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Scientists reassemble the backbone of life using a particle accelerator

13 January 2013

Research published in the journal Nature documents, for the first time, the intricate three-dimensional structure of the backbone in the earliest four-legged animals (tetrapods).

The international team of scientists, led by Dr Stephanie E. Pierce from The Royal Veterinary College and Professor Jennifer A. Clack from the University of Cambridge, bombarded 360 million year old early tetrapod fossils with high energy synchrotron radiation. The resulting high resolution X-ray images allowed the researchers to reconstruct the backbones of the extinct animals in exceptional detail.

Ichthyostega
Julia Molnar's illustration of Ichthyostega showing changes of its spine from front to back, with neural arch/spine in pink, twin pleurocentra in yellow, and intercentrum in green. These four parts, three kinds of bones, made up the backbone of the first land animals. These parts evolved in different ways in later animals, but formed one main bone in all living lineages of animals.

The backbone, also known as the spine or vertebral column, is a bony structure found in all tetrapods, along with other vertebrates such as fish. It is formed from many elements or vertebrae all connected in a row - from head to tail. Unlike the backbone of living tetrapods (e.g. humans), in which each vertebra is composed of only one bone, early tetrapods had vertebrae made up of multiple parts.

Lead author Dr Pierce says: "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. But, by peering inside the fossils using synchrotron X-rays we have discovered that this traditional view literally got it back-to-front."

For the analysis, the European Synchrotron Radiation Facility (ESRF) in France, where the three fossil fragments were scanned with X-rays, used a new protocol to reveal tiny details of the fossil bones buried deep inside the rock matrix.

Using this new technology, the team of scientists discovered that what was thought to be the first bone - known as the intercentrum - is actually the last in the series. And, although this might seem like a trivial oversight, this re-arrangement in vertebral structure has over-arching ramifications for the functional evolution of the tetrapod backbone.

Dr. Pierce explains: "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".

But, the findings didn't end there. One of the animals - known as Ichthyostega - was also found to have an assortment of hitherto unknown skeletal features including a string of bones extending down the middle of its chest.

Professor Clack says: "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."

Ichthyostega
(a,b) How we used to think the vertebrae were composed in the early tetrapod Ichthyostega. (c) How we found that Ichthyostega's posterior thoracic vertebrae actually tend to look. (d) Ichthyostega's anterior lumbar vertebral morphology. (e) Acanthostega according to Coates's important description. (f) Our revision of the anatomy of Acanthostega (anterior dorsal). (g) Our new interpretation of Pederpes's morphology, from a posterior dorsal. Focus on the yellow vs. green elements. In a, b and e they are in different positions (reversed) compared with our new versions in c, d, f, g.

This unexpected discovery supports recent work done by the same authors that showed Ichthyostega probably moved by dragging itself across flat ground using synchronous 'crutching' motions of its front legs - much like that of a mudskipper or seal.

Dr Pierce adds: “The results of this study force us to re-write the textbook on backbone evolution in the earliest limbed animals."

The next step, the researchers say, is to understand how the backbone aided locomotion in these early tetrapods using sophisticated biomechanical analysis.

The study was funded by the Natural Environment Research Council.
Additional support was provided by the European Research Council and the ESRF, of which the Science and Technology Facilities Council (STFC) is the UK shareholder.

Movies

These are rotating images of the anatomy, colour-coded, of the three species of early tetrapod that we examined for this study. Each shows the same basic pattern of having a "reverse rhachitomous" (precentra in the front, intercentrum in the back) anatomy. This is opposite to the pattern that essentially all studies since Edward Drinker Cope – who coined the term "rhachitomous" in 1878 - have portrayed these and related animals as having, which forces a re-examination of how the backbone first evolved and which parts (intercentra? pleurocentra?) formed the spines of later animals.

You can also see the sternebrae (sternal elements; parts of the sternum that evolved independently in later land animals) in the first movie.


The segmented PPC-SRµCT of Ichthyostega stensioi MGUH VP 6115 spinning in yaw and roll.


The segmented PPC-SRµCT of Ichthyostega eigili MGUH VP 29017a spinning in yaw and roll.


The segmented PPC-SRµCT of Acanthostega gunnari MGUH f.n. 1227 spinning in yaw.


The segmented µCT of Pederpes finneyae GLAHMS 100815 spinning in yaw.

Notes for editors

  • The Royal Veterinary College is the UK's first and largest veterinary school and a constituent College of the University of London. In the recent Research Assessment Exercise the RVC ranked as England’s best school in the Agriculture, Veterinary and Food Science unit of assessment, for institutions whose research is exclusively veterinary related, with 55% of its submitted academics viewed as producing ’world class’ and ’internationally excellent’ research.  The College provides support for veterinary and related professions through its three referral hospitals, diagnostic services and continuing professional development courses. www.rvc.ac.uk
  • The University of Cambridge’s mission is to contribute to society through the pursuit of education, learning and research at the highest international levels of excellence. Cambridge’s reputation for excellence is known internationally and reflects the scholastic achievements of its academics and students, as well as the world-class original research carried out by its staff.  Some of the most significant scientific breakthroughs occurred at the University, including the splitting of the atom, invention of the jet engine and the discoveries of stem cells, plate tectonics, pulsars and the structure of DNA.  From Isaac Newton to Stephen Hawking, the University has nurtured some of history’s greatest minds and has produced more Nobel Prize winners than any other UK institution with over 80 laureates. www.cam.ac.uk
  • The Natural Environment Research Council (NERC) funds world-class science, in universities and its own research centres, that increases knowledge and understanding of the natural world. It is tackling major environmental issues such as climate change, biodiversity and natural hazards. NERC receives around £400m a year from the government's science budget, which is used to provide independent research and training in the environmental sciences. www.nerc.ac.uk/

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