As the weather begins to cool, the ‘dig’ season starts for us (palaeontologists) as we venture off along the coast and into the outback heart of Queensland. Over the last ten years we have been investigating a series of fossil sites at South Walker Creek located near the town of Nebo, west of Mackay. It is here that we are finding some of Australia’s last tropical ice age megafauna.
Our dig team usually consists of palaeontologists, along with other scientists and specialists who contribute and volunteer their time. During the year of 2016 one of our volunteers, Noel Sands, who specializes in caving (and their fossil deposits!) called speleology, found a very large fossilized bone. Using an array of brushes and dig tools, Noel carefully excavated the sediment from around the bone to expose its shape and size. It was identified as a tibia (shinbone) [Sketchfab 1] from a kangaroo, but not just any kangaroo, the world’s largest species of kangaroo!
Once the position of the bone was established, we trenched around the specimen to create a pedestal so it could be isolated, with its surrounding sediment, from the rest of the dig site. The bone was then recorded and photographed in situ (the original place of deposition). We also place a temporary paddle pop stick [Sketchfab 6, 7)] with an identifying field number on the bone so it can be plotted in 3-D using a process called photogrammetry.
To begin the process of extraction, we first cover the bone and pedestal with foil to act as a protective layer. It is then covered with strips of wet newspaper, which provides cushioning for the jacket we are going to make to contain the bone. To make the jacket we use strips of hessian dipped in a plaster mix (casting plaster and water) and wrap them around the pedestal with the bone and wait for it dry. This is always the fun job!
Once the plaster jacket is dry, the field number and a directional north arrow are written on it so we know which specimen it is and its position in the site. The next step of the process is always tricky and is about getting it just right to roll the jacket over with the specimen kept in one piece. We use a hammer to bang in chisels at the base of the pedestal to loosen it from the underlying sediment. When it becomes loose it is ready to be quickly rolled over. If we have made a good jacket the specimen should stay all in one piece. On rare occasions we are not so lucky, but this time it went without a hitch! You can see this whole process in the video below, check it out!
Finally, the other side of the plaster jacket is sealed with the same plastering process to form a lid. Now the specimen is protected in a hard, egg-like shell to be transported back to the Queensland Museum and stored temporarily in the Geosciences collection with other unprocessed specimens awaiting preparation. There was also a distal tibia epiphyses (end cap) bone [Sketchfab 3] sitting on the shaft of the tibia and a bone shard [Sketchfab 4] nearby that were carefully collected so they were out of the way of extracting the tibia. We will be able to see if these bones are associated (connected) to the tibia.
Another challenge of the fieldwork is getting large jackets from the dig site back to the field vehicle. As the terrain is quite rough and our excavations occur in an eroded creek bed we cannot drive very close to the dig site. We have to use trollies, stretchers and manual handling to slowly walk the jackets with their specimens out of the dig site. Whilst excavating fossils by hand is exciting, it is also a lot of hard work. It involves good fitness, experience, precision, problem solving and most importantly team work.
Check out Part 2: In the Lab as we go behind the scenes to investigate the giant kangaroo leg further.
Top Image – The dig team sit proudly around the tibia of the world’s largest species of kangaroo, all ready to be extracted. Image Credit: Rochelle Lawrence.