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Dinosaur tree of life still elusive


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.

This ordering of dinosaurs has stood the test of time for 130 years, weathering the onslaught of cladistics in the 1980s, when palaeontol­ogists began using computers to 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.

This 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.

None of this changes what we know for sure about what dinosaurs evolved which traits and when. But the key point is that accurately depicting the tree of life matters. If you care about modern biodiversi­ty, it’s important that all species are not equal. Some are more distinctiv­e than others, possessing more unique features, and having a longer independen­t history. Working this out requires an accurate tree.

On a broader scale, getting the tree right affects our calculatio­ns of rates of trait evolution, extinction and post-extinction recoveries. We will never find the very first dinosaur but we can establish some things about it by estimating the ancestral states of different species from a correct tree.

We invest enormous efforts into constructi­ng testable systems for categorisi­ng different species, and their size is increasing as computing power grows. When I ran my first cladogram in 1982, I had to use punch cards on a mainframe computer, and I could include only 10 or 12 species and 50 or so characteri­stics.

Today, I was able to run all the data for this new paper through my desktop computer and get an answer in 33.21 seconds, while writing this article at the same time. Recent publicatio­ns have sported trees of all 10 000 species of birds, and even summary trees of all life. The dream is to run such trees with all 1.5 million named species, using data about both genes and physical shape.

Is this new paper the true answer for the evolutiona­ry origins of dinosaurs? The data we have is riddled with question marks, and so the algorithms still struggle to calculate the one true tree. This is no criticism of the researcher­s, just a statement of the practicali­ties. We don’t know yet whether we can see the wood for the trees.

Benton is professor of vertebrate palaeontol­ogy at the University of Bristol.

 ??  ?? The old family tree. Graphic: Zureks/Wikimedia, CC BY-SA
The old family tree. Graphic: Zureks/Wikimedia, CC BY-SA

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