Wooster’s Fossil of the Week: Ammonite septa from the Upper Cretaceous of South Dakota

January 27th, 2017

This week we have an ammonite from the Pierre Shale (Upper Cretaceous, Campanian-Maastrichtian) of southwestern South Dakota. It was collected on a wonderful field expedition in June 2008 with my friend Paul Taylor (The Natural History Museum, London) and my student John Sime. Ammonites are extremely common in this interval, but I like this one because it is broken in such a way to expose its complex internal walls, called septa. We are looking at a cross-section of a coiled ammonite showing an early whorl in the upper left surrounded by a later whorl. The septa are fluted at their margins as they meet the outer wall. The wiggly boundary line between a septum and the outer wall is called a suture.

Ammonite septa are remarkably complex, showing fractal patterns. Why did these animals, extinct for 66 million years, evolve such complexity in their septa? This is a hotly debated topic in paleontology. The most popular explanations include strengthening the walls of the shell to resist hydrostatic pressure at depth, buttressing the shell against the crushing pressures of biting predators, and increasing soft-tissue (mantle) surface areas for physiological advantages. Klug and Hoffman (2015) have an excellent summary of these ideas. Lemanis et al. (2016) have a fascinating mathematical study that suggests the answer in many cases complex sutures “seem to increase resistance to point loads, such as would be from predators.”

The astonishing English polymath Robert Hooke (1635-1703) took considerable interest in ammonites and their complicated septa. We have no contemporary images of him, but based on descriptions, Rita Greer painted the above portrait in 2004. Hooke’s life was as complex as the suture patterns he studied, so I leave you to other sources on him. Note in the portrait above, though, the ammonite!

These are drawings by Robert Hooke of ammonites and their suture patterns (from Kusukawa, 2013). It is a single image mirror-reversed. Beautiful.

References:

Derham W. 1726. Philosophical experiments and observations of the late eminent Dr. Robert Hooke, S.R.S. and Geom. Prof. Gresh., and other eminent virtuoso’s in his time. London: Derham.

Garcia-Ruiz, J.M., Checa, A. and Rivas, P. 1990. On the origin of ammonite sutures. Paleobiology 16: 349-354.

Klug, C. and Hoffmann, R. 2015. Ammonoid septa and sutures. In: Ammonoid Paleobiology: From anatomy to ecology (p. 45-90). Springer Netherlands.

Kusukawa, S. 2013. Drawings of fossils by Robert Hooke and Richard Waller. Notes Rec. R. Soc., 67: 123-138.

Lemanis, R., Zachow, S. and Hoffmann, R. 2016. Comparative cephalopod shell strength and the role of septum morphology on stress distribution. PeerJ 4:e2434

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