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