Nicknamed the “T-Rex of the seas,” Mosasaurs were formidable marine predators of the Cretaceous period. These monstrous, snake-like reptiles could grow to more than 17 meters (59 ft) in length and consequently weren’t afraid to take on a challenging dinner, like sharks or plesiosaurs.

While we know that these animals were not to be messed with, little was understood about their breeding biology due to a scarce fossil record of babies or infants. Now, thanks to some newly identified fossils, Yale scientists have finally shed some light on how these iconic animals began their lives. According to their new Paleontology study, these powerful carnivores gave birth in the open ocean, rather than travelling to the shores like many other marine reptiles.

“Mosasaurs are among the best-studied groups of Mesozoic vertebrate animals,” explains lead author Daniel Field, “but evidence regarding how they were born and what baby mosasaur ecology was like has historically been elusive.”

For the investigation, scientists examined the youngest and smallest mosasaur fossils so far discovered. But these weren’t new specimens; they had been collected more than 100 years ago and were part of the Yale Peabody Museum’s collection. They managed to remain under paleontologists’ radars because they were mistakenly identified as ancient marine birds, but a reexamination concluded that they were indeed mosasaurs due to a variety of unique jaw and teeth features.

Since these extremely young specimens were recovered from pelagic deposits, the researchers concluded that neonatal mosasaurs likely lived in the open ocean and were probably also born there.

“Contrary to classic theories,” says study author Aaron LeBlanc, “these findings suggest that mosasaurs did not lay eggs on beaches and that newborn mosasaurs likely did not live in sheltered nearshore nurseries.”

Mosasaurs lived during the late Cretaceous period, some 85-65 million years ago, spending their days gobbling down pretty much anything they could get their fierce jaws around. Although these widespread animals were extremely successful killers with few, if any, predators of their own, they became extinct during the K-T extinction, or Cretaceous-Tertiary extinction, which saw the loss of around three-quarters of plant and animal species across the globe, including the dinosaurs.