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Achelousaurus, Late CretaceousTwo Medicine Formation, Montana · named 1994
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Achelousaurus

ah-KEL-oh-SORE-us

"Achelous's lizard"

Type species Achelousaurus horneri

Lived
84 to 72 million years ago, Late Cretaceous
Diet
Herbivore
Length
about 6 m (20 ft)
Group
Ceratopsians, Ceratopsidae
Found in
United States
Named by
Sampson 1994

Size comparison

To scale

Where its fossils were found

The evidence

Today

2 finds

80 million years ago

Map: C. R. Scotese, PALEOMAP

Habitat

Rivers crossed a floodplain that experienced seasonal dry spells in the Two Medicine Formation. Maiasaura, Einiosaurus and Daspletosaurus lived in the wider region.

Field notes

About Achelousaurus

What it was like

Achelousaurus was a plant-eating dinosaur that walked on four legs and carried a large head with a beak at the front, much like other ceratopsians. It grew to about 6 meters (20 feet) long. Where many of its relatives had horns above the eyes and on the nose, Achelousaurus instead had rough, thickened bosses in those spots.

A bony frill extended from the back of its skull, with a pair of curved spikes at the edge. The bosses were probably covered in a tough layer of keratin in life, and some researchers think animals may have used them to butt heads or to show off to rivals.

Its world

Achelousaurus lived from about 84 to 72 million years ago, during the Late Cretaceous, in what is now Montana in the United States. Its fossils come from the Two Medicine Formation.

It shared its world with other plant eaters such as Maiasaura and Einiosaurus, as well as predators including Daspletosaurus and Troodon. Armored Euoplocephalus and fast-moving Saurornitholestes also lived in the same region.

How it was found

Sampson named Achelousaurus in 1994, based on fossils collected in Montana. Only two finds are known, mostly skull material from animals at different ages, from juveniles to adults. Its closest relatives include Styracosaurus, Pachyrhinosaurus, and Triceratops, and some researchers think Achelousaurus may represent a stage between horned and bossed ceratopsians, though this idea is debated.

Field gallery

More views of Achelousaurus

Scenes reconstructed from the available fossil evidence.

Achelousaurus in its habitat, photo 1

1 of 7

Lived alongside

Found in the same rock formations

A rock formation can hold fossils laid down millions of years apart, so not every animal here met Achelousaurus.

The evidence

The science behind Achelousaurus

Which parts of the picture come from its own bones, and which come from its relatives.

From its own fossils

  • Skulls have rough bony bosses on the nose and above the eyes, in place of long facial horns.
  • A broad frill carries a pair of long spikes at its rear edge.
  • The texture of the facial bosses supports a covering of thick, tough keratin pads.

Borrowed from relatives

  • Einiosaurus and Pachyrhinosaurus help fill gaps in its heavy, four-legged body.
  • Body scales are inferred from other horned dinosaurs.

Still unknown

  • The exact shape and thickness of the pads over its facial bosses.
  • How the bosses and frill spikes developed in the youngest animals.
  • Its color and how it used its head ornaments.

Often drawn wrong

  • Long brow horns like those of Triceratops replace its low, rough bosses.
  • A pointed nose horn replaces its broad nasal boss.
  • The openings in its bony frill were covered by tissue, rather than being open holes through the living animal.

Discovery timeline

  1. 1994Named itSampson
  2. 1995Full descriptionSampson
  3. 1997Growing upSAMPSON, RYAN & TANKE
  4. 2009SkinHieronymus et al.
  5. 2017Same rocksCarr et al.

Key papers

The research behind what we know of Achelousaurus

Close relatives

Other members of Ceratopsidae

Sources: Wikidata