Enigmacursor mollyborthwickae: Small and speedy dinosaur recognised as a new species

A newly discovered species of dinosaur is going on display in London’s Natural History Museum.

Enigmacursor mollyborthwickae was a speedy, two-legged herbivore, 64 centimetres tall and 180 cm long that lived about 145 million to 150 million years ago, during the Late Jurassic Period.

Its reconstructed skeleton will be on display in the museum’s Earth Hall from 26 June, alongside its contemporary, Sophie the Stegosaurus.

Susannah Maidment and Paul Barrett, both palaeontologists at the Natural History Museum, have analysed the Enigmacursor specimen, which was uncovered from the Morrison Formation in the western US in 2021-22.

Back then, it was thought to be a Nanosaurus – a poorly known species of small herbivorous dinosaur. The Enigmacursor fossil isn’t complete, but using the few teeth – which reveal it ate plants – and portions of the neck, backbone, tail, pelvis, limbs and feet, Maidment and Barrett have defined this fossil as a new species, placed it in an evolutionary tree and reconstructed it for display.

They have based the structure of missing elements, like the skull, on similar small dinosaurs like Yandusaurus and Hexinlusaurus. Generally, we know little about smaller dinosaurs, both because they are less likely to fossilise than bigger animals and because fossil hunters tend to seek larger, more valuable examples.

“This is a two-legged dinosaur and it’s got very small forearms that it probably would have used to grasp food to bring it to its mouth,” says Maidment. “And it’s got incredibly large feet and very long limbs. So, it was probably quite fast by dinosaur standards.”

That is where the “cursor” part of its name comes from: it means “runner”. Maidment says it was probably charging around in the shadows of behemoths like Diplodocus and Stegosaurus.

The specimen’s vertebrae weren’t fused, which implies it wasn’t fully mature when it died. “I think this animal was probably a teenager, but it may well have been sexually mature, so it might not have got that much bigger,” says Maidment.

“Enigmacursor represents one of the rarities from further down the food chain of the dinosaur era,” says David Norman at the University of Cambridge. “This newly described animal was clearly a small, wallaby-sized herbivore that scampered around the Late Jurassic countryside.”

The discovery sheds light on the early evolutionary stages of the herbivorous dinosaurs that would go on to dominate Cretaceous ecosystems in North America, says Maidment, and helps us build a more realistic ecological picture of the life and times of dinosaurs.