Mesoplodon sp.

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 : 147-149

Mesoplodon sp. aff. Mesoplodon layardii (Gray, 1865)

REFERRED SPECIMENS AND LOCALITIES. — Partial cranium MNHN.F.COI5 including rostrum and right supraorbital region ( Fig. 8 A-C); geographic coordinates 48°08’43.8”S, 71°59’13.8”E (150 km NE to Kerguelen Islands) ; depth 1720 m ( Fig. 1 ). Partial cranium MNHN.F.COI6 including rostrum and right supraorbital region ( Fig. 8 D-E); geographic coordinates 45°53’49.2”S, 67°19’40.8”E (330 km NNW to Kerguelen Islands) ; depth 1959 m ( Fig. 1 ). Rostrum fragment MNHN.F.COI7 ( Fig. 8F ); geographic coordinates 44°54’39.6”S- 44°25’30.0”E (445 km W to Crozet Islands) ; depth 1222 m ( Fig. 1 ).

BRIEF DESCRIPTION AND COMPARISON

The general aspect of the bone of these three specimens is markedly different from other cranial remains of the sample; the bone is whiter, less compact, and more crumbly in the outer regions. The ventral surface of the rostrum in these three specimens is more damaged than the dorsal surface, a condition most likely due to the presence of more spongy bone ventrally, as observed via computed tomography in several extant ziphiids ( Lambert et al. 2011).

The rostrum is elongated, being incomplete anteriorly at least in MNHN.F.COI7 and COI6; it is higher than wide at estimated mid-length ( Table 6). The mesorostral groove is completely filled with the vomer. The latter is especially dorsally inflated in the proximal region of the rostrum, with a visible median suture. Towards mid-length, the dorsal exposure of the vomer narrows markedly, concomitantly to a conspicuous lowering of its height above the premaxilla as seen in lateral view ( Fig. 8C, E, F ). The lateral margin of the rostrum diverges only slightly towards the corresponding antorbital notch. The maxilla-premaxilla suture approaches the lateral margin of the rostrum at a level 100-150 mm anterior to the antorbital notch, whereas a transverse con- striction of the premaxilla occurs in front of the notch. One large dorsal infraorbital foramen is opening anterolaterally at a level posterior to the antorbital notch. An anteriorly directed, smaller dorsal infraorbital foramen is located medial to the notch in MNHN.F.COI7 and COI5. Slightly posterior to level of antorbital notch, the premaxillary foramen is anterior to the large dorsal infraorbital foramen and located higher dorsally than the latter. There is no rostral maxillary crest and no conspicuous maxillary crest posterolateral to the antorbital notch, the supraorbital region being low and roughly flat, with only a slight elevation of the dorsal surface of the maxilla just anterior to the posterior dorsal infraorbital foramen. Although only the maxilla is preserved in that region, the anteromedial corner of the right bony naris displays a distinct elevation.

Among extinct and extant hyperoodontines, apart from the considerably flatter supraorbital region lacking any high crest, these three fragmentary cranial remains share the highest number of similarities with the strap-toothed whale Mesoplodon layardii for the narrow and high rostrum, the markedly inflated vomer in the proximal part of the mesorostral groove, the topology of the foramina in the rostrum base area, and the elevation of the right anteromedial corner of the bony nares (see Turner 1880; Hale 1931). Even more strikingly, the narrowing of the dorsal exposure of the vomer towards mid-length of rostrum and the related lowering of its height are similarly observed to various extents in specimens of M. layardii , where this region is surrounded by the greatly enlarged and posterodorsomedially curved lower tusks in adult males (e.g. SAM 38236, 40479, USNM 550150; Van Beneden & Gervais 1880: pl. 27; Mead 1989a: fig. 15I; Fig. 9 ).

Comparing measurements of these three specimens with skulls of Mesoplodon layardii (sample of 11 specimens, including males and females, from Australia, Falkland Islands, and South Africa in Ross 1984), a few differences are noted. The general dimensions of the three specimens are smaller than large adults of M. layardii . The ratio between estimated postorbital width and rostrum length is distinctly lower in MNHN.F.COI5, corresponding to a proportionally longer rostrum, whereas the ratio falls in the lower part of the range of M. layardii for the incomplete rostrum COI6. The width and height at mid-length of the rostra COI5 and COI6 are in the upper part of the range for M. layardii , corresponding to a proportionally greater cross section in line with the relatively longer rostrum. Pending the discovery of more complete specimens and/or the extraction of ancient DNA (see discussion on geochronological age below), these three specimens are identified as Mesoplodon sp. aff. M. layardii .

Interestingly, an isolated, poorly preserved ziphiid rostrum found on a beach of Kerguelen Islands and tentatively attributed ( Robineau 1973) to the extant Andrew’s beaked whale Mesoplodon bowdoini shares some similarities with the specimens described here. Characterized among other features by its great length, this beached rostrum may correspond to the same taxon.

Figures

FIG. 8. — Partial crania of Mesoplodon sp.aff.Mesoplodon layardii: A-C, MNHN.F.COI5; D-E, MNHN.F.COI6; F, MNHN.F.COI7; A, B, D, dorsal views; C, E, F, right lateral views. 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.

FIG. 9. — Skull of the extant strap-toothed whale Mesoplodon layardii (unnumbered SAM specimen) showing the relationship between the large mandibular tusks, the depression in the dorsal surface of the rostrum, and the transverse constriction of the dorsal opening of the mesorostral groove. Note that except for the tusks the mandibles are made of plaster, and that the tusks are positioned slightly too posterior along the rostrum as compared to their in vivo position. Photos by G. Bianucci.