Thrinacodus bicuspidatus, Ginter & Sun, 2007
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publication ID |
https://doi.org/ 10.5281/zenodo.13741212 |
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persistent identifier |
https://treatment.plazi.org/id/954A87EC-2C1B-3362-FC8C-FA647A7D7880 |
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treatment provided by |
Felipe |
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scientific name |
Thrinacodus bicuspidatus |
| status |
sp. nov. |
Genus Thrinacodus St. John and Worthen, 1875 Thrinacodus bicuspidatus sp. nov.
Fig. 2 View Fig .
Holotype: Specimen PKUM02−0129 , a tooth ( Fig. 2B View Fig ) from the MH section north of Muhua , Guizhou Province, southern China; sample MH−1, bioclastic limestone, Muhua Formation , Carboniferous , Mississippian, Tournaisian, Siphonodella crenulata conodont Zone.
Derivation of the name: Latin bicuspidatus = bicuspid.
Material.—Nine teeth.
Diagnosis.—A species of Thrinacodus whose dentition is composed at least partly of teeth with only two cusps in the crown. The species resembles Thrinacodus ferox ( Turner, 1982) in the general tooth outline, the shape of the base and the strong crown asymmetry in the lateral teeth, but differs by the loss of one of the cusps.
Description.—Judging from the material from Muhua and elsewhere, and comparing them to the type series of Th. ferox ( Turner 1982) and dentition models proposed by Duncan (2003: fig. 3) we may presume that there are three general morphotypes of teeth in Thrinacodus bicuspidatus sp. nov.
The first, most common, which probably corresponds to lateral tooth families, has a bicuspid crown with both cusps curved lingually. The distal cusp is larger (see Ginter et al. 2002 for identification of sides in Thrinacodus ), standing almost upright in the labial view; its distal divergence from the normal to basal plane does not exceed 10 degrees. The mesial cusp is smaller, clearly divergent mesially from the vertical position in the labial aspect, and also much more stronger directed lingually than the distal cusp. The cusps are smooth labially and ornamented with gentle subparallel cristae lingually. A weak lateral carina separates the two faces of the cusps. This carina connects the cusps, but in some specimens it forms a straight line ( Fig. 2C View Fig , partly covered by dirt), and in others, the carinae of the two cusps join at the midline at an angle ( Fig. 2A View Fig 1, B 1 View Fig , B 4). In the latter case, there occur a few short cristae, on the lingual side of the medial area, which do not belong to any of the cusps.
The base is extended lingually, almost flat and thin lingually and thicker, with a low hump, nearer to the crown. The lateral margins of the base are almost parallel or the lingual part is wider. The distal lingual angle is oblique, and the mesial is acute. A large foramen occurs almost at the centre of the oral surface. There is a triangular attachment area on the labial side ( Fig. 2B View Fig 1 View Fig , B 3 View Fig ) devoid of enameloid, bearing a few minor foramina. Its shape corresponds to the route of the carina in the medial area.
The second tooth type ( Fig. 2E View Fig ), if correctly ascribed to this species, probably represents the anterior region of the jaw. It has a long base and three slender cusps of different sizes. Such teeth are commonly found among the dentition of Th. ferox (see e.g., Turner 1982: fig. 3F), but might also have been retained in Th. bicuspidatus . In such a case, based only on this type of tooth it is impossible to distinguish these two species.
The third form, not found in Muhua, but co−occurring with the first morphotype in a sample from the middle Tournaisian of Martin Spring (Nevada; Dr Charles A. Sandberg’s collection, deposited at IGPUW) is tricuspid, compact, with short and thick cusps and a relatively short base. It resembles the teeth usually placed in the symphyseal ( Turner 1982: figs. 2D, 3A, 5) or parasymphyseal ( Duncan 2003: figs. 3A, B, 4) positions in the restorations of Th. ferox dentition.
Remarks.—The existence of bicuspid thrinacodonts in the Tournaisian of NW Australia was signalled by Ginter et al. (2002: 202), but this is the first time that such teeth have been illustrated and described. It was supposed by Ginter et al. (2002) that the disappearance of the mesial cusp was due to an advanced stage of crown rotation combined with size reduction of the mesial and central cusps in the lateral teeth of Thrinacodus ferox . However, the size difference and the angle between the distal and central cusps in the most asymmetrical teeth of the type series of Th. ferox ( Turner 1982: fig. 2A–C; see also Ginter 2001: fig. 3D, E) is greater than between the two cusps of Th. bicuspidatus sp. nov., so in this aspect the asymmetry is more advanced in Th. ferox .
It is largely assumed that the bicuspid Thrinacodus teeth developed from tricuspid forms, but a controversy exists as to whether it was the lateral or central cusp that was lost in this process. Our preferred hypothesis is that the lateral (probably mesial) cusp became phylogenetically less involved in food aprehension due to the vertical torsion of the crown. Almost all of the clutching function was taken by the distal, enlarged cusp, with a minor help from the central one, and finally the mesial cusp became obsolete. However, an alternative opinion is presented by Vachik Hairapetian (personal communication 2006) who suggested, based on specimens of Th. bicuspidatus from Iran, that the central cusp was reduced due to the pressure from a cusp of the next tooth. In his concept, the angular connection of carinae and the presence of short cristae in the medial area are remnants of the suppressed central cusp.
Stratigraphic and geographic range.—Middle Tournaisian of southern China (Muhua), NW Australia (Canning Basin), Iran, and Nevada.
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