A team of paleontologists from the University of Cambridge and the Natural History Museum, London, UK, has proposed radical changes to the dinosaur family tree, based on their careful analysis of dozens of fossil skeletons and tens of thousands of anatomical characters.
The research by Baron et al provides important new insights about the origins and evolution of dinosaurs. Image credit: Sergey Krasovskiy.
Dinosaurs were first recognized as a unique group of fossil reptiles in 1842 as a result of the work of the anatomist, Professor Richard Owen, who later went on to found the Natural History Museum in London.
Over subsequent decades, various species were named as more and more fossils were found and identified.
During the latter half of the 19th century it was realized that dinosaurs were anatomically diverse and attempts were made to classify them into groups that shared particular features.
It was Harry Govier Seeley, a paleontologist trained in Cambridge under the renowned geologist Adam Sedgwick, who determined that dinosaurs fell quite neatly into two distinct groupings, or clades: Saurischia or Ornithischia.
This classification was based on the arrangement of the creatures’ hip bones and in particular whether they displayed a lizard-like pattern (Saurischia) or a bird-like one (Ornithischia).
As more dinosaurs were described it became clear that they belonged to three distinct lineages: Ornithischia, Sauropodomorpha and Theropoda.
In 1887 Seeley placed the sauropodomorphs (which included the huge ‘classic’ dinosaurs such as Diplodocus and Brontosaurus) together with the theropods (which included T. rex), in the Saurischia.
The ornithischians and saurischians were at first thought to be unrelated, each having a different set of ancestors, but later study showed that they all evolved from a single common ancestor.
Phylogenetic relationships of early dinosaurs: time-calibrated strict consensus of 94 trees from an analysis with 73 taxa and 457 characters. A – the least inclusive clade that includes Passer domesticus, Triceratops horridus and Diplodocus carnegii — Dinosauria, as newly defined. B – the least inclusive clade that includes P. domesticus and T. horridus — Ornithoscelida, as defined. C – the most inclusive clade that contains D. carnegii, but not T. horridus — Saurischia, as newly defined. All subdivisions of the time periods (white and grey bands) are scaled according to their relative lengths with the exception of the Olenekian (Early Triassic), which has been expanded relative to the other subdivisions to better show the resolution within Silesauridae and among other non-dinosaurian dinosauromorphs. Image credit: Baron et al, doi: 10.1038/nature21700.
The new analysis of dinosaurs and their near relatives concludes that the ornithischians need to be grouped with the theropods, to the exclusion of the sauropodomorphs.
It has long been known that birds (with their obviously ‘bird-like’ hips) evolved from theropod dinosaurs (with their lizard-like hips).
However, the re-grouping of dinosaurs proposed by the authors shows that both ornithischians and theropods had the potential to evolve a bird-like hip arrangement — they just did so at different times in their history.
“When we started our analysis, we puzzled as to why some ancient ornithischians appeared anatomically similar to theropods,” said study lead author Matthew Baron, a Ph.D. student in earth science at the University of Cambridge.
“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 carnivorous theropods were more closely related to the herbivorous ornithischians and, what’s more, some animals, such as Diplodocus, would fall outside the traditional grouping that we called dinosaurs,” he added.
“This meant we would have to change the definition of the ‘dinosaur’ to make sure that, in the future, Diplodocus and its near relatives could still be classed as dinosaurs.”
The revised grouping of Ornithischia and Theropoda has been named the Ornithoscelida which revives a name originally coined by the evolutionary biologist, Thomas Henry Huxley in 1870.
“The repercussions of this research are both surprising and profound,” said co-author Dr. David Norman, also from the University of Cambridge.
“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.”
While analyzing the dinosaur family trees, the researchers arrived at another unexpected conclusion.
For many years, it was thought that dinosaurs originated in the southern hemisphere on the ancient continent known as Gondwana. The oldest dinosaur fossils have been recovered from South America suggesting the earliest dinosaurs originated there.
But as a result of a re-examination of key taxa it’s now thought they could just as easily have originated on the northern landmass known as Laurasia, though it must be remembered that the continents were much closer together at this time.
“This study radically redraws the dinosaur family tree, providing a new framework for unraveling the evolution of their key features, biology and distribution through time,” said study co-author Prof. Paul Barrett, from the Natural History Museum, London.
“If we’re correct, it explains away many prior inconsistencies in our knowledge of dinosaur anatomy and relationships and it also highlights several new questions relating to the pace and geographical setting of dinosaur origins.”
The research appears today in the journal Nature.
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Matthew G. Baron et al. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature, published online March 23, 2017; doi: 10.1038/nature21700
This article is based on a press-release from the University of Cambridge.
