So far diverse, current crocodiles are remains of a group that walked side by side with dinosaurs – 01/31/2021 – science

The reptiles known as crocodiles, alligators, and gavials (animals with a very long and thin snout found in India) are today a pale representation of the diversity that existed in the past.

While the current 24 species are all freshwater species (with the exception of two species that have also adapted to saltwater), there have been terrestrial, amphibious, freshwater, and marine forms in the past for the past 250 million years.

The group, called crocodilomorphs, included both 12-meter reptiles (such as the Sarcosuchus imperator, which lived 110 million years ago) and land animals that were no larger than a bush dog and could walk – such as the species of the genus Mariliasuchus found in rocks where Today the city in the interior of São Paulo, Marília, lies about 90 million years ago.

It is difficult to estimate species, but in the past there have been at least ten times more phyla of these animals. This is why today’s crocodile morphs are known as relics – living species are much closer together today than any of those that lived in the past. But why were all these forms extinct?

The most likely hypothesis is that environmental changes played a key role in this. However, a new article recently published in the journal Communications Biology (from the Nature group) also points to another cause: a low rate of evolution in these animals.

Since the days of Darwin, the father of evolutionary theory, scientists have tried to understand how biodiversity occurred in the past.

The theory of evolution is usually associated with small, continuous, and gradual changes over time. The speed at which these small changes occur, or the rate of evolution, varies by species.

However, in the 1970s, paleontologists Stephen Jay Gould and Niles Eldredge suggested that evolution of species could be accelerated and sudden even in a short period of time, which is usually triggered by environmental pollution. Called point equilibrium, the idea was that species would diversify very quickly, followed by a long period of evolutionary equilibrium.

The problem is that this is a difficult theory to test as it relies on a large fossil record to ensure that the diversity observed is the result of an explosion in a relatively short time.

See you. Max Stockdale and Michael Benton from the University of Bristol (United Kingdom) assessed the fossil record of crocodilomorphs and used computer models to assess the rate of evolution of these animals.

What they observed was that crocodiles had evolved steadily over the past 200 million years, with some peaks of variation marked by differences in body size, in the mid-Jurassic and early Cretaceous periods.

“One of the most important results of our study was that the low evolution rate of crocodilomorphs was not constant throughout their evolution. The periods [de baixa evolução] were interrupted by brief episodes of rapid change. These changes allowed the ancestors of the crocodiles to take on unknown shapes, such as terrestrial species with long legs that are suitable for walking, shapes with soft snouts such as pigs that are suitable for digging in the ground, and marine species with fins, similarly like fish. He said to Folha Stockdale.

The explosion of new species at some point is followed by the extinction of some of these groups while others remain “intact”. From there, it is environmental changes that determine which groups survive or not.

For Caio Geroto, paleontologist and professor of biology at Unip who did not take part in the study, the work is fantastic as she is the first to actually test the broken equilibrium.

For crocodile scientists, the idea has always been that the evolution of these reptiles was climate-driven, he says. “Crocodilomorphs are considered indicators of the paleoclimate – where there are fossils of these animals, the environment was hot and humid. But no one has ever been able to determine when the environment has affected these animals, and in this sense this work is very relevant, as it gives evidence of the broken equilibrium at work, that is, a moment of stability followed by rapid development .

It is clear that environmental changes were important, but not the only ones, in determining the evolution of these groups. The smaller and terrestrial forms, which became extinct at the end of the Cretaceous Period, would not have found such a different environment at the beginning of the Paleocene, immediately after the great extinction of a meteor that decimated the dinosaurs and about 75% of all the diversity on the planet.

“The climate at the beginning of the Paleocene (about 60 million years ago) was not that different from the end of the Cretaceous, but only the current tribes and three others, including two freshwater tribes, survived the extinction. There is even evidence of a hot and humid climate that lasted for the next 20 million years, leaving an open question as to why this diversity has disappeared, ”says Diego Pol, curator of paleontology in the collection of the Egidio Feruglio Museum in Argentina and one of the leading experts in this field.

For Pol, one possible explanation would be that the environmental pressures in marine and land environments are greater than in freshwater, where there are usually more stable conditions for food supply and temperature. “This is a hypothesis, but the surviving groups may have managed to find shelter in areas of low energy flow,” he concludes.

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