Open Paleo

Sphaerotholus

Williamson & Carr, 2003 SFEE-ro-THO-luss

From Greek 'sphaira' (sphere, ball) + Greek 'tholos' (dome, domed building), meaning 'spherical dome' or 'ball dome', referring to the characteristic rounded domed skull of this pachycephalosaurid.

83.6–72.2 million years ago Campanian, Late Cretaceous
Late Cretaceous stages

Sphaerotholus is a genus of pachycephalosaurid dinosaur from the Upper Cretaceous of the western United States and Canada. To date, five species have been described: the type species, S. goodwini, from the Den-na-zin Member of the Kirtland Formation and possibly the Fossil Forest Member of the Fruitland Formation (Late Campanian) of San Juan County, New Mexico, USA; S. buchholtzae, from the Hell Creek Formation (Late Maastrichtian) of western Carter County, Montana, USA and the Frenchman Formation of Saskatchewan, Canada; S. edmontonensis, from the Horseshoe Canyon Formation of Alberta, Canada; S. lyonsi, from the Dinosaur Park Formation (Campanian) of Alberta, Canada; and S. triregnum from the Hell Creek Formation of Garfield County, Montana, USA.

Diagnosis

  1. Parietosquamosal bar that reduces in depth laterally to a lesser extent than S. buchholtzae
  2. Parietal reduced to a thin slip between the squamosals in caudal view
  3. High, wide, and convex dome that incorporates the lateral marginal bones
  4. One row of large and conical nodes along the caudodorsal and lateral margins of the squamosal
  5. Dorsotemporal fenestrae closed
  6. Parietosquamosal shelf small, inflated, and does not include the parietals
  7. Broad nuchal shelf that overhangs the occipital region

Species (5)

Sphaerotholus goodwini Williamson & Carr, 2003

Type species

Named for paleontologist Mark Goodwin, in recognition of work with pachycephalosaurian dinosaurs.

Holotype
NMMNH P-27403 , New Mexico Museum of Natural History and Science
Material
Partial skull with nearly complete frontoparietal, right supraorbital 2, right postorbital, right squamosal, and much of the basicranium
Horizon
De-na-zin Member, Kirtland Formation, Campanian, Late Cretaceous , 83.6–72.2 million years ago
Locality
Willow Wash, New Mexico, United States

Sphaerotholus buchholtzae Williamson & Carr, 2003

Honors Emily A. Buchholtz for contributions to pachycephalosaurian research.

Holotype
TMP 1987.113.0003 , Royal Tyrrell Museum of Paleontology
Material
Partial skull
Horizon
upper Hell Creek Formation, Maastrichtian, Late Cretaceous , 72.2–66.0 million years ago
Locality
SE 1/4, Sec. 5, T1S, R55E, Montana, United States
Diagnosis
  1. Parietal widely exposed between the squamosals and wide enough to bear parietosquamosal nodes.
  2. Caudal margin of the parietosquamosal shelf shallows to a greater degree than in S. goodwini.
  3. Lateral corner node reduced in size relative to S. goodwini and positioned above the ventral margin of the parietosquamosal bar.
  4. Nodes along the lateral margin of the parietosquamosal shelf are reduced on the squamosal and coalesce into a ridge on the postorbital.

Sphaerotholus edmontonensis Brown & Schlaikjer, 1943

Named for the Edmonton Formation (now the Horseshoe Canyon Formation) of Alberta, the horizon of the type series.

Holotype
GSC 8830 , Canadian Museum of Nature
Material
Frontoparietal (dome) region of an adult skull
Horizon
Horseshoe Canyon Formation, Maastrichtian, Late Cretaceous , 72.2–66.0 million years ago
Locality
opposite the mouth of Big Valley Creek, below Scollard ferry, Red Deer River, Alberta, Canada
Diagnosis
  1. Frontoparietal dome oval in outline in dorsal view, low and gently arched in the adult.
  2. Parieto-squamosal shelf only slightly developed.
  3. Frontals low and narrow where in contact with the nasals, not set off from the rest of the frontals by lateral grooves.
  4. Dorsal region of the endocranial cavity relatively longer and wider than in other species of the genus.
  5. Base of the parietals behind the endocranial cavity not downcurved.
  6. Inner walls of the lateral temporal openings indicate these were not as dorsoventrally compressed as in other species.

Sphaerotholus lyonsi Woodruff et al., 2023

Honors Randy Lyons, a long-time TMP Field Experience Program participant who discovered the holotype in 2002.

Holotype
TMP 2002.012.0063 , Royal Tyrrell Museum of Paleontology
Material
Nearly complete right squamosal, missing only the lateral surface and ventral processes.
Horizon
upper Dinosaur Park Formation, Campanian, Late Cretaceous , 83.6–72.2 million years ago
Locality
north side of the Red Deer River, in the northeast region of Dinosaur Provincial Park, Alberta, Canada
Diagnosis
  1. Squamosal bears two posterior node rows, with a ventral row occurring below the primary row and medial to the corner node, versus a single posterior row in S. buchholtzae and S. goodwini and three rows in S. triregnum.
  2. Posterior region of the squamosal bar is thickened ventrally to support the accessory second row of nodes.
  3. Strongly posteriorly pronounced 'lip' on the parietosquamosal shelf that tapers laterally more gradually than in other Sphaerotholus.
  4. Medialmost node is not bisected by the parietosquamosal suture, indicating the squamosal lacks a true parietosquamosal node, unlike S. buchholtzae.

Sphaerotholus triregnum Woodruff et al., 2023

From triregnum, the three-crowned Papal tiara, referencing the domed frontoparietal and the three crown-like rows of peripheral nodes.

Holotype
ROM 53583 , Royal Ontario Museum
Material
Nearly complete, well-preserved isolated left squamosal; no frontoparietal dome known
Horizon
Hell Creek Formation, Maastrichtian, Late Cretaceous , 72.2–66.0 million years ago
Locality
Garfield County, ~45 km southwest of Jordan, Montana, United States
Diagnosis
  1. Squamosal bears three posterior and three lateral node rows, versus a single row of each in S. buchholtzae, S. goodwini and S. edmontonensis
  2. W-shaped (rotated) postorbital sulcus on the squamosal, an autapomorphy unique among known pachycephalosaurines
  3. Medialmost node of the primary posterior node row has a prominent pyramidal medial process but is not bisected by the parietosquamosal suture, unlike S. buchholtzae and S. edmontonensis
  4. Lacks a parietosquamosal node, as in S. lyonsi and S. goodwini, rather than possessing one as in S. buchholtzae and S. edmontonensis

References

  1. Williamson, T. E.; Carr, T. D. (2003). A new genus of derived pachycephalosaurian from western North America. Journal of Vertebrate Paleontology 22(4): 779-801. Informa UK Limited. doi:10.1671/0272-4634(2002)022[0779:angodp]2.0.co;2

    Erects the genus Sphaerotholus with type species S. goodwini and the second species S. buchholtzae.

    williamson2003a
  2. Brown, B.; Schlaikjer, E. M. (1943). A study of the troödont dinosaurs with the description of a new genus and four new species. Bulletin of the American Museum of Natural History 82(5): 115-150.

    Erects the basionym Troodon edmontonensis (now Sphaerotholus edmontonensis) on domes from the Edmonton (Horseshoe Canyon) Formation.

    brown1943a
  3. Woodruff, D. C.; Schott, R. K.; Evans, D. C. (2023). Two new species of small-bodied pachycephalosaurine (Dinosauria, Marginocephalia) from the uppermost Cretaceous of North America suggest hidden diversity in well-sampled formations. Papers in Palaeontology 9(6): e1535. Wiley. doi:10.1002/spp2.1535

    Erects S. lyonsi and S. triregnum and revises the genus.

    woodruff2023a