''Sphaerotholus'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of pachycephalosaurid

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

from the Upper

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

of the western

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

and

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

. To date, five

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

have been described: the

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

, ''S. goodwini'', from the Den-na-zin Member of the

Kirtland Formation The Kirtland Formation (originally the Kirtland Shale) is a Sedimentary rock, sedimentary geological formation. Description The Kirtland Formation is the product of alluvial muds and overbank sand deposits from the many channels draining the ...

and possibly the Fossil Forest Member of the Fruitland Formation (Late

Campanian The Campanian is the fifth of six ages of the Late Cretaceous epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campa ...

) of San Juan County,

New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...

, USA; ''S. buchholtzae'', from the

Hell Creek Formation The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana. The Formation (stratigraphy), formation s ...

(Late

Maastrichtian The Maastrichtian ( ) is, in the International Commission on Stratigraphy (ICS) geologic timescale, the latest age (geology), age (uppermost stage (stratigraphy), stage) of the Late Cretaceous epoch (geology), Epoch or Upper Cretaceous series (s ...

) of western Carter County,

Montana Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...

, USA and the

Frenchman Formation The Frenchman Formation is a stratigraphic unit of Late Cretaceous (late Maastrichtian) Geochronology, age in the Western Canadian Sedimentary Basin, Western Canada Sedimentary Basin. It is present in southern Saskatchewan and the Cypress Hills (C ...

Saskatchewan Saskatchewan is a Provinces and territories of Canada, province in Western Canada. It is bordered on the west by Alberta, on the north by the Northwest Territories, on the east by Manitoba, to the northeast by Nunavut, and to the south by the ...

, Canada; ''S. edmontonensis'', from the Horseshoe Canyon Formation of

Alberta Alberta is a Provinces and territories of Canada, province in Canada. It is a part of Western Canada and is one of the three Canadian Prairies, prairie provinces. Alberta is bordered by British Columbia to its west, Saskatchewan to its east, t ...

, Canada; ''S. lyonsi'', from the

Dinosaur Park Formation The Dinosaur Park Formation is the uppermost member of the Belly River Group (also known as the Judith River Group), a major geologic unit in southern Alberta. It was deposited during the Campanian stage of the Late Cretaceous, between about 7 ...

(Campanian) of Alberta, Canada; and ''S. triregnum'' from the Hell Creek Formation of Garfield County, Montana, USA.

History of discovery

The

etymology Etymology ( ) is the study of the origin and evolution of words—including their constituent units of sound and meaning—across time. In the 21st century a subfield within linguistics, etymology has become a more rigorously scientific study. ...

of ''Sphaerotholus'' is a combination of the

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

''sphaira'', meaning "ball", and ''tholos'', meaning "dome", and is a reference to the characteristically dome-shaped pachycephalosaurian

skull The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...

. The survival of ''Sphaerotholus'' from the Campanian of New Mexico to the end of the Maastrichtian of Montana demonstrates that this

taxon In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

had both a relatively long duration (approximately 7-8 million years) and a widespread distribution. Williamson and Carr, who first described the

in 2002, diagnose it as follows: "Differs from all other pachycephalosaurids where known in the possession of a parietosquamosal bar that decreases in depth laterally as seen in caudal view and is bordered by a single row of nodes and one lateroventral corner node." ''Sphaerotholus'' is considered a highly derived pachycephalosaur.

Species

''Sphaerotholus goodwini''

The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

of the type species (NMMNH P-27403, New Mexico Museum of Natural History) consists of an incomplete skull lacking the facial and palatal elements. The species is diagnosed as follows: "''Sphaerotholus'' which in caudal view possesses a parietosquamosal bar that reduces in depth laterally to a lesser extent than in ''S. buchholtzae'' and the parietal is reduced to a thin slip between the

squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestra ...

s." The species name honors

paleontologist Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geolo ...

Mark Goodwin for his work with pachycephalosaurian dinosaurs. It has been suggested that ''Stegoceras novomexicanum'' is a juvenile representative of this species, because the parietal shelf that morphologically distinguishes ''S. novomexicanum'' from ''Sphaerotholus goodwini'' disappears through growth in '' Stegoceras validum.''

''Sphaerotholus edmontonensis''

''Troodon edmontonensis'' was described by Brown and Schlaikjer in 1943 on the basis of three domes from the Horseshoe Canyon Formation of Alberta. Williamson and Carr considered the species to be invalid, but in 2010 Nicholas Longrich et al. created the novel combination ''Sphaerotholus edmontonensis'' that could be distinguished from ''S. goodwini'' by the paired hornlets on the back of the dome, and from ''S. buchholtzae'' by the elongate parietals. It was once considered a species of ''

Stegoceras ''Stegoceras'' is a genus of Pachycephalosauria, pachycephalosaurid (dome-headed) dinosaur that lived in what is now North America during the Late Cretaceous Period (geology), period, about 77.5 to 74 million years ago (mya). The first specim ...

'' and was about the same size, reaching in length and in body mass.

''Sphaerotholus buchholtzae''

The holotype of ''S. buchholtzae'' (TMP 87.113.3) consists of an incomplete skull, found in the Hell Creek Formation. The species was diagnosed as having a parietal that is widely exposed between the squamosals and wide enough to bear parietosquamosal nodes, a shallower caudal margin of the parietosquamosal shelf, the

lateral Lateral is a geometric term of location which may also refer to: Biology and healthcare * Lateral (anatomy), a term of location meaning "towards the side" * Lateral cricoarytenoid muscle, an intrinsic muscle of the larynx * Lateral release ( ...

corner node is reduced in size and located above the ventral margin of the parietosquamosal bar, and the nodes in the lateral margin of the parietosquamosal shelf reduced on the squamosal and coalescing into a ridge on the postorbital. The specific name honors Emily A. Buchholtz for her extensive work with pachycephalosaurians. Sullivan (2003) considered ''S. buchholtzae'' a junior synonym of ''

Prenocephale ''Prenocephale'' (meaning "sloping head") is a genus of small pachycephalosaurid dinosaur from the Late Cretaceous Nemegt Formation of Mongolia. It was similar in many ways to its close relative, ''Homalocephale''. Discovery The holotype specime ...

edmontonensis'' (or ''Sphaerotholus edmontonensis''). However, Mallon et al. (2015), in their description of a new ''S. buchholtzae'' specimen from the

, noted that ''S. edmontonensis'' was distinct from ''S. buchholtzae'' based on comparative morphology and morphometrics. Additionally, a redescription of ''S. buchholtzae'' used geometric morphometrics to distinguish the two species.

''Sphaerotholus lyonsi''

Described by Woodruff, Schott, and Evans in 2023 based on an immature specimen. Lived in the Dinosaur Park Formation in Canada, c. 76 million years ago. It is characterized by a double row of small bony nodes along the back of its skull, as opposed to the single row of large nodes in ''S. goodwini'', ''S. edmontonensis'', and ''S. buchholtzae''.

''Sphaerotholus triregnum''

Described by Woodruff, Schott, and Evans in 2023 based on a subadult specimen. Lived in the middle section of the Hell Creek Formation in

, USA. It is characterized by a triple row of small bony nodes along the back of its skull, as opposed to the single row of large nodes in ''S. buchholtzae'' and ''S. edmontonensis''. It is named for the resemblance of its domed skull with a triple "crown"-like appearance of nodes to the

Papal tiara The papal tiara is a crown that was worn by popes of the Catholic Church from as early as the 8th century to the mid–20th century. It was last used by Pope Paul VI in 1963, and only at the beginning of his reign. The name ''tiara'' refers t ...

or "Triregnum".

Classification

In their 2023

description Description is any type of communication that aims to make vivid a place, object, person, group, or other physical entity. It is one of four rhetorical modes (also known as ''modes of discourse''), along with exposition, argumentation, and narr ...

of ''S. lyonsi'' and ''S. triregnum'', Woodruff, Schott & Evans analyzed the placement of the genus ''Sphaerotholus'' within the Pachycephalosauria. They recovered all proposed species of ''Sphaerotholus'' as a monophyletic group of derived pachycephalosaurines, as the

sister taxon In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

to the Pachycephalosaurini. The results of their

phylogenetic analyses In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as Computational phylogenetics, phylogenetic inference. It infers the relationship among organ ...

are shown in the

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

below:

References

Sources

External links

dinosaur.net.cn
(includes photograph of the type skull of ''Sphaerotholus goodwini'' and a life restoration of same) {{Taxonbar, from=Q134104 Pachycephalosauria Dinosaur genera Campanian dinosaurs Maastrichtian dinosaurs Dinosaur Park Formation Kirtland Formation Horseshoe Canyon Formation Hell Creek Formation Frenchman Formation Fossil taxa described in 2002 Dinosaurs of Canada Dinosaurs of the United States