial surface below the enameloid of the cusps. This specimen has features in common with specimens of Phoebodus gothicus from the Famennian of the Holy Cross Mountains of Poland described by Ginter and Ivanov (1996) that have strongly recurved cusps and a long narrow base with a button less pronounced than in the type specimens. Ginter and Ivanov commented that these characters make these specimens of P. gothicus similar to the symmetrical teeth of Thrinacodus “so closely that they can be mistaken at first sight, particularly when the tips of the cusps are broken.” However they suggested that the taxa could be distinguished because “the base of Thrinacodus is slightly twisted, devoid of a button, and flattened or concave at the lingual end; the cusps are not sigmoidal, as in P. gothicus .” TCD.36808 however, shows one lateral cusp that has a tendency to be sigmoidal.