Before the asteroid strike that caused their extinction, dinosaur species were already in decline

[The Conversation] - Published on June 30, 2021 in Science and Society

66 million years ago, an asteroid approximately 12 kilometers in diameter struck the Yucatán Peninsula in Mexico. The impact triggered an explosion of a magnitude that is hard to imagine: equivalent to several billion times the power of the atomic bomb dropped on Hiroshima. Most of the animals on the American continent were killed instantly.

Fabien Condamine, University of Montpellier

The impact also triggers global tsunamis. In addition, tons and tons of dust are ejected into the atmosphere, plunging the planet into darkness. This “nuclear winter” thus leads to the extinction of a vast number of plant and animal species. Among the latter, the most iconic are the dinosaurs. But before this cataclysm, how was this group faring? That is the question we sought to answer in our study, the results of which have just been published in the scientific journal *Nature Communications*.

We focused on six families of dinosaurs—the most representative and diverse of the Cretaceous period, particularly the last 40 million years. Three were carnivorous: the Tyrannosauridae, the Dromaeosauridae (which included the famous velociraptors, made famous by the Jurassic Park movies), and the Troodontidae (small, bird-like dinosaurs). The other three families we studied were herbivores: the Ceratopsidae (represented notably by Triceratops), the Hadrosauridae (the most diverse of all the families), and the Ankylosauridae (represented in particular by the ankylosaur, a sort of “tank” with bony armor and a club-like tail).

Our goal was to determine the rate at which these families were diversifying (species formation) or going extinct (species extinction). We knew that all of these families had survived until the end of the Cretaceous Period, marked by the asteroid impact.

1,600 fossils analyzed

Over the course of five years, we compiled all known information on these families in an effort to determine how many and which species each group included. The work was tedious, as we cataloged most of the known fossils for these six families, which amounted to more than 1,600 individuals representing approximately 250 species. Several challenges arise with each fossil: accurately classifying the species and dating it correctly.

Fortunately, this work was carried out in collaboration with eminent paleontologists (Guillaume Guinot, Mike Benton, and Phil Currie). In the scientific community, for the sake of traceability, each fossil is assigned a unique number, which allows us to track it in the scientific literature over time. It was painstaking work, because one author might assign a date and a species to a fossil, then another author would reexamine it and perform a different analysis, and so on. So we had to make decisions. If we had too many doubts, we excluded the fossil from the study.

Once each fossil had been properly classified, we used a statistical model to estimate the number of species over time for each family. We were thus able to track, over millions of years (from –160 to –66 million years ago), which species appeared and which went extinct, and to estimate—again for each family—the rates of speciation and extinction over time.

To estimate speciation and extinction rates, several biases had to be taken into account. The fossil record is, in fact, biased. It is uneven over time and space, and certain groups do not fossilize well. This is a well-known problem in paleontology when attempting to estimate the dynamics of past biodiversity. Given these issues, modern, sophisticated models can account for the heterogeneity of preservation over time and across species. In doing so, the fossil record becomes a more reliable tool for estimating the number of species at a given time. But it is important to remain cautious, as we are dealing with estimates, and these estimates may change—for example, as the fossil record becomes more complete or as new analytical models are developed.

The decline of herbivores preceded that of carnivores

In this study, our results show that 10 million years before the asteroid impact—starting 76 million years ago—the number of dinosaur species was in sharp decline. This decline is particularly interesting because it was global in scope and affected both carnivorous groups, such as tyrannosaurs, and herbivorous groups, such as triceratops. It was also observed in the Old World (Europe, Asia, Africa, and Australia) and the New World (the Americas). There is still some variation in the response. Some groups, such as ankylosaurs and ceratopsians, declined sharply, while only one family (the troodontids) out of six shows a very slight decline, which occurred within the last 5 million years.

What could have caused this sharp decline? At that time, the Earth experienced a global cooling of 7 to 8 °C.

We know that dinosaurs needed a warm climate for their metabolism. As is often said, they were not ectothermic (cold-blooded) animals like crocodiles or lizards. Nor were they endotherms (warm-blooded), like mammals or birds. They were mesotherms—a system intermediate between reptiles and mammals—and needed a warm climate to maintain their body temperature and thus carry out basic biological functions such as metabolic activities. This drop in temperature must therefore have had a very significant impact on them.

It should be noted that we found a time-delayed decline between herbivores and carnivores. Herbivores declined slightly before carnivores did. The decline of herbivores is thought to have triggered that of carnivores. Thus, according to our model, as soon as herbivores go extinct, carnivores disappear—this is what is known as a cascade of extinctions. Indeed, herbivores are keystone species in ecosystems (even today in the African savannas, for example). Many species “rely on” herbivores. Their extinction often leads to the extinction of other species that depend on these herbivores.

The big question that remains unanswered: What would have happened if the asteroid hadn’t struck? Would the dinosaurs have gone extinct anyway, due to the decline that had already begun, or could they have rebounded? It’s very hard to say. Many believe that if they had survived, primates—and therefore humans—would never have appeared on Earth. An important point is that a potential rebound in biodiversity can be highly heterogeneous and depend on the specific group, meaning that some groups might have survived while others did not. Hadrosaurs, for example, showed a certain degree of resilience in the face of decline and might, perhaps, have diversified again after the decline.

In any case, one could say that ecosystems at the end of the Cretaceous were under pressure (climate deterioration, major changes in vegetation), and that the asteroid was the final blow. This is often the case with species extinction: species are first in decline and under pressure, and then another event occurs that finishes off a group that may have been on the brink of extinction before that event.

Fabien Condamine, CNRS Researcher in Phylogeny and Molecular Evolution, University of Montpellier

This article is republished from The Conversation under a Creative Commons license. Readthe original article.

Keywords: Environment, History, Science