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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. |
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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. |
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Illustration of ammonoids: Young,
1963, Upper Cretaceous Ammonites from the Gulf Coast of the United
States
UT pub#6304, BEG |
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The simple suture
patterns indicated by dark red arrows on this specimen are much less
complex than the example in the illustration below. |
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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. |
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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. |
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