Otodus (Otodus), 1843

Ebersole, Jun A., Cicimurri, David J. & Stringer, Gary L., 2019, Taxonomy and biostratigraphy of the elasmobranchs and bony fishes (Chondrichthyes and Osteichthyes) of the lower-to-middle Eocene (Ypresian to Bartonian) Claiborne Group in Alabama, USA, including an analysis of otoliths, European Journal of Taxonomy 585, pp. 1-274 : 26-28

Otodus (Otodus) sp.

Fig. 9 A–L

Otodus obliquus – Maisch et al. 2014: 189 , fig. 3:1–2.

Cretolamna sp. – Cappetta & Case 2016: 55, pl. 6, figs 1–3.

Material examined

UNITED STATES OF AMERICA – Alabama • 9 isolated teeth; Claiborne Group; ANSP 23413, MSC 2998.3, MSC 2999.2, MSC 3008.1, MSC 35761.1–2, MSC 38528.1–3.

Description

Our sample includes anterior, lateral, and posterolateral teeth. All teeth have a triangular, unserrated main cusp and single pair of triangular, divergent, lateral cusplets. Labial and lingual crown faces smooth. Labial crown face flat; lingual face convex. Tooth root holaulacorhizous. Root lobes range from rounded to angular and thin basally. One or more nutritive foramina present on lingual root face. Crown taller and more erect in anterior files, but lower, broader, and distally inclined on lateral teeth. Root bilobate, with narrow, elongate, diverging lobes on anterior teeth. Lateral teeth with short, broad, sub-rectangular lobes. Interlobe area U-shaped, deep in anterior files but shallow on lateral teeth. In basal view, anterior teeth have a prominent, box-like, well delineated, lingual boss.

Remarks

Paleogene specimens from Alabama with a similar morphology to the teeth in our sample have traditionally been referred to the genera Otodus or Cretalamna (see Ehret & Ebersole 2014; Maisch et al. 2014; Cappetta & Case 2016). For many years authors have recognized the morphological similarity of Cretalamna and Otodus teeth, lending strong support to the idea that the two genera are part of a single lineage where Cretalamna gives rise to the Otodus group. It is readily accepted that the transition from Cretalamna to Otodus occurred in the Paleocene ( Siverson 1992; Zhelezko 2000; Ward 2010; Siversson et al. 2015;), but it is unclear as to how late in the Paleogene members of the genus Cretalamna persisted. Cretalamna specimens have been reported in Ypresian deposits elsewhere ( Cappetta 2012) and deposits as young as the Lutetian in Alabama ( Cappetta & Case 2016), and often these teeth have been referred to the Late Cretaceous taxon Cretalamna appendiculata (Agassiz, 1843) (see Noubhani & Cappetta 1997; Adnet 2006; Carlsen & Cuny 2014; Vasquez & Pimiento 2014). This later species, however, has become a ‘waste-basket’ taxon, and recent studies have shown that C. appendiculata sensu stricto is, at present, stratigraphically and geographically restricted to the Turonian of England ( Siverson 1999; Siversson et al. 2015). A redescription of the genus by Siversson et al. (2015) has shown a good degree of morphological variability among the teeth of the various Late Cretaceous Cretalamna species, making it difficult to determine what characteristics can be utilized to separate them from those of the closely related Otodus . This lack of disparity can be seen among the unserrated Paleocene and Eocene otodontid teeth recovered in Alabama, as it remains uncertain whether they should be assigned to Otodus or Cretalamna . Although much more clarity is needed to resolve the taxonomy of these unserrated Paleogene otodontids, it is our opinion that the Claiborne specimens are likely best placed within Otodus (Otodus) because it seems more parsimonious to extend the Paleocene-derived Otodus (Otodus) lineage into the middle Eocene, as opposed to further extending the range of the Late Cretaceous-derived Cretalamna sensu stricto. Furthermore, we have observed that, towards the commissure, the teeth of several fossil lamniform sharks become broader, low-crowned, more recurved, and with wider lateral cusplets than teeth in more anterior positions. Therefore, Eocene specimens identified as Cretalamna could represent more lateral/ posterior teeth of Otodus (Otodus) .

Two species of Otodus have previously been described from Paleogene deposits in Alabama, Otodus mediavius ( Leriche, 1942) and O. obliquus Agassiz, 1843 ( Ehret & Ebersole 2014). The taxonomic validity of O. mediavius , however, is in question because Leriche (1942) did not designate a holotype, and his syntypes included a range of morphologies from both the Danian Midway Group of Wilcox County, AL, and the Thanetian Cannonball Formation of North Dakota, USA. Furthermore, Arambourg (1952), Cvancara & Hoganson (1993), and later Müller (1999) considered O. mediavius a junior synonym of O. obliquus , a species originally described from the Ypresian London Clay in England. As part of this study, the specimens in our sample were compared directly to the O. mediavius syntypes at the USNM (USNM 8256, USNM 25957-59, USNM 25961–67, USNM 25970), as well as illustrated O. obliquus specimens from the London Clay (i.e., Agassiz 1843: fig. 208a–e; Casier 1966: pl. 6–7). The teeth in our sample ( Fig. 9 A–L) differ from both of these taxa in being of smaller size (not exceeding 3.0 cm in greatest height for anterior teeth, 1.5 cm for lateral teeth), and by having a main cusp that is not as wide (especially those in the anterior files). Furthermore, the mesial and distal cutting edges on the main cusp on the anterior teeth in our sample ( Fig. 9 A–F) are concave at the base, as opposed to straight or often bi-convex on the teeth of O. mediavius and O. obliquus . Moreover, the lateral teeth in our sample ( Fig. 9 G–L) have a shorter crown and never more than a single pair of lateral cusplets (as opposed the occasional presence of up to two pairs on both O. mediavius and O. obliquus ). Although some of the teeth in our sample may indeed represent a stratigraphically young (upper Ypresian) occurrence of O. obliquus , here we choose to remain conservative and assign them to Otodus (Otodus) sp. as to resolve the taxonomy of middle Eocene Otodus specimens in Alabama would require many more specimens than what is currently available. For the purposes of this report, we utilize the subgenus Otodus (Otodus) to differentiate unserrated Otodus teeth from those with serrations (see below).

Stratigraphic and geographic range in Alabama

All of the specimens in our sample were collected from the Tallahatta Formation at sites ACon-6 and ADl-1. Cappetta & Case (2016) reported additional Otodus teeth from the contact of the Lisbon and Tallahatta formations at site ACov-11. These specimens were likely derived from the Tallahatta Formation (see our discussion of these specimens below). Upper Ypresian to lower Lutetian, zones NP12 to NP14.

Figures

Fig. 9. Otodontidae Glikman, 1964, teeth. A–L. Otodus (Otodus) sp. A–C. MSC 2999.2, lower Tallahatta Formation. A. Labial view. B. Lingual view. C. Mesial view. D–F. MSC 3008.1, lower Tallahatta Formation. D. Labial view. E. Lingual view. F. Mesial view. G–I. MSC 35761.2, lower Tallahatta Formation. G. Labial view. H. Lingual view. I. Mesial view. J–L. MSC 35761.1, lower Tallahatta Formation. J. Labial view. K. Lingual view. L. Mesial view. — M–X. Otodus (Carcharocles) sp. M–O. MSC 37019, basal Lisbon Formation, courtesy of Carl Sloan. M. Labial view. N. Lingual view. O. Distal view. P–R. MSC 37172, basal Lisbon Formation, courtesy of James Lowery. P. Labial view. Q. Lingual view. R. Mesial view. S–U. MSC 2371, Gosport Sand. S. Labial view. T. Lingual view. U. Mesial view. V–X. MSC 2370, Gosport Sand. V. Labial view. W. Lingual view. X. Distal view. Scale bars: A–F, M–X =1 cm; G–L=5 mm.