Trilobites from the Permian

Fort Apache Limestone East of

Payson

Fort Apache Limestone
The rarest trilobites in Arizona are Permian in age
Updated 9/1/17

Arizona has a rich sedimentary rock record both in the northern part of the State and south of Tucson. Trilobites, which are only found in Paleozoic aged strata can be found in many of them, although they are rare except in Cambrian rocks. The rarest and hardest to find trilobites are in Permian aged rock. This includes in northern Arizona the Kaibab and the Fort Apache Limestones. But the Fort Apache is a very thin bed in the Supai formation - only tens of feet thick west of Payson, and at a maximum of 100 feet thick in the inaccessible areas on the Fort Apache Indian reservation which is the easternmost exposure. East of Payson, the limestone contains virtually NO fossils. This starts to change around Strawberry north of Payson where very rare gastropods can be found.

Finally, the easternmost exposures on Highway 260 just before you top the Mogollon Rim (southern edge of the Colorado Plateau) are the last you will find until a hundred miles to the east on the reservation. Fortunately, there are areas along the High line trail that cut right into the scree slopes of the Fort Apache Limestone outcrops. Here, for the very first time we can find fossiliferous limestones with silicified invertebrates which can be released by treatment in muriatic acid. (about 10% dilution)

Very little has been published on the Fort Apache Limestone recently. Up until recently, the best I had was the 1964 paper "Stratigraphy of the Fort Apache Member Supai Formation (Permian) East-Central Arizona" by Thomas Gerrard of the University of Arizona. Although he discusses the stratigraphy and geology at length, very little information on the enclosed fossils is to be had.

Fortunately, I was able to get through a very kind fossil forum member a copy of Winters epochical paper from the GSA Memoir 89 - "Supai Formation Permian of Eastern Arizona" written in 1963. (Nothing significant has been written since.) This outstanding 99 page memoir covers not only what is now called the Schnebly Hill formation which is a Sahara type dune complex that brackets the limestone above and below, but a superb treatment of the rare fossils which they found in the Fort Apache Limestone - From the richest 6 areas directly on the Indian reservation. What Winters did was truly inspiring as far as collection and field work. They spent 1947 collecting one ton of limestone from the best localities, and then trucked this huge crate of rock back to the Smithsonian where they spent years acid reducing the rock down to its insoluables! And it took them over a decade to write the Memoir in 1963 on the geology and paleontology of the formations. From that one ton of rock, they obtained a few fragmentary trilobite fossils and many other more common types such as mollusks.

And this leads me up to the present. We have brought back so far over a hundred pounds in promising rock in our backpacks, and spent many hours dissolving it in the masonry muriatic to free the specimens. Then three sizes of sieves were used to sort them according to size. Amongst the countless tiny gastropods which dominate the fauna present, two or three trilobite pygidia and many fragments were found in the acid fines. The preservation is key - unlike all the other fossils present which are preserved in a white silica that is fairly opaque, the trilobites are always preserved as casts of a yellow translucent opal like silica, making pieces easier to spot in the sorting trays under the stereo microscopes. You can see this unusual caramel colored preservation here in this set of images from the first 120 pounds of rock we have processed.

Winters partial specimens allowed him to get a good approximation of the genus and possibly the species. Below is what a complete pygidium of Anisopyge sp. looks like from the Treatise volume O. So rare are Permian trilobites, only a page is devoted to all of them!

 

10x microscope shot (field about the size of a dime) of many of the common trilobite fragments found. Winters identifies this trilobite as Anisopyge cf. A. inornata. (cf. is a Latin abbreviation meaning "compares to")

In the most recent trip with 60 pounds of rock, this all we got - a few pieces of the outer rim of the pygidium.

Most complete specimen so far a nice pygidium with the ribs visible. it is roughly 1 cm tall. The trilobite would have been less than an inch in size.

Oblique view same specimen.

Another pygidium (tail) still attached to some matrix. The specimens are so delicate, I am afraid to even try to pick them up with tweezers, as they will fall apart. To move them I use a wet toothpick and stick them to the fossils.

A glabella (nose) both front lip and top part which fell apart.

Here I am at the locality this morning (Sept. 1) looking for promising limestones. Usually, the ones with the most urchin spines on the surface seem to have the best fossils inside!

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