Africanacetus sp.

Gol’Din, Pavel E. & Vishnyakova, Karina A., 2013, Africanacetus from the sub-Antarctic region: The southernmost record of fossil beaked whales, Acta Palaeontologica Polonica 58 (3), pp. 445-452 : 446-448

Africanacetus sp.

Material.—YugNIRO 408–409, partial skulls including portions of the rostrum.

Age and horizon.—Unknown. Previous studies derived tentative Neogene age estimates for trawled specimens of fossil ziphiids from associated phosphorites ( Bianucci et al. 2007). However, no phosphate concretions are present on the surface of the specimens described here, and no Neogene events of phosphorite formation have been described for Banzare Bank ( Burnett and Riggs 1990; Baturin 2004).

Description.—The two skulls are larger than Africanacetus ceratopsis ( Bianucci et al. 2007, 2008) and comparable in size to the extant Indopacetus pacificus ( Dalebout et al. 2003) , suggesting a total body length of about 6 m ( Table 1). All of the sutures are well fused, indicating that the specimens represent adult individuals.

The rostrum is high and robust, and only slightly narrows anteriorly in dorsal view ( Fig. 1 ). In lateral profile ( Fig. 3), it is similar to that of the extant Mesoplodon densirostris and M. europaeus , but much wider transversely and more robust. While damage to the ventral surface prevents any precise estimate of the height of the rostrum for either specimen, its proximal portion seems to have been at least as high as wide. The vomer is pachyosteosclerotic (i.e., both dense and swollen) within the rostrum, as well as in the ventral narial region ( Fig. 5 ), and completely fills the mesorostral groove. As in A. ceratopsis , the vomer bears a median suture and is strongly elevated above the dorsal margin of the premaxilla, overhanging the latter along its entire preserved length. In lateral view, the part of vomer elevated above the premaxilla and maxilla accounts for about one half of the total height of the rostrum.

The rostral part of the premaxilla is transversely narrow and fused to the vomer along a barely visible suture. As in A. ceratopsis , the premaxillary sac fossa is shallow and slopes laterally. In anterior view, the premaxillary foramina are located slightly dorsal to the maxillary (dorsal infraorbital) foramina. The suture between the maxilla and premaxilla is visible, but mostly obliterated owing to fusion of these two bones. Posterolaterally, the rostrum terminates in a shallow prominential notch bordered laterally by a small maxillary tubercle. The lateral margin of the maxilla is distinctly keeled along the rostrum and ascends towards the prominential notch.

As in A. ceratopsis , a low crest runs posteriorly from the maxillary tubercle, joining a more posteromedially situated, dome−shaped maxillary crest at its posterior end. The dome−shaped crest has a steep medial and a gradually sloping lateral margin, giving the maxilla a “bird−wing” profile in anterior or posterior view. This profile is distinct from the symmetrical, semi−rounded maxillary crests in the extant Hyperoodon planifrons . The anteromedial portion of the facial region of the skull is slightly asymmetrical, with the facial portion of the right maxilla being larger than its counterpart on the left. This results in the right maxillary crest being located more posterolaterally than the left one, as can be seen when comparing measurements 12/13 and 14/15 ( Table 1).

http://dx.doi.org/10.4202/app.2011.0097

On the ventral surface of the rostrum, the pterygoid sinus fossa extends to a point approximately 90–100 mm anterior to the prominential notch. Extensive wear and damage has obliterated most of the other morphological details. The maxilla−palatine suture is not visible, likely owing to the poor state of preservation of the bone surface.

Figures

Fig. 1. The beaked whale Africanacetus sp., skull in dorsal view. A. YugNIRO 409, photograph (A1) and explanatory drawing (A2). B. YugNIRO 408.

Fig. 5. The beaked whale Africanacetus sp. (YugNIRO 408), narial region of the skull in ventral view.