Imprints of ancient bones or footprints are often found in rock, but that's not the only way they form – in much rarer cases they can also be encased in opal. Now palaeontologists in Australia have uncovered the most complete "opalized" dinosaur ever found, which also happens to be a new species.

Opal naturally forms inside cavities in rocks when silica dissolved in water fills a space and hardens into a solid, colorful gemstone. While most of the time opals are amorphous, taking on whatever random shape a given cavity has, in rare cases it might form a perfect cast of a bone, if it finds itself filling a gap left behind after bones have rotted away

Opalized fossils are most commonly small things like shells or teeth, but sometimes larger bones turn up too. Lightning Ridge in outback Australia is one of only a handful of sites in the world that produces opalized fossils, and this newest discovery is arguably the most impressive haul so far.

Not only is it one of the largest batches of opalized bones, it includes the most complete opalized fossil dinosaur ever found, totaling almost 60 bones. The other pieces come from at least three other individual animals, including small juveniles and larger adults, making it the first dinosaur "herd" found in Australia.

Even more intriguing, the bones belong to a brand new species, dubbed Fostoria dhimbangunmal.

"We initially assumed it was a single skeleton, but when I started looking at some of the bones, I realized that we had four scapulae (shoulder blades) all from different sized animals," says Phil Bell, lead researcher on the study. "Fostoria has given us the most complete opalized dinosaur skeleton in the world. For opalized dinosaurs we generally have only a single bone or tooth or in rare instances, a few bones. To recover dozens of bones from the one skeleton is a first."

The new species was a two-legged herbivore of the Iguanadontia family, which were basically the cows of the dinosaur era. From the bones of the adult specimens, the team estimated that the animals grew to be up to 5 m (16.4 ft) long.

Although they were originally dug up in the 1980s, the bones have only now been identified. That's after they were donated to the Australian Opal Centre for study, after spending 30 years in the private collection of opal hunter Robert Foster. The new species has been named in his honor.

This new discovery of the most complete opalized dinosaur fossil joins a range of bizarre finds in recent years. That includes pickled dinosaur brains, a specimen so well preserved it looks like it's sleeping, a feathered dinosaur tail preserved in amber, and the bones of the largest land animalto ever walk the Earth.