May 29. Ammonites

Today is my 150th daily podcast. I very much appreciate your continued interest. As I said back on episode 100, I’ll be trying my darndest to keep the daily postings going, but just be aware that the summer is my busy season doing history tours and such. Please accept my apologies in advance if I miss a day or so here and there. And also, let me encourage you to post questions on the Question of the Week page on the blog, and also if you have suggestions for ways I can improve the program, feel free to post a review on iTunes or send me an email at rigibson@earthlink.net.

Goniatite

I think we’ve only talked about cephalopods once in this exploration of the history of the earth. Cephalopods are mollusks, a diverse group that includes clams and snails as well as cephalopods. In contrast to clams and snails, the cephalopods have an internal shell or none at all. They also have an array of arms or tentacles extending from their heads. Cephalopods today include octopuses, squid, and cuttlefish, and the chambered nautilus, a spiral-shelled marine invertebrate.

Spiral cephalopods became abundant during mid- to late Devonian time, around 400 million years ago. They included the first ammonites, coiled animals that lived in chambers within a spiral shell. Each chamber was separated from the next by a wall, called a septum, that had amazingly complex convolutions. Some of the traces of septa on fossil shells’ surfaces, called suture patterns, form an intricate fractal design and can be used to identify various species. This helps make ammonites excellent index fossils, fossils that are characteristic of a specific and typically short interval of geologic time.

Ammonites get their name from the Egyptian god Amun, who was often portrayed with tightly coiled ram’s horns, which resemble ammonites.

Early ammonites such as those from the Devonian often have simpler suture patterns than later species. The commonest general type from the Paleozoic is called goniatitic, for the genus Goniatites. Most Devonian goniatites have a gentle waving or zigzag suture pattern rather than the incredibly complex patterns that came later.

Ammonites survived until the extinction at the end of the Cretaceous, a run of about 330 million years. We’ll talk about them several more times in the course of our journey through the history of the earth.

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Today’s birthday is Hollis Hedberg, born May 29, 1903, in Falun, Kansas. Hedberg worked in the petroleum exploration business and he made major contributions to understanding sedimentation and stratigraphy. He was employed mostly by Gulf Oil Company, and worked extensively in Venezuela. Gulf Oil named one of its marine seismic exploration vessels for him.

—Richard I. Gibson

Goniatite photo by Rama under terms of the CeCILL license.