Odontoceti aff. Amphidelphis bakersfieldensis (Wilson, 1935)

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 : 394-400

Odontoceti aff. Amphidelphis bakersfieldensis n. comb.

( Figs 21-25 )

REFERRED SPECIMENS FROM THE EAST PISCO BASIN. — MUSM 602, partial cranium lacking the anterior part of the rostrum, part of the roof of the temporal fossae, ear bones, and teeth, with the ventral surface of the basicranium and palate damaged. This specimen was collected in 1993 by M. Urbina in an undetermined level of the Lower Miocene Chilcatay Formation from the locality of Zamaca ( Fig. 1B ). MUSM 4961, partial cranium lacking most of the rostrum, ear bones, and teeth, with superficial bone being either damaged or covered with a thin layer of hard sediment in many regions of the neurocranium. This specimen was collected at an unknown date by M. Urbina in an undetermined level of the Chilcatay Formation from the locality of Roca Negra ( Fig. 1B ; see Lambert et al. 2018). Considering the above-mentioned dates obtained through 40Ar/39Ar radiometric analyses and Strontium Isotope Stratigraphy, and in agreement with silicoflagellate and diatom biostratigraphy, the age of MUSM 602 can be restricted between 19.3 and 17.95 Ma and the age of MUSM 4961 between 19.3 and 18.3 Ma (both Burdigalian) ( Lambert et al. 2018; Di Celma et al. 2019; Bosio et al. 2020b).

DESCRIPTION OF REFERRED SPECIMENS

Most of the description is based on the better-preserved cranium MUSM 602 ( Figs 21-24 ), except for the few parts that are only preserved in MUSM 4961 ( Fig. 25 ; Appendix 5 ), that differ between the two specimens, or that display shared differences with Amphidelphis bakersfieldensis n. comb. Both specimens have similar cranial dimensions, with MUSM 4961 being only slightly smaller. Except for the vertex of the cranium that is transversely broader than in YPM 13406, no marked differences are noted with specimens of A. bakersfieldensis n. comb.

Premaxilla

Though the rostrum of MUSM 602 appears to be slightly transversely crushed in its anterior portion, right and left premaxillae most likely contacted each other dorsomedially for at least 100 mm, before a gradual divergence of the medial margins towards the bony nares ( Fig. 21 ). In both specimens, and to an even greater extent in MUSM 4961 ( Fig. 25A ), the resulting dorsal opening of the mesorostral groove is narrower than in MUSM 4691 and YPM 13406 (and also ‘ Argyrocetus ’ joaquinensis , as well as squaloziphiids) and is not spindle-shaped. In MUSM 602, the premaxillary foramen is 15 mm posterior to the antorbital notch on the right side and 20 mm posterior on the left side. As in Amphidelphis bakersfieldensis n. comb., the anteromedial sulcus is long (110 mm on the right side and 120 mm on the left). At least in MUSM 602, the prenarial triangle is longer than in A. bakersfieldensis n. comb. The posteromedial sulcus is poorly defined, and the posterolateral sulcus is narrow, becoming nearly indiscernible before the anterior margin of the bony nares. The premaxillary sac fossae are generally less dorsally elevated in MUSM 602 (but not in MUSM 4961) than in A. bakersfieldensis n. comb.; they are transversely concave, with an anterior portion that is also longitudinally concave while the thicker posterior portion has a roughly flat surface. At the level of the anterior margin of the bony nares, the lateral margin of each premaxilla is laterally concave, with an abrupt posteromedial turn at mid length of the bony nares. Better preserved on the right side of MUSM 602, the ascending process of the premaxilla is narrower than in MUSM 4691 and YPM 13406, with parallel lateral and medial margins, a condition that is reminiscent of Squaloziphius emlongi and Yaquinacetus meadi . Posteromedially, each premaxilla broadly contacts the corresponding nasal. Due to the complex and not fully resolved outline of the nasal-frontal suture, the presence of a contact between premaxilla and frontal on the vertex cannot be assessed.

Maxilla

Along the rostrum base, the lateral margin of the maxilla is incomplete in MUSM 602. In MUSM 4961, this margin is directed more posterolaterally than in MUSM 4691 and YPM 13406, suggesting that both MUSM 602 and MUSM 4961 had a broader rostrum base compared to A. bakersfieldensis n. comb. In both MUSM 602 and 4961, the lateralmost portion of the dorsal surface of the maxilla is moderately thickened just anteromedial to the antorbital notch (maxillary flange). In both MUSM 602 and MUSM 4961, the antorbital notch is much more laterally open and shallow than in MUSM 4691 and YPM 13406; differing from the latter (and also Chilcacetus spp. , Macrodelphinus kelloggi , Perditicetus yaconensis , Squaloziphius emlongi , and Yaquinacetus meadi ), the antorbital region does not extend anterior to the bottom of the antorbital notch, in a way more similar to ‘ Argyrocetus ’ joaquinensis . The dorsal surface of the maxilla is flat in this area. In MUSM 602, four dorsal infraorbital foramina are present in the right antorbital notch region and three in the left region. Along the vertex, the dorsomedial margin of the maxilla overhangs the more ventral part of the bone. Posteriorly, the maxilla similarly raises abruptly to contribute to the acute nuchal crest. Roughly rectilinear along the vertex, the nuchal crest turns moderately posterolaterally towards the posteriormost part of the roof of the temporal fossa. Roughly complete in MUSM 4961, the posterolateral corner of the maxilla is more medial than the postorbital process in dorsal view, indicating that the posterior part of the temporal fossa remained dorsally open to an extent similar to YPM 13406 (at least for the right side of the latter).

In ventral view, a few small maxillary alveoli are visible in the posterior portion of the alveolar groove ( Fig. 23A ). The first posterior alveolus is 50 mm anterior to the antorbital notch, but it cannot be excluded that a few more posterior alveoli have been abraded. The mean diameter of the preserved alveoli is 3 mm and interalveolar septa are short, 1-2 mm long, a condition that does not differ substantially from Amphidelphis bakersfieldensis n. comb.

Vomer

In MUSM 602, the vomer is exposed in the vomerine trough from a level 62 mm anterior to the antorbital notch ( Fig. 23A ).

Lacrimojugal complex

Better preserved on the right side of MUSM 602, the lacrimojugal complex sends a thin plate laterally, anteroventral to the preorbital process of the frontal ( Fig. 23 ). A similar plate is instead directed anterolaterally in Amphidelphis bakersfieldensis n. comb. This plate thickens dorsoventrally in its lateralmost portion, up to 7 mm.

Palatine/pterygoid

The pointed anterior end of the joined palatines is 23 mm anterior to the level of the antorbital notch in MUSM 602. The palatine-maxilla suture gradually diverges posterolaterally for about 20 mm before abruptly turning towards the ventral infraorbital foramen. No parts of the pterygoid could be identified on any of the two specimens, except for part of the medial lamina along the pharyngeal crest. The pterygoid sinus fossa is anteriorly short, with an anterior margin 25 mm posterior to the level of the antorbital notch, a feature that is similarly seen in Amphidelphis bakersfieldensis n. comb., Chilcacetus spp. , Macrodelphinus kelloggi , Perditicetus yaconensis , and Yaquinacetus meadi ; the fossa is somewhat anteriorly longer in Squaloziphius emlongi and, to a greater extent, Dolgopolis kinchikafiforo .

Nasal

As in Amphidelphis bakersfieldensis n. comb., the top of the cranium is made by the nasals, with a dorsal surface sloping posteroventrally ( Figs 21 ; 22 ; 24A ). Their anterodorsal portion is damaged in MUSM 602; the break surface suggests that each nasal originally projected anterodorsally, as seen in YPM 13406, overhanging the bony nares for more than 4 mm, in a way similar to Chilcacetus spp. The outline of the nasal-frontal suture is complex and difficult to interpret, due to a series of deep grooves, as in YPM 13406, but also Chilcacetus cavirhinus and Yaquinacetus meadi . The nasals were either as long or longer than the frontals on the vertex. As preserved, the left nasal is wider than long.

Frontal

As in YPM 13406, the dorsal surface of the frontals on the vertex of MUSM 602 and MUSM 4961 is irregular, marked by a series of meandering grooves, pits, and crests, a feature also seen for example in Chilcacetus cavirhinus and Macrodelphinus kelloggi .

As in Amphidelphis bakersfieldensis n. comb., the preorbital process of the frontal is dorsally exposed. In lateral view ( Fig. 22 ), it is only moderately thickened and anteroventrally margined by the lacrimal. Better preserved on the left side of MUSM 602 and right side of MUSM 4961, the postorbital process is robust and directed ventrally. Except for the right side of MUSM 602, the posterior surface of the postorbital process roughly contacts the similarly robust anterior margin of the zygomatic process of the squamosal. A slight degree of post-mortem deformation may have either separated the two bones on the right side of MUSM 602 or, less likely, brought these closer to each other on the left side of the latter and in MUSM 4961. In ventral view, the infratemporal crest is distinct from the tip of the postorbital process to the posterior wall of the frontal groove, as seen in YPM 13406.

Supraoccipital

The dorsomedial part of the occipital shield is anteroporsteriorly concave, this depressed region being anteriorly defined by the high posterior wall of the nuchal crest ( Figs 21 ; 22 ; 24A ). From the nuchal crest, the surface of the shield leaves in a nearly horizontal posterior direction, as in YPM 13406, before becoming markedly anteroposteriorly convex. A broad, longitudinal groove separates two posteriorly swollen regions corresponding to the right and left brain hemispheres. Only partly preserved in both specimens, the temporal crest was originally short but distinct, and directed posterolateroventrally.

Exoccipital

The occipital condyles protrude posterior to the rest of the exoccipital, with a well-defined condylar neck and moderately excavated dorsal condyloid fossae ( Figs 22 ; 24B ). The posterior surface of the exoccipital slopes more posteroventrally in MUSM 602 compared to MUSM 4961 and YPM 13406, but this may be the result of some post-mortem deformation. The jugular notch is too incomplete on the two specimens to be compared to the deep notch observed in Amphidelphis bakersfieldensis n. comb. YPM 13406.

Basioccipital

Only the dorsal part of the basioccipital crests is preserved in both specimens, defining a basioccipital basin that is narrow anteriorly (38 mm at about the posterior tip of the pterygoids in MUSM 602), with a gradual posterior widening (distance between the two crests reaching 45 mm at their posterior end). The medial surface of each crest is marked by a ridge that turns anterodorsally and dorsomedially.

Squamosal

The outline of the zygomatic process of the squamosal varies from one side to the other and from one specimen to the other ( Figs 22 ; 25B ). It is more slender, anteroposterioly elongated on the right side of MUSM 602 and markedly dorsoventrally thicker (22 mm) and shorter on the right side of MUSM 4961, with intermediary conditions on the left side of MUSM 602 and left side of MUSM 4961. The preserved right zygomatic process of Amphidelphis bakersfieldensis n. comb. YPM 13406 ( Fig. 18A ) is more similar to the left side of MUSM 4961. The anterior margin of the process is anteriorly concave in lateral view, with a shallow transverse groove at mid-height possibly corresponding to the surface of contact with the postorbital process of the frontal. The shape of the dorsal margin of the zygomatic process varies also markedly, from roughly rectilinear on the right side of MUSM 602 and left side of MUSM 4961 to concavo-convex on the other sides of these two specimens, with a specially dorsally bulging posterior part on the left side of MUSM 4961. Such a variation may be related to a highly variable development of the sternocephalicus fossa, which is much deeper and anteriorly extended on the right side of MUSM 4961 and, probably, the left side of MUSM 602. Such an extent of morphological variation between the two sides of a single specimen should urge to remain cautious with the use of zygomatic process characters to diagnose new taxa. On the right side of MUSM 602, the sternocephalicus fossa is subdivided in two fossae by a thick oblique crest, a condition that is similarly observed in YPM 13406.

Only preserved in MUSM 4961 ( Fig. 25B ), the postglenoid process is ventrally long, reaching much farther than the posttympanic process, and slightly anteroposteriorly longer (more than 19.5 mm) than transversely thick (18 mm) on the left side. This process appears only slightly anteroposteriorly longer than in YPM 13406 and does not differ much from ‘ Argyrocetus ’ joaquinensis, Caolodelphis milleri, Chilcacetus ullujayensis n. sp., and Perditicetus yaconensis . Based on the preserved proximal part, similar proportions of the postglenoid process can be proposed for MUSM 602. As for the corresponding zygomatic process, the right postglenoid process of MUSM 4961 displays unusual proportions, forming an oblique plate that is 10 mm thick and 22 mm long. It cannot be excluded that such a highly asymmetric condition is pathological.

In ventral view, the tip of the ventral surface of the zygomatic process displays a circular articulation surface for the jugal, with an anteroposterior diameter of 12 mm in MUSM 602 ( Fig. 23 ). The mandibular fossa is broad (24 mm on the right side) and medially margined by a shallow tympanosquamosal recess. Separated from the mandibular fossa by a well-defined step, the latter extends anterolaterally for a short distance beyond the level of the anterior margin of the squamosal fossa, in a way similar to YPM 13406. The falciform process is not adequately preserved in any of the two specimens.

The squamosal fossa is shorter in MUSM 4961 compared to MUSM 602, lacking the anterior part with an anteroposteriorly convex floor seen in the latter.

COMMENTS

These two crania are highly similar in both their dimensions and main cranial features. Several differences were noted above, and those mostly focus on the squamosal (zygomatic process, sternocephalicus fossa, and postglenoid process). Because strong differences are also noted between the left and right sides of one specimen for this region, their diagnostic value can be reasonably questioned. Therefore, it is proposed that MUSM 602 and MUSM 4961 belong to the same species. As they share several differences with Amphidelphis bakersfieldensis n. comb. MUSM 4691 and YPM 13406, including the outline of the rostrum base in dorsal view, the shape of the antorbital notch, and the extent of the dorsal opening of the mesorostral groove at rostrum base, it cannot be excluded that they correspond to another, closely related species. Furthermore, the lack of more anterior sections of the rostrum prevents from investigating the relative anterior extent of the premaxilla and maxilla, a key character to test for eurhinodelphinid affinities. Pending the discovery of more complete and better-preserved specimens to provide diagnostic characters, it is proposed to identify these two specimens as Odontoceti aff. A. bakersfieldensis n. comb.

Figures

FIG. 21. — Partial cranium of Odontoceti aff. Amphidelphis bakersfieldensis MUSM 602 (Chilcatay Formation, East Pisco Basin, Peru) in dorsal view with corresponding line drawing and detail view of the vertex in dorsal view. Hatching for main break surfaces. Scale bar: 100 mm. Photos by Olivier Lambert.

FIG. 22. — Partial cranium of Odontoceti aff. Amphidelphis bakersfieldensis MUSM 602 (Chilcatay Formation, East Pisco Basin,Peru) in left (A) and right (B) lateral view. Scale bar: 100 mm. Photos by Olivier Lambert.

FIG. 23. — Partial cranium of Odontoceti aff. Amphidelphis bakersfieldensis MUSM 602 (Chilcatay Formation, East Pisco Basin, Peru) in ventral view (A) with corresponding line drawing and detail view of the right half of the basicranium in ventral view (B). Hatching for main break surfaces. Scale bars: A, 100 mm; B, 50 mm. Photos by Olivier Lambert.

FIG. 24. — Partial cranium of Odontoceti aff. Amphidelphis bakersfieldensis MUSM 602 (Chilcatay Formation, East Pisco Basin, Peru) in anterodorsal (A) and posterior (B) view. Scale bar: 100 mm. Photos by Olivier Lambert.

FIG. 25. — Partial cranium of Odontoceti aff. Amphidelphis bakersfieldensis MUSM 4961 (Chilcatay Formation, East Pisco Basin, Peru) in dorsal (A), right lateral (B), ventral (C), and posterior (D) view. Scale bar: 100 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 5. — Textured 3D model of the cranium of Odontoceti aff. Amphidelphis bakersfieldensis MUSM 602, available at: https://doi.org/10.7934/P5641

FIG. 18. — Detached left part of the basicranium of Amphidelphis bakersfieldensis n. comb. YPM 13406 (holotype, Kern County, California, United States) in lateral (A), ventral (B), and posterior (C) view. Scale bar: 50 mm. Credits: Photos by Advait Jukar. Photos by Olivier Lambert.