Galeorhinus aff. g. duchaussoisi de Blainville 1816

Galeorhinus aff. G. duchaussoisi Adnet & Cappetta, 2008

Fig. 23 A–F, K–M

Galeorhinus duchaussoisi Adnet & Cappetta, 2008: 435, fig. 2.

cf. Galeorhinus sp. – Clayton et al. 2013: fig. 3k.

Galeorhinus duchaussoisi – Cappetta & Case 2016: 60, pl. 9, figs 7–8.

Material examined

UNITED STATES OF AMERICA – Alabama • 10 isolated teeth; Claiborne Group; ALMNH PV1989.4.219d (3 specimens), MSC 35756.1 – 2, MSC 37353.4, MSC 37691, SC 2012.47.56 (3 specimens).

Description

Teeth mesiodistally wider than tall, with elongated mesial cutting edge that is sinuous in anterolateral jaw positions but slightly convex in lateral files. Distal cutting edge short, vertical or inclined, forms triangular cusp with mesial edge. Cusp distally inclined in all tooth positions; apex on some teeth upturned. Distinct distal heel bears one to six triangular distal cusplets. Cusplets decrease in size laterally; largest, most mesial, cusplet well separated from main cusp. Distal cusplets often less conspicuous than the others, often forming an irregular cutting edge. Irregular serrations occur on lower half of mesial cutting edge of anterolateral teeth but absent on lateral files. Labial and lingual crown faces generally smooth, but faint labial folds observed on some teeth. Labial crown face overhangs the root with a pronounced bulge. Roots low with very divergent lobes. Lingual root face distinctly flat, with deep and wide nutritive groove.

Remarks

Five species of Paleogene Galeorhinus have been reported in the literature, including G. duchaussoisi Adnet & Cappetta, 2008; G. louisi Adnet & Cappetta, 2008; G. mesetaensis Noubhani & Cappetta, 1997; G. minutissimus (Arambourg, 1935); and G. ypresiensis (Casier, 1946).Averianov & Udovichenko (1993) erected Galeorhinus tenius based on specimens from the Eocene of Uzbekistan, but this species has not been formally described or figured so it is considered as a nomen nudum. Cappetta & Case (2016) were the first to report the occurrence of G. duchaussoisi in the Eocene of Alabama, and most of the specimens in our sample appear to fit the type description for this taxon in that they range between 5.0 to 7.0 mm in greatest width and have up to six pairs of lateral cusplets. However, one specimen in our sample, MSC 37353.4 (Fig. 23 K–M), measures only 2.2 mm in width and has only two distal cusplets. This tooth lacks any labial ornamentation, separating it from G. louisi and G. mesetaensis, and also lacks any denticulations on its mesial edge, separating it from G. ypresiensis. Although this tooth is complete, assigning it to either G. duchaussoisi or G. minutissimus has proven to be problematic.

MSC 37353.4 appears to correspond well to the G. minutissimus specimens described and figured by Noubhani & Cappetta (1997: 81, pl. 43, figs 2–14) as it falls within the size range they provided for this species (1.87 to 3.79 mm in greatest width) and, as claimed by the authors, the teeth of this taxon never have more than three distal cusplets. These characteristics, however, contrast with the type description by Arambourg (1935) for G. minutissimus in which he described the teeth as not exceeding 5.0 mm in width and having up to five or six distal cusplets. Arambourg (1952: 155–157, pl. 24, figs 29–37) later provided a more complete description of G. minutissimus and included additional figures. The two aforementioned characteristics were not only reiterated by Arambourg (1952), but are clearly visible on several of the teeth he figured in both 1935 and 1952. Aside from specimen MSC 37353.4, the remaining teeth in our sample exceed 5.0 mm in width, the maximum size for G. minutissimus per Arambourg (1935, 1952).

To further differentiate these species, Adnet & Cappetta (2008) noted that the main cusp on the teeth of G. minutissimus are more slender than those on G. duchaussoisi. However, aside from size, the characteristics provided by Adnet & Cappetta (2008) to separate G. duchaussoisi from G. minutissimus appear ambiguous when dealing with specimens smaller than 5.0 mm in width. In a comparison of the type suites and descriptions for both taxa, the number of distal cusplets can vary from between one and six for both taxa depending on tooth position (see Arambourg 1935: pl. 10, figs 13–15, 1952: pl. 24, figs 29–37; Adnet & Cappetta 2008: fig. 2). Furthermore, their size ranges overlap from 2.0 to 5.0 mm for G. minutissimus and 2.8 to 6.7 for G. duchaussoisi, again depending on tooth position. These characteristics, however, could be a result of ontogenetic heterodonty, as it has been reported that within Recent Galeorhinus specimens, the number of distal cusplets on the teeth increases as the teeth become larger and more robust with age (Compagno 1988). Such changes could be the result ontogenetic dietary shifts, which have been well documented in extant populations of Galeorhinus galeus (Linneaus, 1758) (see Lucifora et al. 2006; Ebert & Stehmann 2013). As a result, there is a distinct possibility that the teeth of G. minutissimus represent the juvenile form of G. duchaussoisi, thus compounding the difficulty in differentiating smaller specimens. Furthermore, both species have been reported from Ypresian and Lutetian deposits (see Noubhani & Cappetta 1997; Adnet & Cappetta 2008; Cappetta & Case 2016), indicating that they have stratigraphic as well as morphological overlap. Regarding the specimens in our sample, teeth with the morphology described above are provisionally assigned to Galeorhinus aff. G. duchaussoisi because a majority are wider than 5.0 mm, exceeding the maximum size of G. minutissimus as reported by Arambourg (1935, 1952).

White (1956) and Thurmond & Jones (1981) reported the occurrence of Galeorhinus recticonus claibornensis White, 1956 from the Gosport Sand from Clarke and Monroe counties in Alabama. This taxon was later placed within the genus Abdounia by Cappetta (1980a), and we refer this species to a new genus described in detailed below. White (1956) and Thurmond & Jones (1981) also reported the occurrence of Galeorhinus cf. falconeri from the Tallahatta Formation and Gosport Sand in Monroe County and the Jackson Group strata in Clarke County. Although neither White (1956) nor Thurmond & Jones (1981) figured their specimens, G. falconeri was subsequently referred to Physogaleus by Adnet & Cappetta (2008) (see Physogaleus secundus below).

Stratigraphic and geographic range in Alabama

The specimens in our sample were collected from the lower Tallahatta Formation at site ADl-1, the Tallahatta Formation at site AMo-8, the basal Lisbon Formation at site ACov-11, and the Gosport Sand at site ACh-21. Upper Ypresian to middle Bartonian, zones NP14 to NP17.