An article published in the journal “Nature” describes a research on Acanthostega gunnari (photo ©Ghedoghedo), that about 360 million years ago was one of the first tetrapods as it was one of the first vertebrates to have limbs recognizable as legs. A team of researchers from the universities of Cambridge and Copenhagen led by Jennifer Clack, who in 1987 discovered a deposit containing many skeletons of Acanthostega, examined those specimens and concluded that they were juveniles.
Acanthostega lived in the late Devonian period and represents one of the transitional forms between fish and tetrapods. The characteristics of its limbs were intermediate between those of fish fins and those of the legs of tetrapods adapted to walk on land. Its limbs were equipped with eight fingers each connected by a membrane. Its breathing was also an intermediate form being equipped with both gills like fish and lungs like terrestrial vertebrates.
The first fossils of Acanthostega were found in 1933 but those were just fragments of a skull. Only in 1987, with the discovery of many fossils preserved very well by Jennifer Clack, it was really possible to study this species. The technological advances that occurred later allowed to perfect tomographic techniques up to the propagation phase-contrast synchrotron microtomography used to perform sophisticated analyzes at the European Synchrotron Radiation Facility (ESRF) in France.
Specifically, the researchers examined the internal structure of the Acanthostega specimens’ shoulders obtaining microscopic detail without damaging them in any way. These structures show growth rings, the same that can be seen in both today’s tetrapods and in ancient fossils and can provide information on the specimens’ age.
The result of the tests was that the Acanthostega fossils studied were juveniles, even if they were at least six years old. At that age we’d expect them to be adults, however the growth doesn’t show the typical slowdown of a specimen that is reaching sexual maturity.
Another characteristic discovered is that its forelimbs remained cartilaginous up to an advanced stage of its development. This means that those young specimens couldn’t support their weight on their legs out of water. This suggests that the fossil deposit discovered by Jennifer Clack harbored a group of juveniles which included few or no adults.
This led researchers to wonder where adults were. It’s a question which at the moment has no answers. Acanthostega is a very interesting animal because it’s one of the earliest known species showing traces of the transition from aquatic to terrestrial life. For this reason, the study on this species and other early tetrapods continues.
