Chilcacetus ullujayensis, Lambert & Muizon & Bennion & Urbina & Bianucci, 2025

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 : 377-388

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 border of the palatines, at 70-80 mm from the level of the antorbital notches.

Palatine and pterygoid

The anteromedial part of the palatine-maxilla suture is visible on both sides of the deeply keeled rostrum base, until a level 25 mm posterior to the last pair of palatine foramina ( Fig. 7 ). The pterygoid is much shorter anteriorly, not reaching the level of the antorbital notch. The anteriorly short pterygoid sinus fossa is laterally defined by a low lateral lamina of the pterygoid. This lamina is directed posterolaterally, but remains low before disappearing anterior to the path for the mandibular nerve V3. This lamina does not contact the falciform process of the squamosal, a condition already proposed for Chilcacetus cavirhinus MNHN.F.PRU 11 and, for example, also observed in Squaloziphius emlongi Muizon, 1991 and Waipatia maerewhenua Fordyce, 1994 ( Muizon 1991; Fordyce 1994). The medial wall of the pterygoid thickens both transversely and dorsoventrally in posterior direction, more gradually than in C. cavirhinus MNHN.F.PRU 11, making a robust hamular process with a ventrolateral edge that is keeled anteriorly and more rounded posteriorly ( Figs 6-8 ). Right and left processes are separated by a deep V-shaped valley. The main body of the hamular process is distinguished from its long, finger-like posteromedial projection by a broad notch that is ventral to the choana. Right and left projections do not contact each other (8 mm apart), and are 9.5 mm broad and 25 mm long. They are broadly similar, though shorter, than in ‘ Argyrocetus ’ joaquinensis , Simocetus rayi Fordyce, 2002 , and Olympicetus thalassodon Velez-Juarbe, 2023 ( Kellogg 1932; Fordyce 2002; Velez-Juarbe 2023), similar but more distant to each other than in Ziphiodelphis abeli ( Pilleri 1985; plates 39, 40), longer than in C. cavirhinus , and more posteriorly directed than in S. emlongi . The medial lamina of the pterygoid thickens gradually towards the posterior region of the basioccipital basin, ending 60 mm from the posterior tip of the basioccipital crest.

Nasal

Though the anteromedial margin of each nasal is abraded in MUSM 2527, considering the better-preserved anterolateral margins the anterior outline of the nasals did most likely not differ much from MUSM 1401, displaying a smoothly rounded outline and a short projection above the bony nares ( Figs 5 ; 6 ; 11B ). The dorsal surface of the joined nasals is transversely convex.The nasal-frontal sutures are difficult to follow laterally, but they indicate that each nasal is transversely wider than medially long and that medially each nasal is shorter than the corresponding frontal. As in MUSM 1401, each nasal sends a posterolateral projection, whose full extent cannot be assessed here. There is no indication of interdigitated nasal-frontal sutures in both MUSM 2527 and MUSM 1401, a possible difference with Chilcacetus cavirhinus .

Frontal

On the vertex, the anteromedial part of the frontals is slightly prominent, as in C. cavirhinus MNHN.F.PRU 11, reaching dorsally the same height as the highest preserved medial portions of nasals. From there, the dorsal surface of the frontals slopes posteroventrally to a narrow depression just anterior to the nuchal crest.

In lateral view ( Fig. 6 ), the preorbital process is moderately thickened, as in Chilcacetus cavirhinus . The orbit roof is highly arched, with a maximum depth of 21 mm on the right side. Better preserved on the left side, the postorbital process is robust and roughly vertical.

Lacrimojugal complex

The lacrimojugal complex makes the ventrolateral wall of the antorbital notch, as a thin oblique plate along the anteroventral margin of the preorbital process of the frontal ( Figs 7 ; 8 ), with no major difference with C. cavirhinus MNHN.F. PRU 11. The base of the styliform process of the jugal extends at least 15 mm more anteromedially than the bottom of the antorbital notch. The styliform process remains thin (transverse thickness 3 mm) for most of its length, only expanding a short distance before the contact with the zygomatic process of the squamosal (better seen on the left side).

Supraoccipital

The nuchal crest is thin anteromedially, but it thickens considerably towards the dorsolateral corners of the occipital shield ( Figs 5 ; 11A ), as in C. cavirhinus MNHN.F.PRU 11 and Macrodelphinus kelloggi , reaching a maximum thickness of 15 mm, measured on the left side, which is greater than in MUSM 1401. In posterior view, most of the outline of the nuchal crest is horizontal, as in MUSM 1401 and C. cavirhinus MNHN.F.PRU 11. The occipital shield is moderately transversely concave and its dorsal region is slightly dorsoventrally concave, while more ventrally a broad sagittal groove is present. The ventrolateral parts of the occipital shield being lost, the cranial endocast is visible, with the outer surface better preserved on the left side.

Exoccipital

The exoccipital does not reach as far laterally as the squamosal ( Fig. 11A ). The region of the paroccipital process is too damaged on both sides to allow for a description of this area, as well as of the details of the narrow jugular notch. The occipital condyles are slightly mediolaterally narrower and dorsoventrally higher than in Chilcacetus cavirhinus MNHN.F.PRU 11, with a transversely narrower foramen magnum. The condylar neck is well defined ( Figs 6 ; 7 ), and the dorsal, condyloid fossa is broad and deep ( Fig. 11A ).

Alisphenoid

The anterior portion of the ventral exposure of the alisphenoid is excavated by the broadest part of the pterygoid sinus fossa, the latter being laterally margined by a moderately developed subtemporal crest ( Figs 7 ; 8 ). Posterior to the sinus fossa, the path for the mandibular nerve V3 extends obliquely from the foramen ovale.

Squamosal

The zygomatic process of the squamosal is long and robust, pointing anterodorsally ( Fig. 6 ). Its preserved position compared to the postorbital process of the frontal on both sides suggests either a long, oblique contact between the two bones, or a near contact. The dorsal margin of the zygomatic process is moderately convex in lateral view, and this margin thickens transversely posteriorly. The contact facet with the jugal is best seen on the left side, as a 23 mm long concave portion of the ventral margin, defined posteroventrally by a low tuberosity. From this point, the concave ventrolateral margin of the zygomatic process remains thin until the base of the robust postglenoid process. Not preserved in C. cavirhinus MNHN.F.PRU 11 and MUSM 1401, the latter is anteroposteriorly long (29 mm from the anterior meatal crest to its anterior margin) and transversely thick in MUSM 2527, similar to the condition in eurhinodelphinids, Eoplatanista spp. , Amphidelphis bakersfieldensis n. comb., ‘ Argyrocetus ’ joaquinensis, Caolodelphis milleri Godfrey & Lambert, 2023, and Perditicetus yaconensis Nelson & Uhen, 2020, but anteroposteriorly shorter than in squaloziphiids and possible relatives Crisocetus lydekkeri Gaetán, Paolucci & Buono, 2023, Dolgopolis kinchikafiforo Viglino, Gaetán, Cuitiño & Buono, 2021 , Squaloziphius emlongi , and Yaquinacetus meadi , as well as in Enigmatocetus posidoni Godfrey & Lambert, 2023 ( Muizon 1991; Lambert et al. 2019; Nelson & Uhen 2020; Viglino et al. 2021; Gaetán et al. 2023; Godfrey & Lambert 2023); it reaches ventrally the same level as the basioccipital crest and the exoccipital. One main sternocephalicus fossa is best seen on the right side, not extending much along the zygomatic process (ending before the level of the anterior margin of the postglenoid process).

As in C. cavirhinus MNHN.F.PRU 11, the tympanosquamosal recess is not clearly separated from the mandibular fossa ( Figs 7 ; 8 ). The recess excavates the base of the postglenoid process’ medial surface and extends anteriorly until the anterior limit of the mandibular fossa, thus not reaching farther along the zygomatic process. Probably complete on both sides of MUSM 2527, the falciform process is pointing ventrally just anterior to the periotic, but extends distally (in the ventral direction) less than in C. cavirhinus MNHN.F.PRU 11. The falciform process lowers abruptly until the posterior margin of the path for the mandibular nerve V3, where it disappears ( Figs 8 ; 9 ).

Basioccipital

Better preserved on the left side of MUSM 2527, each basioccipital crest is thickened along its posteromedial surface ( Figs 7 ; 8 ), but not to the extent of Caolodelphis milleri. Right and left crests draw an angle of 35° in ventral view. A curved ridge runs from the base of each basioccipital crest towards the medial region of the basioccipital basin.

Periotic

Both periotics of the holotype ( MUSM 2527) are preserved in situ ( Figs 7-9 ), with the anterior process closely appressed to the corresponding falciform process of the squamosal and with the posterior process displaying a long dorsal contact with the exoccipital and squamosal. Anatomical details are thus only available for the ventral and ventrolateral regions. The anterior process is short ( Table 1 ) and bears a shallow but transversely broad (4.5 to 5 mm wide) anterior bullar facet. This condition differs from the narrower facet seen in an interpretive drawing of the now lost periotic of the holotype of Chilcacetus cavirhinus MNHN.F.PRU 11 ( Lambert et al. 2015). Part of a well-defined parabullary sulcus can be seen on the medial surface of the anterior process of the left periotic ( Fig. 9 ), along the falciform process of the squamosal. It is much closer to the anterior bullar facet than in Yaquinacetus meadi , and is more similar to Perditicetus yaconensis ( Nelson & Uhen 2020), though the latter shares with Y. meadi an anterior bullar facet that is shifted distinctly medially compared to the lateral margin of the anterior process (condition unknown in MUSM 2527, the lateral margin being hidden by the falciform process).

A medium-size accessory ossicle is preserved in situ on both sides, differing from the right periotic of MNHN.F. PRU 11. The anteromedial outline of the pars cochlearis is rounded, while the posteromedial corner is marked by a prominence just medial to the fenestra rotunda. The lateral tuberosity is large, with a ventromedial surface that is broader than the mallear fossa, extending far laterodorsally and anteriorly defining a deep and narrow hiatus epitympanicus. Though being partly hidden by the posterior process of the tympanic bulla on the left side and by sediment and bone fragments on the left right side, the posterior process is tentatively interpreted as long (estimated length 35 mm), nearly reaching the posterior margin of the exoccipital in a posterolateral direction. It retains a considerable breadth for most of its visible/preserved portion, a condition that is reminiscent of the outline proposed in ventral view for MNHN.F. PRU 11. A transverse break surface tentatively observed in ventral and medial view on the left periotic of P. yaconensis ( Nelson & Uhen 2020: fig. 7) may indicate a similarly elongated posterior process. The exposed proximal surface of the right posterior bullar facet is concave along an oblique axis.

Tympanic bulla

In ventral view, the finely preserved left tympanic of MUSM 2527 is olive-shaped, with the inner posterior prominence being slightly shorter than the outer prominence, and the latter being markedly transversely broader ( Fig. 10 A-E). The ventral surface of the inner prominence is moderately keeled, and the keel extends anteriorly for 21 mm. The median furrow is wide but shallow posteriorly, vanishing anteriorly before mid-length of the tympanic. The anterior spine is incipient, barely thickened, associated with an inconspicuous anterolateral notch. In medial view the posterior part of the involucrum is 14 mm thick. After 5 mm the dorsal surface slopes gradually down to a slight indentation, followed by a second plateau before a more abrupt descent towards the anterior opening of the tympanic cavity. The medial surface of the tympanic is crossed at about mid-height by an anteroposteriorly directed ridge, presumably marking the ventral limit of the peribullary sinus. In dorsal view the sigmoid process is slightly obliquely directed, hiding the conical process in lateral view. The lateral furrow is narrow and nearly vertical. The anterodorsal crest is moderately convex in lateral view. The elliptical foramen is 2.5 mm wide and at least 4 mm high. Preserved in situ in the basicranium, the posterior process of the left tympanic lacks its thin distal part ( Figs 7 ; 8 ), indicating that this process was long and sub rectangular in outline, as proposed for the posterior process of the periotic. Except for the somewhat more swollen outer posterior prominence, the ventral and medial views of this tympanic are highly similar to these views in Chilcacetus cavirhinus MNHN.F.PRU 11. Strong similarities are also noted with Yaquinacetus meadi and eurhinodelphinids, though these generally display a deeper and anteriorly longer median furrow (see Muizon 1988a; Lambert 2005a, b). An even shallower, nearly indistinct median furrow is seen in Eoplatanista spp. ( Muizon 1988a).

Malleus

The left malleus of USNM 2527 displays an articular head that makes 70 % of the height of the bone in posteromedial view, corresponding to a markedly shortened tubercule ( Fig. 10F ), as seen in eurhinodelphinids, Inticetus Lambert, Muizon, Malinverno, Di Celma, Urbina & Bianucci, 2017 , Eoplatanista , Yaquinacetus , physeteroids, and ziphiids ( Muizon 1988a; Lambert 2005b; Bianucci et al. 2010; Lambert et al. 2018, 2019). On the tubercule, which is longer than in physeteroids and most ziphiids, the manubrium is approximately at the same height as the low muscular process; the manubrium is considerably less prominent than in eurhinodelphinids and Inticetus , closer to Eoplatanista and Yaquinacetus . The anterior facet for the incus does not project outside the articular head, drawing a general outline in posteromedial view that is more regularly rounded than in Eoplatanista italica MGP 26166 ( Muizon 1988a: fig. 12).

Mandible

As the mandible of Chilcacetus ullujayensis n. sp. is poorly preserved in MUSM 1401 (see Lambert et al. 2015), our description is mostly based on MUSM 2527 ( Fig. 12 ). The latter only preserves a short (110 mm) posterior part of the symphyseal region. The anterior cross section is semi-circular to subtriangular, with no indication of a lateral groove, as in other Chilcacetus spp. specimens. While the two dentaries remain attached to each other, the suture line is visible dorsally, ventrally, and in cross section, indicating that no strong ankylosis occurred along the symphysis. At the level of the posterior end of the symphysis, alveoli have an anteroposterior diameter of 10 mm, with interalveolar spaces of 8 to 10 mm. This is in the range of MUSM 1401, with alveoli larger than in Chilcacetus cavirhinus MNHN.F.PRU 11 and MUSM 4692. Eight alveoli are posterior to the symphysis; the last alveolus is 135 mm posterior to the end of the symphysis and 255 mm anterior to the posterior end of the mandibular condyle. There is no precoronoid crest and the dorsal margin of the coronoid crest flattens and widens in the posterior direction, with thin longitudinal ridges along the medial and lateral edges of the process. Neither of the coronoid processes are complete in MUSM 2527. The angular process draws an angle of about 90°. The mandibular condyle is laterally convex and slightly medially concave, with a dorsoventral height that is slightly greater than in C. cavirhinus MNHN.F.PRU 11. The condyle is in line with most of the alveolar groove, the last alveolus being roughly at the same dorsoventral level as the upper edge of the condyle. The mandibular foramen has a regularly rounded anterior outline, similar to C. cavirhinus MNHN.F.PRU 11, but ending farther anteriorly, 150 mm from the mandibular condyle.

Teeth

Four teeth were found detached along the skull of MUSM 2527 ( Fig. 13 ). Total length ranges between 27 and 35 mm, with a crown length between 8.7 and 10.9 mm, a maximum diameter at the base of the crown between 4.5 and 5.7 mm, and a maximum diameter of the root between 7.2 and 9.2 mm. These dimensions are close to those of the posterior upper teeth of Chilcacetus cavirhinus MNHN.F.PRU 11. Each crown is moderately curved lingually, is slightly labio-lingually flattened, and bears barely discernible mesial and distal carinae, lacking any accessory denticle, as observed in C. cavirhinus . The enamel is thin, with a nearly smooth surface only marked by very low apicobasal ridges. The crown apex is complete, with no apical wear visible on any of the four teeth. No clear occlusion facet could be identified along the prepared surfaces (but see below for one in situ tooth). The root is somewhat inflated in all teeth, with a slight to moderate curvature along a plane perpendicular to the curvature of the crown. The surface of the root along the root-crown boundary is smoothened, possibly corresponding to the part emerging from the alveolus, where some degree of hydrowear or acid corrosion may have occurred (see Marx et al. 2023). A couple of anterior maxillary teeth are preserved in situ. One of them displays a long but shallow occlusal surface along the distal surface.

Humerus

Previously figured to highlight shark bite marks ( Bianucci et al. 2018: fig. 13b, c), the complete left humerus of MUSM 2527 ( Fig. 14 ) has a total length of 149 mm, a maximum diameter of the humeral head of 62 mm, a minimum mediolateral width of the diaphysis of 36.7 mm, an anteroposterior length of the distal epiphysis of 60 mm, and a mediolateral width of the distal epiphysis of 38 mm. The large, posterolaterally facing head is prominent, with a well-defined neck. The attachment area for the supraspinatus on the greater tubercle reaches proximally as far as the head. The lesser tubercle extends slightly more proximally, but not to the extent seen in Amphidelphis bakersfieldensis n. comb. YPM 13406 (see below), the humerus YPM 13437 attributed to Macrodelphinus kelloggi , and Xiphiacetus bossi ( Kellogg, 1925) USNM 8842 ( Kellogg 1925: pl. 12), where the lesser tubercle extends farther proximomedially (better seen in posterior view). The diaphysis is rectilinear in lateral view, while in anterior/posterior view its long axis is not as curved as in A. bakersfieldensis n. comb., being more similar to M. kelloggi . The narrow (9 mm), proximodistally elongated (21 mm), and well-defined fossa for the infraspinatus is located 15 mm anterodistal to the head. The moderately anteriorly thickened, 20 mm mediolaterally wide deltoid tuberosity is located at about mid-length of the bone, as in A. bakersfieldensis n. comb., M. kelloggi , and Eurhinodelphis cocheteuxi IRSNB M. 299 ( Abel 1931: pl. 28, fig. 12: collection number 3403). The tuberosity is located more distally in X. bossi . A distinct constriction marks the posterior margin of the diaphysis just before the distal epiphysis. Separated by a rectilinear ridge, the radial and ulnar facets are 31 and 28 mm long, respectively. The latter is followed by a distinct, 10 mm long facet for the olecranon.

Thoracic vertebrae

Three partial anterior thoracic vertebrae, corresponding to bicipital ribs, were found associated with MUSM 2527 ( Fig.15 ). They are listed from Ta to Tc based on the increase of centrum length, ranging from 48 to 58 mm ( Table 2 ), but marked differences in the height of the transverse process, especially between Ta and Tc, suggest that intermediate thoracics could be missing. The three thoracics have a centrum that is wider than long, and while Ta has anterior and posterior heights that are roughly equal to its length, Tb and Tc are proportionally longer. Only Ta has moderately well-preserved transverse processes, projecting anterolateroventrally and each bearing a robust facet for the rib tuberculum. The articulation facets for the rib capitula on the anterior epiphysis are located slightly below mid centrum height; facets on posterior epiphysis are just above mid height. Only the pedicles, base of the neural spine, and base of transverse processes are preserved on Tb, defining a pentagonal neural canal. The posterior epiphysis of this vertebra bears facets for ribs on the dorsolateral corner of the epiphysis that are much more developed than the barely visible facets on the anterior epiphysis. On Tc, in addition to the centrum only part of the left pedicle is preserved. The facets for ribs are similarly located on the dorsolateral corner of the posterior epiphysis.

Ribs

As for the humerus, one of the ribs of MUSM 2527 was figured in Bianucci et al. (2018: fig. 13a) to illustrate shark bite marks. In the same paper, a transverse thin section of a rib from that specimen was also figured, showing sediment infilling in cancellous bone ( Bianucci et al. 2018: fig. 11e).

Figures

FIG. 5. — Cranium of Chilcacetus ullujayensis n. sp. MUSM 2527 (holotype, Chilcatay Formation, East Pisco Basin, Peru) in dorsal view with corresponding line drawing. Hatching for main break surfaces; dashed lines for incomplete parts. Scale bar: 200 mm. Photos by Olivier Lambert.

FIG. 6. — Cranium of Chilcacetus ullujayensis n. sp.MUSM 2527 (holotype,Chilcatay Formation,East Pisco Basin,Peru) in left lateral view (A), detail of the neurocranium in right (B) and left (C) lateral view.Dashed lines for main sutures and some bone outlines.Scale bars: A,200 mm; B, C, 100 mm.Photos by Olivier Lambert.

FIG. 7. — Cranium of Chilcacetus ullujayensis n. sp. MUSM 2527 (holotype, Chilcatay Formation, East Pisco Basin, Peru) in ventral view with corresponding line drawing. Hatching for main break surfaces; dashed lines for reconstructed parts and tentative outline of the posterior process of the left periotic. Scale bar: 200 mm. Photos by Olivier Lambert.

FIG. 8. — Cranium of Chilcacetus ullujayensis n. sp. MUSM 2527 (holotype, Chilcatay Formation, East Pisco Basin, Peru). Detail of the left side of the neurocranium in ventrolateral view with corresponding line drawing. Dashed line for tentative outline of the posterior process of the left periotic. Scale bar: 100 mm. Photos by Olivier Lambert.

FIG. 9. — In situ left periotic of Chilcacetus ullujayensis n. sp. MUSM 2527 (holotype, Chilcatay Formation,East Pisco Basin,Peru) in ventral view with corresponding line drawing. Dashed line for tentative outline of the posterior process of the left periotic. Scale bar: 20 mm. Photos by Olivier Lambert.

FIG. 10. — Left tympanic and left malleus of Chilcacetus ullujayensis n. sp. MUSM 2527 (holotype, Chilcatay Formation, East Pisco Basin, Peru). Tympanic in ventral (A), dorsal (B), medial (C), lateral (D), and posterior (E) view; malleus in posteromedial view (F) and corresponding line drawing. Scale bars: A-E, 20 mm; F, 0.5 mm. Photos by Olivier Lambert.

FIG. 11. — Cranium of Chilcacetus ullujayensis n. sp. MUSM 2527 (holotype, Chilcatay Formation, East Pisco Basin, Peru) in posterior (A) and anterodorsal (B) view. Scale bar: 100 mm. Photos by Olivier Lambert.

FIG. 12. — Partial mandibles of Chilcacetus ullujayensis n. sp. MUSM 2527 (holotype, Chilcatay Formation, East Pisco Basin, Peru) in dorsal view (A); partial left mandible in lateral (B) and medial (C) view; partial right mandible in lateral view (D). Scale bar: 100 mm. Photos by Olivier Lambert.

FIG. 13. — Three detached teeth of Chilcacetus ullujayensis n. sp. MUSM 2527 (holotype, Chilcatay Formation, East Pisco Basin,Peru),the two first still partly embedded in sediment. Scale bar: 20 mm. Photos by Olivier Lambert.

FIG. 14. — Left humerus of Chilcacetus ullujayensis n. sp. MUSM 2527 (holotype, Chilcatay Formation, East Pisco Basin, Peru) in lateral (A), anterior (B), medial (C), and posterior (D) view. Scale bar: 100 mm. Photos by Olivier Lambert.

FIG. 15. — Three thoracic vertebrae of Chilcacetus ullujayensis n. sp. MUSM 2527 (holotype,Chilcatay Formation,East Pisco Basin,Peru). Ta in anterior (A) and left lateral (D) view; Tb in posterior (B) and right lateral (E) view; and Tc in anterior (C) and left lateral (F) view.Scale bar:50 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 2. — Textured 3D model of the cranium of Chilcacetus ullujayensis n. sp. MUSM 2527, available at: https://doi.org/10.7934/P5641