A drilling crew in Brisbane, Australia, have found a hoard of fossils believed to be 50 million years old.
The fossils include mammals, frogs, fish, plants and a five-metre crocodile. Experts said they were "unique".
The fossils were found by a crew during the construction of a railway overpass, Suzanne Miller of Queensland Museum said.
"This is unique to science. To find this variety of plants and animals that were alive and co-existing 50 million years ago is phenomenal," she told the Brisbane Times.
"Geologically, around the world there will be a lot of interest in the fossils that have been found."
Miller said the find was hugely significant as there has never been a similar discovery in northern Australia.
"Depending on what we find, because we are just at the beginning of this project, this could be unique in the world. This could be some of the earliest mammals ever found."
The fossils come from a time after dinosaurs became extinct around 65 million years ago. Miller added: "It really is a bit of a missing gap in the scientific record about how animals evolved after that massive extinction that killed off the majority of animals on the planet.
"This is a snapshot back in time 50 million years ago when this was all tropical rainforest."
Scott Hocknull, a senior curator at Queensland Museum, said the crocodile was an "amazing find". The bone found was from the animal's backbone and was discovered in a low swampy area.
"When you compare that to a modern-day crocodile, you are looking at something of the order of five metres long," he said. "It is probably an extinct animal, an extinct species, but we don't know for sure because it has only been discovered in the last couple of weeks."
As well as the crocodile, the thigh bone of what is believed to be Australia's oldest frog was found, as well as a fossilised fish similar to a perch.
The rail overpass project has not been delayed by the find but engineers are now working with a team from the University of Queensland to find more fossils.
Queensland transport minister Scott Emerson said the find offered a "fascinating insight" into what Brisbane was like 50 million years ago.