Reconstruction of the early terrestrial vertebrate Ichthyostega. (Photo: Julia Molnar)
Read aloud Our image of the first four-legged land vertebrates needs to be rewritten - at least as far as their spine and thus a crucial part of their anatomy is concerned. For an international research team has now investigated fossils of the 360-million-year-old four-legged Ichthyostega with new methods and discovered amazing: The three-part vortex of this transitional form of water to land animals are built completely different than for over a hundred years in the textbooks. The first terrestrial vertebrates developed from fish-like ancestors. About 400 million years ago, these began to make shorter, then longer trips on land. Their sturdy fins and stable spine helped them to move in an unfamiliar milieu. While the vertebrae of today's living vertebrates consist of only one bone each, the vertebrae of the first tetrapods were still in three parts.

Inverted bone sequence

"For more than 100 years, it has been thought that the vertebrae of the early tetrapods are composed of a bone in front, a bone above, and a pair of bones behind, " says lead author Stephanie Pierce of the University of Cambridge. But when the researchers re-examined the vertebrae of Ichthyostega with the help of a special X-ray procedure, the opposite was true: "In the traditional presentation, the sequence is exactly reversed, " says Pierce. For the intercentrum, the bone supposed to be in front, is actually the last of the three vertebral bones.

This re-sorting does not seem very revolutionary at first, but has significant implications for the mobility of the spine, among other things, as the researchers explain. "By understanding how these bones intermesh, we can also reconstruct how mobile the spine of the first land vertebrates was and how it supported the body and locomotion in the first stages of the gangway, " says Pierce. The scientists also believe that this arrangement of the vertebral bones could be the basic form for all tetrapods - and thus the configuration that eventually formed the fused vertebrae of the higher terrestrial vertebrates. display

First sternum

The reversed vertebral bones were not the only surprise for the researchers. In their reexamination of the Ichthyostega fossil, they also came across a series of small, roundish bones in the middle of the chest. "These breast bones prove to be the earliest attempt of a bony sternum in evolution, " explains Jennifer Clack, co-author of the study. With us and other terrestrial animals living today, the sternum fused into an elongated plate forms an important support for the ribs and thorax and stabilizes the upper body. Ichthyostega's newly discovered bone series may have had a similar function in the researchers' view: it allowed the prehistoric animal to prop himself up on his chest as he moved forward on land with his fin-like legs.

The fact that these structures are only now found is related to the state of the fossils: much of the vertebral bone and the early sternum were obscured by other parts of the bone or embedded so deeply in the surrounding rock that their structure could not be analyzed by normal methods, as the researchers report. They therefore used a special procedure of X-ray microtomography for their study. In this case, particularly high-energy beams coupled with a special evaluation method enable high penetration with high contrasts and high resolution.

"Without this new method, we would not have been able to reconstruct the spinal elements three-dimensionally at 30 microns resolution, " explains Sophie Sanchez of the European Synchrotron Radiation Facility ESRF in Grenoble, the research center where the research was conducted.

Stephanie Pierce (University of Cambridge) et al .: Nature, doi: 10.1038 / nature11825 © - === Nadja Podbregar


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