Messapicetus gregarius Bianucci, Lambert and Post, 2011

Ramassamy, Benjamin, Lambert, Olivier, Collareta, Alberto, Urbina, Mario & Bianucci, Giovanni, 2018, Description of the skeleton of the fossil beaked whale Messapicetus gregarius: searching potential proxies for deep-diving abilities, Fossil Record 21 (1), pp. 11-32 : 14-15

Messapicetus gregarius Bianucci, Lambert and Post, 2011

3.1 Referred material

Specimen MUSM 2548: three cervical vertebrae including the axis, three thoracic and two thoracic–post-thoracic vertebrae all lacking the neural spine ( Figs. 2–3 ), fused manubrium and left part of the second sternebra ( Fig. 4 ), 11 complete to subcomplete ribs ( Fig. 5 ); skull and mandibles are still in the field. Specimen MUSM 2542: partial right scapula lacking the acromion, the anterior part of the scapular blade, and the broken coracoid process ( Fig. 6 ); partial left and right humeri ( Fig. 7 ), complete left radius ( Fig. 8 ); skull, mandibles, and other vertebrae of this individual are still in the field.

3.2 Horizon and locality

The specimens were discovered in the 200 m thick section of sediments from the Pisco Formation exposed at Cerro Colorado. They were identified under the fieldwork numbers O37 and O 39 in the map provided by Bianucci et al. (2016b) .

Two depositional sequences of the Pisco Formation, P1 and P2, are represented at Cerro Colorado and separated by an angular unconformity ( Di Celma et al., 2016, 2017). The specimens MUSM 2542 and MUSM 2548 originate from the P1 sequence (the “lower allomember” of Di Celma et al., 2016), consisting of nearshore conglomerates and finegrained sandstones, bioturbated sandy siltstones, and mudstones ( Di Celma et al., 2016). The lower allomember is rich in marine fossil vertebrates, such as cartilaginous and bony fish, marine turtles, crocodiles, sea turtles, sea birds, and seals ( Bianucci et al., 2016b; Landini et al., 2017a, b; Parham and Pyenson, 2010; Stucchi et al., 2016). Cetaceans are represented by diverse taxa: Physeteroidea ( Livyatan melvillei and aff. Acrophyseter sp. ), Ziphiidae ( M. gregarius , Chimuziphius coloradensis ), Inioidea ( Brachydelphis mazeasi and an undescribed taxon), two undescribed kentriodontid-like Delphinida, Balaenopteroidea, and Cetotheriidae ( Bianucci et al., 2010, 2016b, c; Collareta et al., 2015; Gioncada et al., 2016; Lambert et al., 2010a). The geological age of the lower allomember is estimated as 9.9– 8.9 Ma (Tortonian, Late Miocene) based on the presence of the diatom species Lithodesmium reynoldsii and radiometric dating of a local ash layer ( Di Celma et al., 2016; Gariboldi et al., 2017). Diatom genera found in the lower and upper allomembers (e.g., Delphineis , Odontella , Rhaphoneis , Diplomenora ) are typical of a neritic environment, whereas open-ocean diatoms species are less frequent ( Di Celma et al., 2016; Gariboldi et al., 2017).

3.3 Systematic attribution

Both specimens were identified based on complete skulls and mandibles that were left on the field. They are attributed to the family Ziphiidae based on the enlargement of the hamular fossa of the pterygoid. In MUSM 2548, this is further confirmed by the presence of an enlarged apical alveolus on the mandible. The two skulls have an extremely elongated rostrum, representing approximately 75 % of the total condylobasal length. Furthermore, MUSM 2548 displays the medial fusion of the premaxillae dorsal to the mesorostral groove along the rostrum. These two characteristics are typical of the genus Messapicetus .

The species M. gregarius is by far the most common ziphiid species in the lower allomember of Cerro Colorado, being represented by at least 13 specimens ( Bianucci et al., 2016b, c) .

Figures

Figure 2. Cervical vertebrae of the specimen MUSM 2548, Messapicetus gregarius. Axis in anterior (a), posterior (b), and ventral view (c); C5–C6 in anterior (d), posterior (e), and dorsal view (f); C7 in anterior (g), posterior (h), and dorsal view (i).

Figure 3. Thoracic and post-thoracic vertebrae of the specimen MUSM 2548, Messapicetus gregarius. Thoracic vertebra A in anterior (a), posterior (b), and dorsal view (c); thoracic B in posterior (d), left lateral (e), and ventral view (f); thoracic C in posterior (g), lateral (h), and ventral view (i); thoracic–post-thoracic D in posterior (j) and lateral view (k); thoracic–post-thoracic E in lateral view (l).

Figure 4. Sternum of the specimen MUSM 2548, Messapicetus gregarius, in ventral view (a) and manubrium element in anterior view (b). Horizontal solid lines indicate major break. The dashed line represents a reconstruction of the second element of the sternebra 2.

Figure 5. Ribs of the specimen MUSM 2548, Messapicetus gregarius, in anterior view. Pair 1 (a) and (b); pair 2 (c) and (d); pair 3 (e) and (f); pair 4 (g) and (h); (i), (j), (k), and (l) cannot be precisely positioned.

Figure 6. Right scapula of the specimen MUSM 2542, Messapicetus gregarius, in dorsal view.

Figure 7. Humeri and left radius of MUSM 2542, Messapicetus gregarius. Right humerus in medial (a), lateral (b), ulnar (c), and radial view (d); left humerus in medial (e), lateral (f), ulnar (g), radial (h), and posterior view (i); left radius in lateral (j), medial (k), and ulnar view (l).

Figure 8. Comparison of muscular insertions along the atlas and axis in Messapicetus gregarius MUSM 2548 in ventral view (a) and posterior view (b); in Ninoziphius platyrostris MNHN SAS 941 in ventral view (c) and posterior view (d); in Mesoplodon densirostris USNM 593522 in ventral view (e) and posterior view (f); in Berardius sp. MNHN 1885-278.