From ScienceDaily:
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 tetrapods are four-limbed vertebrates, which are today represented by amphibians, reptiles, birds and mammals. Early tetrapods of the Devonian period (419-359 million years ago) are of great interest to palaeontologists: they were the earliest vertebrate animals that ventured onto land, paving the way for all future vertebrate life on land. The move from water to land must have affected every aspect of the biology of these animals, but until now there has been no serious attempt to investigate their life histories — how long they lived or whether they had an aquatic juvenile stage, for example. Well-preserved skeletons are rare and it has simply been assumed that they represent adults More. Paper. (paywall) – Sophie Sanchez, Paul Tafforeau, Jennifer A. Clack, Per E. Ahlberg. Life history of the stem tetrapod Acanthostega revealed by synchrotron microtomography. Nature, 2016; DOI: 10.1038/nature19354
They were delinquents. No, but seriously, a friend points out the following, from the release:
… the researchers showed that Acanthostega’s foreleg remained cartilaginous until late during its development.
In contrast to bone, cartilage is a non-mineralised tissue, elastic and far too weak to allow the forelegs to sustain the weight of the animal’s body out of the water.
In contrast to bone, cartilage is a non-mineralised tissue, elastic and far too weak to allow the forelegs to sustain the weight of the animal’s body out of the water. “This suggests that the Acanthostega mass-death deposit represents a school of aquatic juveniles that included few or no adults” says Per Ahlberg from Uppsala University. So where were the adult Acanthostega living? Were there segregated distributions of juveniles and adults at least at certain times? This remains to be discovered. The scans done at ESRF ID19 beamline also show that the absolute size at which limb ossification began differs greatly between individuals, suggesting the possibility of sexual dimorphism, adaptive strategies or competition-related size variation.
That amounts to saying that the ones we have found never actually walked on their legs. By the way, six years is long to be a juvenile. Maybe Acanthostega was the first creature to flop on land but not the first to walk?
See also: Transition to land remake: Now starring … the trilobite
and
Early tetrapod (“fishapod”) sheds light on transition to land—maybe
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