Episode 382 The Riversleigh Lagerstätte

Today’s Episode takes us to Australia, the home of the Riversleigh Lagerstätte. Lagerstätten, you may recall, are fossil assemblages that typically have extraordinary diversity as well as extraordinary preservation. The Riversleigh Lagerstätte fits that definition well, containing fossils of 15- to 25-million-year-old mammals, including an extinct giant platypus, as well as birds, reptiles, frogs, and lungfish. But the story begins about 500 million years ago, not 25 million years ago.

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.

Fast forward about 475 million years, to the Oligocene epoch of the Paleogene Period of the Cenozoic Era. The rocks of northern Queensland were dissolving in a warm, wet, tropical environment, and the limestones of the Thorntonia formation were developing karst topography – caves and surficial pools rich in dissolved calcium carbonate. Starting about 25 million years ago, near the end of the Oligocene, animals began to die in those pools and become trapped in the caves, where ongoing calcium carbonate deposition preserved them remarkably well. So even though the origin of the calcium carbonate was a really ancient rock, the Cambrian Thorntonia formation, it was the much more recent dissolution of those rocks that provided the material that preserved the fossils at Riversleigh. Ages of the fossils extend from late Oligocene time into Miocene, around 15 million years ago, but there are younger fossils as well since the caves are still there and deposition is continuing.  

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

Links:

Cambrian fossils in the Thorntonia

A good overview

Lists of species

July 20. Alice Springs Orogeny

Gondwana

It’s easy to visualize Gondwana as one big unified block with not much going on except around the margins. But in at least some places, there was plenty of action, even though we don’t necessarily understand it too well. Deformation, mountain building, within a tectonic plate is sometimes enigmatic. We know about collisions and subduction and such and what they can do to the earth, but the causes and consequences of intraplate deformation are harder to understand. Today, in central United States in southeastern Missouri and adjacent states, intraplate deformation is occurring, producing the well-known earthquakes in the New Madrid Zone. But the ultimate plate tectonic cause is challenging to pin down with certainty.

During the Pennsylvanian, a long-lived intraplate orogeny was culminating in what is now Central Australia. Australia was firmly attached to the eastern side of Gondwana, but mountain uplifts had been occurring there, well within the tectonic plate, since Devonian time or earlier. It’s called the Alice Springs Orogeny, which appears to represent north-south compression (in modern coordinates) and shortening that rejuvenated some old faults whose heritage dated back to the Cambrian or longer. One of the rejuvenated zones, called the Redbank Shear Zone, may be a really major break that extends all the way to the crust-mantle boundary. It is marked by the most intense gravity gradient in the world, a reflection of the huge contrast in density between the mantle and the crust. Such a prominent weak zone would be the sort of thing that could be activated relatively easily, more easily than a fresh break through unbroken crust. 

It seems that it wasn’t simply a matter of reactivating old faults, though. Central Australia had subsided over time, and the load of sediment eroded into the subsiding basins may have had a role in the tectonic activity as well. The tectonism broke the old basin into several sub-basins.

What was causing the compression? By Pennsylvanian time, Gondwana was rotating in a more-or-less clockwise manner, so that its western parts were moving north to collide with North America. Australia, to the east, would have been moving south. It’s possible that the highly variable thickness of the crust in Australia could have led to differential motion, sort of like one section catching up with another to provide squeezing. Whatever the cause, it seems pretty certain that there was no typical continent-continent collision, so the ultimate cause remains pretty enigmatic.

—Richard I. Gibson

References and Links:
Martin Hand’s abstract

Chris Klootwijk’s “heretical view” 

Modeling the Alice Springs Orogeny 

Cartwright, Buick, Foster, and Lambert (1999), Alice Springs age shear zones from the southeastern Reynolds Range, central Australia: Australian Journal of Earth Sciences: Geological Society of Australia, 46:3, 355-363