New Fossils Illuminate Evolutionary History of Caimans
An incomplete fossil record hampers reconstructing the early evolution of caimans (subfamily Caimaninae). In new research, paleontologists from Germany and the United States have described two previously unpublished, 52-million-year-old fossils of a key caiman species, Tsoabichi greenriverensis, from the early Eocene Green River Formation in Wyoming.
These animals can be found in marshes, swamps, mangrove rivers and lakes of Mexico, Central and northern South America.
They are relatively small-sized crocodilians with an average length of 1.5 to 2.5 m (5-8.2 feet) and weight of 6 to 40 kg, with the exception of the black caiman (Melanosuchus niger), which can grow more than 4 m (13.1 feet) in length.
Caimans are distinguished from alligators, their closest relatives, by a few defining features: a lack of a bony septum between the nostrils, ventral armor composed of overlapping bony scutes formed from two parts united by a suture and relatively longer, more slender teeth than those possessed by alligators.
“In the study, we investigated the question of whether the caimans originally came from North or Central America,” said University of Tübingen’s Dr. Márton Rabi and colleagues.
“Using other caiman fossils from Central America, we determined that these species actually represent extinct species more closely related to caimans living today. However, the caimans originally evolved in North America,” added Dr. Jules Walter, also from the University of Tübingen.
“Caimans probably spread from there to South America in the Cretaceous period about 66 million years ago, around the time of the mass extinction of the dinosaurs.”
“Of all the dinosaur species, only the ancestors of today’s birds survived. However, freshwater species such as crocodiles were not as strongly affected by the great extinction.”
“In the Cretaceous, North and South America were connected only by a chain of islands, so caimans had some difficulties to overcome.”
“Nevertheless, it wasn’t the only dispersal between North to South America during evolution; there must have been further migrations between the two continents.”
The team’s analysis suggests that either a species of caiman developed in South America migrated back to North America, giving rise to Tsoabichi greenriverensis and others; or that there was a later, second wave of migration from North to South America.
“The caiman species living today would then have evolved from this group,” Dr. Rabi said.
“In the more recent geological past, caimans again advanced from the south into Central America, this time the living species.”
“However, since there were no longer any suitable corridors with wetlands to the north during this period, they did not reach North America.”
“The evolutionary history of caimans underscores that the ability to migrate and spread is crucial,”
“A species’ ability to do this — or to diverge into new species — is often the only way it can survive when the environment changes.”
“Today, the destruction of many habitats by humans is leading to isolated populations.”
“Species often cannot spread further even if, for example, a changing climate puts pressure on them to do so.”
The study was published in the journal Historical Biology.
Jules Walter et al. On the origin of Caimaninae: insights from new fossils of Tsoabichi greenriverensis and a review of the evidence. Historical Biology, published online August 19, 2021; doi: 10.1080/08912963.2021.1938563