The Evolutionary Evidence lab in EvoBeaker highlights two famous transitional fossils: Tiktaalik, a 'fishopod', and Microraptor, a dinosaur with feathers. Last month Junchang Lü, David Unwin, and colleagues described a new transitional fossil that's sure to become famous too. It's a Jurassic pterosaur from China.
As the tree below shows, reconstructions of evolutionary history based on fossils indicate that the pterodactyls (more correctly, the Pterodactyloidea) were derived pterosaurs (more correctly, Pterosauria).
Pterodactyls had short tails, like this Zhenjiangopterus:
Pterodactyls also had distinctive features in their heads and necks. These include reduced or absent neck ribs and a single large opening in the skull where other pterosaurs had two smaller ones.
Their ancestors, however, had long tails like this Rhamphorynchus:
They also had necks with ribs, and separate nasal and antorbital fenestra (those openings in the skull I mentioned above).
On this evidence it could be predicted that, somewhere in the Middle Jurrasic, there should be fossils illustrating the transition from the ancestral to the derived form. Which is exactly what Lü, Unwin, and colleagues have found.
Darwinopterus was a flying predator with a wingspan approaching one meter and mouth full of sharp teeth. From the neck up, it looked like a pterodactyl. But it had a body and tail like that of an ancestral pterosaur. You can find a photo of a complete specimen here, and a photo of a skull plus an interpretive drawing here. It's worth a look.
Sources:
The report by Lü, Unwin, and colleagues is: Lü, J., D. M. Unwin et al. 2009. Evidence for modular evolution in a long-tailed pterosaur with a pterodactyloid skull. Proceedings of the Royal Society B. Published online before print, 14 October 2009. (doi:10.1098/rspb.2009.1603.)
The phylogeny is based on Lü, J., D. M. Unwin et al. 2009.
The sketches are by John Conway, and are publshed under the GNU Free Documentation License. Full-size versions are available here and here.
