Anisotremus rambo, Lin & Nolf & Steurbaut, 2022

Lin, Chien-Hsiang & Nolf, Dirk, 2022, Middle and late Eocene fish otoliths from the eastern and southern USA, European Journal of Taxonomy 814, pp. 1-122 : 82-84

Anisotremus rambo sp. nov.

urn:lsid:zoobank.org:act:AC553D36-02F2-4A77-9408-62895B70C194

Fig. 30

Anisotremus sp. – Nolf & Stringer 2003: 6, pl. 5 fig. 13. — Nolf 2013: 101, pl. 251.

Anisotremus ? sp. – Ebersole et al. 2019: 211, fig. 71k–l.

Diagnosis

OL/OH = 1.04–1.35, OsL/CaL = 0.67–1.45. Massive otoliths with strongly convex inner face. Large and oblong ostium filled with colliculum. Elongate and narrow cauda of about 1/2 OL.

Etymology

Named after the famous movie ‘Rambo’, which alludes to the unrestrained, wild, but valiant appearance of the otoliths.

Material examined

Holotype UNITED STATES OF AMERICA • Left otolith; Texas, Alabama Ferry; “ Stone City beds”; Fig. 30A ; IRSNB P 10192.

Paratypes UNITED STATES OF AMERICA • 1 otolith: Fig. 30B ; same collection data as for holotype; IRSNB P 10193 • 3 otoliths, of which one is figured: Fig. 30C ; Texas, Rocky Branch; “ Stone City beds”; IRSNB P 10094 • 1 otolith, Fig. 30D ; Texas, Little Brazos River, confluence with Brazos River; Wheelock Member ; IRSNB P 10095 • 6 otoliths, of which two are figured: Fig. 30E–F ; Texas, Stone City Bluff; Brazos River , sample 2 from bed P; “ Stone City beds”; IRSNB P 10196–10197 .

Type locality and horizon

United States of America, Alabama Ferry (Texas), “Stone City beds”.

Dimensions of the holotype

Length = 15.11 mm; height = 11.40 mm; thickness = 3.90 mm.

Description

The outline of the otoliths of this species changes ontogenetically; from circular in smaller specimens to more oblong and triangular in large individuals, which results from their deeper posterior part and longer rostrum. The postero-dorsal angle becomes prominent in large individuals, but the extension on the postero-ventral part of the otolith already exists in the small specimens. The margins can be considered smooth, but are more irregular at the dorsal rim. The otoliths are rather thin, with their inner face strongly convex and outer face strongly concave. The sulcus is well-developed. The ostium is about as long as the cauda in large specimens but it is shorter in smaller ones. The ostium opens on the anterior rim and is completely filled with a large colliculum. The junction of the ostial and caudal crista inferior is located slightly more caudally with respect to the same junction in the crista superior. In large specimens the cauda is straight for about two-thirds of its length and its posterior part is markedly curved in a ventral direction, but in smaller specimens the cauda widens at the end and is only slightly bent ventrally. The cristae are well-developed, but not ridge-like; the ostial crista superior curves markedly towards the dorsal rim in small specimens. The dorsal area is wide and about of the same height as the ventral one.

Remarks

Anisotremus rambo sp. nov. was first reported from the Yazoo Clay of Louisiana by Nolf & Stringer (2003), who then placed the species in the recent genus Anisotremus (see iconography of comparative Recent material therein). The large specimen from the Yazoo Clay further demonstrates the drastic ontogenetic change within the species. Small individuals are rounder in shape and have a shorter downward directed portion of the cauda, whereas larger ones become more elongate both in the ostium and the overall shape of the otolith, and have a longer and more strongly downward inclined portion of the cauda. The sulcus configuration, however, is more or less conservative and not changing ontogenetically. The transition can be observed from the medium-sized specimens ( Fig. 30D, F ). Ebersole et al. (2019) recently reported this species from the “upper” Lisbon Formation of Alabama, expanding its geographical distribution.

Stratigraphic and geographic distribution

Lutetian: “Stone City beds”, Texas. Bartonian: Wheelock Member, Texas; “upper” Lisbon Formation, Alabama. Priabonian: Yazoo Clay, Louisiana.

Figures

Fig. 30. Otoliths ofAnisotremus rambo sp. nov. from the US middle and upper Eocene.A–B. “Stone City beds”, Alabama Ferry, on Trinity River, North, Texas (IRSNB P 10192–10193). C. “Stone City beds”, Rocky Branch, 1.5 miles SW of Stone City Bluff, sample 2, Texas (IRSNB P 10194). D. Wheelock M., Little Brazos River, confluence with Brazos River, Texas (IRSNB P 10195). E–F. “Stone City beds”, Stone City Bluff, Brazos River, sample 2 from bed P, Texas (IRSNB P 10196–10197). A. Holotype (IRSNB P 10192). B–F. Paratypes (IRSNB P 10193–10197). 1 = ventral view; 2 = inner view; 3 = anterior view. Scale bars = 1 mm.