Arnoglossus dispar, Schwarzhans & Klots & Kovalchuk & Dubikovska & Ryabokon & Kovalenko, 2024

Arnoglossus dispar n. sp.

Figure 6 View FIGURE 6 AL-AT zoobank.org/ A809A94B-9A53-402E-AF75-DCFE566BA442

Holotype. NMNHU-P PI 2719 ( Figure 6 View FIGURE 6 AO-AQ), Staryi Zavod , western Ukraine, late Badenian.

Paratypes. 2 specimens, SMF PO 101.354 , Staryi Zavod ; 1 specimen, SMF PO 101.355 , Skala .

Additional material. 6 otoliths: 2 specimens, NMNHU-P PI 2709 , Novyi Pliazh ; 3 specimens, NMNHU-P 2720 , Staryi Zavod ; 1 specimen, Skala.

Etymology. From dispar (Latin) = dissimilar, referring to the large degree of side dimorphism observed in the otoliths of this species.

Diagnosis (left otolith). OL:OH = 1.35–1.45. Dorsal, posterior and ventral rims regularly curved and continuous. Rostrum short, rounded, 5–11% OL. Inner face more convex than outer face. Ostium anteriorly open. OsL:CaL = 1.1–1.3.

Description (left otolith). Small otoliths with a relatively regular oval shape and robust up to a size of 2 mm in length (holotype 1.85 mm). OL:OH = 1.35–1.45; OH:OT = 2.2–2.4. Dorsal, posterior, and ventral rims are regularly curved and continuous. The rostrum is short and rounded, 5–11% OL. Antirostrum and excisura are minute or absent. All the rims are smooth or slightly undulating. The inner face is distinctly convex, with an axially positioned, long, narrow, and deep sulcus. OL:SuL = 1.35–1.45. The ostium is open anteriorly; ostium and cauda are poorly distinguished, the ostium only slightly longer than cauda (OsL:CaL = 1.1– 1.3). Circumsulcal depression is narrow and deep, close to the sulcus and separated from it by a sharp, crest-like cristae. The outer face is flat, smooth.

Side dimorphism. Only a single otolith is available of A. dispar from the right side, and it differs in several aspects from those of the left side. The ratio OL:OH is 1.1 (vs. 1.35–1.45). The dorsal rim shows clear pre- and postdorsal angles; the posterior is more slanting than rounded. The sulcus is differentiated in a shorter ostium and longer cauda by an elevated collum (OsL:CaL = 0.9).

Discussion. The bothid otoliths show that the fishes of the genus Arnoglossus underwent a rapid endemic evolution in the Paratethys, particularly in its eastern part, during Badenian and Sarmatian s.l. (Bratishko et al., 2015, 2023; Schwarzhans et al., 2017c). Arnoglossus dispar differs from A. bassanianus (Kramberger, 1883) , which has been found with otoliths in situ (Schwarzhans et al., 2017c), in the anteriorly open sulcus and the inner face being more convex than the outer face (vs. outer face being more convex than inner face). Both species occurred in parallel during the late Badenian ( A. bassanianus also in the Sarmatian s.s.) whereby A. dispar appears to have been adapted to reefal environments like the Medobory backreef whereas A. bassanianus occurred in clastic sublittoral settings. The later species A. kerichensis Bratishko and Schwarzhans, 2023 and A. scitulus Bratishko and Schwarzhans, 2023 from the Bessarabian of the Eastern Paratethys differ in the longer rostrum, less regularly curved rims and the ostium being distinctly longer than the cauda resulting in a larger ratio OsL:CaL> 1.35, mostly> 1.5 (vs. 0.9–1.3).