Argentina antarctica Schwarzhans & Mors & Engelbrecht & Reguero & Kriwet 2017, sp. nov.

Argentina antarctica sp. nov.

(Figs 3A—C, 6A)

Holotype. NRM-PZ P.15964 (Figs 3A—C, 6A) (only specimen).

Occurrence. Telm 5 unit; ‘ Natica horizon’, Cucullea I member, La Meseta Formation, late Ypresian, early Eocene. Site IAA 2/95, Seymour Island, Antarctica.

Etymology. Named after its occurrence in the Eocene of Antarctica.

Diagnosis. Dorsal rim anteriorly depressed, without excisura; posteriorly broadly expanded. Ventral rim deep and regularly curved. All rims intensely crenulated. Cauda straight, terminating close to posterior rim of otolith, not connected through postcaudal depression.

Description. A single, thin, large otolith of 5.6 mm in length, mostly well preserved except for the anterior tip of the rostrum. OL:OH = 1.6; OH:OT = 4.3. Dorsal rim anteriorly depressed, nearly straight, inclined, without excisura at upper rim of ostial opening; posteriorly much expanded with broadly rounded postdorsal angle and faint denticle at middorsal position. Ventral rim deeply and regularly curved, deepest at about the middle. Rostrum triangular in shape, moderately long and thin, its ultimate tip broken off. Posterior rim irregularly rounded, dorsally shifted. All rims intensely crenulated, dorsal rim more coarsely than ventral rim, the latter particularly narrowly crenulated anteriorly at rostrum.

Inner face almost flat, with narrow, slightly supramedian, moderately deep sulcus. Ostium anteriorly damaged, originally probably half as long as cauda, somewhat widened; cauda long, narrow, moderately deep, slightly widened at its central section and slightly inclined, terminating close to posterior rim of otolith but not connected via postcaudal depression. Dorsal depression wide; dorsal field with short radial furrows from crenulation of rims; ventral field smooth, without ventral furrow but several short radial furrows particularly along ventral margin of rostrum. Outer face flat with many radial furrows on its ventral half.

Remarks. Argentinid otoliths are common in the temperate to cool realms of the Paleocene and early Eocene of the North Atlantic. Four species have been recorded from the Paleocene of Denmark (Schwarzhans 2003), two from the late Paleocene to early Eocene of England (Stinton 1965, 1966) (see revision in Nolf 2013), one from a similar time interval of Ellesmere Island (Schwarzhans 1986) and one from the Paleocene of West Greenland (Schwarzhans 2004). The four species involved are:

Argentina tricrenulata (Stinton, 1965). The name A. tricrenulata has gained priority over A. erratica (Roedel, 1930) sensu Schwarzhans (2003) after Nolf (2013) assigned a lectotype representing a different species (see below). Other synonymies listed in Schwarzhans (2003) for A. erratica remain with A. tricrenulata. Argentina tricrenulata was widespread in the Northern Hemisphere, known from Denmark, England, West Greenland and Ellesmere Island and it resembles A. antarctica closest of all fossil argentinid otoliths. Argentina antarctica differs from A. tricrenulata in being more elongate (OL:OH = 1.6 vs. 1.4—1.5 in specimens of comparable size), showing a more rounded postdorsal region (vs. sharp postdorsal angle) and intense crenulation of the rims.

Argentina longirostris Schwarzhans, 2003 is clearly more elongate than A. antarctica (OL:OH = 1.75—1.95 in specimens larger than 2 mm in length, vs. 1.6) and shows a caudal tip nearly touching the posterior rim of the otolith and connected to it via a postcaudal depression.

Protargentinolithus extenuatus (Stinton, 1966). The holotype refigured by Nolf (2013) shows that Protargentinolithus procerus Schwarzhans, 2003 is a junior synonym of Argentina extenuata. Protargentinolithus otoliths grow to very large sizes for an argentinid, up to 8 mm in length, and are characterized by a short, robust rostrum and a short ostium.

Protargentinolithus erraticus (Rodel, 1930). Following the selection of a well-preserved lectotype by Nolf (2013), this name replaces P. balticus (Rodel, 1930), which is based on a rather poorly preserved specimen. In addition to the short, massive rostrum, P. erraticus is further characterized by a low, regularly rounded postdorsal rim.

Nowadays, argentinid species are primarily Northern Hemisphere fishes from temperate to tropical realms, but there are also a few southern temperate species found around Chile, Australia and New Zealand. When Schwarzhans (1980) described the first fossil argentinid record from the early Miocene of New Zealand (Argentina subfrigida Schwarzhans, 1980) he assumed that this species would represent an invader species from northern temperate seas. The record of A. antarctica, however, shows that the presence of argentinid fishes in the southern temperate seas dates back much earlier into Eocene times, indicating that argentinids may have had a bipolar temperate distribution pattern during the Palaeogene. They are lacking from the warmer shallow water deposits of the European Palaeogene.