Dinosaur family tree radically rearranged

22 March 2017

Graphic illustration of the old and new versions of the dinosaur family tree illustration

Simplified versions of the dinosaur family tree, showing the major changes proposed in this study

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.

Plateosaurus skull

The research team examined the features of many early dinosaurs - such as Plateosaurus, which this skull belonged to
 

'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.

Coelophysis fossil

Coelophysis was one of the earliest theropods, and was included in the study of 74 early dinosaur species

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.

Dinosaur drawings showing the classic lizard-like hip structure (top) and bird-like hip structure (bottom)

Drawings showing the hip-bone arrangements traditionally used to define saurichians (top) and ornithischians (bottom). Modified from a public domain image from Wikimedia commons.
 

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.'

Heterodontosaurus hand fossil

Fossil hand of Heterodontosaurus, an ornithischian dinosaur. Theropods had very similar hands. Specimen from Iziko South African Museum.
 

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.

Herrerasaurus reconstruction

In the new model, the meat-eating dinosaur Herrerasaurus is no longer considered a theropod © Fred Wierum, via Wikimedia Commons
 

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.

Determining diet

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.

Massospondylus skull

The teeth of Massospondylus were a variety of shapes, a sign of an omnivorous diet. The new family tree suggests the ancestor of all dinosaurs was probably also an omnivore. Specimen from the Evolutionary Studies Institute, Johannesburg.
 

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.

Deinonychus animatronics in the Museum Dinosaur gallery

Models of the feathered theropod dinosaur Deinonychus in the Museum's Dinosaurs gallery

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.

Palaeoart reconstruction of Nyasasaurus parringtoni

Nyasasaurus parringtoni may be the oldest known dinosaur. Although only fossil fragments have been found, they show features unique to dinosaurs. © Mark Witton/The Trustees of the Natural History Museum, London
 

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.

Palaeogeographic reconstruction of the world in the Triassic Period

Reconstruction of the world around 225 million years ago, showing the position of what would become the supercontinents Gondwana and Laurasia. Image courtesy of the US Geological Survey.
 

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

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