Plinthicus undetermined

Cicimurri, David J., Knight, James L. & Ebersole, Jun A., 2022, Early Oligocene (Rupelian) fishes (Chondrichthyes, Osteichthyes) from the Ashley Formation (Cooper Group) of South Carolina, USA, PaleoBios 39 (1), pp. 1-38 : 19-20

PLINTHICUS SP.

FIG. 8I–K

Type species — Plinthicus stenodon Cope, 1869 ; Miocene Kirkwood Formation (?); Cumberland County, New Jersey, USA .

2009a Plinthicus stenodon (Cope) ; Cicimurri and Knight, page 641, fig. E–F.

Referred specimens (n=1) —SC2007.36.48.

Remarks —SC2007.36.48 differs from teeth of Ashley Formation “ Mobula ” by being much larger in overall size, by having a much wider and higher crown, by bearing heavily ridged vertical crown faces (ridges extend from the crown foot nearly to the apex), and by having a concave occlusal surface that lacks elongated and lingually directed projections. This specimen differs from “ Mobula ” fragilis ( Cappetta, 1970) reported from the Chandler Bridge Formation ( Cicimurri and Knight 2009a) by being much larger in size, labio-lingually thicker, having a concave occlusal surface, and by having better developed vertical ridges on the labial and lingual faces. Middle Oligocene teeth of Arnomobula Leder, 2015 differ from Plinthicus sp. by having a smooth lingual face and an occlusal surface consisting of irregular peaks and ridges, as opposed to a flat triturating surface. Eocene Eoplinthicus teeth have a distinctive low crown with an occlusal surface that is smaller in area than the crown foot ( Cappetta and Stringer 2002, Adnet et al. 2012, Ebersole et al. 2019), but in contrast Plinthicus has occlusal and basal crown outlines of roughly the same dimensions.

The gross morphology of SC2007.36.48 is comparable to Plinthicus , but it differs significantly in profile view from the European Rupelian species, P. kruibekensis Bor, 1990 . The crown of the Ashley Formation specimen is lingually inclined, with the labial and lingual faces being relatively straight ( Fig. 8K ). In contrast, the crown of P. kruibekensis is rather erect and the labial face is convex, whereas the lingual face is concave ( Bor 1990). Cicimurri and Knight (2009a) reported several specimens from the Chandler Bridge Formation that they identified as Plinthicus stenodon Cope, 1869 , citing close similarities to Mio-Pliocene teeth of this species occurring elsewhere. The crown morphology is variable in the Chandler Bridge Formation sample we examined, but SC2007.36.48 falls within the range of variation and we consider the samples from the two formations to be conspecific. Müller (1999, page 66) identified Plinthicus stenodon from the Ashley Formation, but he did not illustrate any specimens from this lithostratigraphic unit, and we therefore could not evaluate his material.

Our comparison of the Oligocene Plinthicus to Miocene specimens of P. stenodon (SC98.46) revealed significant differences. The Oligocene taxon exhibits a weakly inclined crown with relatively straight labial and lingual faces. The heavy vertical ridges on the labial and lingual faces are thick, highly irregular, and often interconnected via transverse ridges ( Fig. 8J ). In contrast, the crown of P. stenodon is much thinner labio-lingually, very highly lingually inclined, and near the apex the labial and lingual faces appear to curve labially. In addition, the labial and lingual ridges are very thin, sharp, straight, and not interconnected. Furthermore, the lingual ridges extend beyond the general occlusal plain, forming an irregular ridge that is separated from the occlusal surface by a shallow transverse furrow. On the Oligocene teeth, the lingual ridges terminate at the occlusal margin. The morphological differences between the Oligocene and Neogene specimens indicate that they represent different species. We refrain from naming a new species herein because the Ashley Formation specimen is broken and ablated. However, the Oligocene mobulids of South Carolina are currently under further evaluation (including specimens from the Chandler Bridge Formation). The Rupelian occurrence of Plinthicus in South Carolina is confirmed and thereby extends the temporal range of the genus from 24.5 Ma (Chattian) back to approximately 28.5 Ma.

Figures

Figure 8. Batoid teeth from the Givhans Ferry Member, Ashley Formation (Rupelian), Dorchester County, South Carolina. A, B. “Rhinoptera” sp. tooth, SC2015.29.30 in occlusal (A) and lingual (B) views. C, D. “Mobula” sp. male tooth, SC2007.36.40 in apical (C) and occlusal (D) views. E, F. “Mobula” sp. male tooth, SC2007.36.42 in apical (E) and occlusal (F) views. G, H. “Mobu- la” sp. female tooth, SC2007.36.43 in occlusal (G) and labial (H) views. I–K. Plinthicus sp. tooth, SC2007.36.48 in occlusal (I), lingual (J), and profile (K) views. Scale bar=1 mm in C–H; 5 mm in A, B, I–K.