It was a typical day at the construction site -- until the workers found the ancient crocodiles.

On a site in urban Brisbane, Australia, where a multimillion-dollar rail overpass is being built, workers uncovered fossils dating back 50 million years -- of not only crocodiles but also frogs, fish and plants -- trapped in a layer of oil shale.

“The bones have been identified as from ancient crocodiles, as well as other significant material including fish, freshwater shells and plant impressions,” city mayor Graham Quirk told Agence France-Press.

The ancient crocodile, in particular, was an “amazing find,” Queensland Museum's Senior Curator of Ancient Environments Dr. Scott Hocknull told the Brisbane Times. The fossil of a vertebra from the lumbar region of a 16-foot crocodile might be that of an extinct species.

“We don't know for sure because it has only been discovered in the last couple of weeks,’’ Hocknull said.

The fossils were found 49 feet below ground. Workers notified authorities when they found something unusual in their drills. Geoscientists were asked to examine the findings, and they have been deemed “particularly significant” by Queensland Museum chief executive Suzanne Miller.

“It's very unusual to find materials being uncovered in these urban sites. I mean, most discoveries of this nature are found miles from anywhere,” Miller told the Australian Broadcasting Corporation.

‘‘This could be some of the earliest mammals ever found,’’ she told the Brisbane Times.

Construction workers have given the five tons of soil from the drills to the museum for further study. Miller says the museum now needs to find a site to place the mounds of dirt, as well as volunteers to sift through it in case there are more fossils to be found-- a project that may take two to three years, Quirk said.

In fossil terms, 50 million years is relatively young. Still, Miller says, this discovery could shed light on how young animals survived after the dinosaurs were wiped out.

“After the extinction that killed off the dinosaurs and many other animals, very small things tended to survive,” Miller said. “This is one of those sites that's showing us all these smaller animals that co-existed in one place in one habitat at the same time.”

Despite being discovered in the land Down Under, the fossils may shed light on a universal question of how ancient creatures evolved, said Miller. “It's a bit of a missing link in the geological and fossil history, not just of Australia but within the world.”