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Ordovician
Fossils in the
El
Paso Limestone
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Updated 12/13/20
Dawn collects Ordovician fossils in the El Paso at Dos Cabezas
| The
El Paso Limestone lies unconformably on the Coronado Quartzite
in Greenlee county, and on the Abrigo and Bolsa Quartzite in
the Dos Cabezas Mountains. It can attain thicknesses of several
hundred feet, and is upper Cambrian to lower Ordovician in age.
Of great interest to us was the Ordovician component at Dos Cabezas,
where we found fossils of gastropods, and orthocone cephalopods
replaced with red chert in a limestone matrix. Most of the Ordovician
rocks were removed from Arizona by erosion, and what little remains
is in isolated outcrops in remote localities. |
What the Rocks Looks like:
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| The
fossils occur in chertized layers, often poorly preserved by
the diagenetic process. |
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| This
close up shows many fragments of mostly gastropods, nearly all
so poorly preserved as making it virtually impossible to identify
them beyond the phyllum level. |
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| Close
up of some of the chertized fossils that stick out of the slowly
dissolving limestones. Acid treatment will free most of them,
however they are highly pitted and fragment very easily. |
Larger Macrofossils:
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| Orthocone
nautiloid. We found dozens of this type of invertebrate in the
limestones, many were hollow with thick shells. |
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| Tip
(apex) of an Orthocone nautiloid. |
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| Nice
cross section of an orthocone showing how thick the shells were! |
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| Beautiful
section of an orthocone. The full size would have been about
30cm long. |
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| Nice
side cross section of an orthocone "Orthoceras" type. |
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| Short
length of a orthocone. |
Brachiopods:
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| One
of the best shaped specimens, small and symetrical. |
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| Most
brachs were fragmentary and had large cracks in them. |
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| A
more flattened brachiopod typical of finds here. |
Crinoids:
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| Two
tiny crinoid ossicles were found amongst the acid fines. |
Ostracods:
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| One
Ostracod was found, this tiny bivalved arthropod was found intact,
along with many fragmentary remains. |
Short Spines:
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| While
these could be the terminal apex of a juvinile orthocone, more
likely was the armor points from an enchinoderm. |
Gastropods Type 1:
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| Bottom
side coiling away from the viewer. Beautiful tight coils! |
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| Another
specimen with a top view here. The apex rises toward the viewer
here in this rather flat view. |
Gastropods Type 2:
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| Resembling
a Bellerophontid, these gastropods had ribbed shells. |
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| Tight
coiling shown here. |
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| Robust
coils on this infill. |
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| Another
good specimen showing the central median ridge on the outside
of the coils. Again, very Bellerophontid-esk. |
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| Beautiful
external cast. |
Gastropods Type 3:
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| Turriculate
form gastropods also were found. |
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| One
of the very largest. |
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| External
cast of a very well preserved specimen. |
Gastropods Type 4:
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| Flat
and planispiral. That marks the charecteristics of this largest
gastropod group found. |
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| Large
and smooth exteriors with large flat looping coils. |
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| One
of the most complete found. This species by far was the most
common gastropod in the El Paso. |
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