Plinthicus stenodon Cope, 1869

Cicimurri, David J. & Knight, James L., 2009, Late Oligocene sharks and rays from the Chandler Bridge Formation, Dorchester County, South Carolina, USA, Acta Palaeontologica Polonica 54 (4), pp. 627-647 : 641

Plinthicus stenodon Cope, 1869

Fig. 6E, F .

Referred specimens.—BCGM 9118–9121, SC 2009.18.23.

Comments.— Cappetta (1987) and Cappetta and Stringer (2002) assigned Plinthicus to Mobulinae because of dental similarities to Mobula . Purdy et al. (2001) studied a large sample of teeth and concluded that the arrangement of the teeth within the dentition was more similar to Rhinoptera Cuvier, 1829 than Mobula and placed Plinthicus within Rhinopterinae . Our analysis of Chandler Bridge Plinthicus shows that the mesial side of lateral teeth is lower than the distal side, a dental characteristic seen in Rhinoptera (see Cappetta 1987).

Plinthicus kruibekensis Bor, 1990 is based on a unique tooth recovered from Rupelian strata of Belgium. The tooth of P. kruibekensis slopes outwards and then curves inwards, the occlusal surface is flat, and the enameloid ornamentation consists of anastomosing longitudinal ridges and grooves ( Bor 1990). In contrast, the Chandler Bridge teeth slope inward and curve upward ( Fig. 6E 1, 6F 1), the occlusal surface is concave ( Fig. 6F 2 ), and the enameloid ornamentation consists of parallel ridges and grooves ( Fig. 6E 2 ). These features are identical to those we personally observed on Miocene Plinthicus stenodon from North and South Carolina, and the Chandler Bridge specimens constitute the oldest record of the species.

Stratigraphic and geographic range.—Oligocene (Chattian), USA (South Carolina); Miocene, USA (Atlantic Coastal Plain), France, Malta.

Figures

Fig. 6. Batoids from Summerville, upper Chattian. A. Rhynchobatus pristinus (Probst, 1877), BCGM 9085, occlusal view. B, C. Myliobatinae gen. indet. B. BCGM 9117, lateral tooth, occlusal view. C. BCGM 9116, partial medial tooth, occlusal (C1) and lingual (C2) view. D. Paramobula fragilis (Cappetta, 1970), BCGM 9113, anterolateral tooth, occlusal (D1), labial (D2), and lateral (D3) view. E, F. Plinthicus stenodon Cope, 1869. E. BCGM 9120, partial anterior tooth, lateral (E1) and lingual (E2) view. F. BCGM 9121, lateral tooth, lateral (F1) and labial (F2) view. G. Rhinoptera cf. R. studeri (Agassiz, 1843), BCGM 9123, occlusal (G1) and lingual (G2) view. H. Gymnura sp., BCGM 9107, lateral (H1) and labial (H2) view.

Fig. 2. Stratigraphy of the Chandler Bridge Formation showing facies designations of Katuna et al. (1997) and their correlative units (Beds 1–3) as discussed by Sanders and Weems (1986). Marine/marginal marine facies constitute a coarsening upward sequence from poorly sorted, sandy to silty clay to moderately sorted silty, very fine sand, whereas the bay/estuarine facies is poorly sorted silty to clayey fine quartz sand with occasional phosphate pebbles, and fluvial/estuarine facies consists of poorly sorted, clayey, fine sand with abundant phosphate pebbles.