In Early to Middle Cambrian time, more than half a billion years ago, the supercontinent of Gondwana was pretty much assembled, with South America, Africa, India, Antarctica, and Australia comprising a wide, long continent that stretched from the South Pole to north of the Equator. The prong of the continent including Australia was the part that was in the tropics, and much of what is now northern Australia was covered by a warm, shallow sea.
In what is now northwestern Queensland, where the Riversleigh Lagerstätte is found, the shallow sea transgressed and regressed – meaning it came and went – and among other things, sediments included grainy shallow-water limestones. The resulting Thorntonia Limestone contains fossils that I’m sure are interesting and informative, including trilobites, brachiopods, and stromatolites, pretty typical of Cambrian rocks. But that’s not the lagerstätte.
|Nimbadon, a koala-like tree-dweller. From Wikipedia.|
The Riversleigh site is not just one location, but many, spread out over a hundred-square-kilometer area that was named a World Heritage Site in 1994.
Riversleigh contains the richest assemblage of bat fossils in the world – at least 35 different species. Besides bats, mammal fossils include extinct koalas, marsupial lions, wombats, herbivores the size of sheep, at least 14 species of opossum, and 15 different kangaroo species. While many fossils are remarkably well preserved, some, such as Yalkaparidon, are only known from a few teeth and scattered bones, making their relationships to other families uncertain. In fact, some are grouped colloquially as Thingodonta, meaning “toothed thing”. Some researchers have suggested Yalkaparidon was a mammalian woodpecker.
Bird fossils at Riversleigh range from extinct flightless rails to storks and lyrebirds. Reptiles are represented by tree-dwelling crocodiles, horned turtles, and dragon lizards, which were probably related to iguanas. There are even snakes and frogs and two species of lungfish, all of which are really rare in the fossil record, especially as long ago as the Riversleigh assemblage, 15 to 25 million years.
Some lagerstätten record essentially an instant in geologic time, or a relatively short period. An example of such an assemblage is the fish of the Green River Formation in Wyoming, many of which may have been killed in a single event – a heavy fall of ash into the lake where they lived, from an erupting volcano. So Riversleigh, in addition to its diversity and excellent preservation is also special because it spans such a long period of time – more than 10 million years – offering a remarkable insight into the evolution of the unique fauna of Australia.
—Richard I. Gibson