new fossils shed light on the evolutionary history of caimans | Paleontology

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An incomplete fossil record prevents reconstituting the early evolution of caimans (Caimaninae subfamily). In new research, paleontologists from Germany and the United States have described two unpublished, 52-million-year-old fossils of a key caiman species, Tsoabichi greenriverensis, from the early Eocene Green River Formation in Wyoming.

Tsoabichi greenriverensis, specimen cast in a private collection. Scale bar – 5 cm. Image credit: Christopher A. Brochu, doi: 10.1080 / 02724634.2010.483569.

Caymans are small to large crocodilians belonging to the subfamily Caymanes, one of the two primary lineages within Alligatoridae, the others being alligators.

These animals can be found in swamps, swamps, mangrove rivers, and lakes in Mexico, central and northern South America.

They are relatively small sized crocodilians with an average length of 1.5-2.5m (5-8.2ft) and a weight of 6-40kg, with the exception of black caiman (Melanosuchus niger), which can grow to over 4 m (13.1 feet) in length.

Caymans are distinct alligators, their closest relatives, by some determining characteristics: absence of bony partition between the nostrils, ventral armor composed of superimposed bony scales formed of two parts united by a suture and teeth relatively longer and thinner than those of alligators .

“In the study, we looked at whether the caimans came from North America or Central America,” said University of Tübingenis Dr Márton Rabi and colleagues.

“Using other Central American caiman fossils, we determined that these species actually represent extinct species more closely related to the caimans living today. However, caimans originally evolved in North America, ”added Dr Jules Walter, also from the University of Tübingen.

“The caimans probably spread from there to South America in the Cretaceous around 66 million years ago, around the time of the dinosaurs extinction.”

“Of all the species of dinosaurs, only the ancestors of today’s birds have survived. However, freshwater species such as crocodiles were not as badly affected by the Great Extinction. ”

“In the Cretaceous, North America and South America were only connected by a chain of islands, so caimans had some difficulty to overcome.”

“However, this was not the only dispersion between North America and South America during evolution; there must have been other migrations between the two continents.

The team’s analysis suggests that a species of caiman developed in South America migrated to North America, giving rise to Tsoabichi greenriverensis and others; or that there was later a second wave of migration from North America to South America.

“The caiman species living today would then have evolved from this group,” said Dr Rabi.

“In a more recent geological past, caimans again advanced from south to Central America, this time the living species.”

“However, as there were no longer suitable corridors with wetlands to the north during this time, they did not reach North America.”

“The evolutionary history of caimans emphasizes that the ability to migrate and spread is crucial,”

“The ability of a species to do this – or to transform into new species – is often the only way it can survive when the environment changes. ”

“Today, the destruction of many habitats by humans leads to isolated populations.”

“Species often cannot spread further even if, for example, climate change causes them to do so. ”

The to study was published in the journal Historical biology.

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Jules Walter et al. On the origin of the Caimaninae: glimpses of new fossils from Tsoabichi greenriverensis and a review of the evidence. Historical biology, published online August 19, 2021; doi: 10.1080 / 08912963.2021.1938563


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