Chilcacetus cavirhinus Lambert, Muizon & Bianucci, 2015

Lambert, Olivier, Muizon, Christian de, Bennion, Rebecca F., Urbina, Mario & Bianucci, Giovanni, 2025, New data on archaic homodont odontocetes from the Early Miocene of Peru reveal a second species of Chilcacetus Lambert, Muizon & Bianucci, 2015 and a Southern Hemisphere record for a northeastern Pacific species, Geodiversitas 47 (9), pp. 369-408 : 374-377

Chilcacetus cavirhinus Lambert, Muizon & Bianucci, 2015

( Figs 2-4 )

Chilcacetus cavirhinus Lambert, Muizon & Bianucci, 2015: 81 .

TYPE MATERIAL. — Holotype. Peru • 1 specimen (nearly complete cranium with the associated mandibles and the manubrium; the anterior portion of the rostrum and of the right mandible, and the left mandible were re-discovered in the MNHN collection after the initial publication of this specimen); East Pisco Basin ; 14°34’40”S, 75°38’40”W; Chilcatay Formation; Burdigalian (late Early Miocene); MNHN.F. PRU11 .

TYPE LOCALITY. — Precise locality unknown, but close to Ullujaya , East Pisco Basin, Peru. Approximate geographic coordinates: 14°34’40”S, 75°38’40”W ( Fig. 1A, B ).

TYPE HORIZON AND AGE. — In Ullujaya, cetacean fossil remains only occur, abundantly, in the widely exposed Lower Miocene Chilcatay Formation. More specifically, all cetaceans originate from the Ct 1a facies association, a sub-horizontal 36 m package of interbedded sandstones, sandy siltstones, and siltstones punctuated by conglomerate levels ( Bianucci et al. 2018; Di Celma et al. 2018). The age of the entire Chilcatay Formation exposed at Ullujaya can be restricted between 19.28 and 17.95 Ma ( Burdigalian ) based on radiometric dating of two volcanic ash layers positioned respectively 1.6 m below the contact between the Chilcatay and overlying Pisco Formation (40 Ar /39 Ar : 18.02 ± 0.07 Ma ) and 2 m above the base of the exposed section (40 Ar /39 Ar : 19.0 ± 0.28 Ma ) ( Di Celma et al. 2018; Bosio et al. 2020b) ( Fig. 1D ). This age is consistent with the 87 Sr /86 Sr stratigraphy and silicoflagellate and diatom biostratigraphy ( Di Celma et al. 2018; Bosio et al. 2020a, 2022).

REFERRED SPECIMEN. — MUSM 4692, nearly complete cranium including a few teeth in situ with the associated partial right and left mandibles, the atlas, and two sternum elements. This specimen was reported in the geological map of Zamaca provided by Di Celma et al. (2019) with the field number ZM 12 and referred to Chilcacetus cavirhinus . Geographic coordinates: 14°38’20.2”S, 75°38’26.1”W. Zamaca is a locality a few kilometers south of Ullujaya where the same Burdigalian Chilcatay Formation is extensively exposed ( Fig. 1B ). In detail, the MUSM 4692 skeleton was collected 10.7 m above the contact with the underlying Otuma Formation, near the top of the of the Ct1c facies association (sandstones and conglomerate beds) of the Ct1 allomember ( Fig. 1E ). The age of Ct1c is well defined by two dates: 1) a 19.25 ± 0.05 Ma 40Ar/39Ar radiometric dating of a volcanic ash layers near the base of Ct1c ( Di Celma et al. 2018); and 2) a 19.1-18.7 Ma 87Sr/ 86 Sr dating obtained with the Strontium Isotope Stratigraphy analyzing oyster shells collected a few meters below the MUSM 4692 skeleton ( Bosio et al. 2020a, 2022). Therefore, the age of MUSM 4692 is roughly 19 Ma (lower Burdigalian).

EMENDED DIAGNOSIS. — Chilcacetus cavirhinus differs from Chilcacetus ullujayensis n. sp. in its slightly smaller size (bizygomatic width <260 mm); the rostrum being proportionally shorter (ratio between rostrum length and condylobasal length ≤ 0.7 and ratio between preorbital width and rostrum length> 0.4); 34 maxillary teeth per side; the mesorostral groove being dorsally open for a shorter distance (dorsomedial contact between premaxillae> 190 mm long); the vertex of the cranium being proportionally anteroposteriorly longer and narrower; the occipital shield being transversely broader (minimum posterior distance between temporal fossae clearly higher than width of premaxillae in facial region); the long axis of the zygomatic process of the squamosal being closer to the horizontal plane; and the higher number of alveoli (10) in the post-symphyseal region of the mandible.

COMMENTS

The specimen MUSM 1401 was originally referred to Chilcacetus cavirhinus , but several differences with the holotype were already noted ( Lambert et al. 2015). With the addition of two new Chilcacetus specimens in the sample, some of these differences were observed between two subsets of specimens, leading to the proposal that two species of Chilcacetus are recorded in the Chilcatay Formation. Interestingly, the two specimens referred to Chilcacetus ullujayensis n. sp. originate from close levels of the Ct1 allomember of the Chilcatay Formation, both in the facies association Ct1a, while C. cavirhinus MUSM 4692 was collected in a lower level, from facies association Ct1c ( Fig. 1 ). The latter can thus be considered geologically older. These considerations support the hypothesis that the two Chilcacetus species were not sympatric, with C. cavirhinus preceding C. ullujayensis n. sp. Such scenario would require that the holotype of C. cavirhinus , whose exact locality and stratigraphic provenance are unknown, also came from the oldest layers of the Chilcatay Formation exposed in the Ullujaya-Zamaca area.

BRIEF DESCRIPTION

This description focuses on the skull parts of the holotype MNHN.F.PRU11 recently re-discovered at the MNHN, as well as on some diagnostic regions of the newly referred specimen MUSM 4692.

Ontogenetic stage

Both MNHN.F. PRU 11 and MUSM 4692 have robust cranial and mandibular bones, and neither display detached cranial elements or broadly open cranial sutures. Altogether, these observations point to subadult to adult individuals.

Cranium

The addition of the anterior part of the rostrum to the cranium of the holotype ( MNHN.F. PRU 11; Fig. 2 ) allows for the measurement of the condylobasal length (cbl) and rostrum length (rl) ( Table 1 ). With a rl/cbl ratio of 0.7, corresponding to a longirostrine condition (seeMcCurry & Pyenson 2019; Lambert & Goolaerts 2022), the holotype has a rostrum proportionally slightly shorter than in Chilcacetus ullujayensis n. sp. MUSM 2527 (0.73) and MUSM 1401 (0.72). With a ratio estimated to 0.68, MUSM 4692 ( Fig. 3 ) has a similarly shorter rostrum, as confirmed by the ratio between postorbital width and rl (0.48 and 0.49 for the holotype and MUSM 4692, respectively), which is greater than in MUSM 2527 (0.42). On the complete rostrum of the holotype, 41 alveoli are counted on the left side, a number that falls in the estimated range of C. ullujayensis n. sp. The premaxilla-maxilla suture is visible on both lateral surfaces of the holotype’s rostrum, reaching the lateral margin of the alveolar groove at 65 mm from the tip of the rostrum. The premaxillary part thus makes 12.5 % of the rostrum length, a ratio that is close to that of MUSM 1401 (see Lambert et al. 2015). Seven well-defined alveoli are counted on each side of this premaxillary portion.

In both the holotype and MUSM 4692, the mesorostral groove is dorsally open on a shorter distance in the anterior part of the rostrum ( Figs 2 ; 3 ; Appendix 1 ) when compared to specimens of C. ullujayensis n. sp. Previously observed in the holotype, the relatively long region of contact between the two premaxillae above the mesorostral groove is confirmed in MUSM 4692 (at least 200 mm). As for the holotype, the bony nares of MUSM 4692 are narrower than in C. ullujayensis n. sp. MUSM 2527 and MUSM 1401. The posterior apex of each premaxilla reaches farther along the nasal in MUSM 4692 compared to MUSM 1401, but no clear difference is noted with MUSM 2527. Due to the incompleteness of the anterior part of the nasals and the complex topology of the nasal-frontal sutures (see below), proportions of the vertex in MUSM 4692 cannot be precisely quantified. Still, it was originally proportionally anteroposteriorly longer than in C. ullujayensis n. sp. MUSM 2527 and MUSM 1401, with longer nasals, as in the holotype. Sutures between nasals and frontals appear to be interdigitated in MUSM 4692, with many longitudinal grooves and ridges, as in the holotype. The interpretation of the outline of these sutures is complex, as several lines are observed that may correspond to the suture lines (see Fig. 3B ), a condition reminiscent of the holotype of Yaquinacetus meadi Lambert, Godfrey & Fitzgerald, 2019 , where two different interpretations were provided for the nasal-frontal suture outline ( Lambert et al. 2019). The challenging interpretation of vertex sutures in various groups of odontocetes could be partly due to the complex evolutionary and developmental history of the bones making this region ( Roston et al. 2023). Noteworthily, several ziphiids have been described with an unusual configuration of vertex bones, sometimes showing extreme ossification ( Bianucci et al. 2013) and in other cases including a prominent medial bone identified as the interparietal (e.g., Bianucci et al. 2007). The upper part of the occipital shield of MUSM 4692 is transversely and dorsoventrally concave. The temporal crests are robust and project far posteriorly, more so than in C. ullujayensis n. sp. MUSM 2527.

Mandible

With the anterior tip now added to the previously described posterior portion, the right mandible of the holotype is preserved on its whole length ( Fig. 2A, B, D ). The mandibular condyle is shifted about 30 mm anterior to its original position in the mandibular fossa, and the anterior tip of the mandible reaches about 40 mm more anterior than the rostrum.

The rostrum was thus not longer than the mandible, a major difference with the eurhinodelphinids for which a mandible was found associated to the rostrum (e.g., Kellogg 1925; Pilleri 1985). Thirty-eight alveoli are counted on the nearly complete left mandible of the holotype ( Fig. 4 ); the total count was most likely close to the upper count. As in the holotype, the mandible of MUSM 4692 bears 10 alveoli in the post-symphyseal region, a number that is higher than in Chilcacetus ullujayensis n. sp. MUSM 2527.The symphysis is not ankylosed in MUSM 4692, a feature shared with the holotype.

Atlas

Preserved upside down attached to the left side of the occipital shield, the atlas of MUSM 4692 is 118 mm wide, 73 mm high, and 30.5 mm long on the ventral side along the sagittal plane ( Fig. 3 ). There is no indication of an ankylosis, even partial, with the axis, a difference with most, but not all, ziphiids ( Ramassamy et al. 2018). The lower transverse process is moderately elongated posterolaterally. The base of the ventral tubercle is broad, but this process only extends for a short distance posterior to the ventral edge of the posterior articular facets. The ventral tubercle is for example longer in the eurhinodelphinids Xiphiacetus cristatus and Ziphiodelphis sigmoideus Pilleri, 1985 ( Pilleri 1985; Lambert 2005a) and several early platanistoids ( Tanaka & Fordyce 2015; Kimura & Barnes 2016; Bianucci et al. 2020).

Figures

FIG. 2. — Cranium and right mandible of Chilcacetus cavirhinus Lambert, Muizon & Bianucci, 2015 MNHN.F.PRU11 (holotype, Chilcatay Formation, East Pisco Basin, Peru) in dorsal (A), right lateral (B), left lateral (C), and ventral (D) view. The mandible is shifted slightly anteriorly relative to the mandibular fossa on the cranium, which gives the wrong impression of some degree of underbite. Scale bar: 200 mm. Photos by Philippe Loubry and Lilian Cazes.

FIG. 3. — Cranium and atlas of Chilcacetus cavirhinus Lambert, Muizon & Bianucci, 2015 MUSM 4692 (Chilcatay Formation, East Pisco Basin, Peru) in dorsal view (A) with corresponding line drawing, detail of the vertex of the cranium in dorsal view (B) with corresponding line drawing, and left lateral view (C). Hatching for main break surfaces; dashed lines on line drawing for more tentative interpretations of sutures and edges; dashed lines on photo for maxilla-premaxilla suture. Scale bar: 200 mm. Photos by Olivier Lambert.

FIG. 4. — Partial left mandible of Chilcacetus cavirhinus Lambert, Muizon & Bianucci, 2015 MNHN.F.PRU11 (holotype, Chilcatay Formation, East Pisco Basin, Peru) in lateral (A), dorsal (B), and medial (C) view. Scale bar: 200 mm. Photos by Olivier Lambert.

FIG. 1. — Geographic and geological setting: A, location of the main outcrops of the Chilcatay Formation along the southern coast of Peru; B, simplified map providing the position of Ullujaya (the type locality of Chilcacetus ullujayensis n. sp.), Roca Negra,and Zamaca (the localities of several other odontocetes described here) alongside other highly fossiliferous sites of the Ica desert; C, schematic stratigraphic column of the Cenozoic succession exposed in the East Pisco Basin; D, simplified stratigraphic section of the Chilcatay Formation in Ullujaya, showing the exact position of the two specimens of Chilcacetus ullujayensis n. sp. in the Ct1 allomember, Ct1a facies association; E, simplified stratigraphic section of the Chilcatay Formation in Roca Negra and Zamaca, showing the exact position of Chilcacetus cavirhinus Lambert, Muizon & Bianucci, 2015 MUSM 4692 in the Ct1 allomember, Ct1c facies association, and of Amphidelphis bakersfieldensis n. comb. MUSM 4691 in the Ct1 allomember, Ct1a facies association. Both sections D and E include positions of ash layers dated with 40Ar/39Ar and shell-rich beds dated with 87Sr/86Sr along with the corresponding age estimates (after Bosio et al. 2020a, b, 2022). Maps and sections modified from Bianucci et al. (2018), Di Celma et al. (2018, 2019), Bosio et al. (2020b, 2022), and Lambert et al. (2023).

APPENDIX 1. — Textured 3D model of the cranium of Chilcacetus cavirhinus MUSM 4692, available at: https://doi.org/10.7934/P5641