Scientists thought the answer would be nothing. The two endangered species, whose last common ancestor lived during the age of the dinosaurs, have been evolving separately for more than 184 million years, according to The New York Times. But when Hungarian researchers placed paddlefish sperm and sturgeon eggs together in a lab, they produced babies that combined the genes of each. Thus, the sturdlefish was born.

"I think it's pretty cool that these living fossils can still surprise us," Solomon David, an aquatic ecologist at Nicholls State University in Louisiana, who was not part of the research, told The New York Times.

The scientists, who published their findings in the journal Genes in May, had not been trying to produce a hybrid. Instead, they wanted to breed both species separately for conservation reasons.

The American paddlefish is the last paddlefish left swimming, after the Chinese paddlefish was declared extinct in January. Sturgeon, meanwhile, are the "most threatened group of animals on the IUCN Red List of Threatened Species," the conservation group said.

Specifically, the scientists were trying to breed sturgeon through a process called gynogenesis, in which sperm are placed next to eggs in order to prompt them to reproduce asexually, Science Alert explained.

No DNA was supposed to mix, but it did.

"We never wanted to play around with hybridization. It was absolutely unintentional," Dr. Attila Mozsar, a study coauthor and senior research fellow at the Research Institute for Fisheries and Aquaculture in Hungary, told The New York Times.

The successful hybrids surprised the scientists because previous attempts to combine the species had ended in failure, according to USA TODAY. Sturgeons are carnivores, while paddlefish are plankton-eating filter feeders with large snouts, The New York Times explained.

"This hybrid should die," study coauthor Miklós Bercsényi of the University of Pannonia told USA Today. "The embryonic development should not happen."

After it did, the researchers intentionally tried to replicate it, and saw more success. After a year, more than 100 have survived and many weigh more than six pounds.

"They grow well, they eat well," Mozsár told USA TODAY. "We keep them in a very safe place."

Some of the fish look more like their mothers with scutes, or bony scales. Others have their paddlefish fathers' snouts.

While their birth was surprising, researchers think it was possible because of the species' status as living fossils, animals like the crocodile that have not changed very much over the course of their evolution, Science Alert explained.

"These living fossil fishes have extremely slow evolutionary rates, so what might seem like a long time to us isn't quite as long of a time to them," David told The New York Times.

The sturdlefish are probably sterile, a trait shared by other hybrids like the mule, The New York Times pointed out. And the researchers don't plan to breed anymore, since they could pose a danger to wild fish. However, researchers do plan to study how sturgeon and paddlefish reproduce to see if they uncover anything that could help those species survive, Science Alert reported.