About 130 years ago, Harry Govier Seeley, a paleontologist trained in Cambridge, classified dinosaurs into two distinct groups, or clades, based on the shape of their pelvic bones. The “reptile-hipped” saurischians included carnivorous theropods like the Tyrannosaurus Rex (T-Rex), while the “bird-hipped” ornithischians comprised herbivores such as the Stegosaurus and Triceratops. As more dinosaur fossils were discovered, a third group, dubbed sauropodomorphs, was established. In 1887, Seeley concluded that the long-necked herbivorous sauropods, like the Brontosaurus, were related to theropods and classified them as saurischians.
The broad groupings were not entirely satisfactory given that researchers later found some ornithischians that displayed theropod-like behavior: they were bipeds and carnivorous. However, despite the shortcomings, the classification was never questioned until recently when a team from the University of Cambridge and Natural History Museum decided to revisit the decades-old dinosaur family tree.
University of Cambridge paleobiologist Matthew Baron and his colleagues David Norman and Paul Barrett spent three years examining 74 species, most of which lived within the first 100 million years of the dinosaur era. They then analyzed the differences and similarities in over 457 anatomical features of these ancient creatures.
Their investigation led to the conclusion that it is the ornithischians and saurischians that are related and the sauropodmorphs which need to be separate. The researchers created a new dinosaur tree, grouping the former two under a new classification — ornithoscelidan — and giving the sauropods their own category.
As a result, the meat-eating T-Rex and the vegetarian Stegosaurus now belong to the same family. This arrangement eliminates the confusion of the previous classification since all the members of the ornithoscelidan, which mean “bird-limbed,” possess common physical characteristics such as the shape of their skulls and hind-limbs.
Baron, who led the study, says, "When we started our analysis, we puzzled as to why some ancient ornithischians appeared anatomically similar to theropods. Our fresh study suggested that these two groups were indeed part of the same clade. This conclusion came as quite a shock since it ran counter to everything we'd learned."
The researchers also assert that the first dinosaurs may have inhabited earth slightly earlier than 243 million years ago and suggest that contrary to popular belief, they may have originated in what is now North America, not Gondwana – the southern portion of the supercontinent Pangaea.
Study co-author Barrett of the Natural History Museum says: "This study radically redraws the dinosaur family tree, providing a new framework for unraveling the evolution of their key features, biology, and distribution through time. If we're correct, it explains away many prior inconsistencies in our knowledge of dinosaur anatomy and relationships, and it also highlights several new questions relating to the pace and geographical setting of dinosaur origins."
It will, of course, take many years of debate before experts accept or reject the new theory. So don’t toss out your social studies textbooks yet!
Resources: nature.com, phys.org, guardian.co.uk