Chiloscyllium undetermined

Chiloscyllium sp. Fig. 3D–G Material examined UNITED STATES OF AMERICA – Mississippi • 12 isolated teeth; Catahoula Formation; MMNS VP-8795 (3 teeth), MMNS VP-12034, SC 2013.28.55 (Fig. 3D–E), SC 2013.28.56, SC 2013.28.57 (Fig. 3F–G), SC 2013.28.58 to 28.62. Description SC 2013.28.55 is the best-preserved specimen and has a crown that measures 2 mm in width and approximately 2.2 mm in height (Fig. 3D–E). The other teeth are broken and/or ablated but appear to have had similar dimensions. The crown of SC 2013.28.55 consists of a broadly triangular but sharply tapering main cusp that comprises approximately one-half of the total crown height. The main cusp is flanked by a single pair of short, broad, and slightly diverging lateral cusplets (Fig. 3D). In labial view, the crown is symmetrical, with an erect main cusp, but in profile view the main cusp is slightly lingually inclined. Smooth mesial and distal cutting edges occur on the main cusp that extend onto the lateral sides of the cusplets but do not reach the crown base. The labial face is straight (apico-basally) but slightly convex (mesio-distally), and the crown foot is formed into a broad, bifid protuberance that overhangs the root. The lingual crown face is convex, and a medial protuberance extends onto the upper surface of the root. The root is low in profile view and less wide than the crown (Fig. 3E). In basal view, the root is bilobed, with narrow and widely diverging lobes. The lobes are separated by a deep V-shaped embayment that opens labially, and a very large foramen occurs at the center of the root. Additional foramina occur on the upper root surface on each side of the lingual crown protuberance, and another is located on the lingual-most face of the root. Specimens SC 2013.28.59 and SC 2013.28.60 (and likely SC 2013.28.56) are less well preserved but appear to have been similar to SC 2013.28.55. Specimens SC 2013.28.57 (Fig. 3F–G), SC 2013.28.58, and SC 2013.28.61 differ from the above specimens by their lower overall crown height, lower and distally inclined main cusp, and lower and broader lateral cusplets. Additionally, both mesial and distal cusplets are distally directed (whereas they are diverging on the teeth described above). SC 2013.28.62 is highly ablated but notable for the apparent lack of lateral cusplets and the uniformly convex labial apron. Remarks These Catahoula Formation teeth are like those of fossil species assigned to Chiloscyllium and to extant Stegostoma Müller & Henle, 1837, and species of both genera have smooth crown enameloid and typically a pair of large lateral cusplets. However, one conspicuous difference between the genera is the shape of the teeth, which in extant Stegostoma are all symmetrical, from anterior to posterior files (Herman et al. 1992; Cappetta 2012). In contrast, teeth of Chiloscyllium exhibit monognathic heterodonty, with the main cusp becoming lower and more distally inclined towards the commissure (Herman et al. 1992; Noubhani & Cappetta 1997; Cappetta 2012). Adnet et al. (2020) recently erected a new tooth-based Eocene species of Stegostoma that seemingly contradicts the near homodonty observed in extant representatives. The teeth of their fossil species are comparable to those of Chiloscyllium, particularly to Eocene C. meraense Noubhani & Cappetta, 1997, but Adnet et al. (2020) apparently separated the two genera by the presence or absence of lateral cusplets. In the Eocene Stegostoma species, lateral cusplets are present even in posterior positions, whereas they are reduced or absent in distally located teeth of Chiloscyllium (i.e., Noubhani & Cappetta 1997). Treating this particular feature as taxonomically significant, the Catahoula Formation specimens are referred to Chiloscyllium due to the combination of features that include a single pair of lateral cusplets on anterior and lateral teeth, asymmetrical lateral and posterior teeth, and reduced to absent lateral cusplets on distal lateral and posterior teeth. The teeth we identify as Chiloscyllium sp. bear similarities to those of Hemiscyllium, but they differ from the latter by their larger overall size and significantly larger lateral cusplets, particularly on anterior teeth (Herman et al. 1992; Cappetta 2012; Adolfssen & Ward 2013; Engelbrecht et al. 2017). The Catahoula Formation teeth also differ from the superficially similar Eocene taxon Notorhamphoscyllium Engelbrecht et al., 2017 by having conspicuous mesial and distal cutting edges on the main cusp. The Chiloscyllium sp. teeth clearly differ from the Brachaeluridae gen. et sp. indet. tooth in our sample (SC 2013.28.54) by their shorter cusp, roughly equal crown height and width dimensions, broader (typically bifid) labial apron, and lateral cusplets occurring much higher on the crown. Teeth of Catahoula Formation Nebrius sp. are generally wider than high and have ten or more sets of lateral cusplets that decrease in size distally. Although Chiloscyllium has previously been reported from the Cretaceous Gulf Coastal Plain in Alabama (Nicholls & Russell 1990; Ciampaglio et al. 2013; Ikejiri et al. 2013; Ebersole et al. 2024b) and Mississippi (Cicimurri et al. 2014), to our knowledge the Catahoula Formation specimens represent the first North American Oligocene record of this genus.