The life of a parasite must be a good one, and often successful; the creature at the top of the drawing above is a primitive lamprey from the Devonian, 360 million years ago, and the similarities with the modern lamprey (at the bottom) are amazing. It’s less eel-like and more tadpole-like than modern forms, but it has the same disc-shaped mouth specialized for latching on to the flank of its host, it has similar circumoral teeth for rasping through scales and skin for its blood meal, the same pharyngeal adaptations for a life spent clamped to a fish.
I’ve put a photo of the fossil and a cladogram below the fold.
It’s not a very exciting animal, but put it in context. This creature was swimming about in the late Devonian, in the waning years of the great agnathan radiation. It shared the seas with those exotic armored jawless fishes that are all gone now, with just the hagfish and lampreys remaining. If you’d been alive in those days, would you have picked the lineage of those annoying soft and slimy parasites as the only members of that diverse group that would survive the next few hundred million years? A mere degenerate ostracoderm, thin-skinned and leech-like, is the line with the greatest longevity…perhaps because it was suitably adapted to take advantage of the gnathostome radiation, rather than compete with it.
Gess RW, Coates MI, Rubidge BS (2006) A lamprey from the Devonian period of South Africa. Nature 443(7114):981-984.