Study Reveals First Ever Images Of Early Tetrapod Backbone And How It Helped In Land Evolution

[Watch Video: 3D X-Ray Images Of Early Tetrapod Backbone]

Lawrence LeBlond for – Your Universe Online

Using high-energy X-rays and a new data extraction protocol, an international consortium of scientists have for the first time rendered a 3D model of a prehistoric tetrapod backbone. The new reconstruction has shed new light on how the early animals moved once they made it onto land.

One of the main creatures studied was a fierce-looking ichthyostega that lived from 374 — 359 million years ago and was a transitional species between aquatic and terrestrial animals. The 3D model showed researchers that these new land dwellers moved much like modern seals do.

The researchers believe ichthyostega was more of a shallow water predator, navigating swamps and ponds in search of food, occasionally making landfall to perhaps feed. The researchers think the animal dragged itself across flat ground, using its front legs to push up and forward.

Results of the new study have been published in this week´s issue of the journal Nature.

The international study was led by Stephanie E. Pierce from The Royal Veterinary College in London and Jennifer A. Clack from the University of Cambridge. Other members of the team hailed from Sweden and France.

Tetrapods are four-limbed vertebrates. In our modern world, animals such as amphibians, reptiles, birds, and mammals are all tetrapods. Early tetrapods, such as the ichthyostega, made short excursions across shallow bodies of water and perhaps even shorter jaunts over land, using their underdeveloped limbs for primitive locomotion.

Just how these early tetrapods transitioned from a life at sea to land-dwelling has been a hotly debated topic among paleontologists and evolution biologists for decades.

Not only do all tetrapods have four limbs, but they have backbones (vertebral column) as well. These vertebrates also include fish, from which tetrapods evolved. The backbone is formed from vertebrae connected in a row–from head to tail. But unlike the backbone of modern tetrapods, in which each vertebra is composed of only one bone, early tetrapods had vertebrae made up of multiple parts.

“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 Pierce. “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,” Discovery News quoted Pierce as saying.

To make their analysis, the team relied on the European Synchrotron Radiation Facility (ESRF) in France to scan three fossil fragments of early tetrapods. Using the X-ray scanner, details began to emerge of the fossil bones buried deep inside the rock matrix. Although the rock obscured most of the X-rays, the team was able to decipher the readings using a detailed data extraction method.

“Without the new method, it would not have been possible to reveal the elements of the spine in three dimensions with a resolution of 30 micrometres” noted study coauthor Sophie Sanchez from University of Uppsala and ESRF.

Between the X-ray images and the data extraction tools, the team discovered that what they believed to be the first bone (the intercentrum) was actually the last in the series. The team said that this revelation brings new insight into how the vertebral structure plays out for the functional evolution of the tetrapod backbone.

“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,” noted Pierce.

Aside from the backbone discovery, the team also found that the ichthyostega also had an unusual assortment of previously unknown skeletal formations including a string of bones extending down the middle of its chest.

“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,” Clack explained.

In continuing their research, the team said the next phase will be to further investigate how the backbone aided in the locomotion of these early tetrapodous animals.

Image below shows an artist’s impression of an Ichthyostega Tetrapod, with the cut-out showing the 3-D reconstruction of two vetrebrae from the study. Credit: Julia Molnar