A dinosaur decline was already a problem on the planet long before an asteroid smashed into the Yucatan Peninsula, scientists suggest. The Chicxulub asteroid caused cataclysmic climate change that marked the end of the Cretaceous period, and killed off some three-quarters of animal species.
A small proportion of hardy birds survived, but the other dinosaurs went extinct. They were, however, already in decline, the Los Angeles Times reports.
Manabu Sakamoto from the University of Reading has shown that dinosaur species were going extinct faster than new ones were appearing, for at least 40 million years before the end of the Cretaceous. The dinosaur opera had already been going through a long diminuendo well before the Chicxulub asteroid ushered in its final coda.
Many other researchers had looked at the fates of the dinosaurs before that infamous extinction event and suggested that they were already declining. But most of these studies had simply tabulated raw numbers of species from different blocks of time. This approach has problems: the rocks from certain time periods may simply be better at preserving fossils, or may have been more intensively scrutinized by fossil-hunters.
After a recent attempt to adjust for these biases, using up-to-date information and better statistical techniques, Stephen Brusatte from the University of Edinburgh found “no evidence for a progressive decline in total dinosaur species richness.” Two groups—the horned ceratopsians and the duck-billed hadrosaurs-were fading in both number of species and variation in body shapes, but the others were not, United Press International noted. “Recently, the idea that the dinosaurs were reigning strong has dominated the academic debate,” says Sakamoto.
He begs to differ. Together with Michael Benton and Chris Venditti, he took a recently published family tree, comprising 614 dinosaur species, and modeled the rates at which new species arose and old ones went extinct. “We’re not counting numbers of species throughout the history of dinosaurs, but of speciation events,” he explains.
The titanic long-necked sauropods went through the biggest downturn. They experienced a burst of speciation in the early Jurassic period that culminated in the late-Jurassic arrival of iconic giants like Brontosaurus and Diplodocus. But by 114 million years ago, they were losing species faster than they could replace them. Even the arrival of the record-breaking titanosaurs couldn’t compensate for this evolutionary recession.
The dinosaur went through a similar, but less pronounced, rise and fall, the Ars Technica reported. The Cretaceous period was known for its extremely diverse range of theropods, including sickle-clawed dromaeosaurs like Velociraptor, pot-bellied and scythe-clawed therizinosaurs, ostrich-like ornithomimosaurs, and mighty tyrannosaurs. But many of these lineages originated much earlier in the Jurassic; during their Cretaceous heyday, they were also going extinct faster than they were spectating.
Ironically, the only exceptions to this pattern were the ceratopsians and hadrosaurs-the two groups that Brusatte’s analysis showed were in decline. During the dinosaur decline, these plant-eaters radiated into species with very similar builds, but with subtle variations in skulls and teeth that allowed them to exploit different sources of food.
Then again, scientists may have overestimated the number of species in these two groups, says Susannah Maidment from Imperial College London. For example, there’s a fierce ongoing debate about whether Torosaurus was actually an older version of Triceratops. “Over-splitting of this group relative to other dinosaur groups could produce a false picture of high speciation rates at the end of the Cretaceous,” says Maidment. “The method used in [Sakomoto’s] study requires us to have an accurate dinosaur evolutionary tree, and although we are very happy with the major branches, the arrangements of the twigs at the end is still debated and constantly undergoing change with each new dinosaur discovered.”
NDTV said that even if the ceratopsian and hadrosaur trees are right, these groups represented just one seventh of total dinosaur diversity. For the dynasty as a whole, extinction rates were surpassing speciation rates for roughly 48 to 53 million years before the Chicxulub asteroid impact.