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