The dinosaur family tree is about 130 years old, but it may not stand the test of time. Based on a new study of dozens of fossils and tens of thousands of specific anatomical features, scientists from the University of Cambridge and the London Natural History Museum are proposing that we uproot the old family tree and replace it with one that better reflects how dinosaurs and their descendants evolved.
Palaeontologist Harry Seeley planted the current tree in the late 19th century, when he first described dinosaurs as belonging to two separate groups, according to the structure of their hip bones. Those with lizard-like hips were dubbed Saurischia, while Ornithischia described those with a more bird-like structure. Later, the Saurischia group was further divided into the Theropoda and Sauropodomorpha families.
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If those names are hard to visualize, the Sauropodomorpha include the huge, long-necked dinosaurs like Brachiasaurus, Theropoda boasts the infamous T. Rex, and Ornithischia includes examples like the Stegosaurus.
In the accepted family tree structure, the Ornithischia group branches off earlier, leaving the other two more closely related. But this new study found more similarities between those in the Ornithischia group and theropods, suggesting a new level of relation there that excludes Sauropodomorpha.
"When we started our analysis, we puzzled as to why some ancient ornithischians appeared anatomically similar to theropods," says Matthew Baron, lead author of the paper. "Our fresh study suggested that these two groups were indeed part of the same clade (grouping). This conclusion came as quite a shock since it ran counter to everything we'd learned."
Although ornithischians share both their name origins and hip structure with birds, scientists have long known that our modern feathered friends evolved from the lizard-hipped therapods. But by grouping Ornithischia and Theropoda together under a new title, Ornithoscelida, the anatomical similarities suggest that both lines could have evolved more bird-like hips at different stages.
"The carnivorous theropods were more closely related to the herbivorous ornithischians and, what's more, some animals, such as Diplodocus, would fall outside the traditional grouping that we called dinosaurs," says Baron. "This meant we would have to change the definition of the 'dinosaur' to make sure that, in the future, Diplodocus and its near relatives could still be classed as dinosaurs."
Along with re-drawing the family tree, the study also raises new questions about when and where dinosaurs first appeared. Previously it was believed that dinosaurs originated on Gondwana, an ancient supercontinent that was made up of most of the landmass in the Southern Hemisphere today, but the new study suggests it's just as likely that they came from Laurasia in the north. By the same token, the new model pushes back the beginning of the dinosaur age to 247 million years ago – between four million and 16 million years earlier than previously thought.
"The repercussions of this research are both surprising and profound," says David Norman, co-author of the study. "The bird-hipped dinosaurs, so often considered paradoxically named because they appeared to have nothing to do with bird origins, are now firmly attached to the ancestry of living birds.
"For 130 years palaeontologists have considered the phylogeny of the dinosaurs in a certain way. Our research indicates they need to look again at the creatures' evolutionary history. This is simply science in action. You draw conclusions from one body of evidence and then new data or theories present themselves and you have to suddenly reconsider and adapt your thinking. All the major textbooks covering the topic of the evolution of the vertebrates will need to be re-written if our suggestion survives academic scrutiny."
The research was published in the journal Nature, and the team describes their findings in the video below.
Source: University of Cambridge