Amphidelphis bakersfieldensis Lambert, Muizon, Bennion, Urbina & Bianucci, 2025, n. comb.
Year: 2025
Type: other
Abstract: 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 13
Authors: Lambert, Olivier, Muizon, Christian de, Bennion, Rebecca F., Urbina, Mario, Bianucci, Giovanni
Language: English
Subfield: Paleontology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W6968055003
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