A study published in the journal Nature challenges a dinosaur classification used for more than a century, suggesting that grouping these animals primarily by pelvic structure may have been misleading.
Scientists divided non-avian dinosaurs, which dominated terrestrial ecosystems around 180 million years ago, into two groups: Ornithischia and Saurischia.

The first group included the herbivorous Iguanodon, Triceratops with its bony frill, and Stegosaurus, with its spines. The second included herbivorous titanosaurs, megaraptors and the well-known Tyrannosaurus rex. This second group also included theropods, whose feathered descendants eventually gave rise to birds.
The division into these two groups was based on the structure of the pelvis.

The study, conducted by researchers from the University of Cambridge and the Natural History Museum in London, identifies at least 21 anatomical features, in addition to pelvic structure, that associate theropods with Ornithischia rather than Saurischia.
The proposed regrouping associates theropods with the “bird-hipped” dinosaurs. Although birds were already known to have evolved from theropods, the latter had traditionally been placed within Saurischia, the “lizard-hipped” dinosaurs.
The old definition of dinosaurs “needed an update”, said Matthew Baron, a palaeontology graduate at the University of Cambridge and co-author of the study.
The new interpretation also changes relationships among other subgroups, placing a primitive group of carnivorous dinosaurs alongside long-necked herbivores such as Brontosaurus and Diplodocus.
“This approach has produced a new perspective on the ancestral dinosaur and its way of life,” Baron emphasised.
The earliest dinosaurs, described here as survivors of the mass extinction approximately 250 million years ago, remain poorly understood because Early Triassic fossils are relatively poorly preserved. Fossils of later dinosaurs nevertheless allow researchers to infer the appearance of their ancestors.
The study, which rearranges the dinosaur evolutionary tree, concludes that the earliest dinosaurs were probably small, bipedal omnivores with grasping forelimbs. It also proposes that dinosaurs originated in the Northern Hemisphere rather than the Southern Hemisphere, as previously thought.