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Artist's rendering of Stantonoceras solisense - AmmoniteStantonoceras solisense
Ammonite

TMM UT 53002
Cretaceous
Williamson County, Texas


This ammonite was found by Mr. Bill Jolley in Williamson County, Texas. It was generously donated for research because it is a better example of the species than the actual “type” specimen. A type specimen is the unique, official reference specimen for a species or genus.

Ammonites are invertebrates that can no longer be found in oceans today, they are extinct. They looked very similar to the living (extant), chambered, Nautilus, but are more closely related to squid. They lived on continental shelves and deep ocean environments and were prolific until the end of the Cretaceous. At that point they became extinct. Nautilus survived the stresses that led to the extinction of the ammonites. Why did they survive?  We really are not sure; some people have suggested subtle differences in lifestyles or hatchery location of their larvae.

Coiling styles of ammonites varied, this example is coiled in one plane others coil in two planes. This specimen is quite tightly coiled others are loosely coiled. In some each new coil covers the previous one so that you can only see the last whorl, called involute coiling and this ammonite is of that involute coiling style.

Within the coil is a series of chambers, the animal actually lived in the very last, the outermost, chamber. The chamber walls (septa) were connected with a tube (siphuncle) that allowed the animal to regulate its buoyancy by controlling gases in each chamber. A little bit like regulating a hot air balloon, except in the case of the ammonite control is within a water column and not the atmosphere.

These chambers have been very useful to paleontologists because the way the chamber wall attaches (sutures) to the outer shell has changed in complexity over time. This suture pattern is often preserved in the ammonite fossil. Paleontologists can identify particular species with the help of these suture patterns. The patterns evolve quite rapidly and become a proxy for time. Certain patterns evolved after others and that allows us to place the species in order, to date them relative to each other.


 

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