Saltwater crocodiles crossed the Indian Ocean to the Seychelles before humans arrived and made them extinct.

Saltwater crocodiles once ranged across a vast range from across the Indian Ocean to the Seychelles, a new DNA study confirms.

The extinct crocodile population in the Seychelles, an archipelago in the western Indian Ocean, was not a group of Nile crocodiles (Crocodylus niloticus), nor was it a separate species. Rather, it is probably the westernmost population of saltwater crocodiles (Crocodylus porosus), currently living in India, Southeast Asia, Australia, and islands in the western Pacific Ocean, researchers reported Jan. 28 in the Royal Society Open Science journal.

“The founders of the Seychelles population must have drifted at least 3,000 kilometers.” [1,864 miles] It could cross the Indian Ocean to reach remote archipelagos or even further afield,” study co-author Frank Groh, a reptile expert at the Bavarian State Collection of Natural History, said in a statement.

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According to records from expeditions dating back more than 250 years, Seychelles was once home to many crocodiles. However, with the arrival of human settlers in the late 18th century, all crocodiles on the island became extinct. The remains of some specimens were kept in museums in the Seychelles, London, and Paris.

Initially, Western scientists thought that the Seychellois crocodile was part of the Nile crocodile species that migrated from Africa. But in 1994, researchers reclassified the preserved remains as saltwater crocodiles based on their physical characteristics.

In a new study, another team of scientists used genetic material to confirm that conclusion. They collected mitochondrial DNA from the skulls and teeth of old museum specimens of different types of crocodiles and compared that DNA to modern museum specimens and tissue samples from living crocodiles.

The research team found that the Seychellois crocodile’s genetic markers closely matched those of the saltwater crocodile. This suggests that the saltwater crocodile’s range extended over 7,500 miles (12,000 km) from east to west before the Seychelles population became extinct.

“The genetic patterns suggest that saltwater crocodile populations remained connected over long distances over long periods of time, demonstrating the high mobility of this species,” study co-author Stephanie Agne, an evolutionary biologist at the University of Potsdam in Germany, said in a statement.

For C. porosus to spread as far west as the Seychelles, it would have had to cross thousands of miles of ocean. However, crocodiles are adapted to life in the ocean and have special salt glands in their tongues that allow them to excrete excess salt. This adaptation may have allowed the animals to spread widely across the Indo-Pacific region, limiting further speciation, the researchers said in their paper.

However, future research may still reveal differences between groups of saltwater crocodiles. Because mitochondrial DNA is inherited only from the mother, it may not capture the subtle genetic differences caused by male alligators. Future studies using DNA taken from the nuclei of crocodile cells could help reveal regional differences between populations, the researchers wrote.