Brachycarcharias atlasi

Brachycarcharias atlasi (Arambourg, 1952)

Fig. 12

Odontaspis substriata mut. atlasi Arambourg, 1952: 80, pl. 12, text fig. 18.

Odontaspis atlasi – Nolf 1972: 115, pl. 1, figs 4–6.

Carcharias aff. atlasi – Tabuce et al. 2005: 385.

Brachycarcharias atlasi – Cappetta 2012: 193.

Material examined

UNITED STATES OF AMERICA – Alabama • 113 isolated teeth; Claiborne Group; MMNS VP-8220, MMNS VP-8951, MMNS VP-8953 (10 specimens), MSC 37561.1, MSC 37561.16, MSC 37561.25, MSC 37561.30, MSC 37284.19, MSC 37284.20, MSC 37284.23, MSC 37284.24, MSC 37284.33, MSC 37325.12, MSC 37325.13, MSC 37325.25, MSC 37325.27, MSC 37130, MSC 37189.17, MSC 37189.18, MSC 33269, MSC 33310, MSC 33334, MSC 33343, MSC 33373, MSC 33450, MSC 33452, MSC 33495, MSC 33503, MSC 33549, MSC 33567, MSC 33663, MSC 33705, MSC 33882, MSC 33935, MSC 33938, MSC 35626.1, MSC 35627.2, MSC 35627.6, MSC 35628.1, MSC 35629.5, MSC 35629.6, MSC 35632.6, MSC 35632.10, MSC 35633.6, MSC 35635.1 – 2, MSC 35636.6, MSC 35638.4, MSC 35644.3, MSC 35645.1, MSC 35646.6, MSC 35646.8, MSC 35647.5, MSC 35647.9, MSC 35648.3 – 4, MSC 35650.6, MSC 35651.4, MSC 35652.7, MSC 35653.8, MSC 35653.11, MSC 35654.7, MSC 35655.1, MSC 35655.8, MSC 35662.1, MSC 35663.1, MSC 35665.8, MSC 35667.3, MSC 35672.2, MSC 35675.1, MSC 35675.8 – 10, MSC 35676.3, MSC 35676.7, MSC 35677.6, MSC 35678.2, MSC 35678.6, MSC 35741.13, MSC 37199.7, MSC 37077.2, MSC 37077.6, MSC 37650.1, MSC 37685, MSC 37668, SC 2012.47.85, SC 2012.47.158 (2 specimens), SC 2012.47.254, SC 2012.47.255 (2 specimens), SC 2012.47.256 (3 specimens), SC 2012.47.261 (7 specimens), WSU 3, WSU 7, WSU 16.

Description

Teeth small, most measuring less than 1.0 cm in mesiodistal width. Main cusp tall, narrow, triangular. Labial face of main cusp flat to slightly convex; lingual face strongly convex. Main cusp erect in anterior and anterolateral files; with slight distal inclination in lateral files. Coarse vertical ridges occur on lower one-half to two-thirds of lingual face of main cusp of anterior teeth. Ornamentation on lateral teeth less distinct. Anterior teeth generally with single pair of tall, sharply pointed medially curved cusplets; occasionally two pairs present, with second pair incipient. Two pairs lateral cusplets present on most lateral teeth; cusplets wider, lower than on anterior teeth, divergent. All lateral cusplets with slightly convex labial face, strongly convex lingual face; vertical ridges occur on lingual side, particularly on anterior teeth. Cutting edges extend from main cusp apex across lateral cusplets. Root bilobate. Anterior teeth with elongated, strongly divergent lobes having rounded extremities. Lateral tooth root lobes shorter, wider. Prominent lingual root boss bisected by deep nutritive groove. Interlobe area wide, U-shaped.

Remarks

Three species of Brachycarcharias were identified within our Claiborne Group sample including B. atlasi (Arambourg, 1952); B. lerichei (Casier, 1946); and B. twiggsensis (Case, 1981). The various B. atlasi tooth morphologies identified in our sample appear to be conspecific with many of the syntypes figured by Arambourg (1952: pl. 12, text fig. 18) in having tall and narrow cusplets on the anterior teeth, coarse lingual ornamentation on both the main cusp and lateral cusplets and having two pairs of lateral cusplets on the lateral teeth. In addition to these morphological similarities, the stratigraphic range of Arambourg’s (1952) type material overlaps with those in our sample (Ypresian to Bartonian). The B. atlasi lateral teeth from Alabama were differentiated from those of B. lerichei by the combination of having up to two pairs of lateral cusplets and the presence of coarse striations on the lingual face of the main cusp and on the comparatively larger lateral cusplets. The anterior teeth are similar between the two species, but those of B. atlasi have much coarser ornamentation, the lateral cusplets are mesiodistally narrower, more lingually curved, and have a more conical apex. In addition, the tooth root of B. atlasi is often more labiolingually compressed than that of B. lerichei. Brachycarcharias atlasi teeth differ from B. twiggsensis by their smaller overall size, having a main cusp that is mesiodistally narrower, and having lingual crown ornamentation. The lingual striations on the teeth of B. atlasi are reminiscent of Striatolamia macrota teeth, but the species are easily distinguished by the very prominent lateral cusplets on B. atlasi compared to S. macrota (which are minute on anterior teeth), the lateral teeth are more strongly ornamented, and the anterior teeth have a much shorter and less sigmoidal main cusp. Furthermore, the lingual ornamentation is not present on the lateral cusplets of S. macrota. The lateral cusplets also distinguish teeth of B. atlasi from those of Odontaspis winkleri, whose cusplets are taller, more cylindrical, and lack mesial and distal cutting edges extending across the lateral cusplets.

Stratigraphic and geographic range in Alabama

The B. atlasi teeth in our sample were collected from the Meridian Sand Member and lower Tallahatta Formation at site ADl-1, the contact of the Tallahatta and Lisbon formations and the basal Lisbon Formation at site ACov-11, and the basal Gosport Sand at site ACl-4. Upper Ypresian to middle Bartonian, zones NP12 to NP17.