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.
Most people would class the Cenozoic (the period of time spanning from 66 Ma to the present) as the Age of Mammals. Certainly the diversity of mammals exploded and the majority of modern groups evolved after the demise of the non-avian dinosaurs at the end of the Cretaceous. However, what a lot of people don’t realise when they think about mammals is that they have been around for a lot longer than 66 Ma. The oldest known definite mammals date to around 165 Ma but the actual origins of the group would have been some time previous to that but remains uncertain, primarily due to the fact that most early mammals are known only from isolated teeth.
Two remarkable new finds, both from the Middle – Late Jurassic Tiaojishan Formation in the Hebei Province of China, have provided new food for thought in this debate, whilst not necessarily providing any definitive answers. The two new species, described in separate papers in last week’s issue of Nature preserve not only teeth, but skull material and post-cranial elements such as vertebrae, limb bones and even fur. Both species belong to an extinct group of mammals known as the haramiyids.
The first new species, Arboroharamiya jenkinsi, described by Zheng et al., was an omnivore or herbivore that had several adaptations for living in trees, such as elongated digits. The morphology of its caudal (tail) vertebrae also hints at it possessing a prehensile tail. It has been dated to 160 Ma.
The second new species, Megaconus mammmaliaformis, described by Zhou et al., was herbivore that lived on the ground, with its morphology indicating it had an ambulatory (walking) gait similar to that of a modern day armadillo. Megaconus was more primitive than Arboroharamiya and is also slightly older, dating to around 164 – 165 Ma.
Now, so far so good. But where these two studies provide conflicting opinions about the early mammal evolution is in their phylogenetic analyses. The Zheng et al. paper groups Arboroharamiya and all other haramiyids as the sister group to the multituberculates within Mammalia. This puts the origin of mammals at around 215 Ma, in the Late Triassic, much older than most palaeontologists would estimate, but in agreement with molecular estimates. The Zhou et al. paper on the other hand, placed Megaconus and all other haramiyids outside of Mammalia, meaning they are not closely related to multituberculates and also estimates the origin of mammals at around 180 Ma, a figure more in line with palaeontologists expectations given what fossils are currently known.
So, which tree is the correct one? Well, neither probably. There are several factors why this will most likely turn out to be the case. One is that these two phylogenies don’t contain the other new species, a potential next move for the authors of these two papers is to combine their data a produce a phylogeny with both new taxa to see where the haramiyids place. Another is that although these fossils are relatively well preserved, there is still a lot of anatomical and morphological data missing from them, with Arboroharamiya possessing less than a quarter of the 436 characters used in the Zheng et al. study and Megaconus possessing less than half of the 475 characters used in the Zhou et al. study. A third factor is that whilst these two new taxa might be relatively well preserved, the majority of other early mammal taxa are poorly preserved or are only known from teeth. More fossils of the quality of these two new specimens would help resolve the origin of the mammals.
Finally, there is also a little lesson to be learnt here about cladistics, the method by which phylogenies are now generated. Whilst this method is undoubtedly the best and most powerful tool we possess for distinguishing relationships between species at present, there are many different cladistic techniques that scientists can employ, and it will often depend which technique is used as to which phylogeny they end up obtaining. So don’t always accept the phylogenetic position of taxa just because there’s a phylogeny showing it that way, try to look at what methods they’ve used to obtain their results. Remember, good scientists will question everything!
Cifelli, R. L. & Davis, B. M. 2013. Jurassic fossils and mammalian antiquity. Nature 500, 160–161.
Zheng, X., Bi, S., Wang, X. & Meng, J. 2013. A new arboreal haramiyid shows the diversity of crown mammals in the Jurassic period. Nature 500, 199–202.
Zhou, C.-F., Wu, S., Martin, T. & Luo, Z.-X. 2013. A Jurassic mammaliaform and the earliest mammalian evolutionary adaptations. Nature 500, 163–167.