Everyone loves a great big shark.

The world’s seas bear a greater abundance of small sharks. I learned this as a young shark fan. Almost every shark book stated that there were more than 350 known shark species, only a handful posing any potential risk to people, and most are small. (The count now exceeds 550 living shark species.) But even with my love of the epaulettes and cookie cutters, the lantersharks and spinners, the draw of the big sharks is hard to ignore. To think, out there, swim fish that stretch more than three times my height and weigh multiple tons. Just as there were hundreds of millions of years ago.

The big sharks were found in northern Australia. Not in the water, but in the rock. Ashtray-shaped bones, vertebral centra, are littered through the 115 million-year-old rocks of the Darwin Formation. The largest of these are nearly five inches in diameter, big bones from sizable sharks.

With our persistent focus on what ancient animals looked like, isolated bits of backbone might not seem like much to go on. But even the most unassuming bone carries anatomical clues formed through millions of years of evolution, enough to get a rough image even if not a paleoart-ready view. In this case, paleontologist Mohamad Bazzi and colleagues were able to identify the sharks whose flesh once housed these bones as lamniform. The bones belonged to mackerel sharks, the group that contains great whites and basking sharks and their relatives, and seemed similar to an extinct Cretaceous form called Cardabiodon. And the Darwin Formation sharks were big.

Working from the relationship between centrum diameter and body size, the researchers produced multiple models of how long the Darwin Formation sharks were. (The researchers noted that teeth, despite seeming like obvious choices to estimate shark sizes, are so variable within and between species that they’re not especially reliable for accurate measurements.) Bazzi and coauthors estimate that the fossil sharks at the center of their study were 20 feet long or more, comparable in size to other giant lamniform sharks that evolved later in the Cretaceous like the famous “Ginsu shark” Cretoxyrhina.

The find pushes back the origin of large lamniform sharks by about six million years. Not that such sharks stayed big or became ever-larger over time. The pattern uncovered by the new research suggests that giant-size lamniform sharks, equivalents of today’s great whites, evolved multiple times over tens of millions of years. The question is what led these sharks to repeatedly open the same niche.

The paleoecological question is a large one, and difficult to investigate directly. Understanding paleoecology means understanding biodiversity, and it can be challenging to know whether the organisms of an ancient habitat are more or less completely known or if apparent absences indicate a bias. Still, Bazzi and coauthors point out, the big predatory sharks of the Cretaceous swam through nearshore habitats where the giant predatory pliosaurs, like the famous Kronosaurus, were apparently absent. It’s possible that prehistoric equivalents of today’s great whites evolved along coastlines that were not only abundant in fish but also marine reptiles that could have been prey - waters where shark-eating giants like pliosaurs were scarce.

Perhaps such conditions favor the emergence of giant sharks through the ages. Multiple times during the Cretaceous, it seems, there was enough food to support large carnivorous sharks in habitats that lacked truly superlative apex predators. It’s unlikely that the evolutionary story played out the same way every time, but, a combination of abundant food and a lack of enormous megapredators opened the possibility for big sharks. Perhaps more are out there. If the Cretaceous seas allowed huge sharks to evolve several times over, Bazzi and authors conclude, Mesozoic waters may very well have been “an ocean of giants.”