Dinosaur family tree radically rearranged
22 March 2017
Scientists have proposed a significant reorganisation of the dinosaur family tree, with major implications for our understanding of dinosaur origins and evolution.
The collaborative study between the Museum and Cambridge University regroups the familiar lineages of dinosaurs that have been established for 130 years.
The new arrangement, which is based on analysis of hundreds of fossil features, helps explain some puzzling aspects of early dinosaur evolution and provides new clues as to when key features and behaviours evolved.
The study is published in the journal Nature. Museum palaeontologist and co-author Prof Paul Barrett says:
'If we're correct, this study explains away many prior inconsistencies in our knowledge of dinosaur anatomy and relationships. It also provides new insights and raises several new questions relating to the pace and geographical setting of dinosaur origins.'
Tracing the tree
It's hard to establish how different dinosaurs were related to both each other and their evolutionary history. The only evidence left is bones, with no DNA to offer clues.
Palaeontologists make their deductions by comparing the anatomical features of different dinosaurs and determining how these features could have evolved from one species to another.
The team examined fossils from a wide range of dinosaurs and their close relatives, including examples from all around the world and with varied body shapes.
'We are the first to rigorously test the anatomical similarities and differences between a large number of the earliest dinosaur species,' explains Prof Barrett.
'By focusing on the oldest animals we have come up with this radical rearrangement of the major branches on their evolutionary tree.'
The researchers looked at nearly 35,000 individual anatomical features - based on 457 different features found in 74 early dinosaur species. They used a computer programme to model the most likely way that each feature would have evolved from dinosaur to dinosaur and used the results to place the dinosaurs into a new family tree.
Overturning 130 years of dogma
The branches of the tree are organised by how closely animals are related to each other. Branches arising from a single point on the tree represent an animal or group of animals that all descended from a common ancestor. This is called a clade.
Since 1887, scientists have divided dinosaurs into two main clades, based on the arrangement of their hip bones. Saurischians had a classic reptile-like pelvic anatomy and ornithischians were defined by a bird-like pelvic structure.
Ornithischians were generally herbivorous and included dinosaurs such as Stegosaurus.
The saurischians are split into two further branches: the herbivorous sauropodomorphs (such as Diplodocus) that walked on four legs, and the bipedal (two-legged), often carnivorous theropods such as T. rex.
The new research has changed what we know about the relationships between these three lineages. Theropods are now actually more closely related to ornithischians, while the sauropodomorphs are more distant.
Matthew Baron, lead author of the study and joint Museum and Cambridge University PhD student, 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 revised grouping of Ornithischia and Theropoda has been named Ornithoscelida, meaning 'bird-limbed'.
In addition, a group of carnivorous dinosaurs called herrerasaurids, which have traditionally been classed as theropods, have been moved to join the sauropodomorphs within the Saurischia.
Why work out the dinosaur family tree?
Scientists can gain an idea as to what the common ancestor of all dinosaurs looked and behaved like by examining early dinosaurs and seeing what characteristics they share.
The proposed new family tree helps make better sense of some of these clues.
A two-legged advantage
The new work supports the current consensus that the earliest dinosaurs were relatively small - about one to two metres long - and walked on two legs.
It also suggests that primitive dinosaurs could use their front two limbs, no longer needed for walking, as hands to grasp objects.
The researchers speculate that these grasping hands may have given early dinosaurs an advantage over rival animals, perhaps in feeding.
Scientists have long debated the diet of early dinosaurs. In the new tree, early dinosaurs in both clades have a combination of teeth, indicating that they were probably omnivores. It is therefore likely that the common ancestor of dinosaurs was an omnivore too.
As for later dinosaurs, both herrerasaurids and theropods became carnivores. The new tree puts them in different clades, implying that meat-eating evolved twice.
Following the feathers
While a number of dinosaurs are now known to have had feathers, nearly all examples come from the theropod group. A few examples of feather-like structures have also been found in ornithischians, although some scientists dispute that these are true feathers.
Nonetheless, the new family tree places both groups within the same clade, whereas previously they were separate.
So it's possible that if feathers were present in both ornithischians and theropods, they evolved only once.
The first dinosaurs
The new analysis also gives clues to when dinosaurs first emerged and where palaeontologists should look for evidence.
By examining how the very early dinosaurs were related to each other, the researchers can calculate how long it would have taken for different features to have evolved from a common ancestor - and thus estimate when that ancestor lived.
Although disputed by some, the earliest animal claimed as a dinosaur is Nyasasaurus parringtoni, which lived about 245 million years ago. If Nyasasaurus is indeed a dinosaur, the calculations suggest the very first dinosaur appeared around 247 million years ago.
The origins of dinosaurs
Currently, most scientists think that the first dinosaurs appeared in the southern hemisphere, which at the time was occupied by the supercontinent Gondwana. However, some of the very early dinosaurs on the reworked family tree, as well as their close relatives, were found in the northern hemisphere. This suggests that the first dinosaurs could also have emerged in the north, on the landmass known as Laurasia.
Once they appeared, it seems that dinosaurs diversified very quickly, in geologic terms. Between about 242 to 227 million years ago dinosaurs evolved many of the key characteristics of their main groups. The stage was then set for their later spectacular success and long domination of the Earth.
Co-author Dr David Norman, of the University of Cambridge, comments on the team's findings: 'All the major text-books covering the topic of the evolution of the vertebrates will now need to be re-written if this suggestion survives academic scrutiny and becomes accepted more widely.
'It seems that the dinosaur family tree is being shaken quite firmly - it will be interesting to see what drops from its branches in years to come!'
- By James McNish and Lisa Hendry