Nebrius undetermined

Nebrius sp. Fig. 3H–M Material examined UNITED STATES OF AMERICA – Mississippi • 8 isolated teeth; Catahoula Formation; MMNS VP-6966 (Fig. 3H–I), MMNS VP-12033 (Fig. 3J–M), SC 2013.28.63 to 28.68. Description Teeth are wider (mesio-distally) than they are high, with the largest specimen (SC 2013.28.63) measuring slightly over 6 mm and 5 mm in these dimensions. In labial view, the broadly triangular, low crown bears a medially located main cusp that is flanked by multiple sets of lateral cusplets (Fig. 3K–L). The main cusp is low, triangular, lingually directed, and may be vertical to distally inclined. Lateral cusplets are much smaller than the main cusp, and cusplet size decreases towards the crown base. Typically, there are more cusplets on the mesial side than on the distal side (Fig. 3I, K). The mesial and distal cutting edges extend along the lateral cusplets and the main cusp. The labial face is weakly convex mesio-distally, and there is a conspicuous basally directed protuberance that has a rounded or flattened basal margin (Fig. 3I, K). The lingual crown face is convex and bears a medial protuberance that extends lingually onto the root (Fig. 3L). In profile view, the labial face ranges from sinuous to straight (compare Fig. 3H to J), and the main cusp apex is distally directed. The crown enameloid is smooth. The root is low, extends laterally nearly to the crown margin, and is sub-triangular in basal view. A large central foramen occurs within a deeply convex basal attachment surface (Fig. 3M). Additional foramina are located just below the crown on the upper surface of the lingual root face. Remarks Based on the dentition of extant Nebrius ferrugenius (Lesson, 1831), as illustrated by Herman et al. (1992), the sample available to us, although limited, indicates that gradient monognathic heterodonty was developed in the Catahoula Formation Nebrius species. Rather narrow and somewhat symmetrical teeth are interpreted to represent anterior jaw positions (Fig. 3K), whereas lateral teeth are wider, have a more distally inclined main cusp, and the longer mesial side bears more cusplets compared to the distal side (Fig. 3I). Teeth from distal lateral to posterior positions have a diminutive and sharply distally inclined main cusp, an elongated and highly convex mesial edge with numerous cusplets, and a comparatively shorter distal edge with significantly fewer cusplets. Additionally, the labial protuberance on the teeth in our sample has either a flat or a rounded basal margin, a phenomenon we also observed in the N. ferrugenius dentition, further corroborating the presence of monognathic heterodonty within our sample of fossil Nebrius sp. teeth. The Nebrius sp. teeth clearly differ from specimen SC 2013.28.54 (Brachaeluridae gen. et sp. indet.) by their wider crowns, prominent labial basal apron, and numerous lateral cusplets. Unfortunately, all the specimens in our sample are damaged and/or ablated, and it is difficult to compare them to ginglymostomatid teeth reported from Oligocene deposits elsewhere. Only a crown fragment referred to Ginglymostomatidae was reported from the Rupelian Ashley Formation of South Carolina (Cicimurri et al. 2022), and an incomplete tooth was recovered from the Chattian Chandler Bridge Formation by Cicimurri & Knight (2009). The latter specimen does not differ appreciably from the Catahoula Formation teeth. Müller (1999) identified Ginglymostoma delfortriei Daimeries, 1889 from the Oligo-Miocene Belgrade Formation of North Carolina, but all the specimens shown (pl. 2 figs 2–4) are broken apically. We concur with Yabumoto & Uyeno (1994) and assign the delfortriei morphology to Nebrius because the teeth exhibit significantly more than three sets of lateral cusplets, as opposed to only two or three on Ginglymostoma (Cicimurri & Knight 2009; Ebersole et al. 2019). Cicimurri & Knight (2009) tentatively identified their single specimen as N. serra (Leidy, 1877), but the validity of this taxon is debatable because the stratigraphic and geographic provenance of the original specimen(s) is uncertain. It may be that the Mississippi and North and South Carolina fossil Nebrius specimens are conspecific, but larger samples of well-preserved specimens from all these locations are necessary to make this determination.