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Description

Pachycephalosaurus (/ ˌpækɪˌsɛfələˈsɔːrəs/;[2] meaning "thick-headed lizard", from Greek pachys-/παχύς- "thick", kephale/κεφαλή "head" and sauros/σαῦρος "lizard") is a genus of pachycephalosaurid ornithischian dinosaur . The type species , P. wyomingensis, is the only known species , but some researchers argue that the genus Stygimoloch might be a second species, P. spinifer or a juvenile specimen of P. wyomingensis. It lived during the Maastrichtian age of the Late Cretaceous period in what is now western North America . Remains of it have been excavated in Montana , South Dakota , Wyoming , and Alberta . It is primarily known from a single skull and a few extremely thick skull roofs , at 22 centimetres (9 inches) thick. More complete fossils have been found in recent years. Pachycephalosaurus was among the last non-avian dinosaurs before the Cretaceous–Paleogene extinction event . The genus Tylosteus has been synonymized with Pachycephalosaurus, as have the genera Stygimoloch and Dracorex in recent studies.[3] [4]
Like other pachycephalosaurids, Pachycephalosaurus was a bipedal herbivore with an extremely thick skull roof. It possessed long legs and small arms with five-fingered hands. Pachycephalosaurus is the largest-known pachycephalosaur and the thick skull domes of it and related genera gave rise to the hypothesis that they used their skulls in intra-species combat. This hypothesis has been highly disputed in recent years.
Remains attributable to Pachycephalosaurus may have been found as early as the 1850s. As determined by Donald Baird , in 1859 or 1860 , Ferdinand Vandeveer Hayden , an early fossil collector in the American West, collected a bone fragment in the vicinity of the head of the Missouri River , from what is now known to be the Lance Formation of southeastern Montana.[5] This specimen, ANSP 8568, was described by Joseph Leidy in 1872 as belonging to the dermal armor of a reptile or an armadillo -like animal.[6] It became known as Tylosteus. Its actual nature was not revealed until Baird studied it again over a century later and identified it as a squamosal (bone from the back of the skull) of Pachycephalosaurus, including a set of bony knobs corresponding to those found on other specimens of Pachycephalosaurus.[5] Because the name Tylosteus predates Pachycephalosaurus, according to the International Code of Zoological Nomenclature Tylosteus would normally be preferred. In 1985, Baird successfully petitioned to have Pachycephalosaurus used instead of Tylosteus because the latter name had not been used for over fifty years, was based on undiagnostic materials, and had poor geographic and stratigraphic information.[7] [8] This may not be the end of the story, however. Robert Sullivan suggested in 2006 that ANSP 8568 is more like the corresponding bone of Dracorex than that of Pachycephalosaurus.[9] The issue is of uncertain importance, though, if Dracorex actually represents a juvenile Pachycephalosaurus, as has been recently proposed.[10]
In 1890 , during the Bone Wars between Othniel Charles Marsh and Edward Drinker Cope , one of Marsh's collectors, John Bell Hatcher , collected a partial left squamosal (YPM VP 335) later referred to Stygimoloch spinifer near Lance Creek, Wyoming in the Lance Formation.[11] [12] [13] Marsh described the squamosal along with the dermal armor of Denversaurus as the body armor of Triceratops in 1892 , believing that the squamosal was a spike akin to the plates on Stegosaurus .[13] The squamosal spike was even featured in Charles Knight ’s painting of Cope’s ceratopsid Agathaumas , likely based on Marsh’s hypothesis.[12] Marsh also named a species of now-dubious ankylosaur Palaeoscincus in 1892 based on a single tooth (YPM 4810), also collected by Hatcher from the Lance.[14] The tooth was named Palaeoscinus latus, but in 1990 , Coombs found the tooth to be from a pachycephalosaurid , possibly even Pachycephalosaurus itself.[15] Hatcher also collected several additional teeth and skull fragments while working for Marsh, though these have yet to be described.[16] [17] [18]
P. wyomingensis, the type and currently only valid species of Pachycephalosaurus, was named by Charles W. Gilmore in 1931 . He coined it for the partial skull USNM 12031, from the Lance Formation of Niobrara County , Wyoming . Gilmore assigned his new species to Troodon as T. wyomingensis.[19] At the time, paleontologists thought that Troodon, then known only from teeth, was the same as Stegoceras , which had similar teeth. Accordingly, what are now known as pachycephalosaurids were assigned to the family Troodontidae , a misconception which was not corrected until 1945 by Charles M. Sternberg .[20]
In 1943 , Barnum Brown and Erich Maren Schlaikjer , with newer, more complete material, established the genus Pachycephalosaurus. They named two species: Pachycephalosaurus grangeri, the type species of their new genus, and Pachycephalosaurus reinheimeri. P. grangeri was based on AMNH 1696, a nearly complete skull from the Hell Creek Formation of Ekalaka , Carter County , Montana. P. reinheimeri was based on what is now DMNS 469, a dome and a few associated elements from the Lance Formation of Corson County , South Dakota.[21] They also referred the older species "Troodon" wyomingensis to their new genus. Their two newer species have been considered synonymous with P. wyomingensis since 1983 .[22]
In 2015 , some pachycephalosaurid material and a domed parietal attributable to Pachycephalosaurus were discovered in the Scollard Formation of Alberta , implying that the dinosaurs of this era were cosmopolitan and didn't have discrete faunal provinces.[23]
The anatomy of Pachycephalosaurus itself is poorly known, as only skull remains have been described.[9] Pachycephalosaurus is famous for having a large, bony dome on top of its skull, up to 25 cm (10 in) thick, which safely cushioned its brain. The dome's rear aspect was edged with bony knobs and short bony spikes projected upwards from the snout. However, the spikes were probably blunted, not sharp.[24]
The skull was short and possessed large, rounded eye sockets that faced forward, suggesting that the animal had binocular vision . Pachycephalosaurus had a small muzzle that ended in a pointed beak. The teeth were tiny, with leaf-shaped crowns. The head was supported by an "S"- or "U"-shaped neck.[24] Younger individuals of Pachycephalosaurus might have had flatter skulls and larger horns projecting from the back of the skull. As the animal grew, the horns shrunk and rounded out as the dome grew.[3] [4] Pachycephalosaurus was bipedal and possibly the largest of all pachycephalosaurids.[25] It has been estimated that Pachycephalosaurus was about 4.5 metres (14.8 ft) long and weighed about 370–450 kilograms (820–990 lb).[26] [27] Based on other pachycephalosaurids, it probably had a fairly short, thick neck, short arms, a bulky body, long legs, and a heavy tail that was likely held rigid by ossified tendons .[28]

Description

Pachycephalosaurus (/ ˌpækɪˌsɛfələˈsɔːrəs/;[2] meaning "thick-headed lizard", from Greek pachys-/παχύς- "thick", kephale/κεφαλή "head" and sauros/σαῦρος "lizard") is a genus of pachycephalosaurid ornithischian dinosaur . The type species , P. wyomingensis, is the only known species , but some researchers argue that the genus Stygimoloch might be a second species, P. spinifer or a juvenile specimen of P. wyomingensis. It lived during the Maastrichtian age of the Late Cretaceous period in what is now western North America . Remains of it have been excavated in Montana , South Dakota , Wyoming , and Alberta . It is primarily known from a single skull and a few extremely thick skull roofs , at 22 centimetres (9 inches) thick. More complete fossils have been found in recent years. Pachycephalosaurus was among the last non-avian dinosaurs before the Cretaceous–Paleogene extinction event . The genus Tylosteus has been synonymized with Pachycephalosaurus, as have the genera Stygimoloch and Dracorex in recent studies.[3] [4]
Like other pachycephalosaurids, Pachycephalosaurus was a bipedal herbivore with an extremely thick skull roof. It possessed long legs and small arms with five-fingered hands. Pachycephalosaurus is the largest-known pachycephalosaur and the thick skull domes of it and related genera gave rise to the hypothesis that they used their skulls in intra-species combat. This hypothesis has been highly disputed in recent years.
Remains attributable to Pachycephalosaurus may have been found as early as the 1850s. As determined by Donald Baird , in 1859 or 1860 , Ferdinand Vandeveer Hayden , an early fossil collector in the American West, collected a bone fragment in the vicinity of the head of the Missouri River , from what is now known to be the Lance Formation of southeastern Montana.[5] This specimen, ANSP 8568, was described by Joseph Leidy in 1872 as belonging to the dermal armor of a reptile or an armadillo -like animal.[6] It became known as Tylosteus. Its actual nature was not revealed until Baird studied it again over a century later and identified it as a squamosal (bone from the back of the skull) of Pachycephalosaurus, including a set of bony knobs corresponding to those found on other specimens of Pachycephalosaurus.[5] Because the name Tylosteus predates Pachycephalosaurus, according to the International Code of Zoological Nomenclature Tylosteus would normally be preferred. In 1985, Baird successfully petitioned to have Pachycephalosaurus used instead of Tylosteus because the latter name had not been used for over fifty years, was based on undiagnostic materials, and had poor geographic and stratigraphic information.[7] [8] This may not be the end of the story, however. Robert Sullivan suggested in 2006 that ANSP 8568 is more like the corresponding bone of Dracorex than that of Pachycephalosaurus.[9] The issue is of uncertain importance, though, if Dracorex actually represents a juvenile Pachycephalosaurus, as has been recently proposed.[10]
In 1890 , during the Bone Wars between Othniel Charles Marsh and Edward Drinker Cope , one of Marsh's collectors, John Bell Hatcher , collected a partial left squamosal (YPM VP 335) later referred to Stygimoloch spinifer near Lance Creek, Wyoming in the Lance Formation.[11] [12] [13] Marsh described the squamosal along with the dermal armor of Denversaurus as the body armor of Triceratops in 1892 , believing that the squamosal was a spike akin to the plates on Stegosaurus .[13] The squamosal spike was even featured in Charles Knight ’s painting of Cope’s ceratopsid Agathaumas , likely based on Marsh’s hypothesis.[12] Marsh also named a species of now-dubious ankylosaur Palaeoscincus in 1892 based on a single tooth (YPM 4810), also collected by Hatcher from the Lance.[14] The tooth was named Palaeoscinus latus, but in 1990 , Coombs found the tooth to be from a pachycephalosaurid , possibly even Pachycephalosaurus itself.[15] Hatcher also collected several additional teeth and skull fragments while working for Marsh, though these have yet to be described.[16] [17] [18]
P. wyomingensis, the type and currently only valid species of Pachycephalosaurus, was named by Charles W. Gilmore in 1931 . He coined it for the partial skull USNM 12031, from the Lance Formation of Niobrara County , Wyoming . Gilmore assigned his new species to Troodon as T. wyomingensis.[19] At the time, paleontologists thought that Troodon, then known only from teeth, was the same as Stegoceras , which had similar teeth. Accordingly, what are now known as pachycephalosaurids were assigned to the family Troodontidae , a misconception which was not corrected until 1945 by Charles M. Sternberg .[20]
In 1943 , Barnum Brown and Erich Maren Schlaikjer , with newer, more complete material, established the genus Pachycephalosaurus. They named two species: Pachycephalosaurus grangeri, the type species of their new genus, and Pachycephalosaurus reinheimeri. P. grangeri was based on AMNH 1696, a nearly complete skull from the Hell Creek Formation of Ekalaka , Carter County , Montana. P. reinheimeri was based on what is now DMNS 469, a dome and a few associated elements from the Lance Formation of Corson County , South Dakota.[21] They also referred the older species "Troodon" wyomingensis to their new genus. Their two newer species have been considered synonymous with P. wyomingensis since 1983 .[22]
In 2015 , some pachycephalosaurid material and a domed parietal attributable to Pachycephalosaurus were discovered in the Scollard Formation of Alberta , implying that the dinosaurs of this era were cosmopolitan and didn't have discrete faunal provinces.[23]
The anatomy of Pachycephalosaurus itself is poorly known, as only skull remains have been described.[9] Pachycephalosaurus is famous for having a large, bony dome on top of its skull, up to 25 cm (10 in) thick, which safely cushioned its brain. The dome's rear aspect was edged with bony knobs and short bony spikes projected upwards from the snout. However, the spikes were probably blunted, not sharp.[24]
The skull was short and possessed large, rounded eye sockets that faced forward, suggesting that the animal had binocular vision . Pachycephalosaurus had a small muzzle that ended in a pointed beak. The teeth were tiny, with leaf-shaped crowns. The head was supported by an "S"- or "U"-shaped neck.[24] Younger individuals of Pachycephalosaurus might have had flatter skulls and larger horns projecting from the back of the skull. As the animal grew, the horns shrunk and rounded out as the dome grew.[3] [4] Pachycephalosaurus was bipedal and possibly the largest of all pachycephalosaurids.[25] It has been estimated that Pachycephalosaurus was about 4.5 metres (14.8 ft) long and weighed about 370–450 kilograms (820–990 lb).[26] [27] Based on other pachycephalosaurids, it probably had a fairly short, thick neck, short arms, a bulky body, long legs, and a heavy tail that was likely held rigid by ossified tendons .[28]