The Long Decline of the Dinosaurs – The Atlantic

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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. “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. (A speciation event occurs when one species diverges into two.)

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 meat-eating theropods went through a similar, but less pronounced, rise and fall. 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 speciating.

Ironically, the only exceptions to this pattern were the ceratopsians and hadrosaurs—the two groups that Brusatte’s analysis showed were in 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.

The Long Decline of the Dinosaurs – The Atlantic