Dinosaur buffs will have noticed some tremors propagating through the ether over the past couple of weeks. These followed from the publication of a new study, by Matthew Baron, David Norman and I, in which we proposed a radical rearrangement of the dinosaur family tree. This paper, published in the august journal Nature, provides a deliberate challenge to around 130 years of the palaeontological status quo. The essence of the paper is a simple one: for most of the past century the majority of scientists have regarded the long-necked sauropods, such as Diplodocus, and the carnivorous theropods, such as Tyrannosaurus, as more closely related to each other than either was to the other great group of dinosaurs, the vegetarian ornithischians, such as Stegosaurus.

However, our new study provides the first evidence for an alternative arrangement in which Stegosaurus and Tyrannosaurus become closer evolutionary cousins (and they and their relatives are combined in to a new group we name Ornithoscelida), with Diplodocus relegated to a more distant relationship with these other dinosaurs. A reader weary of regular pronouncements from dinosaur experts might simply shrug and say “so what?”, as our knowledge of dinosaurs, and other extinct animals, is constantly being renewed and updated, with new discoveries quickly falling off of the front pages to languish in the quiet backwaters of academe. These types of adjustments to evolutionary trees might appear trivial, but our study, and many others like it, have a broader relevance that goes beyond their immediate impact. Establishing the interrelationships of animals, plants, fungi and bacteria to each other, to build a great tree of life, forms the basis of all biological science, for reasons I’ll try to make clearer below. As a result, constant efforts are being made to build and refine evolutionary trees so that they depict the best-supported hypotheses of relationships among all living (and once-living) things.

Facebook Twitter Pinterest A classic sauropod ( Diplodocus), a group traditionally considered the closest to the theropods which include things like Tyrannosaurus and were the ancestors of birds. Photograph: Alamy Stock Photo

Evolutionary trees are testable hypotheses that are based on various lines of evidence. They are not immutable and the details often change although many of the major branches in the tree of life have remained stable for centuries. This doesn’t imply that work on these groups has ended, or that we’ve arrived at the ‘correct’ tree, but that teams of researchers have yet to find better explanations for the mass of evidence gathered throughout the history of the subject. For some organisms, such as bacteria, we rely entirely on the information encoded in their DNA and use shared similarities in their genes to establish patterns of relationships. The same techniques are also used when deducing the genealogies of living animals and plants. Recent technological advances allow rapid gene sequencing that provides vast amounts of data that can be used to untangle evolutionary lineages. Animals and plants possess another major source of information that can shed light on their relationships – the numerous features that make up the anatomy of their bodies, as well as behavioural and other cues. In the case of extinct animals, which comprise a substantial proportion of known biodiversity, DNA is usually unavailable as it does not survive for long in the fossil record. In these cases, we rely exclusively on features preserved in fossils, which are usually limited to bones, shells, exoskeletons and the more resistant parts of plants.

In the case of dinosaurs, its necessary to study their skeletons in detail, poring over them and reducing their complex anatomy to hundreds of individual features, each of which can contribute to revealing their relationships. Our study used almost 35,000 individual pieces of data, each one representing a particular anatomical feature in a different species, to try and unravel the shape of the dinosaur family tree. These features might represent a muscle attachment site on a bone, the bone’s overall shape, the number of teeth, or any other characteristic of the animal, as the skeleton represents the sum of its evolutionary history. When reconstructing relationships, we build our trees on the basis of features that are shared between different species. This is because the possession of shared features implies common ancestry, so the more features that are found in common, the higher the chance of a closer relationship. Choices between competing trees with different patterns of relatedness are then based on the philosophical principle of Occam’s Razor, which states, in essence, that, all things being equal, the simplest explanation is usually the best one. In this case this means that we sought the dinosaur family tree that required the fewest evolutionary changes to explain the maximum number of shared similarities between the species included.

Gathering data for an analysis like this can take years, in our case around 2 years of intensive work. This is due to the need to visit far flung institutions to see all of the necessary fossils, trawl through lengthy descriptive monographs, and to compile and define lists of characters that might be of interest. Given the huge volumes of data now included in these analyses, and the mind-boggling number of permutations that can result, powerful computer algorithms are used to build and compare these trees.



Building the tree is just the starting point, however. Not only do these trees provide a genealogy that identifies whom is related to who, but they are also frameworks that enable us to study evolutionary history. For example, our new tree, if correct, goes beyond identifying some new groupings to provide alternative explanations for many major events in dinosaur history. It suggests, for instance, that feathers, which some of our colleagues have regarded as characteristic of dinosaurs as a whole, were confined to the ornithosceldian group instead. Previous ideas predicted that sauropods should have feathers, as they were regarded as closely related to the feathered theropods, but feathers have not been found in any sauropod species to date. Our study provides a simple explanation for this by showing that sauropods are more distantly related to feathered dinosaurs than usually thought, so their lack of feathers becomes much less of a problem.







Our new tree also throws up some surprising new ideas. It takes some early dinosaurs that had been included within the meat-eating theropod group and splits these off into a branch of their own called Herrerasauria. It looks as though herrerasaurians might be more closely related to the vegetarian sauropods than to the theropods, which would imply that meat-eating might have arisen not once in dinosaurian evolutionary history, but twice. As a final example, an emerging consensus among palaeontologists suggested that dinosaurs had their origin on the southern continents, as many of the earliest dinosaurs and their immediate relatives had been found in this region. However, our study, which included more relevant fossils than any that had gone before it, found that several fossils from the northern hemisphere were occupying key early positions in the dinosaur tree. This result suggests that the southern hemisphere origin of dinosaurs should come under closer scrutiny and that we should not give up on the search for what might be exciting and informative early dinosaur sites in North America, Asia and Europe just yet.



A quick note of caution before ending. Although it’s exhilarating to propose new ideas and shine a light on old theories a couple of caveats need to be made before we infuriate dinosaur fans and museum exhibition designers by re-writing the textbooks. Although Matt, David and I are convinced that our tree provides a good account of dinosaur history, our colleagues around the world will be examining this from every angle to test these new results. Also, new information on dinosaurs comes to light every day, from new discoveries in the field and more detailed examination of skeletons already housed in museum drawers. Any one of these discoveries could impact our new tree (or, indeed, other versions of the dinosaur tree) and force us to reconsider. This is nothing to be upset about, however: this is how science works. None of us should be afraid to go where the evidence leads us.

Baron, M.G., Norman, D.B. and Barrett, P.M., 2017. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature, 543(7646), pp.501-506.