Rewriting the dinosaur family tree: University of Cambridge experts explore palaeontology mystery over ‘bird-hipped’ dinosaurs

Researchers believe they have answered a long-standing mystery in palaeontology about the origins of ‘bird-hipped’ dinosaurs, or ornithischians.

The group includes iconic species such as Triceratops but currently, there is a gap of more than 25 million years in the fossil record, meaning it is hard to place them in the dinosaur family tree.

Now scientists from the University of Cambridge and the Universidade Federal de Santa Maria in Brazil have conducted extensive new analysis that concludes that they are likely to have evolved from a group of animals known as silesaurs, which were first identified in 2003.

They suggest that silesaurs progressively modified their anatomy during the Late Triassic Period, so that they came to resemble ornithischians by the Early Jurassic Period.

But these ornithischian ancestors have the hip structure of the ‘lizard-hipped’ dinosaurs, or saurischians, suggesting the earliest bird-hipped dinosaurs were, in fact, lizard-hipped.

In 2017, Prof David Norman from Cambridge’s Department of Earth Sciences and his former PhD students Matthew Baron and Paul Barrett had argued dinosaur family groupings needed to be rearranged, controversially suggesting that bird-hipped dinosaurs and lizard-hipped dinosaurs such as Tyrannosaurus had evolved from a common ancestor.

That potentially overturned more than a century of theory about the evolutionary history of dinosaurs.

“It seemed to be that they originated with all other dinosaurs in the Late Triassic but exhibited a unique set of features that could not be fitted into an evolutionary succession from their dinosaur cousins,” said Prof Norman, who is a fellow of Christ’s College. “It was as if they just suddenly appeared out of nowhere.”

In 2003, a Late Triassic dinosaur-like animal was discovered in Poland and was described by Jerzy Dzik and named Silesaurus, or the ‘Silesian lizard’. More Silesaurus-like creatures have since been discovered.

Analysis by Prof Norman and colleagues in Brazil suggests a new family tree that depicts silesaurs as a succession of animals on the stem of the branch leading to Ornithischia.

“Silesaurians progressively modified their anatomy during the Late Triassic, so that they come to resemble ornithischians,” said Prof Norman. “We have been able to trace this transition through the development of the toothless beak, the development of leaf-shaped coarse-edged teeth typical of those seen in the herbivorous ornithischians, modifications to the shoulder bones, changes in the proportions of the pelvic bones, and finally a restructuring of the muscle attachment areas on the hind legs.”

The earliest ornithischians of the Late Triassic had none of the anatomical characteristics of true ornithischians, however.

They lacked a predentary and retained the early saurischian hip construction.

“So, the very earliest ornithischians were, technically, saurischian,” explained Prof Norman.

“From a taxonomic perspective, classifying silesaurs as early ornithischians seems counterintuitive. But, taking a Darwinian perspective, the unique anatomical characteristics of ornithischians had to evolve from somewhere, and where better than from their nearest relatives: their saurischian cousins!”