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Another world

April 5, 2022 - 14:47 -- Admin

I was asked to help crack rocks at a relatively newly discovered fossil locality a few weeks ago. It was a brief trip of only a few days, so as many people as able were ‘dragooned’ to help crack as many rocks as possible in those few days, in the hope of discovering as many fossils as we could. This fossil locality is named the McGraths Flat locality, and was discovered when the soil in the area was tilled by the property owner1,2. The locality is extraordinary in that the preservation of the fossils is almost incomparably good. A deposit like this is termed a Konservat-Lagerstätte, or Lagerstätte for short.

Lagerstätte (plural Lagerstätten) is a German term that refers to a sedimentary layer with unusual occurrences of relatively well preserved organic remains. These often consist of complete bodies or parts with soft tissues. Such occurrences have been the focus of much palaeontological study because of the wealth of information they provide on extinct organisms and also because deciphering the causes of exceptional preservation is intriguing in itself. These deposits reflect unusual circumstances that combine to preserve relatively intact organic remains, and usually represent a fairly short interval of time over a small area3.

There are numerous Lagerstätten in places around the world and include such famous localities as the Burgess Shale in Canada, Chengjiang in China, the Emu Bay Shale in South Australia and Sirius Passet in Greenland, among many others. All of these well known deposits listed above are early or middle Cambrian in age (i.e. about 500-520 million years ago). The McGraths Flat locality is much younger, being about middle Miocene, which is only about 10-18 million years ago1.

In McGraths Flat, the fossiliferous layer is an iron-rich shale overlying a thin sandstone which, in turn, overlies a conglomerate. The iron in the shale is inferred to have come from the basalt, a volcanic rock, which overlies the deposit. The fine grainsize of the shale suggests that it was deposited in a very low-energy water body, but the underlying sandstone and conglomerate suggest a higher energy environment. The small scale of the deposit suggests that it was associated with a river bend which became cut off from the main channel (i.e. an oxbow lake), hence the energy decrease1.

An enormous diversity of fossils has been found and include many, many leaves from over 50 different types of angiosperm (flowering plants) as well as numerous ferns, club mosses, conifers and sundry other plants. The types of leaves suggest that the environment was a mesic rainforest. There have also been many complete flowers found. The preservation is so exceptional that pollen and spores are also preserved in exquisite detail. However, the thing that surprised me most about the deposit was the superbly preserved insects, arachnids and small vertebrates. Quite common are insects that have aquatic larvae, especially those of phantom midges, but there are also larvae of non-biting midges, caddis-flies and alder-flies, along with naiads of dragonflies and mayflies. Other insects present are assassin-bugs, weevils, water-beetles, longhorn beetles, cicadas, frog-hoppers, termites, ants, parasitic wasps, sawflies, craneflies, bees, and lepidopterans (the group that includes moths and butterflies). Also present are spiders, including mygalomorphs (the group that includes trapdoor and funnel-web spiders). Vertebrates are rare but nearly 20 fish, belonging to two different species, have been discovered, along with a single feather1,2. The fish are so well preserved you can see their gut and the content of their last meal.

While I was at McGraths Flat, I found many leaves, one of which excited the knowledgeable. I also found a beetle, numerous phantom midge larvae, a couple of bits of wood, some twigs, and a small (non-mygalomorph) spider. 

Sources

  1. https://www.science.org/doi/10.1126/sciadv.abm1406
  2. https://australian.museum/blog/amri-news/discovery-of-new-Australian-fossil/
  3. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/lagerstatte