Africanacetus ceratopsis 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 : 143

Africanacetus ceratopsis Bianucci, Lambert & Post, 2007

REFERRED SPECIMENS AND LOCALITIES. — Partial cranium MNHN.F.COI12 including rostrum, premaxillary sac fossae, part of both supraorbital regions, and vertex ( Fig. 5 ); geographic coordinates 49°48’S, 66°28’E ( Skiff Bank , SWW to Kerguelen Islands) ; depth 1680 m. Partial cranium MNHN.F.COI2 including rostrum, premaxillary sac fossae, and part of both supraorbital regions ( Fig. 6 ); geographic coordinates 47°07’58.8”S, 65°44’59.4”E (290 km NW to Kerguelen Islands) ; depth 1622 m ( Fig. 1 ).

BRIEF DESCRIPTION AND COMPARISON

The rostrum is not complete anteriorly in both specimens, although it is probably close to the original length in MNHN.F.COI2. Probably half of the rostrum in missing in COI12. The rostrum is robust and nearly as wide as high at half of its preserved length on COI2. The mesorostral groove is completely filled with the vomer, the latter reaching a dorsal level much higher than the bordering premaxilla. A median suture is visible on the dorsal surface along the proximal part of the vomer on COI2. The lateral margin of the rostrum of COI2 is a sharp crest in its proximal part; this crest forms a thin blade markedly diverging towards the prominential notch; the blade is somewhat thicker on the left side. No crest is observed on COI12 (except at the rostrum base, close to the prominential notch) and in dorsal view the rostrum is significantly narrower and more slender than on the other specimen. In lateral view, no indication for individual alveoli could be found on the maxilla. Deeper on COI12, the prominential notch is followed posteriorly by a dome-shaped, moderately elevated rostral maxillary crest (sensu Mead & Fordyce 2009). The crest is higher and more posteriorly located on the right side. Compared to COI2, right and left crests are more elevated, massive, and rounded on COI12. A small dorsal infraorbital foramen is present at the top of each crest (on COI2) and a large foramen pierces the medial base of the crest, being followed anteriorly by a shallow depression. Anterolateral to the rostral maxillary crest, the antorbital notch is widely open anteriorly, barely limited laterally by a short anterior extension of the antorbital part of the maxilla and the lacrimojugal complex. Medial to the antorbital notch a small maxillary tubercle protrudes anteriorly. The maxillary tubercle is more salient in COI12, in relation to the larger maxillary crest in this specimen. Right and left premaxillary foramina are difficult to distinguish, hidden by a concretion. Each premaxillary sac fossa is thick, with the dorsal surface raising dorsomedially.

The ascending process of the premaxilla of MNHN.F.COI12 strongly raises dorsally, with its anterior surface being almost subvertical when reaching the level of the premaxillary crest. Each ascending process is distinctly constricted transversely at the level of the maximum width of the bony nares. The vertex is well preserved on COI12, with intact premaxillary crests. The latter are robust and the right crest diverges posterolaterally, whereas the left crest is directed more laterally. In dorsal view, the nasals are anteroposteriorly long and transversely wide; their anterolateral angle reaches the medial margin of the corresponding premaxillary crest, without being integrated in it. In dorsal view, the median suture of the nasal is salient anteriorly, dorsal to the presphenoid.

The thick vomer in the mesorostral groove, the dome-like rostral maxillary crest, and the dorsomedially raising premaxillary sac fossae are similarly observed in specimens of Africanacetus ceratopsis from South Africa ( Bianucci et al. 2007). The dimensions of the rostrum and facial region ( Table 5) match well the South African sample of A. ceratopsis and the specimens of Africanacetus sp. from the Banzare Bank, southernmost part of the Kerguelen Plateau, described by Gol’din & Vishnyakova (2012). The rostral maxillary crests are significantly lower on MNHN.F.COI2 and more rounded on MNHN.F.COI12. For part of the dimensions, the holotype of Africanacetus gracilis is moderately to markedly smaller, especially for the measurements at rostrum base (see Ichishima et al. 2017). As in the latter, the antorbital notch of both specimens is more anteriorly located than in the holotype of A. ceratopsis and several referred specimens; such differences may be explained by sexual dimorphism or any other intraspecific, populationrelated morphological variation.

Figures

FIG. 5. — Partial cranium of Africanacetus ceratopsis (MNHN.F.COI12): A, dorsal view; B, right lateral view; C, detail of the vertex in dorsal view; D, anterodorsal view. Scale bars: 100 mm.

FIG. 6. — Partial cranium of Africanacetus ceratopsis Bianucci, Lambert & Post, 2007 (MNHN.F.COI2): A, dorsal view; B, anterior view; C, right lateral view. Scale bar: 100 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.