History of the Earth

The ammonites, shelled relatives of octopuses and squids, developed more and more complex suture lines during the Cretaceous. The sutures are the lines where septa, the boundaries between the chambers in the shell in which the animal lived, intersect the shell surface. Their patterns approach modern fractal designs in complexity, but it wasn’t always a smooth, continuous evolution. During the Cretaceous, some groups’ suture patterns changed from simple to complex and back to simple again. Nonetheless, over long periods of geologic time, there is a tendency for ammonite suture patterns to increase in complexity, so Cretaceous versions are generally much more complex than Permian or older patterns.

Scaphites with suture patterns (USGS photos)

The role of the suture and its complexity is not fully understood. One study has suggested that the simpler patterns correlate with animals that could withstand pressures well, but could not control their buoyancy. That leads to the idea that those earlier forms were deep-water animals, where pressures are high, and that they pretty much just hung out there without being too active and not moving up and down much in the water column. The more complex patterns such as those seen in Cretaceous ammonites are interpreted to indicate more fragile chamber boundaries that could not stand high pressures, but all the intricate complexities would allow for excellent control on buoyancy by moving gas bubbles in or out of the spaces the patterns created. Those animals would live in shallow water, but would have been very active and fast moving within whatever zone they were adapted to.

Some Cretaceous ammonites grew to six feet in diameter. The chalk cliffs at Peacehaven, Sussex, England, contain some of that size, although they are typically rather poorly preserved. Ammonites living in near-surface waters would have fallen into the calcareous ooze when they died, to become incorporated into the chalk. 

The genus Scaphites is a really common coiled variety in the Fox Hills formation of the western United States. Some of them are more than a meter across, more than three feet, but there’s a wide range in the size of individual adults of any particular species. You will see some spectacular images of huge ammonites in excellent preservation, often more than 6 or 8 feet across. The vast majority of these images are photoshopped hoaxes.  

Baculites suture pattern photo by The Children’s Museum of Indianapolis, used under Creative Commons license.

Ammonites continued to have two distinct forms – coils, such as those exhibited by Scaphites, and straight, typified by the genus Baculites, which are also common in the Cretaceous rocks of western United States. Some Baculites grew to two meters, 6 feet, in length, but as with other ammonites there was a huge range in the size of individuals. Other adults of the same species only reached a few inches in length. It’s not clear what controlled this variety in adult size. 

All ammonites were extinct by the end of the Cretaceous.


Marshall Kay was born November 10, 1904, in Paisley, Ontario. His geological career at Columbia University focused on studies of Ordovician rocks, but that work led him to develop the concept of geosynclines, depositional basins along the flanks of continents. That, in turn, contributed significantly to the idea that continents had changed their positions over time.


Scaphitoid cephalopods of the Colorado Group (Cobban; USGS Prof. Paper 239) – source of black-and-white image 

Peacehaven fossils 

Suture explanation 

Baculites suture pattern photo by The Children’s Museum of Indianapolis, used under Creative Commons license.  

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.

Photo by Ghedoghedo, used under Creative Commons license.

Cephalopods are the group that includes the octopuses, squids, and chambered nautilus. The extinct ammonites were spiral cephalopods that thrived in the Paleozoic and Mesozoic. Ammonites diversified during the Permian Period, but many varieties including the goniatites were killed off in the end Permian extinction.

Most Permian ammonites were relatively small, a few inches across. Many ammonites are associated with another fossil, called aptychus, which was thought for years to be the shells of a clam-like mollusk. Now it is thought to be part of the ammonite animal, but exactly what isn’t settled yet. It might be a trap-door-like mechanism, an operculum, that sealed the animal inside its shell when it was inactive. Snails have things like that. But it’s also possible that it may be some kind of jaw apparatus that helped the animal munch its prey. Or just maybe, it served both purposes. Paleontologists are still working on that little question.

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.


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.

Goniatite photo by Rama under terms of the CeCILL license.