Izikoziphius rossi Bianucci, Lambert & Post, 2007

Lambert, Olivier, Muizon, Christian de, Duhamel, Guy & Plicht, Johannes Van Der, 2018, Neogene and Quaternary fossil remains of beaked whales (Cetacea, Odontoceti, Ziphiidae) from deep-sea deposits off Crozet and Kerguelen islands, Southern Ocean, Geodiversitas 40 (6), pp. 135-160 : 149-151

Izikoziphius rossi Bianucci, Lambert & Post, 2007

REFERRED SPECIMEN AND LOCALITY. — Partial craniumMNHN.F.COI8 including rostrum, premaxillary sac fossae, and vertex ( Fig. 10 ); geographic coordinates 49°50’44.4”S, 63°19’27.6”E ( Skiff Bank , 390 km SWW to Kerguelen Islands) ; depth 2041 m ( Fig. 1 ).

BRIEF DESCRIPTION AND COMPARISON

The cross section of the long and robust rostrum (not complete apically) is considerably wider than high for most of the rostral length ( Table 7). A marked dorsal and lateral expansion of the thickened lateral margin of the rostrum has a maximum extent

at about mid rostral length, resulting in the lateral margin of the rostrum being distinctly convex in dorsal view in that region. The mesorostral groove is filled by the thickened vomer until the level of the antorbital notches, and the dorsal surface of the vomer is transversely concave in the apical part of the rostrum. On the ventral surface of the rostrum, lateral to the ventral exposure of the vomer the maxillae are pierced by several large palatal foramina, anteriorly followed by deep sulci. The rostrum is broad at its base, with a depressed medial region laterally margined by the rostral maxillary crest. The latter progressively raises past the antorbital notch level, forming a high hump in the antorbital region, best seen in lateral view. Medial to the rostral maxillary crest, a large dorsal infraorbital foramen is followed anteriorly by a wide depression. Several smaller foramina pierce the rostral maxillary crests. The premaxillary foramen is roughly in line with the large dorsal infraorbital foramen, both being posterior to the estimated level of the antorbital notch. The two premaxillary sac fossae are moderately transversely concave, and the right fossa is distinctly wider than the left ( Table 7). The lateral margin of each ascending process of the premaxilla is moderately constricted in anterior view.

The vertex of the cranium is elevated, with the anterior surface of the ascending process of the premaxilla reaching the vertical. This anterior surface is excavated on both sides by a deep fossa, extending somewhat on the lateral part of the corresponding nasal.Each premaxillary crest is directed anterolaterally, and the right crest is wider and dorsally higher than left crest.

Both nasals are longer than wide, extending anteriorly beyond the level of the corresponding premaxillary crest. The right nasal is significantly longer posteriorly than the left. From their anterior apices, the lateral margins of the right and left nasals diverge abruptly until mid-length. The median region of the dorsal surface of the nasals is depressed.

The combination of a mesorostral groove filled with the vomer, a high vertex with anterolaterally directed premaxil- lary crests, and anteriorly elongated nasals point to a ziphiine attribution (sensu Bianucci et al. 2016). Other characters (e.g. the long rostrum, the absence of a prenarial basin, the fossa on the anterior surface of the ascending process of the premaxilla, and the relatively long contact between nasal and premaxilla) contrast with Ziphius (adult males of the latter for the prenarial basin) and unambiguously indicate affinities with Izikoziphius . Within the latter genus, MNHN.F.COI8 shares several characters with Izikoziphius rossi : rostrum wider than high at mid-length, rostral maxillary crest extending on rostrum base, large fossa on the anterior surface of the ascending process of the premaxilla extending on the corresponding nasal, median depression of the dorsal surface of the nasals, and right premaxillary crest much higher than left crest.

However, MNHN.F.COI8 differs from the two currently known specimens of I. rossi in the marked dorsolateral expansion of the lateral margin of the rostrum at mid-length. Pending the discovery of additional specimens this conspicuous difference is interpreted here as intraspecific variation, maybe indicating sexual dimorphism as recorded in several extant ziphiids and inferred in a few extinct species ( Heyning 1989; Mead 1989b; Bianucci et al. 2013). It could nevertheless also indicate a closely related new species of Izikoziphius .

Interestingly, although much less developed this expansion of the lateral margin of the rostrum is to some degree reminiscent of the huge rostral maxillary crests observed in SAM PQ 2717 and 2719, two specimens from the South African fauna(s) previously attributed to Odontoceti indet. due to their fragmentary state of preservation ( Bianucci et al. 2007). In addition to rostral maxillary crests with the same position and orientation as the expansion of the lateral margin of the rostrum in MNHN.F.COI8, SAM PQ2717 and 2719 share with the latter a thickened vomer in the mesorostral groove, the dorsal surface of the vomer being transversely concave in the apical region of the rostrum, and several large palatal foramina.Together with the high compactness of rostral bones, all these similarities point to ziphiid (and maybe even ziphiine) rather than physeterid affinities for these two mysterious specimens (G. Bianucci pers. comm. 2017).

Figures

FIG. 10. — Partial cranium of Izikoziphius rossi (MNHN.F.COI8): A, dorsal view; B, same view with interpretive line drawing; C, detail of the vertex in dorsal view; D, anterodorsal view; E, right lateral view; F, ventral view. Scale bars: A, B, D-F, 100 mm; C, 50 mm.

FIG. 1. — Locality maps: A, schematic map providing the position of Crozet and Kerguelen islands in the Southern Ocean; B, schematic map of the Crozet and Kerguelen islands area, including the main localities where fossil ziphiid specimens («) described in this work were found via longline fishing,as well as two ODP sites (l). Light grey shading for areas with a water depth lower than 1000 m; dark grey shading for islands and continents.