Messapicetus cf. longirostris Bianucci, Landini, and Varola, 1992

Bianucci, Giovanni, Llàcer, Sergio, Cardona, Josep Quintana, Collareta, Alberto & Florit, Agustí Rodríguez, 2019, A new beaked whale record from the upper Miocene of Menorca, Balearic Islands, based on CT-scan analysis of limestone slabs, Acta Palaeontologica Polonica 64 (2), pp. 291-302 : 294-300

Messapicetus cf. longirostris Bianucci, Landini, and Varola, 1992

Figs. 2–9 .

Material.— MDM-2029, an incomplete skull (cranium and mandibles) preserved within two limestone slabs. The cranium lacks most of the rostrum, the vertex area, the posteroventral walls of the braincase, the whole right squamosal, part of the left squamosal and all the ear bones. The two incomplete mandibular rami (of which only the posteroalveolar portions are preserved) are still in anatomical position with respect to the cranium. From lower Tortonian N16 (Miocene), 2.5 km East of Ciutadella de Menorca, Spain.

Description. — Premaxilla: In dorsal view, the premaxilla appears as transversely narrow on the preserved portion of the rostrum, whereas it widens abruptly posterior to the antorbital notch ( Fig. 3 ). The medial margins of the premaxillae do not contact each other, reaching the maximum distance of 27 mm ca. 100 mm anterior to the base of the rostrum. The transverse sections of the rostrum indicate that the premaxskull pterygoid hamuli vomer

filled mesorostral canal including mineralised mesorostral cartilage illae are clearly thickened and more dorsally convex than the maxillae close to the anterior end of the preserved portion of the rostrum, whereas they are thin and dorsally excavated near the base of the rostrum ( Fig. 4 ). This proximal excavation of the rostral portion of the premaxillae, further emphasised by the lateral thickening of the maxillae (forming a prenarial basin sensu Lambert 2005), is also clearly visible in dorsal and anterodorsal views of the cranium. The premaxillary sac fossae are weakly asymmetrical, with the right fossa being slightly transversely wider than the left (ratio between the widths of the left and right premaxillary sac fossae = 0.77). At least three small foramina are visible near the anterolateral margin of the right premaxillary fossa, the anteriormost one being located 18 mm anterior to the corresponding antorbital notch. It is not clear whether one of these foramina might represent the main premaxillary foramen.

Maxilla: The maxilla is widely exposed on the dorsal surface of the rostrum, being transversely wider than the premaxilla for all the preserved portion of the rostrum ( Fig. 3 ). The lateral margin of the preserved rostral portion of the maxilla rises progressively toward the antorbital notch, thus forming an elevated rostral maxillary crest, which is also clearly visible in the transverse sections of the rostrum ( Fig. 4 ). This crest does not extend posterior to the antorbital notch in the supraorbital region. Clusters of large (5–9 mm wide) and small (2–3 mm) dorsal infraorbital foramina are visible on both maxillae, both anterior and posterior to the antorbital notches. Their size and position are different on the right and left maxilla. Indeed, on the right maxilla, there are three large foramina located from 5 to 22 mm anterior to the antorbital notch and at least two large foramina located from 23 to 62 mm posterior to the antorbital notch; in turn, on the left maxilla, only one of the five foramina located near the antorbital notch is large in size, whereas two large foramina are located, close to each other, 60–70 mm posterior to the antorbital notch. Posterior to the antorbital notch, the thin ascending process of the maxilla almost completely covers the frontal. No trace of a maxillary crest is observed on the lateral margin of the ascending process of the maxilla. Just anterior to the nares, a narrow strip of the maxilla is apparently exposed medial to each premaxilla. On the palatal surface of the maxillae, there is no trace of dental alveoli or an alveolar groove, indicating that no teeth were present in the posteriormost portion of the rostrum, until a level at least 160 mm anterior to the antorbital notches ( Fig. 5 ). The ventral infraorbital foramen is visible on the posteromedial surface of the maxilla.

Frontal: In dorsal view, the frontal is almost totally covered by the ascending process of the maxilla, being visible for a narrow extension, located at the lateral margin of the orbit and, with a small portion, posterolateral to the nares ( Fig. 3 ). In lateral view, the preorbital and supraorbital processes are dorsoventrally thin, whereas the postorbital process is triangular, with a wide ventral tip that points posteroventrally ( Figs. 6 , 7 ). The orbit is anteroposteriorly elongated. On the ventral surface of the skull, the frontal is widely exposed, forming the dorsal roof of the orbit and of the temporal fossa ( Fig. 5 ).

Presphenoid and vomer: On the cut surface of the two slabs, a section of the ossified nasal septum is observed Fig. 2A 3 , A 4 ); it is formed by the ossified presphenoid and, laterally, by the vomer. Anterior to the nasal septum, the mesorostral canal is filled internally by sediment, as also confirmed by the CT-scan, showing that this region of the slabs has density values lower than in the surrounding areas formed by dense maxillae ( Fig. 8 ). Nevertheless, the CTscan reconstruction of the dorsal surface of the skull ( Fig. 3 ) and the CT cross sections of the rostrum ( Fig. 4 ) highlights a dorsal relief at the middle of the mesorostral canal that could represent the mineralised mesorostral cartilage (i.e., cartilaginous nasal septum), perhaps in origin partially ossified. The vomer extends anterior to the nasal septum, forming a narrow and elevated keel that is visible in ventral view in a narrow fissure between the medial margins of the pterygoids ( Fig. 5 ). The vomer is not pachyostotic and does not fill the mesorostral canal.

skull mandibles pterygoid hamuli vomer filled mesorostral canal endocranial cast filled internal nares and internal oral cavity

Jugal and lacrimal: The anterior process of the jugal is dorsally exposed, forming a distinct tubercle between the antorbital notch and the antorbital process of the frontal (“lateral process” sensu Lambert et al. 2013) ( Fig. 3 ). The preserved anterior and posterior portions of the styliform process of the left jugal are slender ( Fig. 7 ). In ventral view, the suture between the jugal and the lacrimal is not clearly visible ( Fig. 5 ). Compared to the anteroposterior length of the orbit, the anteroposterior extension of the lacrimojugal complex is small.

Pterygoid: The hamular process of the pterygoid is large, extending anteriorly onto the palatal surface of the rostrum for ca. 110 mm from the antorbital notch and ventrally covering the wide pterygoid sinus fossa at its anterior end ( Fig. 5 ). Together, the posterior margins of the hamular processes form a medial point that is posteriorly directed (a plesiomorphic condition that is absent in all other ziphiids for which this feature is known). Nevertheless, it is possible that this shape is due to the fact that the posterolateral portions of the hamular processes are missing or too delicate to be imaged by means of CT-scan analyses. As in Ninoziphius platyrostris and in a few skulls of extant ziphiids ( Lambert et al. 2013), the inner surface of the hamular process is crossed by clearly distinct transverse crests ( Fig. 6A 3 ).

Squamosal: The sections of both zygomatic processes of the squamosals are exposed on one of the cut slab surfaces Fig. 2B 2 ). The left zygomatic process, partially reconstructed using the CT-scans, appears short and directed anterodorsally. Its anterior apex articulates with the posterior preserved portion of the styliform process of the jugal ( Figs. 5 , 7 ).

Mandible: The dorsal surface of the preserved posterior portion of the right mandible (which is more complete than the left) does not exhibit hints of either distinct dental alveoli or an alveolar groove, thus suggesting that the dentigerous portion of the bone is completely missing. In lateral view, the dorsal margin is weakly concave, with an anteroposteriorly elongated coronoid crest anteriorly limited by a precoronoid crest located 21 mm anterior to the posterior end of the mandibular condyle ( Fig. 9A 2 ). Posterior to the precoronoid process, the coronoid crest slopes down progressively towards the coronoid process, which is consequently lower than the precoronoid process. The mandibular condyle is protuberant, being ventrally separated from the angular process by a deep posterior notch. The mandibular foramen extends for more than 20 cm anterior to the posterior end of the mandibular condyle ( Fig. 9A 4 ).

Remarks.—The Menorca skull exhibits a clear ziphiid synapomorphy: the wide hamular fossa of the pterygoid sinus, extending anterior to the antorbital notch onto the palatal surface of the rostrum ( Lambert et al. 2013; Bianucci et al. 2016c: character 35, state 1). Moreover, among odontocetes, transverse crests on the inner surface of the hamular process are only known for a few ziphiid species ( Lambert et al. 2013). Among ziphiids, MDM-2029 shares with all the members of the “ Messapicetus clade” (sensu Bianucci et al. 2016c, including also Dagonodum ; Ramassamy 2016) except Notoziphius the thickening of the compact premaxillae (pachyosteosclerosis) along the rostrum ( Bianucci et al. 2016c: character 30, states 1–3). Within the Messapicetus clade,MDM-2029shares with Aporotus , Beneziphius , Dagonodum , Messapicetus , and Ziphirostrum another derived character: the presence of a prenarial basin bounded laterally by a thick stripe of the maxilla (comparison with Ziphirostrum is based on Ziphirostrum marginatum , the best-known of the three species included in this genus, as well as the only one for which a specimen with precise stratigraphic provenance is known; Lambert 2005). Aporotus and Beneziphius differ from MDM-2029 by the greater thickening of the premaxillae along the rostrum ( Bianucci et al. 2016c: character 30, state 2). Aporotus further differs from MDM-2029 by having a rostral maxillary crest that extends posteriorly on the antorbital process, thus delimiting a wide and deep dorsal depression ( Lambert 2005).

MDM-2029 Messapicetus longirostris

Messapicetus gregarius Ziphirostrum marginatum

Beneziphius further differs from MDM-2029 by its joined premaxillae that limit anteriorly the prenarial basin and by the dorsal surface of maxilla being horizontal lateral to the prenarial basin ( Lambert 2005). Therefore, with respect to MDM-2029, Dagonodum , Messapicetus , and Ziphirostrum appear as the closest genera. Interestingly, all the fossils referred to Messapicetus spp. , the holotype and only known specimen of Dagonodum mojnum , and the only Ziphirostrum marginatum skull for which precise information about the stratigraphical provenance is known come from Tortonian strata, as does the Menorca skull.

Although comparisons between MDM-2029 and the sole known specimen Dagonodum are complicated by the fragmentary status of both skulls, clear differences reside in the shape of the posterior portion of the rostrum, as Dagonodum exhibits a well-developed maxillary tubercle and associated prominential notch that is absent in MDM-2029. Messapicetus and Ziphirostrum marginatum are very similar to each other, differing essentially for the degree of elongation of the rostrum—a feature that is not useful for comparisons with the Menorca skull, which only preserves the posterior portion of the rostrum. Nevertheless, two characters, possibly related to the degree of elongation of the rostrum, attribute MDM-2029 to Messapicetus rather than to Z. marginatum : (i) the length of the dorsally open posterior portion of the mesorostral groove is at least 180 mm in the Menorca skull, close to Messapicetus longirostris (206–230 mm) and in the range of Messapicetus gregarius (163–210 mm), but consistently larger than in Z. marginatum (roughly 100 mm); (ii) the posteroalveolar portion of the rostrum is at least 160 mm long in MDM-2029, close to M. longirostris (180 mm) and M. gregarius (150–255 mm), but consistently larger than in Z. marginatum (roughly 100 mm) ( Fig. 10 ).

Finally, the aforementioned absence of a distinct maxillary tubercle and of a prominential notch, coupled with the presence on the right maxilla of a cluster of two-three dorsal infraorbital foramina (contra only one foramen in M. gregarius ; Bianucci et al. 2016a), also facilitates attribution of MDM-2029 to M. longirostris rather than to M. gregarius . Consequently, MDM-2029 is most closely related to Messapicetus longirostris , both the former and the latter being found in Tortonian deposits of the Mediterranean basin. Indeed, there is a good overlap in size and shape between MDM-2029 and M. longirostris ; moreover, several preserved portions of the Menorca skull appear to be very similar to the corresponding features in M. longirostris (e.g., the shape of the rostral maxillary crest, the slender antorbital process, the thin supraorbital process, the anteroposteriorly elongated coronoid crest of the mandible and the protuberant mandibular condyle). The only substantial differences between the Menorca skull and Messapicetus longirostris (and also M. gregarius ) is the more anteroposteriorly elongate orbit ( Table 2) and the greater dorsoventral thickening of the lateral margin of the preserved rostral portion of the maxilla ( Figs. 3A 2 , 4 ). The first difference could partly be due to some inaccuracy in the CT-scan 3D reconstruction, whereas the different degrees of thickening of the maxilla could be due to intraspecific ontogenetic variation. Nevertheless, considering also the incompleteness of MDM-2029, we prefer to refer this specimen to Messapicetus cf. longirostris , pending the discovery of more complete material.

Figures

Fig. 2. The two limestone slabs showing longitudinal cross sections of the Messapicetus cf. longirostris skull (MDM-2029) from the Tortonian of Menorca (Balearic Islands, Spain). A. Surface of slab 1 (A1, top; A2, bottom), details of A2 (A3, A4, explanatory drawing). B. Surface of slab 2 (B1, top; B2, bottom).

Fig. 3. 3D reconstruction resulting from the CT-scan of the ziphiid cetacean Messapicetus cf. longirostris Bianucci, Landini, and Varola, 1992 skull MDM-2029) inside two limestone slabs from the Tortonian of Menorca (Balearic Islands, Spain), in dorsal (A1) and anterodorsal (A2) views.

Fig. 4. 3D reconstruction resulting from the CT-scan of the ziphiid cetacean Messapicetus cf. longirostris Bianucci, Landini, and Varola, 1992 skull (MDM-2029) inside two limestone slabs from the Tortonian of Menorca (Balearic Islands, Spain); in anterior view, showing transverse cross sections of the rostrum (A1–A4) made at different distances from the rostrum base. Not to scale.

Fig. 5. 3D reconstruction resulting from the CT-scan of the ziphiid cetacean Messapicetus cf. longirostris Bianucci, Landini, and Varola, 1992 skull (MDM- 2029) inside two limestone slabs from the Tortonian of Menorca (Balearic Islands, Spain); in ventral view (A1), ventral view without the mandibles (A2).

Fig. 6. 3D reconstruction resulting from the CT-scan of the ziphiid cetacean Messapicetus cf. longirostris Bianucci, Landini, and Varola, 1992 skull (MDM-2029) inside two limestone slabs from the Tortonian of Menorca (Balearic Islands, Spain); in right lateral view (A1), right lateral view without the mandibles (A2), detail of the pterygoid hamuli showing the well-preserved transverse crests (A3).

Fig. 7. 3D reconstruction resulting from the CT-scan of the ziphiid cetacean Messapicetus cf. longirostris Bianucci, Landini, and Varola, 1992 skull MDM-2029) inside two limestone slabs from the Tortonian of Menorca (Balearic Islands, Spain); in left lateral view (A1), left lateral view without the mandibles (A2), detail of the orbital area (A3).

Fig. 8. CT-scan of the ziphiid cetacean Messapicetus cf. longirostris Bianucci, Landini, and Varola, 1992 skull (MDM-2029) inside two limestone slabs from the Tortonian of Menorca (Balearic Islands, Spain). A. Transverse section of the rostrum. B. Dorsal outline of the cranium showing the position of the section. C. Variation of the gray-value density along the transverse section of the rostrum.

Fig. 9. CT-scan 3D reconstruction of the posterior portion of the left mandible of the ziphiid cetacean Messapicetus cf. longirostris Bianucci, Landini, and Varola, 1992 skull (MDM-2029) inside two limestone slabs from Tortonian of Menorca (Balearic Islands, Spain); in anterior (A1) lateral (A2) posterior (A3), and medial (A4) views.

Fig. 10. Comparison of the dorsal view of the preserved cranium of Messapicetus cf. longirostris (MDM-2029) with some stem beaked whales. The dotted ovals indicate the extent of the prenarial basin.