University of Tübingen: An early caiman from North America

Two fossil finds from the Green River Formation in Wyoming, USA, around 52 million years old, have been classified into the evolutionary history of crocodiles in a new study: the biogeologists Jules Walter and Dr. Márton Rabi from the University of Tübingen identified the extinct species Tsoabichi greenriverensis together with other colleagues as an early caiman. Caiman species living today are not found in North America. Their family tree also reflects the history of their migrations and ways of spreading. The study was published in the journal Historical Biology.

The caimans living today are about 1.5 to 2.5 meters long crocodiles that are found in tropical freshwater wetlands, rivers, lakes and swamps in Mexico, Central and South America. Their closest living relatives are the alligators in North America and Asia. “Complete skeletons of crocodiles from 50 million years ago, like the finds we examined, are very rare in what is now the United States,” says Márton Rabi.

Escaped mass extinction
In their study, the researchers wanted to approach the question of whether the caimans originally came from North or Central America. “Using other caiman fossils from Central America, we determined that these extinct species actually belong to the same group as the caimans living today. However, the whole group of species developed in North America, ”says Jules Walter. The caimans probably would have spread from there to South America in the Cretaceous period around 66 million years ago – at the time of the mass extinction of the dinosaurs.

“Of the dinosaur species, only the ancestors of today’s birds have survived. However, freshwater species such as crocodiles were not so badly affected by the great extinction, ”explains Walter. In the Cretaceous Period, North and South America were only connected by a chain of islands, so that the caimans had to overcome some difficulties. “Still, the spread from North to South America was not an isolated event; there must have been further migrations between the two subcontinent, ”he says.

Survival by spread
The researchers’ family tree study suggests that either new species of caimans developed in South America migrated back to North America and there, among other things, Tsoabichi greenriverensis emerged, or that there was a later second wave from North to South America. “The caiman species living today would have developed from this group,” explains Rabi. In the more recent geological past caimans, this time still living species, have advanced from the south to Central America. However, since there were no more suitable corridors with wetlands in the north during this period, they did not reach North America.

“The evolutionary history of the caimans underscores how crucial the opportunities for migration and spread are. It often depends on whether species can ensure their survival when the environment changes or whether they fan out into new species, ”says Rabi. Today the destruction of many habitats by humans leads to isolated populations. Species are often unable to spread further if, for example, climate changes force them to do so.

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