A Broken Bone and the Origins of Life on Land

Around 300 million years ago the very first vertebrates made their way from the seas onto the land. They weren’t the first animals to make the transition, insects had beaten them to it several million years before, but they were the first animals with a backbone to make the evolutionary leap and at least one of those species was also our ancestor. It was a momentous moment in the history of life on Earth and one that until recently palaeontologists thought they understood fairly well. We have a beautiful series of fossils, mostly from the northern hemisphere, that seemed to show ancient fishy animals gradually being replaced by more amphibious creatures, and with the discovery of Tiktaalik rosae in 2004 it seemed as though the transition from sea to land had been well documented. Of course nothing in palaeontology is ever so simple and at the heart of this new discovery is a single broken bone.

800px-Ossinodus
Reconstruction of Ossinodus pureri. Source: Wikimedia Commons

Palaeontologist find broken bones all the time. Fossilisation is not an easy process and being buried in the ground for millions of years is inevitably going to cause damage but to find a bone that was broken while the animal was still alive is far more unusual.  The bone we’re interested in comes from Australia, and was announced in research published last month by a team from Queensland University of Technology. In life it belonged to an extinct species of salamander-like animal called Ossinodus pueri, estimated to have been about 2 metres long. The bone is the radius, the front-leg bone, and the top part of the bone shows a clear vertical fracture. More interestingly then that though the fracture has partially healed. This is proof positive that the break happened when the animal was still alive and is not damage caused after it died. More importantly a lack of tooth marks or other damage associated with the fracture seems to rule out a bite from a predator as the probable cause. Instead the researchers are convinced that this kind of damage could only have been caused by a fall, and that means it happened on land.

Ossinodus lived 333 million years ago during a time called the early Carboniferous (sometimes referred to as the Mississippian) when Australia was still part of the southern super-continent, Gondwana. At the time worldwide sea levels were far higher than today and the climate was warmer too with plenty of low lying swamps for animals to experiment with life on land. The early terrestrial pioneers, like Tiktaalik, are a little older then Ossinodus but it was widely assumed that at this time most vertebrates were still amphibious at best and were poorly adapted to spending a lot of time roaming around on land. This is what makes Ossinodus so important. Before this the oldest animal from the fossil record that was totally adapted to life on land was called Casineria and it is known exclusively from Scotland. It is also two million years younger than Ossinodus.

In order to examine the fracture in detail the researchers first CT scanned the bone, and then analysed it using a computer programme that allowed them to work out the force required to cause the break originally. From the results it was clear that the fracture was the result of a high-force impact, something nearly impossible to replicate underwater. The most likely explanation is that this particular Ossinodus was walking around on land when it fell and the bones of its front leg impacted, causing a fracture down from the top of the radius. The fall and injury weren’t fatal however, and the individual lived at least long enough for the break to begin healing. Exactly what eventually killed this animal though is impossible to say.

There is a second line of evidence that Ossinodus was spending a great deal of its time on land as well and that is something called bone ‘remodelling’. Basically your bones will change their thickness or structure depending on how much you use them during life. This happens all the time quite naturally and is usually a good indicator of behaviour. For example so professional tennis players actually have slightly thicker bones in one arm then the other. In the case of Ossinodus there was evidence of remodelling to make the bones better able to support the animal’s body weight. If an animal spends most of its time immersed the water supports its body weight and there is little need for strong bones, unlike on land where they take up the weight. This was an important adaptation that the new terrestrial vertebrates had to make and finding evidence of it in Ossinodus is strongly suggestive.

The upshot of this is a radical rethink of when and how vertebrates first left the water. Until now all our best evidence for the transition came from the northern hemisphere and from relatively small animals. This new discovery of a large, terrestrial animal from the southern hemisphere 333 million years ago hints that perhaps it was on this continent that the first pioneers emerged rather than the north. It also highlights the part that luck can play in our understand of ancient life. It is only by chance that this particular individual was preserved with its unique evidence of a life on land. Although the bone remodelling alone would have hinted at the same facts the fracture provides far more direct evidence and is extremely compelling.

Reference: Bishop, P. et al. 2015. Oldest Pathology in a Tetrapod Bone Illuminates the Origin of Terrestrial Vertebrates. PlosOne. DOI: 10.1371/journal.pone.0125723

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