Chilcacetus ullujayensis Lambert, Muizon, Bennion, Urbina & Bianucci, 2025, n. sp.

Chilcacetus ullujayensis n. sp. (Figs 5-15) urn:lsid:zoobank.org:act: 7CFE27F3-EDC1-4572-B5DD-F1662483258D TYPE MATERIAL. — Holotype. Peru • 1 specimen (nearly complete cranium including the two in situ periotics, the detached left malleus, left tympanic bulla, and several teeth, the posterior part of mandibles, the left humerus, three thoracic vertebrae, and a few rib fragments; this specimen has been previously attributed to Chilcacetus cavirhinus, in a paper including a field photo of the specimen, as well as photos of one rib and the humerus (Bianucci et al. 2018: figs 8, 13)); East Pisco Basin, Ullujaya locality; 14°34’2.2”S, 75°38’54.8”W; Chilcatay Formation; Burdigalian (late Early Miocene); MUSM 2527. TYPE LOCALITY. — Ullujaya, East Pisco Basin, Peru (Fig. 1A, B). Geographic coordinates: 14°34’2.2”S, 75°38’54.8”W. This specimen was reported in the geological map of Ullujaya provided by Di Celma et al. (2018) with the field number O5. TYPE HORIZON AND AGE. — Lower Miocene Chilcatay Formation. More precisely, the holotype MUSM 2527 was collected 9.7 m above the base of the exposed section in the sandstones of the Ct1c facies association of the Ct1 allomember (Fig. 1E). The age of the type horizon is constrained to the 18.9-18.5 Ma interval based on the 87Sr/ 86 Sr values calculated from barnacle and pectinid samples collected just below the holotype skeleton (Bosio et al. 2020a, 2022). This dating is consistent with the roughly 19-18 Ma age interval for the whole Chilcatay Formation exposed at Ullujaya as indicated by 40 Ar/ 39 Ar radiometric dating, 87 Sr/ 86 Sr stratigraphy, and silicoflagellate and diatom biostratigraphy (Di Celma et al. 2018; Bosio et al. 2020a, b, 2022). REFERRED SPECIMEN. — MUSM 1401, nearly complete cranium with associated fragments of both mandibles, eight partly preserved vertebrae, and rib fragments. This specimen was previously tentatively attributed to Chilcacetus cavirhinus (Lambert et al. 2015). It was reported in the geological map of Ullujaya provided by Di Celma et al. (2018) with the field number O6 (see also Bianucci et al. 2018: table 1, fig.13). Geographic coordinates: 14°34’36.00”S, 75°38’38.30”W. MUSM 1401 was collected 15 m above the base of the exposed section and 5.3 m above the level of the holotype of Chilcacetus ullujayensis n. sp., in the same sandstones of the Ct1c facies association of the Ct1 allomember of the Lower Miocene Chilcatay Formation (Fig. 1D). Two meters above MUSM 1401, sampled barnacles and oyster shells gave a 87Sr/86Sr dating between 18.8 Ma and 18.6 Ma (Bosio et al. 2020a, 2022). Therefore, both the holotype and referred specimen of C. ullujayensis n. sp. fall in the 18.9-18.4 age interval, i.e. lower Burdigalian. DIAGNOSIS. — Chilcacetus ullujayensis n. sp. differs from Chilcacetus cavirhinus in its slightly greater size (bizygomatic width> 270 mm); the rostrum being proportionally longer (ratio between rostrum length and condylobasal length> 0.71 and ratio between preorbital width and rostrum length <0.4); 37-39 maxillary teeth per side; the mesorostral groove being dorsally open for a longer distance (dorsomedial contact between premaxillae absent to very short); the vertex of the cranium being proportionally anteroposteriorly shorter and wider; the occipital shield being transversely narrower (minimum posterior distance between temporal fossae approximates maximum width of premaxillae in facial region); the long axis of the zygomatic process of the squamosal being farther to the horizontal plane; and the lower number of alveoli (8) in the post-symphyseal region of the mandible. ETYMOLOGY. — The species name refers to the East Pisco Basin fossil-rich locality of Ullujaya, where both the holotype MUSM 2527 and referred specimen MUSM 1401 of this new species were collected. DESCRIPTION This description is mostly based on the holotype MUSM 2527, focusing on parts that are not preserved or differently shaped in the previously described MUSM 1401 (Lambert et al. 2015; at that time referred to Chilcacetus cavirhinus). Ontogenetic stage All epiphyses of the three preserved thoracic vertebrae and left humerus of MUSM 2527 are fully ankylosed, indicating an adult individual (Galatius & Kinze 2003; Moran et al. 2015). A similar interpretation was proposed for MUSM 1401, based on a series of 8 vertebrae (Lambert et al. 2015). This is in agreement with the robustness of cranial and mandibular parts in these two specimens. General cranial features Displaying a complete rostrum and well-preserved facial region as in MUSM 1401, the cranium of MUSM 2527 is slightly more damaged on the roof of the temporal fossae, and its temporal crests are lost (Figs 5-11; Appendix 2). Compared to MUSM 1401 (Appendix 3), it has better-preserved orbits and ventral region in general, including well-defined maxillary and premaxillary alveoli, and complete hamular processes and jugals. Its in situ periotics as well as one detached tympanic bulla, one malleus, and teeth, are other parts not preserved in MUSM 1401 (and any other Chilcacetus spp. specimen with regards to the ear bones). Apart from a somewhat longer rostrum, cranial dimensions of MUSM 2527 are remarkably similar to MUSM 1401 (Table 1). The robust rostrum makes 73 % of the condylobasal length, a proportion that is slightly higher than in MUSM 1401, and the postorbital width is greater than in the two specimens of Chilcacetus cavirhinus. As in other Chilcacetus spp. specimens, the temporal fossa is anteroposteriorly longer than high, and its posterior part was not fully dorsally covered by the frontal and maxilla. The vertex is only slightly shifted to the left side compared to the sagittal plane, indicating a minor degree of asymmetry. Premaxilla As for Chilcacetus cavirhinus, the premaxilla is alone on the anterior tip of the rostrum for a relatively short distance, about 50 mm (Fig. 6A). In MUSM 2527, this premaxillary portion held 4 to 5 teeth, and the corresponding alveoli have an anteroposterior diameter of 9 mm and interalveolar spaces of 2.5 mm (Fig. 7). The premaxilla-maxilla suture is distinct until its tip on the lateral surface of the rostrum. More posteriorly, this suture is not located in a broad lateral groove, though a narrow sulcus occupies the intermediate third of the rostrum, starting from a foramen that is located along the suture. As in all other Chilcacetus spp. specimens, the dorsolateral surface of the premaxilla is marked along the anterior half of the rostrum by numerous shallow and sinuous sulci, leaving from a main longitudinal sulcus that start at the tip of the rostrum (Fig. 5). Right and left premaxillae contact each other above the mesorostral groove from a point at mid rostrum length, before diverging 200 mm anterior to the antorbital notch level. The premaxillary foramen is located at the level of this notch. The anteromedial sulcus is moderately long (about 80 mm) and the posteromedial sulcus is deep and broad; these two sulci define a roughly flat, dorsomedially sloping prenarial triangle. Each premaxillary sac fossa is transversely and anteroposteriorly concave. The left premaxillary sac fossa is only slightly wider than the right, while the opposite condition occurs in MUSM 1401, so no clear signal can be detected for the asymmetry of this region in Chilcacetus ullujayensis n. sp., as in C. cavirhinus MNHN.F.PRU 11. No prominence is seen on the posterolateral corner of the thick ascending process of the premaxilla. The premaxilla contacts the anterolateral and lateral margin of the nasal for most of the length of the latter. The presence of a contact between premaxilla and frontal could not be assessed due to uncertainties about the lateral extent of the nasal-frontal suture on both sides of the vertex. Maxilla Five and four dorsal infraorbital foramina are observed in the area anteromedial to the antorbital notch of MUSM 2527, on the right and left sides, respectively (Fig. 5). This count does not differ much from MUSM 1401. At least one additional foramen is found on the left side, 35 mm posteromedial to the notch. Only the right posteriormost foramen is preserved, level to the posterior edge of the bony nares and 18 mm lateral to the premaxilla-maxilla suture. Better preserved in MUSM 2527 than in MUSM 1401, the antorbital notches are deep and wide, slightly more anterolaterally open than in Chilcacetus cavirhinus MNHN.F.PRU 11. As in the latter, the right notch is narrower than the left, due to a more anteriorly projected antorbital process, reaching a level 12 mm anterior to the preorbital process of the frontal. The dorsal surface of the right antorbital process makes an oblique, prominent blade as in MNHN.F. PRU 11. Along the vertex, the raised medial wall of the maxilla is distinctly shorter than in MUSM 1401 and, to an even greater extent, than in the two specimens of C. cavirhinus, corresponding to an anteroposteriorly shorter posterior part of the vertex. With a total count of 42 to 43 upper alveoli per row, MUSM 2527 had 37 to 39 maxillary teeth per row (Fig. 7), a count that is higher than in C. cavirhinus MNHN.F.PRU 11 (34). The alveolar groove ends 60 mm anterior to the level of the antorbital notch, and the last alveoli are shifted slightly medially compared to the lateral margin of the rostrum. Most maxillary alveoli have an anteroposterior diameter of 9 to 10 mm, with a slight decrease in posteriormost alveoli. Interalveolar spaces range from 2.5 mm anteriorly to 7 mm at mid rostrum length, 11 mm at 3/4 of rostrum length, and 3-4 mm between posteriormost alveoli. 170 mm from the rostrum tip, the vomerine trough widens and deepens backwards with a V-shaped cross section, until 420 mm from tip, where it starts shallowing. A first pair of palatine foramina is located 500 mm from rostrum tip, each followed anteriorly by a well-defined sulcus. A second pair of smaller foramina is observed 15 mm more posteriorly, and a third pair is along the anteromedial bord