Imocetus piscatus, Bianucci & Miján & Lambert & Post & Mateus, 2013

Bianucci, Giovanni, Miján, Ismael, Lambert, Olivier, Post, Klaas & Mateus, Octávio, 2013, Bizarre fossil beaked whales (Odontoceti, Ziphiidae) fished from the Atlantic Ocean floor off the Iberian Peninsula, Geodiversitas 35 (1), pp. 105-153 : 131-136

Imocetus piscatus n. sp.

( Figs 14-17 ; Table 4)

HOLOTYPE. — ML 1358, partial skull including rostrum, facial area and vertex.

REFERRED SPECIMENS. — ML 1359, partial skull including rostrum and anterior portion of facial area and ML 1360, partial skull including rostrum and anterior portion of facial area, south of Nazaré Canyon off the Portuguese coast, exact locality unknown, but likely around 39°18’N, 9°47’W.

ETYMOLOGY. — From Latin “piscatus”, fished, because the holotype and the referred specimens were collected at sea by fishermen.

TYPE LOCALITY. — South of Nazaré Canyon off the Portuguese coast, exact locality unknown, but likely around 39°18’N, 9°47’W.

DIAGNOSIS. — Imocetus piscatus n. gen., n. sp. differs from all other ziphiids in a wide and anteroposteriorly elongated facial depression laterally margined by acute longitudinal maxillary crests, a rostral maxillary crest forming a posterodorsally directed spur, and premaxillary foramen located very posterior to the level of the antorbital notch. Among other ziphiines it further differs from Izikoziphius and Ziphius in the medial fusion of the premaxillae closing the mesorostral groove; it shares with Globicetus n. gen. and Tusciziphius the anterior part of the nasal contacting the premaxillary crest, and the extreme ossification and fusion of the vertex elements, but it differs from Globicetus n. gen. in lacking a large spherical medial premaxillary rostral prominence and in the presence of a distinct antorbital notch. It differs from Tusciziphius in the less transversally expanded vertex (lower width between the premaxillary crests and lower distance between the maxillae posterior to the vertex), and in the posterolateral direction of the right premaxillary crest. It further differs from the possible ziphiine Caviziphius in the less excavated premaxillary sac fossae.

DESCRIPTION ffle skull is large for a fossil ziphiid; based on the width of the rostrum it is close to the size of the modern Mesoplodon layardii . At the apex, the tapered rostrum is cylindrical, whereas it is wider at its base. ffle anteroposterior length of the rostrum is less than half the estimated condylobasal length. ffle rostrum is proportionally longer in the holotype, in which the antorbital notch is more posteriorly located than in ML 1359 and ML 1360. ffle rostrum base is also considerably wider in the holotype. ffle facial area, between the antorbital notch and the vertex, is very long. ffle vertex is proportionally low, overhanging the bony nares. ffle supraorbital area is only partly preserved and the basicranium is completely missing.

Premaxilla

On the anterior portion of the rostrum, the halfcylindrical robust premaxillae display a sutural contact above the mesorostral groove. ffleir medial suture is sinuous on the holotype and more rectilinear in ML 1359 (not preserved in ML 1360). 195 mm posterior to the apex of the rostrum of the holotype, an artificial medial opening between the superficial layers of the premaxillae exposes bone that we interpret as deeper regions of the premaxillae. Alternatively this element could correspond to the thickened vomer, but we prefer the first hypothesis taking into account the anteriorly open mesorostral groove. Considering the preserved parts, the premaxillae seem considerably thicker on the rostrum of ML 1359 than in the holotype, and even more than in ML 1360. From the level of the antorbital notch, the premaxillae descend in a depression between the more prominent maxillae, until the premaxillary sac fossae. fflis depression seems homologous to the prenarial basin described in Beneziphius , Messapicetus , and Ziphirostrum ( Lambert 2005; Bianucci et al. 2010). ffle premaxillary sac fossae, only well preserved on the holotype, are even more depressed compared to the maxillae, with a distinctly concave surface. ffle posteromedial portion of the right fossa is slightly elevated. ffle left fossa is lower than the right, and narrower. However, the asymmetry at this level is less pronounced than in Caviziphius , Globicetus n. gen., and Tusciziphius . If present (area partly covered with phosphorite concretions), right and left premaxillary foramina are close to each other and much more distant from the level of the antorbital notch than in any other known ziphiid, even more than in Hyperoodon . fflis condition stresses the unusual elongation of the facial area between antorbital notches and bony nares in Imocetus n. gen. An additional foramen is present on the medial margin of the left premaxillary sac fossa of the holotype and ML 1360, in the same position as in Globicetus n. gen., Choneziphius leidyi n. sp., and Tusciziphius atlanticus n. sp.

ffle ascending process of the premaxilla is short and erected. Its upper part overhangs the corresponding premaxillary sac fossa. On the vertex, the premaxillary crests are highly asymmetric: the right crest is more voluminous, distinctly higher and more anteriorly bulging. Similar to Globicetus n. gen., the right premaxillary crest is posterolaterally directed whereas the left crest is anterolarally directed.

Maxilla

In dorsal view the maxilla is visible along the premaxilla on the whole length of the rostrum. On the second third of the rostrum, the lateral margin is acute. fflis margin only thickens considerably before the antorbital notch, in relation with the development of the rostral maxillary crest located medial to the notch. Compared to the holotype, this spur-like posterodorsally projecting crest is more laterally directed, larger, and more posteriorly located compared to the antorbital notch in ML 1359 and ML 1360. ffle crests are asymmetric in the holotype and ML 1359; in the former the left crest is slightly higher, whereas in the latter the incomplete right crest was originally more robust than the left crest. ffle antorbital notch is deep and narrow in the three specimens, medial to the robust and long preorbital process. ffle notch is more slit-like in the holotype and V-shaped in the two other skulls. From the bottom of the notch, the maxilla forms an acute longitudinal crest on the supraorbital area of the skull. fflis crest is probably not homologous to the huge rostral crest observed medial to the antorbital notch in adults of Hyperoodon spp. (see Mead & Fordyce 2009 for terminology). fflis maxillary crest is rectilinear and posteriorly diverging in ML 1359 and ML 1360, whereas it is medially convex in dorsal view and slightly overhangs the medial area of the maxilla in the holotype. Right and left crests limit a vast and depressed facial area, where dorsal infraorbital foramina are present along the prenarial basin (two on each side of the holotype, one on each side of ML 1359 and ML 1360). In addition to these large foramina, the surface of the maxilla is covered with shallow and narrow anastomosed sulci and tiny foramina. Between the dorsal infraorbital foramina and the premaxillary sac fossa, the maxilla is distinctly thickened, forming a platform with a convex surface, much wider on the right side (and also more elevated on the right side of the holotype and ML 1359). ffle lateral flank of the maxillary crest is a wide and slightly concave surface with a steep slope. Due to the non-preserved lateral part of the supraorbital process, in lateral view the crest of the holotype displays a high triangular section.

No alveoli could be detected on the heavily postmortem worn alveolar groove. In relation with the anterior shift of the preorbital process and antorbital notch compared to other ziphiids, the position of the ventral infraorbital foramen is strongly modified. fflis foramen is about 200 mm posterior to the antorbital notch in the three specimens, whereas it is a short distance from the notch in other ziphiids. ffle foramen is followed anteriorly by a deep and long groove, edging the pterygoid sinus fossa laterally until the antorbital notch. ffle location of the ventral infraorbital foramen also gives a clue about the level of the non-preserved orbit. Indeed, in other ziphiids and other odontocetes the frontal groove and optic canal are posterior to the infraorbital foramen.

Nasal ffle nasals are wide anteriorly. Each of them occupies a considerable portion of the corresponding premaxillary crest and the nasal-premaxilla suture reaches the anterior surface of the crest, a condition more similar to Hyperoodon . ffle part of the nasal thrusted in the premaxillary crest is more compact than the smoothly depressed medial area. ffle difference of compactness only partly explains this internasal fossa, extending posteriorly on the left frontal and maxilla.ffle premaxilla-nasal suture being hard to distinguish in this very ossified vertex, an alternative interpretation would be a more medial suture more similar to the condition in Globicetus n. gen. and Tusciziphius (hatched line in Fig. 14B ).

Frontal

Only fragments of the frontals are preserved posterior to the nasals on the vertex. ffle frontal sends a short anterior projection between the nasals. In lateral view, the frontal is the main element of the robust anterior part of the preorbital process, covered by a thin sheet of maxilla. More posteriorly the bone is incomplete laterally; the whole orbit area is lost and the extent of the temporal fossa cannot be assessed.

Vomer

Our interpretation of the rostral bones suggests that the vomer does not fill the mesorostral groove of Imocetus n. gen., dorsally covered by the joined premaxillae.

Palatine ffle palatine is preserved on its rostral portion in the three specimens. ffle palatine-maxilla suture is visible at some levels, but the best seen structure is the surface of suture with the lost pterygoid, marked by grooves, and limiting a large depression corresponding to the vast anterior part of the pterygoid sinus fossa. ffle palatine-pterygoid suture extends far anterior to the antorbital notch in the three specimens, even more in the holotype, which is characterized by a more posteriorly located notch.

Lacrimal

Fragments of the lacrimal are preserved on the holotype, but its original outline cannot be precisely drawn.

REMARKS ffle significant morphological variations between the holotype and the more fragmentary referred specimens is interpreted here as due to intraspecific variation (possibly related to sexual dimorphism, see below); pending the discovery of new more complete specimens, we choose to maintain ML 1359 and ML 1360 in the same species as the holotype. Despite similarities of Imocetus n. gen. with some hyperoodontines (nasal probably included in the premaxillary crest and reaching the anteromedial margin of the crest; excavation of the dorsomedial surface of the nasal; right premaxillary crest posterolaterally directed) and more specifically with Hyperoodon (low vertex overhanging the premaxillary sac fossae; premaxillary foramen posterior to the level of the antorbital notch), this new genus is interpreted here as a member of the subfamily Ziphiinae . Indeed, it shares with all ziphiines, except Izikoziphius and Ziphius , the medial fusion of the thickened premaxillae dorsally closing the mesorostral groove, with Globicetus n. gen. and Tusciziphius the extreme ossification and fusion of the vertex elements, and with Choneziphius , Globicetus n. gen., and T. atlanticus n. sp. the deep premaxillary sac fossae.

Figures

FIG. 14. — Skull of Imocetus piscatus n. gen., n. sp. (ML 1358, holotype): A, dorsal view; B, corresponding line drawing. Parallel lines indicate a break surface. Scale bar: 10 cm.

FIG. 15. — Skull of Imocetus piscatus n. gen., n. sp. (ML 1358, holotype): A, lateral view; B, corresponding line drawing; C, ventral view. Scale bar: 10 cm.

FIG. 16. — Skull of Imocetus piscatus n. gen., n. sp. (ML 1358, holotype): A, anterior view; B, corresponding line drawing; C, antero- lateral view. Parallel lines indicate a break surface. Scale bar: 10 cm.

FIG. 17. — A, skull of Imocetus piscatus n. gen., n. sp. (ML 1359), dorsal view; B, skull of I. piscatus n. gen., n. sp. (ML 1360), dorsal view. Parallel lines indicate a break surface. Scale bar: 10 cm.