Chilcacetus 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 : 372

Genus Chilcacetus Lambert, Muizon & Bianucci, 2015

TYPE SPECIES. — Chilcacetus cavirhinus Lambert, Muizon & Bianucci, 2015 by original designation.

OTHER REFERRED SPECIES. — Chilcacetus ullujayensis n. sp.

EMENDED DIAGNOSIS. — The differential diagnosis focuses primarily on differences with taxa that were found to be closely related to Chilcacetus in our phylogenetic analysis and comparison (other members of the Chilcacetus clade, Eoplatanistidae Muizon, 1988 , Eurhinodelphinidae , and Squaloziphiidae ( Muizon, 1991)). Chilcacetus is a medium-size (bizygomatic width ranging from about 240 to 280 mm), longirostrine (ratio between rostrum length and condylobasal length from 0.68 to 0.73), and homodont dolphin genus differing from all other odontocetes except Eoplatanista Dal Piaz, 1916 , Eurhinodelphinidae, Physeteroidea , Yaquinacetus Lambert, Godfrey & Fitzgerald, 2019 , and Ziphiidae Gray, 1850 in the tubercule of the malleus being shorter than the articular head; from all other odontocetes except ‘ Argyrocetus ’ joaquinensis , Ninoziphius Muizon, 1983 , Olympicetus Vélez-Juarbe, 2017 , Simocetus Fordyce, 2002 , Squaloziphius Muizon, 1991 , and Ziphiodelphis abeli Dal Piaz, 1908 in having a finger-like posterior projection of the hamular process of the pterygoid; from all other odontocetes except Amphidelphis bakersfieldensis n. comb., ‘ Argyrocetus ’ joaquinensis, Crisocetus Gaetán, Paolucci & Buono, 2023 , Dolgopolis Viglino, Gaetán, Cuitiño & Buono, 2021 , Eoplatanista , Eurhinodelphinidae , Perditicetus Nelson & Uhen, 2020, Squaloziphius and Yaquinacetus in the postglenoid process of the squamosal being anteroposteriorly long and transversely thick (this process being even anteroposteriorly longer in Crisocetus, Dolgopolis , Squaloziphius , and Yaquinacetus ); from Dolgopolis , Squaloziphius and Yaquinacetus in the dorsal opening of the mesorostral groove being more gradual anterior to the bony nares; from most members of other longirostrine to hyper-longirostrine homodont extinct families ( Allodelphinidae , Eoplatanistidae , Eurhinodelphinidae , and Platanistidae ) in the absence of a deep lateral groove along the rostrum, the posterior process of the periotic being elongated in a posterolateral direction and transversely broad, 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, in the mandible being roughly as long as the rostrum, in the nasals partly overhanging the bony nares, in the more anteriorly elongated zygomatic process of the squamosal (ratio between the length and the height of the process ≥ 1.10), in the neurocranium being distinctly longer than wide, in the lesser transverse widening of the occipital shield (ratio between maximum width of supraoccipital at the lateral corners of the nuchal crest and postorbital width <0.70), in the posterior margin of the postorbital process being vertical, in the top of the temporal fossa being nearly as high as the nuchal crest, in the palatines being separated anteromedially for a long distance at rostrum base, and in the longer and more laterally directed posterior process of the periotic.

It 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, in the transversely concave and less anteriorly projected anterodorsal portion of the supraoccipital shield, in the deep anterior bullar facet of the periotic, and in bearing a conspicuous median furrow on the tympanic bulla.

It differs from Argyrocetus patagonicus in the premaxillae being transversely wider at rostrum base, in lacking a wide dorsal opening of the mesorostral groove, in the premaxillary foramen being roughly at the level of the antorbital notch, in the angle formed by the basioccipital crests in ventral view <50°, in the top of the temporal fossa being nearly as high as the nuchal crest, and in the absence of ankylosis for the symphysis of the mandibles. It differs from Amphidelphis bakersfieldensis n. comb. in its larger size, in lacking a deep sulcus anterior to the main dorsal infraorbital foramen at rostrum base, and in the palatines being separated anteromedially for a long distance at rostrum base. It differs from ‘ Argyrocetus ’ joaquinensis in the dorsal opening of the mesorostral groove anterior to the rostrum base being narrower than the premaxilla, in the presence of more than one dorsal infraorbital foramen at rostrum base, in lacking a deep sulcus anterior to the main dorsal infraorbital foramen at rostrum base, in the proportionally shorter and wider nasal, in the nasal partly overhanging the bony nares, in the wide exposure of the frontal on the vertex, in the posterior margin of the postorbital process being vertical, in the palatines being separated anteromedially for a longer distance at rostrum base, and in the ventral margin of the postglenoid process of the squamosal being approximately at the same level as the ventral margin of the exoccipital in lateral view. It differs from Caolodelphis Godfrey & Lambert, 2023 in its larger size, the frontals not being separated anteromedially on the vertex, and the basioccipital crests being transversely thinner. It differs from Macrodelphinus in its smaller size, in the premaxillary portion of the rostrum making less than 10 % of its total length, in the premaxillary foramen being roughly at the level of the antorbital notch, in the nasal proportionally longer compared to the frontal on the vertex, in the palatines being separated anteromedially for a longer distance at rostrum base. It differs from Perditicetus in in the premaxillary foramen being roughly at the level of the antorbital notch and the zygomatic process of the squamosal being dorsoventrally more slender. It differs from Papahu Aguirre-Fernández & Fordyce, 2014 in the rostrum being proportionally dorsoventrally thicker in its proximal part, in the single premaxillary foramen being roughly at the level of the antorbital notch, in the dorsal exposure of the premaxilla being wider than the dorsal exposure of the maxilla at rostrum base, in the proportionally wider ascending process of the premaxilla, in the anterodorsal elevation of the dorsal surface of the nasal, in the posterolateral projection of the nasal, in the elongate postorbital process of the frontal, in the longer and deeper anterior bulla facet of the periotic, and in the posterior elevation of the dorsal margin of the mandible being progressive.