Astonishing 3D images reveal the first four-legged land animals in amazing detail - and overturn a century of research

High resolution X-ray images expose rock-encased fossils

Findings have implications for understanding how backbones evolved

Textbooks will have to be re-written, say scientists behind the study



Early backboned creatures have been reconstructed the wrong way around for the last 100 years, astonishing 3D X-ray images have revealed.

By using high resolution X-ray imaging researchers have been able to look inside rocks to establish how the backbone of the first four-legged land animals was constructed.

To the consternation of palaeontologists the 3D view of the rock-encased 360 million-year-old remains revealed that for a century they have been putting the fossilised backbones together back-to-front.

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A reconstruction of the early tetrapod Ichthyostega showing the skeleton within the body.

The discovery overturns the accepted way of reconstructing tetrapod fossils and means that scientists will have to rethink how the backbone developed in creatures such as Ichthyostega, Acanthostega and Pederpes.

‘For more than 100 years, early tetrapods were thought to have vertebrae composed of three sets of bones - one bone in front, one on top, and a pair behind,’ said Dr Stephanie Pierce, of the Royal Veterinary College.

‘But, by peering inside the fossils using synchrotron X-rays we have discovered that this traditional view literally got it back-to-front.

‘The results of this study force us to re-write the textbook on backbone evolution in the earliest limbed animals.’

Modern-day tetrapods, including humans, have a spine in which each vertebra is a single bone.

In early tetrapods each vertebra was composed of multiple bones but until now scientists reconstructed them the wrong way around.

Dr Pierce, who led the international study alongside Professor Jennifer Clack of the University of Cambridge, subjected rock-encased fossils to high energy synchrotron radiation.

Most known fossils of the earliest tetrapods, and those of the first backboned fish, are trapped in rock but the X-ray process allows scientists to see them without damage the delicate remains.

3D images showing how the backbone of one of the first four-legged land animals fits together.

Using the European Synchrotron Radiation Facility (ESRF) in France the team saw the fossilised animals in amazing detail and realised that what had been assumed to be the first bone – the intercentrum - in each vertebra was actually the last.

The finding has ‘over-arching ramifications for the functional evolution of the tetrapod backbone’ the team concluded.

Dr Pierce added: ‘By understanding how each of the bones fit together we can begin to explore the mobility of the spine and test how it may have transferred forces between the limbs during the early stages of land movement.’

X-rays revealed the structure of the backbone of Ichthyostega.

One animal was also found to have ‘hitherto unknown skeletal features’ that included a string of bones extending down the middle of its chest and represents evolution’s first attempt at a sternum.

Professor Clack said: ‘These chest bones turned out to be the earliest evolutionary attempt to produce a bony sternum.

‘Such a structure would have strengthened the ribcage of Ichthyostega, permitting it to support its body weight on its chest while moving about on land.’

The early sternum strengthened a theory that Ichthyostega moved by dragging itself across flat ground using 'crutching' motions of its front legs, similar to the way mudskipper and seal pull themselves across land.

In their report on the study, published in the journal Nature, the research team added that ‘inaccurate reconstruction’ of tetrapods ‘have been replicated in the literature for more than half a century.