October 6. Belemnites and Eryon

Belemnites photo by Wilson44691, Mark A. Wilson
(Department of Geology, The College of Wooster),
donated to public domain 

Because of all the hoopla about dinosaurs, Jurassic invertebrates don’t get a lot of attention. But there were plenty of them, and some were pretty interesting. A few days ago when I was talking about ichthyosaurs, I mentioned one fossil with 200 belemnites in its stomach. Belemnites were squid-like cephalopods, related to octopuses and modern squids. They had an elongate, nearly cylindrical internal shell which is often the only vestige of the animal preserved as a fossil – but they are sometimes remarkably common. The fossils look like little cigars or bullets, and the name belemnite comes from a Greek word meaning a dart.

Belemnites appeared during the late Triassic but proliferated in the Jurassic. Since they survived the major extinction at the end of the Triassic, and because they were likely important elements in the marine food chain, the evolution of belemnites may be related to the evolution of marine reptiles too. When numbers of prey expand, that can help the numbers of predators expand as well.

Although belemnites looked very much like modern squids, fossils that preserve their soft parts do reveal some differences. They did have ten grasping arms like modern squids, but they did not have the pair of specialized tentacles modern squids have. Belemnites went extinct at the end of the Cretaceous.

Eryon photo by Didier Descouens,
used under Creative Commons license

Another Jurassic invertebrate that looked a lot like its modern cousins was a crustacean called Eryon. Crustaceans are arthropods, and include the crabs, lobsters, and shrimp. Eryon looks a lot like a crab-headed lobster with a short tail, and it likely represents a link in the evolution of crabs, which have no tails as adults, but the tail is seen in the embryonic development of modern crabs. This is an example of ontogeny recapitulating phylogeny, the concept that the embryonic development of individuals repeats in many ways the evolutionary history of the species. The details of Eryon can only be seen in fossils of exceptional preservation. It was found in the Solnhofen Limestone in Germany, which we’ll talk more about later this month.

—Richard I. Gibson

Late Triassic belemnites

Eryon photo by Didier Descouens, used under Creative Commons license 

Belemnites photo by Wilson44691, Mark A. Wilson (Department of Geology, The College of Wooster), donated to public domain