Life history of the 360-My-old tetrapod Acanthostega rewrites the tetrapod move to land
This week in the journal Nature, a team of researchers from Uppsala University in Sweden, the European Synchrotron Radiation Facility (ESRF) in France and the University of Cambridge in the United Kingdom shows that fossils of the 360 million-year-old tetrapod Acanthostega, one of the iconic transitional forms between fishes and land animals, are not adults but all juveniles. This conclusion, which is based on high-resolution synchrotron X-ray scans of fossil limb bones performed at the ESRF sheds new light on the life cycle of Acanthostega and the so-called conquest of land by tetrapods.
|The Acanthostega fossil [Credit: Jennifer CLack]|
The single richest locality for Devonian tetrapods is a so-called mass-death deposit of Acanthostega, discovered in 1987 in Greenland by Jennifer Clack, one of the authors of the study and leading teams from the Universities of Cambridge and Copenhagen, where dozens of skeletons lie packed together like sardines in a tin. It looks like the tetrapods all died together when a small stream within an "inland delta" (like the modern Okavango in Botswana) dried out. The team decided to look at the life history of these fossils by investigating the internal structure of their humeri (upper arm bones). "Using the tremendous power of synchrotron X-rays, we were able to access microscopic details in these dense specimens as on real histological slices, but without damaging these unique fossils" says Paul Tafforeau from the ESRF.
The powerful X-ray beam of the ESRF revealed that all studied fossils of Acanthostega were immature individuals, even though they were at least 6 years old and probably older. Their growth had not yet begun to slow down as it does at sexual maturity. In addition, the researchers showed that Acanthostega's foreleg remained cartilaginous until late during its development.
The tetrapods' move onto land was arguably one of the most radical adaptive shifts in vertebrate evolutionary history. "Our study provides a first glimpse of the life-history traits of an early tetrapod. We plan to undertake a more complete survey of early-tetrapod life histories which should have a significant impact on theories depicted in all textbooks" concludes Sophie Sanchez.
Source: European Synchrotron Radiation Facility [September 07, 2016]