Fossil Ammocoetes!

Recently (in October) fossils of lampreys were reported in the Proceedings of the National Academy of Science (PNAS; link to article). Chang et al report the first fossil ammocoetes ever found, as well as fossil transformers and adult lampreys, all from the same beds! Below is a picture from their article of an ammocoete (A), a transfomer (B and C), and closeups of the head of an ammocoete (D and E) and a transformer (F and G).

Below is the the top panel A with one of my ammocoetes imposed over top of a fossil ammocoete and a picture of the live ammocoete below.

These fossils were found in Lower Cretaceous rocks 125 million years old, and as you can see they appear very similar to modern ammocoetes. This is important for understanding when lampreys evolved and if the ammocoete stage is useful for developmental biology studies. If the ammocoete stage evolved long after lampreys broke away from the rest of the fish ancestors it is not as useful at letting us understand how early fish lived. If on the other hand it has been around since lampreys first diverged we are literally looking back in time at an early fish.

This first ammocoete fossil lets us know the modern pattern of development in lampreys goes back quite a way. Lampreys probably diverged from other fish between 350 and 450 million years ago, but without more fossils we cannot narrow down the time more precisely. However, current evidence suggests that lampreys diverged from other fish (including ourselves) closer to the 450 million year ago mark. This makes lampreys (as a group) close to a half billion years old! For comparision dinosaurs don't appear until ~230 million years ago, meaning that they are only half as old as lampreys.

Citation
Chang, M., Wu, F., Miao, D., and Jiangyong, Z. 2014. Discovery of fossil lamprey larva from the Lower Cretaceous reveals its three-phased life cycle. PNAS 111 (43) 15486-15490.