Aplodinotus gemma ( Koken, 1888 )

Aplodinotus gemma ( Koken, 1888)

Fig. 26N–S View Fig

Otolithus (Sciandarum) gemma Koken, 1888: 281 .

Material examined

UNITED STATES OF AMERICA – Mississippi • 46 sagittae; Catahoula Formation ; MMNS VP-7447 , MMNS VP-7452 , MMNS VP-7456.2 , MMNS VP-7458.3 , MMNS VP-7459.3 , MMNS VP-8201.1 , MMNS VP-8430 (2 specimens), MMNS VP-8430.1 ( Fig. 26P–Q View Fig ), MMNS VP-8430.2 ( Fig. 26R–S View Fig ), MMNS VP-8712.2 ( Fig. 26N–O View Fig ), SC 2013.28.771 , SC 2013.28.779 , GLS otolith comparative collection (33 specimens).

Description

The sagitta outline is somewhat square to discoidal (sensu Smale et al. 1995). Larger specimens (greater than 5 mm) have a greater dorso-ventral height, and a more discoidal shape compared to smaller ones. The margins are generally smooth. The inner face is generally strongly convex and characterized by a very large, prominent heterosulcoid-type sulcus. The ostium extends from near the antero-dorsal margin well down into the ventral field and is largest at the posterior portion. The ventral margin of the ostium curves only slightly upward toward the anterior margin. The posteroventral portion of the ostium extends underneath the cauda. Generally, there is a very short distance between the posterior of the ostium and the downturned portion of the cauda. The height of the cauda constitutes approximately 25% of the height of the ostium, and the cauda has a horizontal and downturned component. The downturned portion of the cauda tends to be slightly curved. The outer face is only slightly convex, and that of larger specimens is often more irregular than on smaller specimens.

Remarks

Koken (1888) mentioned this species from the “Vicksburg; Red Bluff; Jackson River, Mississippi ” and “Jackson and Vicksburg Beds,” but the exact stratigraphic occurrence(s) for his specimens cannot be ascertained. However, Aplodinotus gemma is known from Oligocene formations (Mint Spring and Byram formations) in the vicinities of Vicksburg and Jackson in Mississippi ( Nolf 2003, 2013; Stringer et al. 2020c). Specimens of A. gemma are also known from the Rosefield Marl Beds of the Rosefield Formation (Oligocene, Rupelian) in Catahoula Parish, Louisiana ( Stringer & Worley 2003; Worley 2004) and from the Glendon Limestone Member of the Byram Formation of Alabama ( Ebersole et al. 2021). The 46 A. gemma specimens from the Catahoula Formation constitute 11.2% of the total number of otoliths in the sample. This abundance is only exceeded by Sciaena? pseudoradians (see below).

Schwarzhans (1993) proposed the otolith-based genus Frizzelithus to accomodate A. gemma . However, this designation was based on only 20 large specimens, some of which were eroded. The present study has the advantage of including hundreds of sciaenid otoliths from the Oligocene of Alabama, Mississippi, and Louisiana, as well as an ontogenetic series. Whereas fig. 144 in Schwarzhans (1993) represents Aplodinotus gemma , figs 145–146 are significantly different in the shape of the ostium and the salient inframedian posterior tip, and are attributed to Sciaena? radians ( Nolf 2003, 2013). The otolith in fig. 147 is much more elongate, has a greater distance between the ostium and downturned portion of the cauda, and the shape of the ostium differs greatly from that in fig. 144. The former matches Sciaena ? aff. pseudoradians from the upper Eocene and Sciaena? pseudoradians from the Oligocene of the Gulf Coast. Several of the sciaenids of the Oligocene appear to be closely related to the extant Aplodinotus grunniens Rafinesque, 1819 ( Nolf 2003, 2013; Ebersole et al. 2021; Fuelling et al. 2022), which is extremely common in eastern North America, and the Catahoula specimens are placed in this genus.