It’s not often you find a new species of 3-meter (10-foot) crocodile hiding in plain sight. But that’s exactly what happened to two zoologists after one species of croc that lives in New Guinea turned out to be two.

New Guinea, a vast island to the north of Australia, has long been home to a large species of freshwater crocodile known as Crocodylus novaeguineae, first described in 1928. However, for some time scientists have suspected the species is not quite what it seems. Inspired by the work of the late scientist Philip Hall, who noticed strange differences in the nesting and mating behavior of the species, researchers Chris Murray and Caleb McMahan set out to investigate. They reported their findings in the journal Copeia. 

They analyzed 51 skulls kept in various museums to try to spot any differences between crocs found in the northern parts of the island and those found in the south. The two groups are divided by the New Guinea Highlands, a long chain of mountain ranges and river valleys. The duo noticed physical differences between the skulls and so headed to Florida’s St Augustine Alligator Farm Zoological Park to see if the same differences could be seen in living crocodilians.

"They have live individuals of what's called novaeguineae, and we were able to look at those and say, 'Oh yeah, this matches the north and this matches the south!' I thought that was super cool," said McMahan, who’s an assistant professor at Southeastern Louisiana University, in a statement.

"We could even look at a skull that they had there and tell what river it came from. So our analyses really did a good job at teasing apart where these things are from," added Murray, a scientist at the Field Museum in Chicago.

Crocodiles from the north and south differ in their bone morphology. For example, northern crocs have longer jaw and nose bones than their southern counterparts. And they behave differently too. Southern females nest in the rainy season and produce fewer eggs than northern crocs, which nest in the dry season.

From their analyses and observations of living animals, the team concluded the two groups are different enough to be officially classed as separate species. They named the southern species Crocodylus halli in memory of Philip Hall, whose ideas led to their study. 

"The nice thing is that here we've got differences in the morphology, we've got ecological differences, they're separated by a mountain range, I think the synthesis of all of that is what really builds the case that these two crocodile entities are very different from each other," explained McMahan.

The researchers note that being aware that there are two species of croc on the island is important for conservation. Separated geographically, the two species inhabit slightly different environments, so if a particular habitat is threatened, that could spell disaster for one species but not the other.

"It could be that when we consider crocs on the whole island, they might be okay, but if we start looking at a species north of the highlands and one south of the highlands you might find more habitat degradation and population threats in one over the other,” McMahan said.