Gymnosarda, Gill, 1862

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 : 177-178

Gymnosarda sp.

Fig. 65

Material examined

UNITED STATES OF AMERICA – Alabama • 1 isolated tooth; Claiborne Group ; MSC 37271 .

Description

Hypural plate roughly triangular in outline, consisting of fused hypurals, parahypural, and urostyle. Dorsal and ventral edges flat, straight, of equal length, wedge-shaped in dorsal and ventral views. Lateral faces flat; hypural sutures worn but visible; posterior margin slightly concave; basal notch nearly closed. Uroneural projecting dorsally from urostyle; face of urostyle ovoid, concave.

Remarks

A diagnostic characteristic of the members of the Scombridae is the fusion of the hypurals and urostyle into a unique hypural plate ( Johnson 1986; Monsch 2005). The hypural plate identified in our sample (MSC 37271; Fig. 65 ) also possesses a fused parahypural and has a basal notch that is nearly closed. According to Monsch (2000, 2005), Acanthocybium and the members of the Scomberomorinae have hypural plates with a basal notch that is large and distinct. However, a caudal notch is completely absent on the hypural plates of the members of the Tribe Thunnini Starks, 1910 , and nearly absent on those within the Sardini Collette & Chao, 1975 . The presence of a reduced notch and fused parhypural on MSC 37271 are characteristic of a single member of Sardini , Gymnosarda Gill, 1862 ( Monsch 2000, 2005). MSC 37271 is very similar to the hypural plate described on the extant Gymnosarda unicolor (Rüppell, 1838) and also very similar to the hypural of the only Eocene member of this genus, Gymnosarda prisca Monsch, 2000 (see Monsch 2000: fig. 1c, 2005: fig. 9c). However, the hypural plate of G. prisca has a clear caudal notch, a purported plesiomorphic character within this genus. Monsch (2000) described another Eocene Gymnosarda sp. hypural plate that, although damaged along the posterior edge, exhibits a basal notch that appears nearly closed (see fig. 10) and is more elongated dorsoventrally than MSC 37271, suggesting the plates represent two different species. Although MSC 37271 likely represents an undescribed species within Gymnosarda , we refrain from erecting one because our sample consist of only a single specimen.

Stratigraphic and geographic range in Alabama

The lone specimen in our sample was collected from the basal Lisbon Formation at site ACov-11. Lower Lutetian, Zone NP15.

Figures

Fig. 65. Gymnosarda sp., hypural plate. A–C. MSC 37271, basal Lisbon Formation. A. Right lateral view. B. Anterior view. C. Dorsal view. Scale bar = 1 cm.