Paleontologists have uncovered yet another specimen in the lineage leading to modern tetrapods, creating more gaps that will need to be filled. It’s a Sisyphean job, working as an evolutionist.
This creature is called Tiktaalik roseae, and it was discovered in a project that was specifically launched to find a predicted intermediate form between a distinctly fish-like organism, Panderichthys, and the distinctly tetrapod-like organisms, Acanthostega and Ichthyostega. From the review article by Ahlberg and Clack, we get this summary of Tiktaalik‘s importance:
First, it demonstrates the predictive capacity of palaeontology. The Nunavut field project had the express aim of finding an intermediate between Panderichthys and tetrapods, by searching in sediments from the most probable environment (rivers) and time (early Late Devonian). Second, Tiktaalik adds enormously to our understanding of the fish-tetrapod transition because of its position on the tree and the combination of characters it displays.
Here’s the beautiful beastie as preserved:
The analysis of the fossil clearly positions it as an intermediate: it has a more mobile skull/neck than a fish, and although its limbs are clearly fin-like, they also have features that presage the digits of tetrapods.
The limbs alone have a whole paper dedicated to them. Tiktaalik‘s limb is third from the right in the diagram below, and you can see how it still has the fin rays of its predecessor, Panderichthys, but also has a robust bony axis with smaller bones branching off of it—it’s not quite the clear digits of Acanthostega, but it’s a step in that direction.
Those limbs tell us something about the evolution of limbs. Tiktaalik was definitely not a terrestrial animal, but had developed muscular, bony limbs and a strong pectoral girdle that had helped it prop itself up on the substrate, perhaps even holding itself partly out of the water. Those jointed digits were capable of extension and flexion, splaying out when they were pressed against the ground. That simple function, of spreading out to increase the surface area of limb contact, could be the precursor to the flexibility we now have in our hands.
As does Carl Zimmer, of course.
Ahlberg PE, Clack JA (2006) A firm step from water to land. Nature 440:747-749.
Daeschler EB, Shubin NH, Jenkins FA (2006) A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature 440:757-763.
Shubin NH, Daeschler EB, Jenkins FA (2006) The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature 440:764-771.