Macrorhizodus praecursor (Leriche, 1905)

Fig. 20

Otodus lawleyi Bassani, 1877: 80, pl. 11, figs 3–5.

Oxyrhina desori praecursor Leriche, 1905: 128.

Oxyrhina praecursor americana Leriche, 1942: 45, pl. 3, figs 6–13.

Isurus desori praecursor – White 1931: 47.

Macrorhizodus praecursor – Zharkov et al. 1976: 132.

Isurus praecursor americana – Thurmond & Jones 1981: 55, fig. 21.

Isurus oxyrhincus – Case 1980: 82, 99, pl. 2, figs 4–8.

Cosmopolitodus praecursor – Mustafa & Zalmout 2002: 82, pl. 1, figs 7–11.

Material examined

UNITED STATES OF AMERICA – Alabama • 20 isolated teeth; Claiborne Group; ALMNH PV1989.4.32.3, ALMNH PV 1989.4.15 (2 specimens), ALMNH PV1989.4.161.4 (2 specimens), ALMNH PV1989.4.6.1.1, ALMNH PV1989.4.97.2, ALMNH PV2013.4.56, ALMNH PV2016.3.142,

GSA-V696, GSA-V709 (3 specimens), MSC 2372.25, MSC 2374.1, MSC 2386.1–2, MSC 34585, MSC 35759, MSC 37174, MSC 37500.

Description

All teeth with large triangular crown and lack lateral cusplets. Labial crown face flat; lingual crown face moderately to strongly convex; both crown faces smooth. Main cusp on lower anterior teeth erect and triangular; mesial and distal cutting edges bi-convex. Roots robust with foramina located on large lingual boss (indistinct nutritive groove sometimes observed). Roots on lower anterior teeth slightly higher than the crown. Lower anterior teeth sigmoidal and lingual crown face strongly convex. Lower anterior teeth with robust root protuberance and rounded root lobes; interlobe area deep and U-shaped. Anterolateral teeth with tall triangular crown; height of the crown exceeds the height of the root. Crown on anterolateral teeth have a slight distal inclination and bi-convex mesial and distal cutting edges. Lingual crown face less convex than on anterior files and have a shallower interlobe area. Root lobes on anterolateral teeth range from rounded to angular. Mesial and distal cutting edges on upper lateral teeth distinctly concave. Crown is shorter than those on anterior or anterolateral files, and more triangular. Lateral crown edges slope to the lateral edges of root. Root lobes short, distinctly angular, and flattened basally. Have shallow V-shaped interlobe area.

Remarks

Leriche (1905) erected Oxyrhina desori praecursor for what he believed were Eocene teeth belonging to the stratigraphically younger O. desori Agassiz, 1843. Based on specimens from Priabonian deposits in Choctaw County, Alabama, Leriche (1942) later erected Oxyrhina praecursor americana for teeth that he thought were similar to the praecursor morphology, but differed by having a higher root, narrower crown, and more divergent root lobes. These morphologies were later placed within Isurus by White (1931), then referred by Glikman (1964) to a new genus, Macrorhizodus. Subsequent authors later recognized both the praecursor and americana morphologies as distinct species (see White 1956; Ward & Wiest 1990; Zhelezko & Kozlov 1999; Case & Borodin 2000a).

We reexamined the syntypes of O. praecursor americana of Leriche (1942: 45, pl. 3, figs 6–13) to determine the validity of this subspecies. A comparison of these eight syntypes (USNM 366462 to USNM 366469) to teeth within recent jaw sets of the extant Isurus oxyrinchus (Rafinesque, 1810) suggests to us that Leriche’s (1942) differential characteristics (higher root, narrower crown, and more divergent root lobes) can all be attributed to heterodonty (dignathic, monognathic, and ontogenetic) and intraspecific variation. Thus, it is our opinion that the americana morphology is a junior synonym of Macrorhizodus praecursor, and we dispense with the subspecies name and refer all the specimens in our sample to M. praecursor. The specimens in our sample differ from those of Macrorhizodus nolfi Zhelezko & Kozlov, 1999 by having smooth enameloid shoulders and by lacking vestigial cusplets (Carlsen & Cuny 2014). Finally, although Glickman (1964) placed M. praecursor within its own family, the Lamiostomatidae, we follow Nelson et al. (2016) in placing this taxon within the Lamnidae.

Stratigraphic and geographic range in Alabama

The specimens in our sample were collected from the lower Tallahatta Formation at site ADl-1, the contact of the Tallahatta and Lisbon formations at site ACov-11, the “upper” Lisbon Formation at site ACl-3, the contact of the Lisbon Formation and Gosport Sand at site AMo-4, and the Gosport Sand at sites ACl-15 and ACh-21. Upper Ypresian to middle Bartonian, zones NP14 to NP17.