A Juvenile Crocodilian Jaw

A JUVENILE CROCODILIAN JAW WITH ROOTED TOOTH  

Crocodilia indet. (probably Borealosuchus sp.)  

Late Cretaceous Period (approximately 76 million years ago)  

Judith River Formation, Montana, USA

2 1/4 x 1 1/8 x 7/16 inches (5.7 x 2.9 x 1.1 cm)

Crocodilians are, alongside the turtles, one of the great architectural successes of vertebrate evolution. Their direct lineages reach back more than 200 million years; the body plan you would recognize today — long, low-slung, semi-aquatic, armoured — has been functional for over 80 million years. They were among the few large predators to survive the catastrophe that ended the dinosaur age, and they remain, in their modern descendants, one of the closest living windows onto the deep Mesozoic past.

The present jaw more likely belonged to the Borealosuchus, a mid-sized crocodilian known from the Judith River and adjacent formations. Adult Borealosuchus could reach lengths of around nine feet — a respectable river predator in any era — but the dimensions of the present specimen, together with the proportions of the rooted tooth, indicate a juvenile animal. Juvenile material of any crocodilian taxon is markedly less common in the fossil record than adult: fragile young bones rarely survive intact, and a partial jaw retaining a tooth still in its socket is rarer still.