November 10. Cretaceous ammonites

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.

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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.

—Richard I. Gibson

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.