Ziphirostrum turniense du Bus, 1868

Lambert O., 2005, Systematics and phylogeny of the fossil beaked whales Ziphirostrum du Bus, 1868 and Choneziphius Duvernoy, 1851 (Mammalia, Cetacea, Odontoceti), from the Neogene of Antwerp (North of Belgium), Geodiversitas 27 (3), pp. 443-497 : 461-465

Ziphirostrum turniense du Bus, 1868

Ziphirostrum turniense du Bus, 1868: 622 , 623. — Van Beneden & Gervais 1880: pl. 27bis, fig. 6.

Mioziphius belgicus Abel, 1905: 104 , partim.

LECTOTYPE. — IRSNB 3785-M.539, a partial skull (identified by du Bus [1868] as Ziphirostrum turniense , figured by Van Beneden & Gervais [1880: pl. 27bis, fig. 6, reversed], and placed in Mioziphius belgicus by Abel [1905]).

PARALECTOTYPE. — IRSNB 3784-M.1880, a partial skull (also referred to Ziphirostrum turniense by du Bus [1868] and placed in Mioziphius belgicus by Abel [1905]).

TYPE HORIZON. — “Crag gris” ( du Bus 1868). Following Misonne (1958), the subdivisions “crag gris”, “crag rouge” and “crag jaune” in the lower Pliocene of Antwerp indicate types of alteration more than stratigraphic levels. Furthermore, some levels in the upper Miocene have a similar colour. This species might therefore be late Miocene or early Pliocene. Several circular holes, with a diameter of approximately 14 mm and a depth less than 5 mm, pierce the premaxillary sac fossae of the holotype, in a way identical to the excavations on several specimens of Z. marginatum interpreted as bivalve drillings. This feature, added to the similar external aspect of the bones in specimens of the two species, might indicate an origin in the same stratigraphic unit.

TYPE LOCALITY. — Antwerp, Belgium, exact locality uncertain.

EMENDED DIAGNOSIS. — Species of the genus Ziphirostrum with a size of the skull close to Z. marginatum , differing from that species in: most elevated point of the premaxillae on the rostrum more anteriorly positioned, roughly at mid-length of the rostrum; posterior branch of the premaxillary longitudinal ridge on the rostrum laterally divergent with a small median depressed surface, indicating a less excavated and shorter prenarial basin laterally margined by the premaxilla (and not the maxilla as in Z. marginatum ); wider and flatter dorsal exposure of the maxilla lateral to the premaxilla at the base of the rostrum and for some distance anteriorly. Z. turniense differs from Z. recurvus n. comb. in its less massive and lower rostrum, and an anteriorly open mesorostral groove.

DESCRIPTION ( FIGS 13 ; 14 )

Only one of the two known rostra of the species is associated with the anterior part of the cranium. The vertex is therefore unknown, but the preserved parts provide enough information to separate this species from Ziphirostrum marginatum , contrary to the assertion of Abel (1905).

The bones of the straight rostrum are dense and thick. The rostrum of the lectotype is roughly complete anteriorly: its total length is estimated at 550 mm, which is close to that inferred for Z. marginatum .

Premaxilla

The premaxillae compose the main part of the dorsal view of the rostrum. They are closely sutured on their dorsomedian margin until a progressive anterior separation, 200 mm before the apex ( Fig. 13A ). The maximum width of the premaxillae occurs at mid-length of the rostrum, more anteriorly than in Z. marginatum , and at the level of maximum elevation. The height of this prominence progressively reduces posteriorly, with a separation of the premaxillae forming thick diverging crests. These crests reach the anterior margin of the premaxillary sac fossae, with posterior extremities separated on the lectotype by 60 mm. Medially to the low crest, each premaxilla shows an elongated triangular and depressed surface pierced by a large premaxillary foramen. The foramen extends posterolaterally in a short posterolateral sulcus, and anteriorly in a longer anteromedian sulcus on the median edge of the crest, until the level of the median junction of the two premaxillae. Numerous thin anastomosed vascular grooves run anterolaterally from the sulcus, on the dorsal surface of the premaxilla. The grooves spread on the whole surface; a major groove runs posteriorly from the anterior opening of the mesorostral groove. The left premaxilla is somewhat narrower than the right at the level of the premaxillary sac fossa (respectively 50 and 55 mm for the maximum width).

Maxilla

Lateral to the premaxilla, the maxilla is nearly invisible in dorsal view on most of the length of the rostrum. Posteriorly, it widens to form a dorsally exposed platform, better developed on the lectotype. The posterior part of the platform, at the base of the rostrum, slopes medially from a high and prominent lateral edge ( Fig. 13B ). This structure limits laterally the shallow prenarial basin, characterized by a weaker premaxillary resorption than in Ziphirostrum marginatum . The supraorbital process is similar to Z. marginatum .

In lateral view, the left maxilla ends 92 mm before the apex of the rostrum on the lectotype ( Fig. 13C ). On the lateral side of the maxilla, the surface is excavated by a shallow vestigial alveolar groove beginning 120-130 mm anteriorly to the antorbital notch. On the paralectotype, the groove is almost totally filled by irregular bone ( Fig. 14B ); the alveoli are weakly excavated on the lectotype and only recognizable on the medi- an portion. This groove continues anteriorly on the premaxilla and divides into two sulci for the last 25 mm.

In ventral view, a large foramen is anteriorly extended by a long and distinct sulcus along the median suture of the maxilla, 250 mm before the apex of the rostrum. An additional smaller foramen is present more posteriorly. The vomer appears between the maxillae for 190 mm on the lectotype, but not on the paralectotype.

Palatine

The palatine is not preserved anteriorly, but the marks of the foliated suture with the maxilla reach forward more than 150 mm anterior to the antorbital notch. Small lateral portions of the palatine are present, with a morphology similar to Ziphirostrum marginatum .

SYSTEMATIC DISCUSSION

These two skulls differ from Ziphirostrum marginatum in: more anterior position of the prominence of the premaxillae on the rostrum, roughly at mid-length of the rostrum; the poorly developed prenarial basin, with longer posterior extremity of the dense premaxilla contacting the premaxillary sac fossa; the wider flat dorsal surface of the maxilla at the base of the rostrum; the thinner lateral maxillary wall of the prenarial basin.

The most significant differences between Ziphirostrum marginatum and these specimens are therefore related to the shape and development of the prenarial basin and of the thickened premaxillae on the rostrum. As discussed above, the prenarial basin is subject to a strong sexual dimorphism in the extant Ziphius , and the protuberances on the dorsal face of the rostrum are also sexually dimorphic in extant ziphiids (e.g., the maxillary crests much more developed in the adult males of Hyperoodon sp. ). However, this sexual dimorphism is often based on the size of the structures: width and depth of the prenarial basin in Ziphius , height of the protuberances in Hyperoodon . What is observed here is also shape differences: the elevation of the premaxillae on the rostrum occupies a different position and has a different shape from Ziphirostrum marginatum , and the margins of the reduced prenarial basin are occupied by the premaxillae, not the maxillae (which is the case for Z. marginatum ). Even if those specimens are fragmentary, the characters discussed above are regarded here as sufficiently diagnostic to support the species Z. turniense .

Ziphirostrum recurvus ( du Bus, 1868) n. comb.

Belemnoziphius recurvus du Bus, 1868: 630 . — Van Beneden & Gervais 1880: pl. 27bis, fig. 2.

Mesoplodon longirostris Abel, 1905: 113 , partim.

HOLOTYPE. — IRSNB 3805- M.544, incomplete rostrum (single specimen of Belemnoziphius recurvus sensu du Bus, 1868 , and referred to Mesoplodon longirostris by Abel [1905]).

anterior margin of pterygoid sinus fossa

D

TYPE HORIZON. — No data available, probably Miocene or early Pliocene.

TYPE LOCALITY. — Antwerp, Belgium, exact locality uncertain.

EMENDED DIAGNOSIS. — A large species of the genus Ziphirostrum characterized by: a very massive rostrum, higher and relatively narrower than in Z. marginatum and Z. turniense ; a mesorostral groove dorsally closed by the premaxillae, as in the two other species of the genus, but completely filled by the dense vomer.

DESCRIPTION ( FIG. 15 )

IRSNB 3805-M.544 is a massive rostrum, lacking the apex, the base, and fragments of the right side. It is much higher than wide; the maximum posterior height is 113 mm for a width of 69 mm at the same level. The tapering towards the apex is associated with a distinct dorsal curvature ( Fig. 15A ). At the apex, the height is 41 mm, and the width, 24 mm.

This rostrum has a mesorostral gutter completely filled with dense bone, without an anterior opening. The placement of this specimen in the species Mesoplodon longirostris by Abel (1905) implied that the gutter was filled by the vomer only, as it is the case in several fossil and extant species of Mesoplodon . The sutures of the different bones are difficult to distinguish, because of the strong ossification. However, a V-shaped suture is clearly present on the apical portion of the dorsal surface ( Fig. 15B, D ). By comparison with Ziphirostrum turniense and Z. marginatum , this suture can be related to the dorsomedian margins of the thickened premaxillae, anteriorly diverging in Ziphirostrum . This suture is interpreted here as the premaxillary-vomer contact. This implies that at least the most apical part of the mesorostral groove is filled by the vomer, and that the rest of the groove is roofed by the joined dense premaxillae. This last feature is characteristic of the genera Ziphirostrum and Choneziphius . The premaxillary eminences are, however, closer to Ziphirostrum than to Choneziphius , and even more similar in size and position to Z. turniense : on the rostrum of Z. turniense IRSNB 3784- M.1880, the dorsal margin of the apex exhibits the same kind of curvature, even if less pronounced. On both skulls of Z. turniense , a sulcus starts on the lateral side of the apex, runs posterodorsally, and divides in smaller branches; this kind of sulcus is present, even deeper and longer on the rostrum IRSNB 3805-M.544. By comparison with Z. turniense , it is possible to establish the path of the indistinct lateral suture between maxilla and premaxilla. This confirms the above hypothesis of the roofing of the mesorostral groove by the premaxillae. Ventrally to this suture, the alveolar groove is only slightly visible under a deep longitudinal sulcus, lacking alveoli.

The anterior margin of the pterygoid sinus fossa reaches a level 475 mm posteriorly to the incomplete apex (lacking at least 50 mm). This indicates a rostrum longer than in Z. turniense ; the holotype of Z. turniense IRSNB 3785- M.539 has a distance between the pterygoid sinus fossa and the nearly complete apex of 440 mm .

The keeled vomer wedges between the maxillae on the ventral side for at least 242 mm, a feature present in the lectotype but not the paralectotype of Z. turniense .

DISCUSSION

This rostrum probably belongs to a distinct species of the genus Ziphirostrum , Z. recurvus ( du Bus, 1868) n. comb., closer to Z. turniense than to Z. marginatum . The complete filling of the mesorostral groove is an interesting combination of a dorsal covering by the joined thickened premaxillae, as in Ziphirostrum , associated with the filling of the remaining aperture by t h e d e n s e v o m e r, i n a w a y r e m i n i s c e n t o f Mesoplodon .

Figures

FIG. 13. — Rostrum and anterior part of the cranium of Ziphirostrum turniense du Bus, 1868 (IRSNB 3785-M.539, lectotype); A, dorsal view; B, detail of the base of the rostrum in dorsal view; C, left lateral view. Scale bars: A, C, 100 mm; B, 50 mm.

FIG. 14. — Rostrum of Ziphirostrum turniense du Bus, 1868 (IRSNB 3784-M.1880, paralectotype); A, dorsal view; B, right lateral view. Scale bar: 100 mm.

FIG. 15. — Rostrum of Ziphirostrum recurvus (du Bus, 1868) n. comb. (IRSNB 3805-M.544, holotype); A, left lateral view; B, detail of the apex in dorsolateral view; C, line drawing of the apex in dorsolateral view, indicating the filling of the mesorostral groove by the dense vomer; D, dorsal view. Scale bars: A, D, 100 mm; B, C, 50 mm.