Found: 1 Ammonoid Fossil, Colorado River, Austin, TX.

-Photo credit: Michelle Bright

 

Last week, Michelle and I went to a beach below the Town Lake Dam on the Colorado River. She was photographing the refraction of light in water in relation to depth for a project about hot springs.

So I set about looking for neat objects and stumbled on a lime stone fossil of what I believe is an Ammonite from some 251 – 65 million years ago! Based on its shape I thought it was a ancestor of the Nautilus, turns out its not. Its one of the earliest cephalopods in the phylum Mollusca,who’s lineage is now extinct. Needless to say, I spent the next hour or so scouring for more fossils. Alas, I was only to find patches of crumbling limestone with the occasional partial shell of some tasty clam of the past. Nothing to warrant exhuming from its rocky confines.

Below are a few pictures of the fossil after I found it and after I cleaned it out only to discover more rounded sawtooth ridges which helped me identify its likely time of existence.

The ridges are called sutures. Read the description below and pick which one you think it is, if enough people agree than we probably picked the right one!

Three major types of suture patterns in Ammonoidea have been noted.

Suture patterns:

  • Goniatitic – numerous undivided lobes and saddles; typically 8 lobes around the conch. This pattern is characteristic of the Paleozoic ammonoids.
  • Ceratitic – lobes have subdivided tips, giving them a saw-toothed appearance, and rounded undivided saddles. This suture pattern is characteristic of Triassic ammonoids and appears again in the Cretaceous “pseudoceratites.”
  • Ammonitic – lobes and saddles are much subdivided (fluted); subdivisions are usually rounded instead of saw-toothed. Ammonoids of this type are the most important species from a biostratigraphical point of view. This suture type is characteristic of Jurassic and Cretaceous ammonoids but extends back all the way to the Permian.

Post your guesses!!

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