Diaphus Eigenmann & Eigenmann 1890

Diaphus ? marambionis sp. nov.

(Figs 3D—I, 6B, C)

Material. Holotype: NRM-PZ P.15966 (Figs 3D—F, 6B, C). Paratypes: two specimens NRM-PZ P.15967 (Fig. 3G—I). Other material: two fragmentary specimens NRM-PZ P.15968.

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 Marambio, the Argentinian research station on Seymour Island.

Diagnosis. OL:OH about 1.3. Ventral rim shallow. Dorsal rim regularly rounded, slightly depressed postdorsally. Rostrum about 10% of OL. OCL: CCL = 1.5. Inner face convex. Dorsal margin of ostium curved. Caudal pseudocolliculum well developed.

Description. One well-preserved and four incompletely preserved or partly encrusted otoliths; moderately thick and up to nearly 6 mm in length (holotype 5.2 mm long). OH:OT = 3.5—4.0. Dorsal rim rather regularly curved, slightly undulating, somewhat depressed postdorsally and regularly inclined predorsally, sometimes with slight postdorsal angle. Ventral rim shallow, slightly undulating, with (eroded) denticles or protuberances indicated by up to eight grooves seen in ventral view (Fig. 3E, G). Rostrum slightly projecting, blunt, about 10% of OL. Posterior rim rounded, with its tip above caudal tip.

Inner face markedly convex, with moderately wide, median, shallow sulcus. Ostium about 1.5 times the length of cauda and equally wide, its dorsal margin slightly curved; cauda slightly bent upwards, terminating at moderate distance from posterior rim of otolith, its colliculum narrower than ostial colliculum; caudal pseudocolliculum well developed. Dorsal depression wide; ventral field with distinct ventral furrow moderately close to ventral rim of otolith and few faint radial furrows underpinning the marginal crenulation or inter-denticle incisions. Outer face flat to slightly concave, rather smooth, with faint postcentral umbo.

Remarks. The degree of variability appears to be relatively low in this species and confined to minor variations of the expression of the postdorsal depression and the thickness of the otoliths.

A number of myctophid otoliths have been described from the Eocene strata of south-west France (Nolf 1988) and Australia (Schwarzhans 1985) as well as the early Oligocene of Italy (Nolf & Steurbaut 1988) and have mostly been placed in the genus Diaphus. Most of the Eocene species are characterized by compressed roundish otoliths without denticles at the ventral rim or some degree of crenulation and we consider the majority of those to represent the fossil skeleton-based genus Eomyctophum (otoliths in situ figured by Schwarzhans 1985), while those of the early Oligocene and a few from the late Eocene exhibit all the characters considered to be diagnostic for modern Diaphus otoliths (Schwarzhans 2013). These characters include the shallow ventral rim with denticles, the shape of the dorsal rim with the depressed postdorsal region, the dorsally shifted tip of the posterior rim and the proportions of the sulcus.

The otoliths of D. ? marambionis sp. nov. share those diagnostic features with otoliths of extant species of the genus Diaphus and its close relative Lobianchia, except for the indistinctly preserved denticles at the ventral rim, and thus represent the earliest record of the Diaphus / Lobianchia lineage known to date. The two genera are difficult to distinguish by means of otoliths (Schwarzhans 2013) and it is quite possible that D. ? marambionis could represent a taxon predating the dichotomy of the two extant genera. We therefore consider the placement of D. ? marambionis as preliminary until a more detailed review of Palaeogene myctophid otoliths has been performed or more otoliths in situ have been retrieved. Diaphus ? marambionis is also remarkable for achieving a considerable size, though not quite reaching the size of the largest early Oligocene species of Diaphus.