New ancient DNA-based Madagascar crocodile study suggests modern crocodiles likely originated in Africa – Science

A study by scientists at the American Museum of Natural History resolved a long-standing controversy over an extinct “horned” crocodile that likely lived among humans in Madagascar. Based on ancient DNA, research shows the horned crocodile was closely related to “real” crocodiles, including the famous Nile crocodile, but on a separate branch of the crocodile family tree. The study, published today in the journal Communications biology, contradicts the most recent scientific thought on the evolutionary relationships of the horned crocodile and also suggests that the ancestor of modern crocodiles probably originated in Africa.

“This crocodile was hiding on the island of Madagascar when people were building the pyramids and was probably still there when the pirates ran aground on the island,” said lead author Evon Hekkala, assistant professor at Fordham University and a researcher. associated with the American Museum of Natural History. “They blinked just before we had the modern genomic tools available to make sense of the relationships of living things. And yet, they were the key to understanding the history of all living crocodiles today.”

The arrival of modern humans in Madagascar around 9,000 to 2,500 years ago preceded the extinction of many of the island’s large animals, including giant tortoises, elephants, dwarf hippos, and several species of lemurs. A lesser-known extinction that occurred during this time was that of an endemic “horned” crocodile, Voay robustus. Madagascar’s early explorers noted that Malagasy peoples consistently referred to two types of crocodiles on the island: a large, robust crocodile and a more slender form with a preference for rivers. This suggests that both types persisted until very recently, but that only the slender form, now recognized as an isolated population of the Nile crocodile (Crocodylus niloticus), is currently found on the island.

Despite nearly 150 years of investigation, the position of the horned crocodile in the tree of life has remained controversial. In the 1870s, it was first described as a new species within the group of “true crocodiles”, which includes crocodiles from the Nile, Asia and America. Then, at the start of the 20th century, it was thought that the specimens were simply very old Nile crocodiles. And finally, in 2007, a study based on the physical characteristics of fossil specimens concluded that the horned crocodile was in fact not a real crocodile, but in the group that includes dwarf crocodiles.

“Demonstrating the relationships of modern crocodiles is really difficult because of the physical similarities,” Hekkala said. “A lot of people don’t even realize that there are several species of crocodiles, and they see them as that animal that doesn’t change over time. But we’ve tried to get to the bottom of the great diversity that exists among them. “

To fully examine the place of the horned crocodile in the evolutionary tree, Hekkala and his Museum collaborators have made a number of attempts to sequence the DNA of fossil specimens, including two well-preserved skulls that are at the Museum. since the 1930s.

“It’s a project that we tried to do every now and then for many years, but the technology just wasn’t advanced enough so it always failed,” said the study co-author. George Amato, Director Emeritus of the Museum’s Institute for Comparative Genomics. “But over time we had both the computational setup and the paleogenomic protocols that could actually extract this DNA from the fossil and finally find a home for this species.”

The results place the horned crocodile right next to the true crocodile branch of the evolutionary tree, making it the closest species to the common crocodile ancestor living today.

“This finding was surprising and also very informative about how we think about the origin of real crocodiles found around the tropics today,” Amato said. “The placement of this individual suggests that the real crocodiles are from Africa and from there some went to Asia and some went to the Caribbean and the New World. We really needed the DNA to get the right answer. to this question.”


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