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| Xenorophus simplicidens Boessenecker & Geisler, 2023 |
Abstract
The early diverging, dolphin-sized, cetacean clade Xenorophidae are a short-lived radiation of toothed whales (Odontoceti) that independently evolved two features long thought to be odontocete synapomorphies: the craniofacial and cochlear morphology underlying echolocation and retrograde cranial telescoping (i.e., posterior migration of the viscerocranium). This family was based on Xenorophus sloanii, which, for the past century, has been known only by a partial skull lacking a braincase and tympanoperiotics, collected around 1900 from the Ashley Formation (28–29 Ma, Rupelian) near Ladson, South Carolina. A large collection of new skulls and skeletons (ChM PV 5022, 7677; CCNHM 104, 168, 1077, 5995) from the Ashley Formation considerably expands the hypodigm for this species, now the best known of any stem odontocete and permitting evaluation of intraspecific variation and ontogenetic changes. This collection reveals that the holotype (USNM 11049) is a juvenile. Xenorophus sloanii is a relatively large odontocete (70–74 cm CBL; BZW = 29–31 cm; estimated body length 2.6–3 m) with a moderately long rostrum (RPI = 2.5), marked heterodonty, limited polydonty (13–14 teeth), prominent sagittal crest and intertemporal constriction, and drastically larger brain size than basilosaurid archaeocetes (EQ = 2.9). Dental morphology, thickened cementum, a dorsoventrally robust rostrum, and thick rugose enamel suggest raptorial feeding; oral pathology indicates traumatic tooth loss associated with mechanically risky predation attempts. Ontogenetic changes include increased palatal vomer exposure; fusion of the nasofrontal, occipito-parietal, and median frontal sutures; anterior lengthening of the nasals; elaboration of the nuchal crests; and blunting and thickening of the antorbital process. The consistent deviation of the rostrum 2–5° to the left and asymmetry of the palate, dentition, neurocranium, mandibles, and vertebrae in multiple specimens of Xenorophus sloanii suggest novel adaptations for directional hearing driven by the asymmetrically oriented pan bones of the mandibles. A second collection consisting of a skeleton and several skulls from the overlying Chandler Bridge Formation (24–23 Ma, Chattian) represents a new species, Xenorophus simplicidens n. sp., differing from Xenorophus sloanii in possessing shorter nasals, anteroposteriorly shorter supraorbital processes of the frontal, and teeth with fewer accessory cusps and less rugose enamel. Phylogenetic analysis supports monophyly of Xenorophus, with specimens of Xenorophus simplicidens nested within paraphyletic X. sloanii; in concert with stratigraphic data, these results support the interpretation of these species as part of an anagenetic lineage. New clade names are provided for the sister taxon to Xenorophidae (Ambyloccipita), and the odontocete clade excluding Xenorophidae, Ashleycetus, Mirocetus, and Simocetidae (Stegoceti). Analyses of tooth size, body size, temporal fossa length, orbit morphology, and the rostral proportion index, prompted by well-preserved remains of Xenorophus, provide insight into the early evolution of Odontoceti.
Keywords: Xenorophidae; Odontoceti; Cetacea; Oligocene; asymmetry
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| Geographic and geologic context of Xenorophus fossils from South Carolina. (A) Map of South Carolina. (B) Simplified geologic map of Oligocene rocks within the Charleston Embayment. (C) Stratigraphic column of the Ashley and Chandler Bridge formations of South Carolina, showing stratigraphic position of age determinations (stars) and silhouettes of the xenorophid assemblage known from each stratum. Modified from Boessenecker et al. ([2023]: Figure 1). |
Systematic Paleontology
Mammalia Linneaus,1758
Cetacea Brisson, 1762
Odontoceti Flower, 1867
Xenorophidae Uhen, 2008
Xenorophus Kellogg, 1923
Xenorophus simplicidens sp. nov.
Xenorophus n. sp. Sanders, 1996
Xenorophus sp. Sanders and Geisler, 2015
Holotype: CCNHM 8720, a partial skeleton including a nearly complete but crushed skull, right mandible, nearly complete upper and lower dentition, five cervical vertebrae, eight or nine thoracic vertebrae, nine or ten lumbar vertebrae, and at least six ribs, collected in 2017 by Dean Rogers, Everett White, and Joshua Basak from Bed 2 of the Chandler Bridge Formation in the vicinity of North Charleston, Charleston County, South Carolina.
Diagnosis of Species: A large xenorophid dolphin, with approximate adult condylobasal length of 68–74 cm and bizygomatic width of 27–30.8 cm and differing from Xenorophus sloanii in possessing anteroposteriorly shorter nasals (16–19% of bizygomatic width vs. 22–29% in X. sloanii), nasal process of premaxilla lacking lateral overhanging crest adjacent to bony nares in adult specimens, right antorbital fossa longer than left (left longer than right in X. sloanii), rounded anterior margin of bony nares, anteroposteriorly shorter supraorbital process (distance from antorbital notch to posterior edge of supraorbital process 34% of bizygomatic width vs. 43% in X. sloanii), slightly narrower nasals with posterior end narrower than bony nares, left and right palatines separated by an anteroposteriorly long median triangular exposure of maxilla, paroccipital processes not extending posterior to occipital condyles, longer median furrow on the tympanic bulla (except ChM PV 4266), fewer teeth with accessory denticles (only on PC 7–9, as opposed to PC 5–9 in X. sloanii), fewer accessory denticles per tooth (three distal cusps vs. five in X. sloanii; one–three mesial cusps vs. three–four in X. sloanii), widespread striated enamel throughout dentition and nodular enamel only present on PC7–9 and less rugose than in X. sloanii (nodular enamel on PC5–9).
Etymology: simplicidens—simplex + dens—meaning simple teeth in Latin, referring to the reduced cingula and smoother enamel of this species. This name was coined by Albert Sanders and conveyed to the second author prior to his passing.
Locality and Age: All specimens of Xenorophus simplicidens were collected from the upper Oligocene Chandler Bridge Formation in the Charleston Embayment, ranging in age from 24.7 to 23.5 Ma.
Robert W. Boessenecker and Jonathan H. Geisler. 2023. New Skeletons of the Ancient Dolphin Xenorophus sloanii and Xenorophus simplicidens sp. nov. (Mammalia, Cetacea) from the Oligocene of South Carolina and the Ontogeny, Functional Anatomy, Asymmetry, Pathology, and Evolution of the Earliest Odontoceti. Diversity. 15(11), 1154. DOI: 10.3390/d15111154
(This article belongs to the Special Issue Evolution of Crown Cetacea)
