Plinthicus sp.

Cicimurri, David J., Ebersole, Jun A., Stringer, Gary L., Starnes, James E. & Phillips, George E., 2025, Late Oligocene fishes (Chondrichthyes and Osteichthyes) from the Catahoula Formation in Wayne County, Mississippi, USA, European Journal of Taxonomy 984 (1), pp. 1-131 : 70

Plinthicus sp.

Fig. 21A–D

Material examined

UNITED STATES OF AMERICA – Mississippi • 1 isolated tooth; Catahoula Formation ; SC 2013.28.522 .

Description

The tooth is wider than long (4.5 mm and 2.5 mm in these dimensions), and the crown measures 2 mm in height. In occlusal view, the crown has a roughly oval outline, and the occlusal surface is concave. The labial margin is thickened and forms a conspicuous rim around the depressed oral surface, but the lingual margin is thin and developed into a thin, lingually directed, shelf-like projection. In profile view, the labial and lingual faces are highly inclined ( Fig. 21D ), and the surfaces bear numerous robust, irregular vertical ridges. The labial crown foot is formed into a sharp projection that overhangs the root ( Fig. 21C ), whereas there is a thick and rounded transverse ridge at the lingual crown foot ( Fig. 21A ). The root is not well preserved but appears to have been smaller in area than the crown.

Remarks

Specimen SC 2013.28.522 is easily distinguished from the superficially similar myliobatiform teeth described above by its concave occlusal surface, thinner profile, and coarse vertical ridges on the labial and lingual faces. Plinthicus has been reported from Oligocene strata of South Carolina ( Cicimurri & Knight 2009; Cicimurri et al. 2022), but it is difficult to accurately compare the ablated Catahoula Formation specimen to the South Carolina material. However, Cicimurri et al. (2022) indicated that the Ashley Formation specimens (ca 28.5 Ma) differed from Mio-Pliocene P. stenodon Cope, 1869 and could represent a new species. The Catahoula Formation tooth clearly differs from P. kruibekensis Bor, 1990 from the Rupelian Boom Clay Formation of Belgium by its inclined labial and lingual faces that bear coarse but few vertical ridges. In contrast, the Belgian taxon has a convex labial and concave lingual face that bears finer and more numerous vertical ridges. Although recent taxonomic rankings place extant filter-feeding “devil rays” within Mobulidae (i.e., Notabartolo di Sciara 2020), it may not be correct to include all extinct mobulid-like taxa within this family. Villalobos-Segura & Underwood (2020) presented molecular divergence times for various batoid taxa that indicate that the clade containing Mobulidae did not diverge from its common ancestor until the Early Miocene. Therefore, it does not appear to be prudent to refer Paleogene mobulid-like teeth of presumed planktivorous rays to Mobulidae . However, for the purposes of this report we tentatively follow convention for familial assignment of this genus.

Figures

Fig. 21. Plinthicus sp. (A–D) and Paramobula fragilis (Cappetta, 1970) (E–N), teeth. A–D. SC2013.28.522, Plinthicus sp., lateral tooth. A. Lingual view. B. Basal view. C. Labial view. D. Profile view. E–I. MMNS VP-8429, Paramobula fragilis, tooth. E. Labial view. F. Occlusal view. G. Lingual view. H. Basal view. I. Distal view. J–N. MMNS VP-11678, P. fragilis, tooth. J. Labial view. K. Occlusal view. L. Lingual view. M. Basal view. N. Distal view. Scale bars = 3 mm.