Discovering the world’s largest kangaroo – Part 2: In the lab

By Rochelle Lawrence, Palaeontological Research Assistant, and Scott Hocknull, Senior Curator, Geosciences, Queensland Museum 

The giant kangaroo tibia (shinbone) found at the megafauna fossil sites of South Walker Creek, travelled safely back to the Queensland Museum’s Geosciences collection. The specimen is treated like evidence for a case (fossil evidence!) and is processed through a series of stages from field collection (Part 1) and preparation, to research and conservation. The plaster jacket containing the kangaroo tibia was brought into the fossil preparation laboratory where it was prepared by specially trained fossil preparators, like Rochelle and Peter below. The aim of the preparator is to expose the bone while minimizing the loss of information and preserving the specimen for the future.

South Walker Creek’s senior preparator, Rochelle, using an air scribe to remove matrix away from the specimen.
Rochelle and technician, Peter, make sure to have the right protective equipment when cutting open the plaster jacket. Images Credit: Rochelle Lawrence.

To begin preparation, we cut off the lid of the jacket with a plaster saw. Once opened we started to prepare the underside of the kangaroo tibia, the side that was facing down in the ground in the field and couldn’t be seen. We use a range of tools and vibrational air scribes to remove the sediment or matrix surrounding the specimen. As the lab is a technical workspace using electrical tools, equipment and chemicals, we wear a range of safety gear like earmuffs, glasses, dust masks and lab coats. We look pretty cool!

Palaeontology is a little bit like forensic science except we are solving some very, very old cases that involve prehistoric victims. Can you think of the similarities between the two sciences?

Peter uses the plaster saw to cut the lid off the jacket.
Peter begins the bulk removal of matrix surrounding the specimen. Images Credit: Rochelle Lawrence.

We continued to remove the matrix in layers before exposing the kangaroo tibia. During this whole process we record, photograph and photogram any interesting features such as a change in the sediment colour or type, or if we come across any other bones and teeth that have been preserved next to the main specimen. This information can help us understand how the fossil site formed through the processes of sedimentology and stratigraphy (the way in which the sediment was deposited in layers) and taphonomy (how the animal decayed and fossilized) [Sketchfab 4]

The tibia is revealed in the matrix about half-way through preparation. Image Credit: Rochelle Lawrence.

The final phase of preparation for this side of the kangaroo tibia is the surface cleaning and preservation treatment with chemical glues to consolidate [Sketchfab 3] the specimen, keeping it together. It’s during this stage that we see other interesting features on the surface of the bone. With our tibia we found a few pathologies (diseases or injuries). A deep groove [Sketchfab 1] within the shaft indicates an injury or bone disease. However, it also shows secondary bone growth indicating that the disease did not kill the animal because it had time to grow new bone. We then found two puncture marks [Sketchfab 2] which fit the form of crocodile teeth indicating the kangaroo was attacked by a crocodile and this is how the kangaroo died.

Are you starting to see how similar our science is to forensics? We now have an idea of who may have been involved in the death of our victim, the giant kangaroo.

Crocodile teeth of many shapes and sizes found within the South Walker Creek sediment. Images Credit: Rochelle Lawrence.

The next phase involved the creation of a plaster cradle [Sketchfab 2] so that the specimen could be flipped over allowing us to complete the preparation of the other side. We have to make sure the cradle is just right to support the specimen, not grip it too tight, otherwise it may never be taken out without breaking it. We use plastic wrap to protect the bone and plasticine around the specimen to create a wall for the wet plaster mix (casting plaster and a polymer) to settle and dry into a strong cradle.

Rochelle stands next to the tibia which has completed the first stage of preparation.
The tibia is covered in plastic wrap which is tucked into the plasticine wall. The plaster mix is poured over the plastic wrap molding to the surface of the bone to create a form-fitted plaster cradle for the specimen to sit in. Images Credit: Rochelle Lawrence.

Can you see the similarities and differences between the field plaster jacket from Part 1 and the collection plaster cradle here?

Once again, we have to undertake the tricky process of flipping the kangaroo tibia in one piece. If we have properly conserved the specimen it shouldn’t fall apart – which it didn’t! We took away the plasticine wall and cleared away any loose matrix to prepare the side we first saw in the field. When it was finished, we did some final photographs and photograms of the specimen, and completed the documentation of our preparation job. The specimen is now ready for further analyses and research!

The kangaroo tibia survived the flip and is ready for the final stage of preparation.
The tibia fully prepared and completed. Images Credit: Rochelle Lawrence.

A part of this stage involved the transportation of the kangaroo tibia to a hospital so it could be X-rayed (Computed Tomography scanned). Can you imagine what the other patients thought as we wheeled in a fossil leg bone of a giant kangaroo! A radiographer, like Nikki below, takes x-ray images of the bone from different angles to produce digital slices of the specimen [Sketchfab 1]. This allows us to see inside the bone [Sketchfab 4], like x-ray vision, and in this case to study the pathology [Sketchfab 2, 3] in greater detail. Once a specimen has gone through the research stage it is finally ready for conservation, which is the final treatment of the specimen into a safe environment where it is either stored within the collections or placed on display.

The kangaroo tibia is on its new plaster cradle ready to be CT scanned.
Nikki controls the scanner with her computer and uses special algorithms to process images of the fossil bones, which are denser than human bones. Images Credit: Rochelle Lawrence.

We use all of the data collected as evidence to form a profile on our victim. We have found some teeth and other limb bones representing a giant kangaroo that may be associated with our victim. Its unguals (claws) [Sketchfab 3] on its feet also have a unique morphology (form, shape) being long and hoof-like, similar to a deer. It may have used its strong unguals to tip toe, making itself taller, to reach food that was higher up in the trees.

Starting from the back, the white coloured tibia is from a modern kangaroo followed by two fossil tibiae from our giant kangaroo and finally the shinbone of a smaller species of kangaroo. Image Credit: Rochelle Lawrence.

In fact, what stands out about this giant kangaroo, is its size [Sketchfab 5] perhaps reaching up to 3 metres in height when on its toes. We compared it to other known extinct giant kangaroos and it didn’t fit these species. Its tibia [Sketchfab 1] is longer than these species even without the epiphyses (end caps) fused, meaning it wasn’t fully grown adult. We think our giant kangaroo might be a new species and it looks to be the biggest species ever found!

Reconstruction of the giant kangaroo with a joey in its pouch next to an adult human of average height. Image Credit: Vlad Konstantinov, Andrey Atuchin, Scott Hocknull, Rochelle Lawrence.

Make sure to check out Part 1: In the Field as we go behind the scenes to collect evidence of a giant kangaroo.

Project DIG is a partnership between Queensland Museum and BHP that will digitise and scan our collections and research for people worldwide. Check out our Tropical Megafauna in 3D!

Top Image – The stages our kangaroo tibia went through from being excavated in the field, documented through photogrammetry, CT scanned and finally reconstructed. Image Credit: Vlad Konstantinov, Andrey Atuchin, Scott Hocknull.

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