Grow a Triceratops

From baby to adult, horn by horn

In 2006 Jack Horner and Mark Goodwin lined up ten Triceratops skulls from Montana, from a baby to big adults. The horns and frill changed so much that young ones look like a different animal.

What the skulls showed

A study of 10 skulls found four growth stages: baby, juvenile, subadult and adult. Horner & Goodwin (2006), Major cranial changes during Triceratops ontogeny, Proc. R. Soc. B Adult Triceratops skulls in one growth study were about 2 metres long. The smallest, a baby’s, was 38 cm. Horner & Goodwin (2006), Major cranial changes during Triceratops ontogeny, Proc. R. Soc. B

Its brow horns changed shape as it grew: straight stubs in babies, curving backwards in juveniles, straightening in subadults, and curving forwards in adults. Horner & Goodwin (2006), Major cranial changes during Triceratops ontogeny, Proc. R. Soc. B The baby’s brow horns stood just 3.5 cm above the eye. In the biggest adult skull they stood 82 cm. Horner & Goodwin (2006), Major cranial changes during Triceratops ontogeny, Proc. R. Soc. B

A baby’s frill edge was deeply scalloped. Juveniles grew 17 to 19 small triangular bony points along it, which flattened with age and finally merged into the frill in older adults. Horner & Goodwin (2006), Major cranial changes during Triceratops ontogeny, Proc. R. Soc. B Along the back of the frill, the edge went from scalloped to wavy to smooth as Triceratops grew. Horner & Goodwin (2006), Major cranial changes during Triceratops ontogeny, Proc. R. Soc. B

The little nose horn began as a separate bone that later fused on and grew into a nose horn whose shape varied a lot. Horner & Goodwin (2006), Major cranial changes during Triceratops ontogeny, Proc. R. Soc. B

Why change shape?

Horner and Goodwin suggested these changes let others spot a youngster at a glance, and showed when one was ready to breed. Horner & Goodwin (2006), Major cranial changes during Triceratops ontogeny, Proc. R. Soc. B

That is their hypothesis. Nobody has watched a Triceratops grow up, so the reason is an educated guess from the fossils.

Why so few babies?

For a long time small Triceratops skulls were overlooked: by 2006 only four non-adult specimens had been described. Horner & Goodwin (2006), Major cranial changes during Triceratops ontogeny, Proc. R. Soc. B

Did it ever stop growing?

As horned dinosaurs grew up, their ornaments developed first, then the skull roof fused, and finally the small bones on the frill, cheeks and beak fused on. Longrich & Field (2012), Torosaurus is not Triceratops, PLOS ONE

How skull bones fused with age is one of the clues in the Torosaurus debate, where scientists disagree about what the very oldest Triceratops looked like.

Next: horns, frill, beak and teeth →