Euselachii Hay 1902

Euselachii fam., gen. et sp. indet. Fig. 9C–D, 22M–U Material examined UNITED STATES OF AMERICA – Mississippi • 10 placoid scales; Catahoula Formation; MMNS VP-7750 (Fig. 22M–O), SC 2013.28.330, SC 2013.28.331 (2 specimens), SC 2013.28.332, SC 2013.28.333 (2 specimens), SC 2013.28.334 (2 specimens), SC 2013.28.529, SC 2013.28.530 • 10 teeth; Catahoula Formation; MMNS VP-7753 (Fig. 22P–R), MMNS VP-8736, MMNS VP-8741.1 (Fig. 5–U), MMNS VP-8741.2, SC 2013.28.115 (Fig. 9C–D), SC 2013.28.264 (2 specimens), SC 2013.28.265 (2 specimens), SC 2013.28.268. Description Specimens assigned only to Euselachii indet. include teeth and placoid scales. Several small teeth consist of a simple triangular main cusp and bilobate root (Fig. 22P–U). The crown has smooth to weakly serrated cutting edges that may not extend to the apex or crown foot. The labial face is nearly flat, but the lingual face is convex, and both faces have smooth enameloid. The root is robust for the size of the tooth, and very short root lobes are separated by a well-developed lingual nutritive groove. Placoid scales consist of an enameloid-covered crown and dentine base (Fig. 22M–O). The crown on each specimen is apico-basally flattened, and the apical surface may be weakly convex or flat. The apical outline is oval or rhomboidal. Several specimens have smooth enameloid, but others exhibit a series of parallel ridges along the anterior margin (Fig. 22N). These ridges do not extend to the posterior margin. The base has a somewhat triangular outline in profile, and the basal surface is flat. Remarks Teeth like those shown in Fig. 22P and S probably belong to a member of Carcharhiniformes, as they are similar to the symphyseal and parasymphyseal teeth that occur in the jaws of extant Carcharhinus spp. that we examined. It is possible that specimens with serrated cutting edges are from the upper dentition, whereas those with smooth cutting edges are from the lower dentition. Several other shark teeth are too broken or abraded to be confidently identified beyond Euselachii. For example, SC 2013.28.115 (Fig. 9C–D) is an ablated posterior tooth that bears similarity to the teeth of Galeorhinus (i.e., Fig. 9A–B), Physogaleus (i.e., Fig. 7 EE–JJ), and Rhizoprionodon (i.e., Fig. 8A–D). The labial crown foot appears to be thickened as on Galeorhinus teeth, but this may be an artifact of preservation, as the root is abraded. Additionally, the distal heel is not as clearly separated from the cusp as it is in Galeorhinus (Fig. 9A; also see Herman et al. 2003). The short and somewhat pointed distal heel is reminiscent of Physogaleus (Reinecke et al. 2005; Haye et al. 2008) and even Rhizoprionodon (see Ebersole et al. 2023). Isolated scales are rare in the elasmobranch component of the Catahoula Formation compared to isolated teeth. This phenomenon could be related to a collecting bias, but it may be an artifact of winnowing (removal of very small items through current action). At least two scale morphotypes are represented, including those with anterior ridges and those that are smooth. We cannot confidently assign these specimens to any particular elasmobranch genus that we identified by its teeth, nor can we determine whether more than one taxon is represented. However, specimens SC 2013.28.334 and MMNS VP-7750 are similar to scales that Dillon et al. (2017) identified as ridged abrasion strength morphotypes (i.e., with a protective function), potentially of Ginglymostomatidae. We include here for completeness nine isolated calcified cartilage tesserae (SC 2013.28.335). These have a columnar appearance when viewed perpendicular to their length, and the surface is roughened. The outline is six-sided. We cannot determine what skeletal element (probably cranial) nor the species the tesserae represent.