Just a quick one today, to mention sharks, which were very much coming into their own during the Devonian.
|Devonian fish. Shark at top.|
Sharks are remarkably successful fish. Although there have been some changes, the basic plan of sharks has survived, apparently quite well, for more than 400 million years. Sharks are one of the major subdivisions of fish – the ones that have cartilaginous skeletons rather than bony skeletons. Cartilage is a lot more challenging to preserve in rocks than bones, so a lot of sharks are best known from fossils of their teeth. But there are still some body fossils that give us a glimpse into a strange and diverse group.
Stethacanthus, from the late Devonian, had a weird projection on its back – its dorsal fin – that looked like an ironing board or anvil. When you think about the modern hammerhead shark, maybe such an evolutionary adaptation isn’t so weird, but we don’t really know what it was for. Speculation focuses on mating displays.
—Richard I. Gibson
Sharks of the Devonian
Devonian shark teeth
Drawing by Joseph Smit, 1905, public domain