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Invertebrates-Cephalopods

Ammonoids

Ammonite

Ammonoids are common fossils in the Austin area. Their distinctive shapes and rapidly changing form (geologically speaking) has made them invaluable resources for analyzing the relative age of rocks.


Many of the specimens in the collections are from nearby sites, but the first one shown below is from Europe; it was a gift from Spath of the British Museum to Adkins as thanks for 'care packages' sent to him during WW2.

A 'care package' fossil from Spath to Adkins

Taxon: Deshayesites deshayesi
Age: Cretaceous (Lower Aptian)
Location: Bailly - aux- Forges, France
Collected by: Spath, 1930

Taxa may be named after the person who found them, a characteristic of their form, or a variety of other options.  This fossil genus is named after a famous French paleontologist of the mid 19th century named Deshayes.  Species differ in both size and shape. Deshayesites is very small, and has very well-marked (strong) ridges (ribs).


One of the most important characteristics of an ammonoid's shell is the suture pattern, which reflects the way in which the dividing walls between the internal chambers intersect with the outer wall.  Paleontologists use suture patterns to categorize cephalopods.  The sutures are distinctive and evolved over time, making fossil ammonoids good indicators of relative geologic age.


The suture pattern sketch on this page was made from the the lower right hand specimen in this illustration.

Illustration of ammonoids: Young, 1963, Upper Cretaceous Ammonites from the Gulf Coast of the United States
UT pub#6304, BEG

An ammonite with a simple suture pattern

The simple suture patterns indicated by dark red arrows on this specimen are much less complex than the example in the illustration below.

A sketch of a complex suture pattern


Most cephalopods have shells coiled in a flat spiral (referred to as 'planispiral').  However, some fossil cephalopods had straight shells,  some were helically coiled (similar to a snail shell), and some had shells that followed different patterns at different stages of the animal's life.


Block containing specimens of two genera with non-planispiral coiling

Taxa: Nostoceras sp. and Solenoceras sp.
Specimen number:  UT 51802
Age:  Cretaceous

The shells of these two genera began as regular planispiral forms and developed a 'J' shape as they grew older.


 

 

Frequently used abbreviations: NPL  Non-vertebrate Paleontology Laboratory | TNSC Texas Natural Science Center | UTDGS Department of Geological Sciences | BEG  Bureau of Economic Geology | VPL Vertebrate Paleontology Laboratory | JSG  Jackson School of Geosciences | SUPPORT | VOLUNTEER | GLOSSARY


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