Amphidelphis bakersfieldensis (Wilson, 1935) 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 : 388-394

Amphidelphis bakersfieldensis n. comb. ( Wilson, 1935)

( Figs 16-20 )

Acrodelphis bakersfieldensis Wilson, 1935: 58 .

Argyrocetus bakersfieldensis – Barnes 1976: 325.

‘ Argyrocetus ’ bakersfieldensis – Lambert 2005a: 224.

TYPE MATERIAL. — Holotype. United States • 1 specimen (partial cranium lacking the anterior part of the rostrum, most of the occipital shield, the right side of the basicranium, and the ear bones, with two associated cervical vertebrae and the left humerus); California, Kern County, southwest of Woody ; undifferentiated sediments from Freeman Silt and Jewett Sand; Aquitanian to early Burdigalian (Early Miocene); YPM 13406 .

TYPE LOCALITY. — Approximately eight km southwest of the small town of Woody, Kern County, California, United States, the same locality as the holotype of Macrodelphinus kelloggi , and a short distance from the type locality of the allodelphinid Allodelphis pratti Wilson, 1935 ( Wilson 1935; Kimura & Barnes 2016).

TYPE HORIZON AND AGE. — Undifferentiated sediments from the Freeman Silt and Jewett Sand, 50 feet (about 15 m) lower in the section than the holotype of Allodelphis pratti, Woody Local Fauna, Early Miocene (Aquitanian to early Burdigalian), Saucesian benthic foraminifera stage, Arikareean North American Land Mammal age ( Wilson 1935; Barnes 1976; Shimada et al. 2014; Kimura & Barnes 2016).

NEWLY REFERRED SPECIMEN FROM THE EAST PISCO BASIN. — MUSM 4691, partial cranium lacking the occipital shield and basicranium, with associated partial right mandible. This specimen was found at Roca Negra, a locality close to Zamaca ( Fig. 1B ). Geographic coordinates: 14°38’59.5”S, 75°38’51.8”W. This cranium was reported in the geological map of Zamaca provided by Di Celma et al. (2019) with the field number ZM 36 and referred there to Odontoceti indet. It was collected 25.9 m above the contact with the underlying Otuma Formation in the Ct1a facies association (sandstones and conglomerate beds) of the Ct1 allomember of the Lower Miocene Chilcatay Formation ( Fig. 1E ). The age of Ct1a in the Roca Negra-Zamaca areas is constrained between 19.1 and 18.3 Ma (early Burdigalian), based on a Strontium Isotope Stratigraphy analysis of oyster shells collected in the underlying Ct1c facies association and shark teeth in the overlying Ct2a facies association, respectively ( Bosio et al. 2020a, 2022).

DIAGNOSIS. — The differential diagnosis focuses primarily on differences with taxa that were found to be closely related to Amphidelphis in our phylogenetic analysis and comparison (other members of the Chilcacetus clade, Eoplatanistidae , Eurhinodelphinidae , and Squaloziphiidae ). Amphidelphis bakersfieldensis n. comb. is a small (bizygomatic width estimated at 174 mm in the holotype), longirostrine, and homodont dolphin species differing from Argyrocetus patagonicus in its smaller size, in the rostrum being proportionally considerably shorter (ratio between preorbital width and rostrum length estimated at 0.5), and lacking an extended premaxillary portion, in the dorsal opening of the mesorostral groove being narrower than the premaxilla at rostrum base, in the premaxillary foramen being roughly at the level of the antorbital notch, and in the absence of ankylosis for the symphysis of the mandibles; from ‘ Argyrocetus ’ joaquinensis in its smaller size, in the dorsal opening of the mesorostral groove being narrower than the premaxilla at rostrum base, in the presence of more than one dorsal infraorbital foramen at rostrum base, in the proportionally shorter and wider nasals, and in the nasals partly overhanging the bony nares; from Chilcacetus in its smaller size, in the rostrum being proportionally shorter, in possessing a deep sulcus anterior to the main dorsal infraorbital foramen at rostrum base, and in the palatines not being separated anteromedially for a long distance at rostrum base; from Perditicetus in its smaller size, in the premaxillary foramen being roughly at the level of the antorbital notch, and in the zygomatic process of the squamosal being dorsoventrally more slender; from Caolodelphis in its smaller size, the frontals not being separated anteromedially on the vertex, and the basioccipital crests being transversely thinner; from Macrodelphinus in its much smaller size, in the rostrum being proportionally shorter and lacking an extended premaxillary portion, in the premaxillary foramen being roughly at the level of the antorbital notch, and in the exposure of the frontals on the vertex being shorter and narrower.

It differs from Crisocetus, Dolgopolis , Squaloziphius , and Yaquinacetus in the postglenoid process of the squamosal being significantly shorter anteroposteriorly, and from Dolgopolis , Squaloziphius , and Yaquinacetus in the dorsal opening of the mesorostral groove being more gradual anterior to the bony nares. It differs from most members of other longirostrine to hyper-longirostrine homodont extinct families (including Eoplatanistidae and Eurhinodelphinidae ) in the absence of a deep lateral groove along most of the rostrum and in the absence of ankylosis for the symphysis of the mandibles. It further differs from Eurhinodelphinidae in lacking an extended edentulous anterior premaxillary portion of the rostrum and in the nasals partly overhanging the bony nares. It further differs from Eoplatanistidae in the premaxillary foramen being roughly at the level of the antorbital notch, in the thinner and flatter antorbital process, in the acute anterior margin of the nasal partly overhanging the bony nares, and in the less anteriorly projected supraoccipital shield.

REMARK ON THE TAXONOMY AND SYSTEMATICS

OF AcROdeLPHIS ABEL, 1900 AND ARGYROceTUS

Originally placed in the genus Acrodelphis as Acrodelphis bakersfieldensis n. comb. by Wilson (1935), this species was later referred to the genus Argyrocetus by Barnes (1976), together with the type species Argyrocetus patagonicus and the other Californian species ‘ Argyrocetus ’ joaquinensis . Later, due to the fragmentary state of the type specimens and the lack of synapomorphies for this genus, the referral of the two Californian species to this genus was questioned ( Lambert 2005a; Lambert et al. 2015). The description of the new specimen MUSM 4691 allows for the identification of major differences with A. patagonicus , further supporting the removal of ‘ Argyrocetus ’ bakersfieldensis from the genus Argyrocetus . The original genus, Acrodelphis , has been previously identified by Muizon (1988a) as a junior synonym of Champsodelphis Gervais, 1848 , and the latter has been regarded in the same work as an incertae sedis, restricted to the type specimen (a partial mandible) of its type species, Champsodelphis macrogenius (Fischer, 1829) . A new genus name is thus proposed here for ‘ Argyrocetus ’ bakersfieldensis .

DESCRIPTION AND COMPARISON OF THE NEWLY REFERRED SPECIMEN MUSM 4691 WITH NOTES ON THE HOLOTYPE

A detailed description of the holotype YPM 13406 is to be found in Wilson (1935), together with interpretive line drawings of the cranium in dorsal and left lateral view. This description is complemented here with photos of the cranium in two pieces (partial rostrum with facial region and detached left portion of the basicranium), as well as of the left humerus and two cervical vertebrae.

General cranial features

Cranial dimensions of MUSM 4691 differ little from the holotype of Amphidelphis bakersfieldensis n. comb. YPM 13406 ( Table 3 ; Wilson 1935), being smaller than in Argyrocetus patagonicus and ‘ A. ’ joaquinensis . The roughly complete, pointed rostrum is moderately elongated ( Fig. 16 ; Appendix 4 ), with a ratio between preorbital width and rostrum length estimated to 0.51. This ratio is considerably higher than in Chilcacetus spp. and, most likely, Argyrocetus patagonicus , corresponding to a proportionally shorter rostrum. Though the condylobasal length can only be estimated (between 465 and 480 mm), the rostral index (rostral length/condylobasal length) was most likely in the range of longirostrine odontocetes (between 0.6 and 0.78, see McCurry & Pyenson 2019). The antorbital notches are U-shaped and broadly anterolaterally open (more so on the right side), as in the holotype YPM 13406. The vertex is weakly elevated as seen in lateral view.

Premaxilla

The premaxilla-maxilla suture can be followed until near the anterior end of the maxilla ( Fig. 16C ), indicating that the premaxillary anterior portion of the rostrum was not as extended as in eurhinodelphinids. Based on the extent of the maxillary portion of the rostrum and the length of the mandible, Cabrera (1926) proposed for the type of Argyrocetus patagonicus MLP 5-7 a more than 200 mm-long premaxillary portion of the rostrum. He also suggested that this premaxillary portion was edentulous in the latter, as in eurhinodelphinids. The upper alveolar groove of Amphidelphis bakersfieldensis n. comb. MUSM 4691 reaches the tip of the rostrum ( Fig. 16B ), indicating that there was no edentulous premaxillary portion in this species. Premaxillae are incomplete dorsally in the anterior part of the rostrum. The dorsal opening of the mesorostral groove in this region is thus at least partly artificial. From a level 115 mm anterior to the antorbital notches, both premaxillae contact each other dorsomedially for 25 mm, before a broad, spindle-shaped dorsal opening of the mesorostral groove, as in YPM 13406 ( Fig. 17A ). The premaxillary foramen is 15 mm posterior to the antorbital notch on the right side and 25 mm posterior on the left side, while in YPM 13406 the right premaxillary foramen is only 7.5 mm posterior to the corresponding antorbital notch. The anteromedial sulcus is long (about 82 mm on the right side and at least 76 mm on the left side). The dorsal surface of the narrow prenarial triangle is approximately flat and horizontal. The relatively deep posterolateral sulcus runs until the anterior margin of the bony nares, as in YPM 13406, and the posteromedial sulcus is indistinct. As in YPM 13406, on the lateral edge of the transversely and anteroposteriorly concave premaxillary sac fossa, the premaxilla is dorsoventrally thick, forming a pedestal medial to the posterolateral sulcus. While the dorsal part of the ascending process is abraded on the right side, the roughly complete left side reveals a moderately pointed posterior end of the premaxilla, displaying an extensive contact with the corresponding nasal.

Maxilla

In dorsal view, the maxilla is wider than the premaxilla at the level of the antorbital notch ( Fig. 16A ), a condition that differs from YPM 13406. As noted by Wilson (1935), the maxilla of YPM 13406 is marked by a shallow and broad longitudinal groove extending on the maxilla, below the premaxilla-maxilla suture, for 100-120 mm anterior to the anteriormost dorsal infraorbital foramen. A similar groove extends forwards for only about 40 mm in MUSM 4691. In both specimens the groove is less developed than the deep lateral groove following the premaxilla-maxilla suture, on a much longer distance, in various clades of longirostrine to hyper-longirostrine odontocetes. On the right side, a small (transverse diameter 1.5 mm) dorsal infraorbital foramen is just anterior to the antorbital notch; a larger foramen (diameter 4.5 mm) is slightly posterior to the notch; and a smaller foramen (diameter 2 mm) is 25 mm posterior to the notch. Three dorsal infraorbital foramina are also observed on the left side: the first is posteromedial to the notch (diameter 4 mm); the second is just posteromedial (diameter 2.5 mm); and the third is posterolateral (diameter 3 mm). In the slightly dorsally domed antorbital region, the maxilla only partly covers the frontal, with a dorsal exposure of the latter that has an outline strikingly similar to YPM 13406 ( Fig. 17A ). As in the latter, a deep oblique groove directed towards the antorbital notch marks the dorsal surface of the right maxilla in this region.

In lateral view, the maxilla-premaxilla suture raises posterodorsally from about mid rostrum length, becoming slightly dorsally convex in the posterior part of the rostrum ( Fig. 16C ). The ventral surface of the rostrum is relatively flattened, and the alveolar groove is close to the lateral margin ( Fig. 16B ), a condition that is reminiscent of Yaquinacetus meadi . Part of the upper alveoli are preserved on both sides. As mentioned above, the anteriormost alveoli may correspond to premaxillary teeth, though most preserved alveoli are located in the maxilla. 31 small, circular alveoli with a diameter ranging from 2.5 to 3 mm are counted on a length of 155 mm, with interalveolar septa ranging from 2 to 3 mm in length. This corresponds to 20 alveoli for 100 mm, a number that is close to YPM 13406 (20 alveoli for 108 mm; Wilson 1935; Fig. 17B ). One pair of tiny palatine foramina is located 65 mm anterior to the level of the antorbital notches, each followed anteriorly by a narrow sulcus; a second pair may be present 8 mm posteriorly, but the surface is too damaged for a clear identification.

Nasal

The nasal extends dorsally higher than the frontal, making the top of the cranium. The anterodorsal portion of each nasal is strongly abraded, indicating that it may have been as dorsally elevated as in YPM 13406 ( Fig. 17C ) and Argyrocetus patagonicus (nasals of the type MLP 5-7 are now lost, but original illustrations of the skull by Lydekker (1893: plate 5) reveal anterodorsally elevated nasals).

Frontal

The frontal-supraoccipital suture is possibly preserved, 10 mm anterior to the preserved posterior edge of the specimen ( Fig. 16A ); if correct, this interpretation indicates a 10 mm-long frontal exposure on the vertex, with a smooth, slightly transversely convex dorsal surface. In lateral view, the preorbital process is dorsoventrally thin, as in YPM 13406 ( Fig. 17C ). Both postorbital processes are missing, and the preserved lateral margin of the orbit roof is slightly ventrally concave in lateral view.

Jugal

Only the preserved base of the styliform process is visible posteromedial to the antorbital notch ( Fig. 16B ).

Squamosal

Not preserved in MUSM 4691, the squamosal displays a relatively short and dorsoventrally high zygomatic process with a distinctly convex dorsal outline in the holotype YPM 13406 ( Fig. 18A ). The irregular outline of its anterior tip both in ventral and lateral view suggests that it may be incomplete. Two deep sternocephalicus fossae extend to a level slightly anterior to the ventral tip of the postglenoid process. Both fossae are visible in posterior view, not being hidden by the exoccipital. The postglenoid process is moderately anteroposteriorly thickened, with a rounded ventral outline in lateral view. Its medial wall is invaded by the posterolateral part of the tympanosquamosal recess ( Fig. 18B ). The latter extends anteriorly along the medial side of the broad mandibular fossa, ending approximately at the level of the anterior margin of the squamosal fossa.

Exoccipital/basioccipital

In the holotype, the left paroccipital process is separated from the transversely thin basioccipital crest by a deep and narrow jugular notch, in which the hypoglossal foramen is visible ( Fig. 18C ).

Mandible

The right mandible of MUSM 4691 is preserved on a length of 310 mm, including 114 mm of the posterior part of the symphyseal region ( Fig. 19 ). The symphysis is not ankylosed, a major difference with eurhinodelphinids. This region is narrow, with height and width of the bone at the end of the symphysis being 27 and 10 mm, respectively. Posterior to the symphysis, the ventral margin of the bone is moderately concave in lateral view. Approximately 34 small alveoli (diameter between 3 and 3.5 mm) are counted on a length of 172 mm, with interalveolar septa 2 to 2.5 mm long. This count for lower alveoli matches well the upper count.

Humerus

The left humerus of the holotype YPM 13406 ( Fig. 20 A-D) is characterized by a low greater tubercle which does not extend as far proximally as the robust lesser tubercle, a proximo-distally long but moderately prominent deltoid tuberosity that is centered at about mid-length of the diaphysis, a deep fossa for the infraspinatus, and a short facet for the olecranon (see comparison with humeri of Chilcacetus ullujayensis n. sp. and Macrodelphinus kelloggi above).Measurements are provided inWilson (1935:69).

Cervical vertebrae

A description and measurements of the two unfused cervical vertebrae of the holotype YPM 13406 ( Fig. 20E, F ), identified there as C4 and C6, are provided in Wilson (1935: 67, 68). Though the centra are described as anteroposteriorly short by Wilson (1935) (ratio between width and length without epiphyses = 3.9 in C4 and ratio between width and length with one epiphysis = 2.6 in C6), their proportions do not differ much from the cervicals of Xiphiacetus cristatus , comparable to the extant freshwater dolphins Inia Orbigny, 1834 and Platanista Wagler, 1830 ( Lambert 2005a).

COMMENTS

Despite a few minor differences with the holotype YPM 13406, including a slightly wider dorsal opening of the mesorostral groove, a relatively broader dorsal exposure of the maxilla at rostrum base, and a slightly more anterior premaxillary foramen, MUSM 4691 is morphologically similar enough to YPM 13406 to allow for its attribution to the same species, Amphidelphis bakersfieldensis n. comb. Differences between these two individuals may be explained by intraspecific variation; YPM 13406 represents for example a relatively young animal, as indicated by unfused epiphyses for two cervical vertebrae ( Fig. 20E, F ; Galatius & Kinze 2003)

The attribution of MUSM 4691 to A. bakersfieldensis n. comb. provides information on cranial regions that are not preserved in the holotype. The anterior portion of the rostrum is especially informative for the comparison with Argyrocetus patagonicus and eurhinodelphinids.

Figures

FIG. 16. — Partial cranium of Amphidelphis bakersfieldensis n. comb. MUSM 4691 (Chilcatay Formation, East Pisco Basin, Peru) in dorsal view (A) with corresponding line drawing, in ventral view (B), and in right lateral view (C). Hatching for main break surfaces; dashed lines on photo for maxilla-premaxilla suture. Scale bar: 200 mm. Photos by Olivier Lambert.

FIG. 17. — Partial cranium of Amphidelphis bakersfieldensis n. comb. YPM 13406 (holotype, Kern County, California, United States) in dorsal (A), ventral (B), and right lateral (C) view. Scale bar: 100 mm. Credits: Photos by Advait Jukar. Photos by Olivier Lambert.

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.

FIG. 19. — Partial right mandible of Amphidelphis bakersfieldensis n. comb. MUSM 4691 (Chilcatay Formation, East Pisco Basin, Peru) in lateral (A), dorsal (B), and medial (C) view. Scale bar: 100 mm. Photos by Olivier Lambert.

FIG. 20. — Left humerus and two cervical vertebrae of Amphidelphis bakersfieldensis n. comb. YPM 13406 (holotype, Kern County, California, United States). Humerus in lateral (A), anterior (B), medial (C), and posterior (D) view;?4th cervical in posterior view (E); 6th cervical in posterior view (F). Scale bar: 50 mm. Credits: Photos by Advait Jukar.

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 4. — Textured 3D model of the cranium of Amphidelphis bakersfieldensis n. comb. MUSM 4691, available at: https://doi.org/10.7934/P5641