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Chrysocetus
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Chrysocetus
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.
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".
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.
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.
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Chrysocetus
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.
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".
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.
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.
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