Dino evolutionary tree replanted
Dinosaur ancestry was long thought to be settled. New research suggests a radical revamp. Stephen Fleischfresser reports.
A new hypothesis is set to completely rewrite the dinosaur family tree, overturning 130 years of conventional taxonomic wisdom, and potentially resolving a long-running conundrum in the field.
Our current understanding of the evolutionary genealogy of dinosaurs comes from Richard Owen, Darwin’s greatest foe, and the student of another Darwin-antagonist, Adam Sedgwick.
Owen first recognised the large fossilised reptiles as a distinct group in 1842, calling them Dinosauria (‘terrible lizards’).
Sedgwick’s student Harry Seeley further recognised three distinct groupings within Dinosauria in 1887: Ornithischia (‘bird-hipped’ dinosaurs such as triceratops and stegosaurus), Theropoda (‘beast-footed’ animals such as velociraptor and tyrannosaurus) and Sauropodomorpha (‘lizard-footed’ species such as diplodocus and brontosaurus).
He placed the latter two in the category of Saurischia (‘lizard-hipped’), on the assumption that they were closely related.
The classification, however, eventually produced its own contradiction: despite the name, bird-hipped beasts did not appear to be direct ancestors of birds, which evolved instead from the lizard-hipped category.
Yet that remained the accepted understanding. Until now.
In a paper published in Nature, a team of scientists from the University of Cambridge and the Natural History Museum in London has put forward a radical hypothesis that changes everything.
Matthew Baron and palaeobiologist colleagues David Norman and Paul Barrett present findings from extensive research into dinosaur skeletal anatomy, and argue for the rearrangement of these categories.
Their argument is based in the taxonomic school known as cladistics. Cladists hold that the classification of organisms should be based solely on ancestry. The method centres on divisions known as a ‘clades’: groups of organisms that share biological characteristics derived from the most recent common ancestor.The researchers dismantle Seeley’s Saurischia, and instead forms a new system, comprising a new group called Ornithioscelida – containing theropods, the ancestors of modern birds – and the old Ornithischia. Sauropodomorpha becomes a distinct group by itself.
The name of the new clade comes originally, and ironically, from Darwin’s greatest ally and Owen’s most public detractor, T. H. Huxley.This new hypothesis begins to explain features of dinosaur evolution that had been confusing until now.
As co-author Norman states, “The repercussions of this research are both surprising and profound. 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”.Although the authors seem to believe that the clades they identify are real, there are both scientific and philosophical reasons to think that classifications such as these are nothing more than a way of organising our knowledge of the world. For one group of people, however, it has enormous significance: academic publishers.
If the researchers’ hypothesis is well received, as Norman says, “All the major textbooks covering the topic of the evolution of the vertebrates will need to be re-written”.