April 22. Fish fins get strong

 


First off, I’m pleased to announce that you can now subscribe or listen to our podcasts in Stitcher. Stitcher is a free, mobile-optimized app that you can download, so if you prefer to listen that way, it’s now available. Thanks to listener Max for pointing me to Stitcher.

* * *

A couple days ago we talked about fish getting jaws. They were also getting other things during the late Silurian, including stronger fins. The crossopterygians – that means fringe-finned – and sarcopterygians – that one means fleshy fin – are most closely related to today’s lungfish and the famous “living fossil,” the coelacanth, as well as the tetrapods – four-limbed animals including amphibians, reptiles, birds, and mammals. “Living fossil” isn’t really a very useful term, but it’s out there – it mostly means that whatever we have today isn’t too much different from fossil versions.

Reconstruction of Guiyu oneiros, late Silurian of China

The earliest known sarcopterygians are from the very late Silurian, about 418 million years ago. The Silurian ended about 416 million years ago, and it’ll be next month, the Devonian, when the fish really took off and diversified.

The modern coelacanth, which was thought to have been extinct since the Cretaceous Period, was first caught off the coast of South Africa in 1938. Its four strong fins are the main connection to the Silurian and Devonian forms that also gave rise to tetrapods, land-walking animals. And obviously, to live on land animals had to be able to breathe air directly rather than taking it from water, and the closely related lungfish, which also probably originated in the very late Silurian but survived to the present, are probably examples of early stages of that evolution.

A related footnote – Sharks are in the news, fossil sharks. A report in the journal Nature last week described a fossil shark with significant differences from modern sharks – in the jaws and gill structures. It’s common to say that sharks haven’t really evolved all that much for millions of years, but this discovery reminds us that there has indeed been considerable change even in things that seem to remain the same for long periods of time. Sharks have evolved, too.

—Richard I. Gibson

Image by ArthurWeasley via Wikipedia under Creative Commons license.