Tag: vertebrae

Dugong it, where are all the Australasian fossil sea cows?

Posted on by Travis Park in Australian palaeo, Fossil mammals, Marine mammals

Well, time certainly flies when you’re busy and before you know it, it’s been almost a month since you’ve last written a blog post. At least that’s what has just happened to me! I’ve been busy doing research on fossil whales, fossil penguins, talking fossil penguins at Museum Victoria’s latest SmartBar, giving a talk on Australian fossil seabirds as well as preparing and submitting abstracts for an upcoming conference, whew! But I haven’t been blogging and bringing you, dear readers, new and cool fossil discoveries. So let’s rectify that situation then shall we?

As you may have guessed from the title above, this post is about fossil dugongs, or more precisely, the lack of them in the Indopacific region. Whilst today the region is the centre of sirenian abundance and fossils are known from areas such as Madagascar, Somalia, India, Sri Lanka and Indonesia, fossil evidence from the Indopacific has been lacking with the only reported finds being a partial mandible from the Pliocene of South Australia, a partial rib from the Miocene-Pliocene boundary of Victoria and fossils of the extant Dugong dugon from the Quaternary of Papua New Guinea and Holocene of southeast Australia. There is no clear explanation for the scarcity of dugong fossils in the Indopacific region as the find from South Australia shows they were present in the area in the past. Furthermore, there are plenty of available outcrops of sediments of the correct age, the sediments also indicate the climate would have been suitable for dugongs to be present and the high densities of sirenian bones make them favourable for preservation.  Therefore any new finds would be crucial to gaining a more detailed understanding of sirenian evolution in the Indopacific.

The single living species of dugong, Dugong dugong (that's a lot of dugongs for one sentence). Image source habitatadvocate.com.au.
The single living species of dugong, Dugong dugon (that’s a lot of dugongs for one sentence). Image source habitatadvocate.com.au.

One such find was made but it was actually 30 years ago, with the fossils not being studied until only recently and published in the Journal of Vertebrate Paleontology this July by Erich Fitzgerald (who also happens to be one of my PhD supervisors) and colleagues from the Smithsonian, Howard University College of Medicine and Flinders University. The recovery of the fossils (consisting of three posterior vertebrae, one anterior caudal vertebra and seven partial ribs) is a story in itself. The fossils were found in a cave in the remote Hindenburg Range of the New Guinea Highlands, Papua New Guinea, but when the fossils were being recovered the cave suddenly flooded meaning the crew had to make a quick exit leaving some fossil material behind!

Some of the vertebrae of the fossil seacow found in Papua New Guinea, being held by lead author Dr. Erich Fitzgerald. Image source MV.
Some of the vertebrae of the fossil seacow found in Papua New Guinea, being held by lead author Dr. Erich Fitzgerald. Image source MV.

The fossils date to between 11.8–17.5 Ma, giving a minimum age of just before 12 Ma for sirenians being present in Australasian coastal marine ecosystems, and by implication their primary food source: seagrasses. As Dr. Fitzgerald explains, “Modern-day dugongs are major consumers of sea-grass, and, by doing so, have a tremendous impact on the structure of the ecosystem,” said Dr Fitzgerald. “They participate in a delicate balancing act: their feeding allows diversity in sea-grass and animal species that would otherwise be lacking. Previously, it was thought that sea cows were fairly new arrivals in Australasia, and that their relationship with sea-grass ecosystems here was a recent event. This new evidence suggests sea cows have been an important component of Australasia’s marine ecosystems for at least 12 million years and that their role in the long-term health of these environments may be substantial.”

So whilst we are still in the dark about an awful lot of the history of sirenians in Australasia, this new find does shed a little light their evolution and now we know that they were there around 12 Ma, researchers can start looking in shallow marine sediments of similar age to find the next illuminating discovery.

Dr. Fitzgerald’s comments are taken from the Museum Victoria media release.

Reference

Erich M. G. Fitzgerald, Jorge Velez-Juarbe & Roderick T. Wells (2013) Miocene sea cow (Sirenia) from Papua New Guinea sheds light on sirenian evolution in the Indo-Pacific. Journal of Vertebrate Paleontology 33: 956–963.

Back to front stem tetrapods

Posted on by Travis Park in CT scanning, Stem tetrapods

Relatively complete, articulated (preserved in life position) fossil specimens are extremely precious items. But sometimes the fact that the specimen is articulated can mean that matrix (surrounding rock/soil/gravel) has to be left in place to ensure it holds together. This remaining matrix can occasionally obscure important morphological features on the bones of prehistoric creatures, leading to incorrect interpretations about the animals morphology.

Fortunately however, this is becoming a less frequent occurrence with increasing use of x-ray scanning technologies that allow palaeontologists to virtually strip away the surrounding rock and see the fossil as it actually is, and realise that previous interpretations may have been incorrect.

This is exactly what has happened in the case of the new paper by Pierce et al. in the prestigious journal Nature. To make matters even more interesting the taxon they were looking at was none other than Ichthyostega, the classic example of a stem tetrapod. More specifically, they have realised that the structure of the vertebrae are actually back-to-front compared to the traditional interpretation.

Using CT scanning, Pierce et al. have realised that what was thought to be the structure of Ichthyostega vertebrae (A & B) is incorrect. C & D show the actual structure of the vertebrae. Neural arches = pink; pleurocentra = yellow; intercentra = green; ribs = blue. Modified from Pierce et al. 2013.
Using CT scanning, Pierce et al. have realised that what was thought to be the structure of Ichthyostega vertebrae (A & B) is incorrect. C & D show the actual structure of the vertebrae. Neural arches = pink; pleurocentra = yellow; intercentra = green; ribs = blue. Modified from Pierce et al. 2013.

As you can see from parts A & B of the figure above, the traditional understanding of Ichthyostega vertebrae was that the paired pleurocentra sat behind the intercentrum with the neural arch on top (known as rhachitomous vertebrae). However when Pierce et al. looked at the CT scans they realised that everyone had been wrong all along. In fact the pleurocentra are actually fused to the intercentrum and are in front of them (reverse rhachitomous vertebrae)!

Intrigued by this finding, the team proceeded to look at two other stem tetrapods, Acanthostega and Pederpes. Both of these were also thought to possess the traditional rhachitomous vertebrae, but… you guessed it, they found that they both possessed the reverse rhachitomous vertebrae too! What this finding does is raise new questions about what we thought we knew about the early evolution of the tetrapod spine. It looks like the textbooks will have to be changed again, but that’s exactly the way it should be in science.