October 18. Ammonites

While we are in the Jurassic, I wanted to mention that most of today’s oceanic crust was formed during the Jurassic, or more recently. The oceanic crust at a spreading center, like the Mid-Atlantic Ridge, is essentially brand new – new crust is forming in Iceland right now, and in a gradual way all along the mid-ocean ridges. Because oceanic crust is consumed by subduction, the oldest areas of crust out there today are the first crust formed in the opening of the Atlantic, which began in Jurassic time, about 180 or so million years ago. There’s a patch of oceanic crust in the western Pacific that’s about the same age. The only exceptions are pieces of older crust that get stranded by complex collision and subduction processes. Some parts of the oceanic crust in the Mediterranean offshore Greece are probably as old as late Permian. But all the older crust has been subducted. 

Today’s episode brings us back to the ammonites. 

Ammonites, you recall, were shelled relatives of squids and octopuses. 

Photo by Ghedoghedo, used under Creative Commons license.

Jurassic ammonites represent a spectacular recovery from the extinction event at the end of the Triassic. Ammonites were nearly wiped out by that poorly understood extinction, so that only one single family of ammonites is known to have survived. But that one family seems to have been extremely adaptable, because during the Jurassic it expanded into more than 200 genera and many hundreds of species. As we’ve seen before, extinction events, while they spell doom for some species, they also open up ecological niches where opportunistic survivors can proliferate. This process is called adaptive radiation, a result of environmental pressures driving evolution, resulting in diversity.

By later Jurassic time, one genus, Titanites, had species that attained sizes of more than 1.3 meters (6 feet). But some Jurassic ammonites were the size of a small coin.

Ammonites evolved rapidly, with individual species appearing and disappearing over periods of a few million years or less. Consequently they serve as excellent index fossils that define particular intervals of Jurassic time. They lived in oceans all over the world, and were free-swimming carnivores. Because they often swam in the shallows above abyssal ocean plains, when they died they often sank into poorly oxygenated waters where their chemical decomposition was limited and scavengers were less abundant, which explains the fact that they are common in the fossil record.

—Richard I. Gibson

Photo by Ghedoghedo, used under Creative Commons license.