Normally the dinosaurian world is rocked by a new fossil -- the biggest, fastest, or toothiest. But the latest dinosaur research threatens to change our understanding of how dinosaurs evolved at a much deeper level, and blow aside 130 years of agreement on the topic.A new paper published in the journal Nature suggests that scientists need to reorganise the major groups used to classify dinosaurs. This means we may have to revisit what we think we know about the first dinosaurs, what features they evolved first, and where in the world they came from. The way we classify dinosaurs goes back to the 19th century. In 1887, Harry Govier Seeley, a classic, hard-working Victorian palaeontologist, divided dinosaurs into two major suborders based primarily on their hip structure. Saurischia comprises the flesh-eating theropods such as Tyrannosaurus and the ponderous, long-necked sauropodomorphs such as Diplodocus. Ornithischia comprises all the rest, including the two-legged Iguanodon, and the armoured, four-legged Stegosaurus, Triceratops, and Ankylosaurus.
analyse and categorise groups of animals based on features that pointed to a common ancestor. There are now thousands of diagrams (cladograms) of dinosaur subgroups, and ever-growing data matrices, that closely document the anatomical features of each species.The new paper completely disrupts the consensus over Seeley's categories. The researchers ran a cladistic analysis of 457 characteristics across 74 species (that is a data matrix of 33,818 bits of information recorded from skeletons). They concluded that, based on 21 unique characteristics of the fossils, the theropods were more closely related to the Ornithischia group and should be moved into that category. This would create a new group named Ornithoscelida and leave behind the Sauropodomorpha. The trick in cladistics is to find a unique anatomical feature that evolved at a specific time and can indicate a particular subgroup. For example, Seeley noted that the hip bones of ornithischians were arranged with pubis and ischium running backwards (superficially, like modern-day birds). Meanwhile, the hip bones of saurischians (including theropods) matched other reptiles, with pubis forwards and ischium back.
Matthew Baron/NatureThis suggests the two groups split from a common ancestor and evolved different hip shapes. This was a massive anatomical change or novelty, and palaeontologists until now have assumed that it happened only once in evolutionary history. Grouping the theropods with the ornithischians suggests that the hip change occurred later and raises the question of whether some early theropods had this feature. The researchers also suggest that the new analysis can reset our understanding of where dinosaurs originated and what their diet was. The classic view was that the first dinosaur was a carnivore living in what is now South America. The new analysis makes this more of an open question and suggests they might have evolved as omnivores in the northern hemisphere.