Chrysocetus is a genus of extinct archaeocete ceteceans belonging to the Basilosauridae that is known from the Late Eocene of the eastern United States and western Africa. Across its range two species are known, the American Chrysocetus healyorum and the African Chrysocetus fouadassii, named in 2001 and 2015 respectively. Chrysocetus was a comparably small basilosaurid, larger than Saghacetus and Tutcetus but smaller than most other members of this group. In addition to being noted for its well-preserved hip bones, Chrysocetus is unique amongst basilosaurids for possibly having only had a single set of teeth. Though the fusion of the skeleton indicate that the holotype was a juvenile or subadult, the teeth represent the adult condition, having led to the hypothesis that unlike other basilosaurids and like modern whales, Chrysocetus did not replace its teeth but rather kept a single set throughout its life.
History and naming The type species, Chrysocetus healyorum, is based on a single subadult specimen from the late middle or early late Eocene of Orangeburg County, South Carolina. Initially, Uhen and Gingerich list the Cross Formation as being the source of the type material, tho later studies regard this horizone as a member of the Tupelo Bay Formation. The holotype, SCSM 87.195, consists of a partial skull with lower jaws, ten teeth, and the hyoid apparatus; 21 vertebrae, some ribs and a sternum; a partial left forelimb; and partial innominates. This species was discovered by Craig and Alice Healy, for which the type species was named, and scientifically described in 2001 by Mark D. Uhen and Phillip D. Gingerich. In their study, the two paleontologists took particular note of the innominate bones, as Chrysocetus was the first dorudontine with preserved pelvic material though its phylogenetic position remains variable across studies.
A second species, Chrysocetus fouadassii, was described from Bartonian-age Aridal Formation in the Western Sahara in 2015 alongside the pachycetine Antaecetus aithai (then a species of Platyosphys). C. fouadassii is known from fragmentary material of four individuals, namely a partial mandible, various isolated teeth and some postcranial elements. Though fragmentary and in some regards similar to the type species, a new species was erected in light of some proportional differences primarily concerning the neck vertebrae and limb bones.
According to Uhen and Gingerich, the name Chrysocetus is derived from the Greek words "chrysous", 'golden' and "ketos", 'whale'. The name was chosen in reference to the colouration of the holotype fossils, which are described as "rich gold".
Species Chrysocetus healyorum The type species of Chrysocetus was named in 2001 based on fossil remains from the Late Bartonian to Early Priabonian Tupelo Bay Formation, located in South Carolina. It was named for its discoverers, Craig and Alice Healy.
Chrysocetus fouadassii Named in 2015, C. fouadassii is exclusively known from the Bartonian Gueran locality of the Aridal Formation in Western Sahara. It chiefly differs from the type species through having longer neck vertebrae and limb bones. The species name honors the collector M'Barek Fouadassi, who led a paleontological expedition to the region in 2014.
Description
Skull The skull of Chrysocetus is only poorly known, with much of it damaged or lost during excavation. What remains are parts of the occipital, temporal and frontal regions of the skull, parts of the premaxillae, a squamosal and lacrimal bone as well as a single tympanic bulla (the fused inner ear bones). The preserved elements of the premaxillae represent the very tip of the snout and contain the first three incisors. These bones show that Chrysocetus clearly had a tapering snout with a rounded tip. The alveoli (toothsockets) are separated by evenly spaced diastemas, which bear the marks left by the teeth of the mandible when the jaws were closed. Since the two individual premaxillae appear to have been unfused, they may have been separated by a grove or gap, something that was also the case for other basilosaurids.
The frontal bones are only fused towards the front of the skull but become unfused further towards the back where they connect to the parietal bones. The frontoparietal suture is curved, with the frontals projecting back into the parietals along the midline of the skull. Like the frontals, the parietals are only partially fused to each other, with the suture open towards the frontals and closed towards the back where they connect with the supraoccipital bone along an open suture. No interparietal bone is known from Chrysocetus and the sagittal crest is described as somewhat convex. The supraoccipital is not especially elevated above the parietal and a low boss projects back near its dorsal surface.
The lacrimal was found in isolation and was therefore likely unfused. Three grooves are present on the outer surface of the lacrimal, leading to the edge of the eyesocket. The jugal is missing both ends, but about halfway down its length the sides become noticeably flattened while the element expands in its height, thinning the further back it goes. The tympanic bulla is similar to that of other basilosaurids. Some parts of the lower jaw are also known.
Dentition The teeth of Chrysocetus are described as broadly resembling those of other basilosaurids, consisting of simple caniniform incisors and canines and more complex premolars and molars that feature numerous accessory cusps before and after the main apex. The incisors are single-rooted and curve back, with their mesial side bearing strong carinae. The lingual surface, the surface facing inwards, bears vertical striations. Chrysocetus possessed four premolars in its upper jaw, the first three of which gradually increase in size until the fourth becomes smaller again. The first premolar is the most distinct of the four, only possessing a single root and appearing much more caniniform like the incisors. Still, unlike the mesial teeth, the root of the first premolar features a pronounced grove on both the buccal and lingual side of the root, a pronounced distal carina and a tiny cusp at its base. However, Chrysocetus''' premolars lack the denticles on the cingula characteristic of Zygorhiza. The cheek teeth are overall described as narrow and gracile than those of Dorudon with comparably smooth tooth enamel. As with other basilosaurids, the premolars and molars feature accessory denticles that precede and follow the main apex of each tooth. In the case of C. fouadassii the fourth premolar of the dentary bears five such denticles on either side of the main apex, with the subsequent first dentary molar only bearing three and the second molar only featuring these accessory denticles behind the main apex. In C. healyorum the distal denticles, those behind the main apex are noticeably larger than those mesial to the main apex. The third lower premolar is thought to be the largest tooth in the jaw and possesses tall denticles that are well distinguished from the main apex.
Vertebrae and ribcage The cervical vertebrae appear similar to those of other basilosaurids. The dorsal arch of the atlas is perforated by large foramina on either side and the axis is notably taller than all other neck vertebrae, owing to its robust neural spine. The subsequent vertebrae show a gradual increase in size and differ from those of protocetids by having less compressed vertebral bodies. The epiphyses are attached to their respective vertebrae, but not fully fused, nor are the cervicals fused to each other. The sixth cervical vertebra has been noted for having enlarged parapophyses. Both the cervicals of C. healyorum and those of C. fouadassii are similar in most aspects of their anatomy, but can be distinguished by the fact that the African species has significantly longer vertebral centra.
The Chrysocetus holotype preserves 12 thoracic vertebrae, with Uhen and Gingerich arguing that many more were likely to have been present and were simply not recovered alongside the other material. This is supported by the number of thoracic vertebrae in other dorudontines, as Dorudon atrox possessed 17, while 15 thoracic vertebrae are known from Zygorhiza kochii. According to Uhen and Gingerich, the first 10 of these recovered vertebrae appear to have formed a continuous series. Each of these early vertebrae preserves two costal fovea, articular surfaces that receive the ribs, with the tenth vertebra showing that they were much more closely set to each other than in any of the prior ones. The next known vertebra is only preserved through a pair of transverse processes. Similar transverse processes are known from the later thoracic vertebrae of Dorudon, specifically the earliest thoracic with only a single costal fovea. Therefore, its hypothesized that this specific vertebra was either the 11th or 12th in Chrysocetus. Finally, the caudal-most thoracic vertebra known from Chrysocetus has wide transverse processes and a singular articular facet for the ribs on either side. Unlike in the previous vertebrae, the height and width of the body are nearly the same, which resembles the anatomy of the lumbar vertebrae. Though the precise position of this vertebra is uncertain, it may have been the 13th vertebra in the thoracic series or positioned even further back.
The lumbars are only poorly preserved, but appear to be as long as they are wide and tall as already hinted at by the posterior thoracic vertebrae. This is an important feature that clearly distinguishes Chrysocetus and other dorudontines from basilosaurines, which have highly elongated lumbar vertebrae. The transverse processes emerge from the lower section of the lumbars and are angled downward. The pedicles that sit atop the lumbar vertebrae are almost as long as the vertebral body, but shifted towards the front. The number of lumbars is unknown and no sacral or caudal vertebrae were found along the holotype specimen. Overall the thoratic, lumbar and caudal vertebrae are noted for being relatively "normal" in their proportions, lacking the elongation seen in animals like Saghacetus and most prominent in Basilosaurus.
Three fragments of the sternum are part of the holotype specimen, one belonging to the manubrium and two more of the main sternal body. Overall elements of the sternum are thickened and have articular surfaces that are covered by spongy bone. The manubrium specifically, the front-most element of the sternum, is elongated and widens in the direction of the animals head. About two thirds down its length the manubrium is slightly bendt and its caudal end, which connects to the rest of the sternum, is described as U-shaped. The elements of the sternal body are only fragments of the entire bone, but appear flattened towards the front with concave upper and lower surfaces. The second fragment, which would have been further away from the manubrium than the other known piece, has articular surfaces that nearly wrap all around the element. Unlike the manubrium, these elements are straight and do not feature a dorsal bend.
The ribs are not too different from those of other dorudontines. The first few ribs are short and stout, but towards the back of the body they grow gradually longer until reaching their greatest length around the fifth or sixth rib. The early ribs further feature both heads and tubercles, with the latter being lost somewhere around the tenth rib. Internally, the ribs lack the medullary cavity that would contain bone marrow and are instead filled with spongy bone. While the ribs are slightly pachyostotic, meaning the layers of bone are thickened, they are not osteosclerotic, so the bone itself is not denser than in other archaeocetes as is the case in the subfamily Pachycetinae.
Appendicular skeleton