Allomorone undetermined

Allomorone sp. Fig. 26A–B Material examined UNITED STATES OF AMERICA – Mississippi • 1 sagitta; Catahoula Formation; MMNS VP-12060. Description MMNS VP-12060 has an elliptic outline (sensu Smale et al. 1995) with a rather prominent, somewhat blunt rostrum with excisura. The inner face is broadly and evenly convex with a conspicuous sulcus (heterosulcoid type). The sulcus extends approximately 80% of the length of the inner face (Fig. 26A). The ostium is relatively narrow and subquadrate in shape. The height of the ostium is only slightly greater than the height of the cauda, but the length of the ostium is significantly shorter than the cauda. The narrow cauda is approximately 2.5 times as long as the ostium. The dorsal and ventral margins of the cauda are horizontal and essentially parallel. The posterior portion of the cauda is bent downward. A ridge-like crista superior occurs above the cauda and a linear, depressed area is located above the crista superior. There may have been a ventral furrow, but this region is not well preserved on the specimen in our sample. The outer face is concave and has a slightly irregular surface (Fig. 26B). Remarks The Catahoula Formation Allomorone sp. sagitta appears to be very closely related to what was previously reported as Orthopristis americana (Koken, 1888) from the Gulf Coast upper Eocene (Jackson Group), a taxon that was first reported by Koken (1888) as Otolithus (Carangidarum) americana. However, Schwarzhans et al. (2024) considered Allomorone different from Orthopristis because of the much shorter, downward section of the cauda that terminates away from the post-ventral margin on the sagitta of Allomorone. Unfortunately, the somewhat eroded specimen available to us is not sufficient to make a specific determination. Allomorone sp. otoliths are much more common in the Gulf Coastal Plain compared to the Atlantic Coastal Plain, as Müller (1999) only reported three specimens from the Eocene (as “Genus aff. Orthopristis sp. ”) in a sample of over 12 000 fossil otoliths collected from middle Eocene to Pliocene deposits in the Atlantic Coastal Plain. In contrast, Allomorone americana was more abundant in the upper Eocene (primarily Priabonian) Yazoo Clay otolith assemblage from Copenhagen, Louisiana, representing 4.35% of the total specimens in a sample (Nolf & Stringer 2003; reported as Orthopristis americana). Green (2002) reported a significant occurrence of A. americana (18.17% of the total number of otolith specimens) in the middle Eocene (Bartonian) Moodys Branch Formation at the Heison Landing locality along the Ouachita River in Caldwell Parish, Louisiana (shown as O. americana). Ebersole et al. (2021) did not report O. americana from the Rupelian Glendon Limestone Member of the Byram Formation in Alabama, and it has not been reported from younger formations in the Gulf and Atlantic coastal plains (Stringer & Bell 2018; Stringer & Shannon 2019; Stringer & Hulbert 2020; Stringer & Starnes 2020; Stringer et al. 2020a, 2022b).