Dinosaurs were declining before asteroid death blast finished them off

Better to burn out than fade away.


Rae Hodge
June 30, 2021


Ecological factors had already begun mounting their own opposition to Cretaceous creatures, say researchers, well before the species' final betrayal by that fateful asteroidal blast.Getty Images

For decades now, paleontologists have debated whether -- if spared the fiery finality of an asteroidal death-blast -- the 160-million-year reign of the world's most fearsome creatures could've continued unbroken on a planet whose ecology had already begun turning its back on them. In a study released Tuesday, scientists assert that long before a space rock tore a 115-mile-wide crater in the Yucatán Peninsula, the fate of the dinosaurs may've already been sealed.

The new research shows how cooling climate and the decline of herbivores created a domino effect that turned the ecosystem against the Cretaceous creatures. The study, published in the journal Nature Communications, comes from an international team of scientists, including researchers from the University of Bristol in England and the Institut des Sciences de l'Evolution de Montpellier in France.

"We looked at the six most abundant dinosaur families through the whole of the Cretaceous, spanning from 150 to 66 million years ago, and found that they were all evolving and expanding and clearly being successful," the study's lead author, CNRS researcher Fabien Condamine, said in a Tuesday release.

"Then, 76 million years ago, they show a sudden downturn. Their rates of extinction rose and in some cases the rate of origin of new species dropped off."

Graphs from the study show how speciation rate (in blue) dropped off just as the extinction rate (in red) spiked in the last 10 million years of the dinosaurs' reign. Scientists say that together, this corresponds to a rapid reduction in the number of species just before the impact of the asteroid 66 million years ago.University of Bristol / Fabien L. Condamine

A swirl of variables previously dogged the quest to nail down the condition of the dinosaurs before the asteroid's impact, including incomplete fossil records, uncertainties over age-dating the fossils, and doubts about the evolutionary models. So, using more than 1,600 dinosaur records, the team ran each model millions of times to consider all possible sources of error. And, ultimately, to find out whether the analyses would converge and point toward the most probable answer.

"This means that the data are getting better all the time," said co-author Phil Currie, of the University of Edmonton. "The decline in dinosaurs in their last ten million years makes sense, and indeed this is the best-sampled part of their fossil record as our study shows"

In the end, the dinosaur data pointed toward a handful of key causes for the species' decline, according to co-author Mike Benton, of the University of Bristol.

"It became clear that there were two main factors. First, that overall climates were becoming cooler, and this made life harder for the dinosaurs which likely relied on warm temperatures," Benton said.

"Then, the loss of herbivores made the ecosystems unstable and prone to extinction cascade. We also found that the longer-lived dinosaur species were more liable to extinction, perhaps reflecting that they could not adapt to the new conditions on Earth."

In the meantime, scientists said, the scurrying of tiny burrowing mammals in the undergrowth -- the ultimate inheritors of the postapocalyptic planet we know today -- likely went unnoticed by the massive dinosaurs.

Just as, 66 million years ago, a tiny ball of fire in the sky likely went unnoticed by the Cretaceous titans until -- growing larger and hotter -- the hurtling rock immersed the dinosaurs in one all-consuming final explosion, and it was too late for them to notice anything ever again.

First published on June 30, 2021