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

66 million years ago, on Mexico's Yucatán Peninsula, an asteroid measuring around 12 kilometers in diameter crashed into the Earth. The impact provoked an explosion whose magnitude is hard to imagine: equivalent to the power of billions of times that of the atomic bomb dropped on Hiroshima. Most of the animals on the American continent were killed immediately.

Fabien Condamine, University of Montpellier

The impact also triggers global tsunamis. In addition, tons and tons of dust were ejected into the atmosphere, plunging the planet into darkness. This "nuclear winter" saw the extinction of a huge number of plant and animal species. The most emblematic of these were the dinosaurs. But before this cataclysm, how was this group faring? This is the question we set out to answer in our study, the results of which have just been published in the scientific journal Nature Communications.

We looked at six dinosaur families, the most representative and diverse of the Cretaceous period, particularly over the last 40 million years. Three were carnivorous: the Tyrannosauridae, the Dromaeosauridae (including the famous velociraptors made famous by the Jurassic Park films) and the Troodontidae (small bird-like dinosaurs). The other 3 families we studied were herbivorous: Ceratopsidae (represented in particular by Triceratops), Hadrosauridae (the richest of all families in terms of diversity) and Ankylosauridae (represented in particular by Ankylosaurus, a kind of "tank" in bone armour with a club-like tail).

Our aim was to determine the rate at which these families diversified (species formation) or became extinct (species extinction). We knew that all these families had survived until the end of the Cretaceous, marked by the fall of the asteroid.

1,600 fossils analyzed

Over a period of five years, we compiled all known information on these families in an attempt to determine how many and which species were in each group. The work was tedious, as we inventoried most of the known fossils for these six families, representing over 1,600 individuals for around 250 species. Each fossil posed a number of challenges: categorizing 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, each fossil is given a unique number for traceability, enabling us to track it in the scientific literature over time. It was a painstaking task, as one author might assign a date and species, then another would re-study it and make another analysis, and so on. So we had to make decisions. If we had too many doubts, we eliminated the fossil from the study.

Once each fossil had been properly categorized, we used a statistical model to estimate the number of species for each family as a function of time. We were thus able to retrace over millions of years (from 160 to 66 million years ago) which species appeared and which disappeared, and to estimate speciation and extinction rates over time for each family.

To estimate speciation and extinction rates, several biases had to be taken into account. The fossil record is biased. It is uneven in time and space, and some groups do not fossilize well. This is a well-known problem in paleontology when estimating the dynamics of past diversity. Given these problems, sophisticated modern models can take into account the heterogeneity of preservation over time and between species. In so doing, the fossil record becomes more reliable for estimating the number of species at any given time. But it is important to remain cautious, as we are talking about estimates, and these estimates may 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 hit, from 76 million years ago, the number of dinosaur species was in steep decline. This decline is particularly interesting because it was worldwide, affecting both carnivorous groups such as tyrannosaurs, and herbivorous groups such as triceratops. It is also found in the Old World (Europe, Asia, Africa and Australia) and the New World (the Americas). There is still some heterogeneity in the response. Some have declined sharply, such as the ankylosaurs and ceratopsians, and only one family (the troodontids) out of the six shows a very slight decline, which is in 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 we often hear, they were not ectothermic (cold-blooded) animals, like crocodiles or lizards. Nor were they endothermic (warm-blooded), like mammals or birds. They were mesotherms, a system between reptiles and mammals, and needed a warm climate to maintain their temperature and perform basic biological functions such as metabolic activities. This decline must therefore have had a major impact on them.

Note that we found a staggered decline between herbivores and carnivores. Grass-eaters declined slightly before meat-eaters. The decline of herbivores would have triggered that of carnivores. So, according to our model, as soon as herbivores become extinct, carnivores disappear, in what we call a cascade extinction. Indeed, herbivores are key species in ecosystems (even today in the savannahs of Africa, for example). Many species "gravitate" around herbivores. Their extinction often leads to the extinction of other species dependent on these herbivores.

The big question is: what would have happened if the asteroid hadn't crashed? Would the dinosaurs have gone extinct anyway, with their decline already underway, or could they have bounced back? Very difficult to say. Many believe that if they had survived, primates, and therefore humans, would never have appeared on Earth. An important fact is that any rebound in diversity can be very heterogeneous and group-dependent, so that some groups would have survived and others not. Hadrosaurs, for example, show a certain resilience to decline and could, perhaps, have rediversified after the decline.

In any case, we could say that ecosystems at the end of the Cretaceous were under pressure (climatic deterioration, major vegetation change), and that the asteroid was the coup de grâce. This is often the case in the disappearance of species: first they are in decline, under pressure, then another event intervenes and finishes off a group that may have been on the brink of extinction before this 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. Read theoriginal article.