Dinosaurs A to Z
Pachycephalosauridae

Stegoceras validum steg-OSS-eh-ras

"Roof horn"

Stegoceras silhouette
You 1.8 m (5.9 ft) tall
Stegoceras 2.5 m (8.2 ft) long
as long as 2 people standing fingertip to fingertip
silhouette · Gareth Monger (CC-BY) via PhyloPic
Length
2.5 m (8.2 ft) — About 2.5 m long, ~40 kg — much smaller cousin of Pachycephalosaurus.
Period
Late Cretaceous (77–74 Mya)
Place
North America · Alberta, Canada · Montana
Food
Omnivore — Plants with possible insects and small animals — varied teeth suggest a mixed diet.
Clade
Pachycephalosauridae Dinosauria Ornithischia Cerapoda Marginocephalia Dome-headed bipedal herbivores.

A goat-sized dome-head whose thick skull-roof launched decades of "what is it?" debate.

Stegoceras reconstruction by TotalDino
Reconstruction · TotalDino (CC0)

Stegoceras was a small pachycephalosaur with a thinner dome than Pachycephalosaurus, but the same basic head-bashing equipment. Several well-preserved skulls show how the dome grew thicker with age — young Stegoceras had flatter heads. Stegoceras is sometimes mistaken for Stegosaurus by tourists because of the similar name, but they aren’t closely related.

More about Stegoceras

Discovery

Named by Lawrence Lambe · 1902 · the Belly River Group in the Red Deer River district of Alberta, Canada

Lambe collected partial skull domes from two animals in 1898 and a third dome in 1901, then described the new genus and species in 1902. The first remains were only domes, so the rest of the animal stayed a mystery for years.

The genus name means "horn roof", from the Greek stege ("roof") and keras ("horn"). The species name validus means "strong" in Latin, possibly a reference to the thick skull-roof.

Lawrence Lambe. A palaeontologist who collected and named Stegoceras in 1902 and was initially unsure what kind of dinosaur the domes belonged to, even entertaining the idea of a single-horned animal.

What was the famous dome actually for?
The function of the thick dome has been debated. Competing theories include use in intra-specific combat, such as head or flank-butting, and use as a sexual display. The skull is thought to have been flat in juveniles and to have grown into a dome with age.
Are both named species really Stegoceras?
Only two species currently remain: S. validum and S. novomexicanum, the latter named in 2011 from New Mexico fossils. The validity of S. novomexicanum has been debated, and it may not even belong to the genus Stegoceras. It is also disputed whether its known specimens are adults or juveniles.

What we know

  • Skull dome thickened with age — juveniles had flatter heads.
  • Smaller than Pachycephalosaurus, with a less massive dome.
  • Lived in the Dinosaur Park Formation, alongside many ceratopsians and hadrosaurs.
  • Several well-preserved skulls and partial skeletons known.

What we guess

  • Whether the dome was used in combat like Pachycephalosaurus or mainly for display.
  • Whether young animals fought differently with their flatter heads.
  • Why two pachycephalosaurs coexisted in the same ecosystem — different diets are likely.