Before the asteroid strike that caused their extinction, dinosaur species were already in decline
Sixty-six million years ago, an asteroid approximately 12 kilometers in diameter struck the Yucatán Peninsula in Mexico. The impact triggered an explosion of unimaginable magnitude: 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: 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 dinosaurs closely related to birds). 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).
Mariana Ruiz Villarreal LadyofHats/Wikimedia, CC BY
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 available 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 categorizing the species and dating it correctly.
Fortunately, this work was carried out in collaboration with leading 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 through the scientific literature over time. It was painstaking work, because one author might assign a date and a species to a fossil, then another 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 disappeared, and to estimate, again for each family, the rates of speciation and extinction over time.
Fabien Contamine, Provided by the author
To estimate rates of speciation and extinction, several biases had to be taken into account. The fossil record is indeed 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 more reliable for estimating the number of species at a given time. But it is important to remain cautious, as we are talking about estimates, and these estimates can change with a more complete fossil record, for example, or with new analytical models.
The decline of herbivores preceded that of carnivores
In this study, our results show that, 10 million years before the asteroid impact—starting around 76 million years ago—the number of dinosaur species was in sharp decline. This decline is particularly interesting because it is global in scope and affects both carnivorous groups, such as tyrannosaurs, and herbivorous groups, such as triceratops. It is 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 declined sharply, such as ankylosaurs and ceratopsians, 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? During that period, the Earth experienced a global cooling of 7 to 8 °C.
We know that dinosaurs needed a warm climate to sustain 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 between reptiles and mammals, and needed a warm climate to maintain their body temperature and thus perform basic biological functions such as metabolic activities. This drop in temperature must therefore have had a very significant impact on them.
It is worth noting that we observed a time-lagged decline between herbivores and carnivores. Herbivores declined slightly before carnivores. The decline of herbivores appears to have triggered that of carnivores. Thus, according to our model, as soon as herbivores go extinct, carnivores follow—a phenomenon known as cascading extinctions. Indeed, herbivores are keystone species in ecosystems (even today in the savannas of Africa, 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 bounced back? 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 could have been highly uneven and dependent on the species group, meaning that some groups might have survived while others did not. Hadrosaurs, for example, showed a certain resilience to the decline and might, perhaps, have diversified again after the decline.
In any case, one could say that ecosystems at the end of the Cretaceous period were under pressure (climate deterioration, major changes in vegetation), and that the asteroid strike was the final blow. This is often the case with species extinction: they are first in decline, 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.