Marc Behrendt - Fossil Preparation

The Rochester Shale is one of the best-known fossil units in North America. Its fossils are found in nearly every natural history museum, rock and fossil show, and general fossil guide. Why? There are 3 primary reasons: Spectacular preservation; excellent displayability due to the contrasting dark-colored fossils that set on a lighter-colored shale; amazing species diversity. More than 200 species of invertebrate fossils have been reported from the Rochester Shale. Remarkably, more than 80 species of bryozoa have been identified. Of interest to collectors, the Rochester is famous for its trilobites, crinoids, cystoids and sea stars. This paper will focus on just one group, the trilobites.

During the Lower Silurian, the region east of today’s Mississippi River was a tropical sea located just south of the equator. Coral reefs provided a foundation for a rich and varied ecosystem. The large faunal diversity that included crinoids and cystoids suggests a quiet, well-oxygenated, clear water offshore community. Fossils within the Rochester Shale tend to be found in pockets, suggesting they were carried by currents and buried by storm sediment.

The Middle Silurian Rochester Shale was first discovered in Lockport during the construction of the Erie Canal and was the first designated North American rock unit described in 1839 by James Hall, who studied the shale exposures in the Genesee River gorge near Rochester, New York. Stratigraphically, the Rochester Shale is part of the Clinton Group, lying over the crinoidal Irondequoit Limestone, and under the DeCew Dolostone. The shale is exposed in many creeks and roadcuts throughout its range in New York and Ontario. It is a significant geological feature of Niagara Falls.

The Rochester Shale is a dark-grey shaley mudstone with thin interbedded limestone layers. Its greatest thickness is over 37 meters (122 feet) in Wayne County, New York. Rainwater tends to turn the affected shale into mud, thus shale exposed to the weather is not a good source for museum grade fossils. Fresh material is a must when in search of quality specimens.

In the western exposures, the Rochester shale is divided into 2 member units. The lower half is the highly fossiliferous Lewiston Member; the upper half is the Burleigh Hill Member, a dolomitic shale unit in which fossils are rare. The Lewiston contains the layers that are so well known and collected. The lower and upper portions of the Lewiston contain a diverse and well-preserved fossil community, including the trilobites Arctinurus, Dalmanites, Calymene and Trimerus. The Burleigh Hill shale is dark gray and mostly barren indicating it was a deeper water habitat, however there are occasional thin layers and pockets of mostly disarticulated fossils, though complete Dalmanites and Trimerus can be found.

Further east, the far-from-shore Rochester Shale changes over to a near shore shallower unit designated as the Herkimer Sandstone. This stratum has yielded many excellent trilobite trace fossils, as well as an occasional Dalmanites and Trimerus.

Interestingly, trilobite genera and species differ from whether collected from the east or the west of Rochester, New York.

The following is a list of trilobites reported from the Rochester Shale:
Acanthopyge sp.
Arctinurus boltoni
Bumastus ioxus
Calymene niagarensis
Calymene sp.
Cheirurus sp.
Dalmanites limulurus
Dalmanites sp.
Decoroproetus corycoeus
Diacalymene sp.
Deiphon pisum
Dicranopeltis nereus
Illaenoides sc. I. trilobite
Maurotarion sp.
Odontopleurid
Proetid
Radnoria sp.
Staurocephalus sp.
Trimerus delphinocephalus
Trochurus halli

Family Lichida- Arctinurus boltoni is one of the most beautiful and highly sought after trilobites in the Rochester Shale. Though small specimens are found, they are often 100 mm or longer in length. Arctinurus tend to be 2 dimensionally flat, though the surface of the thin preserved exoskeleton is pustular. Complete specimens of this species are quite rare, most are found from the private trilobite quarry in Middleport, New York.

Family Lichida – Dicranopeltis nereus is another very rare lichid trilobite from the Rochester Shale. D. nereus has a small head with short, genal spines jutting out away from the cephalon. The pygidium has curved short-notched spines in contrast to Arctinurus’ slightly longer straight spines. The exoskeleton is highly pustulose. Dicranopeltis tend to be smaller than Arctinurus, most measuring less than 70 mm.

Family Lichida – Trochurus halli is a small and extremely rare trilobite, so rare that even fragments are rare. Trochurus has long outreaching genal spines, the thoracic segments end in long spines, and the pygidium has 4 long thick spines. The exoskeleton is covered in large pustules or tubercles. Only a couple of these most beautiful trilobites have ever been found. (Not pictured)

Family Styginidae - Bumastus ioxus is easily identified by its 3 dimensional oval shape and smooth texture. The eyes are set far apart, towards the edges of each side. These trilobites range from very small to about 40 mm, though rare larger specimens have been found. The outer edge of exoskeleton preserved the best, the central portions of the trilobite seemed to possess no or very thin exoskeleton. Often specimen molts lacking free cheeks are found.

Family Styginidae – Illaenoides cf. I. trilobite superficially looks like Bumastus, however the cephalon to Illaenoides is much larger, and the pygidium has a flat broad border that Bumastus does not possess. The exoskeleton seems to be thicker as well.

Family Calymenidae – Calymene niagarensis is one of the common Rochester Shale trilobites. Specimens rarely exceed 40 mm in length. The exoskeleton of C. niagarensis is typically black and finely pustular. In my experience, complete specimens are more common than molts. Often the free cheeks are pushed underneath the cephalon, giving the appearance that the specimen is cheekless.

Family Calymenidae – Calymene sp. is found in the eastern portion of the Rochester Shale in Wayne County. Compared to C. niagarensis, this Calymene sp. possesses smaller eyes, the glabella is more tapered anteriorly, and the cephalon is covered pustules of varying sizes.

Family Calymenidae – Diacalymene sp. is represented by 2 different undescribed species, both collected from the eastern portion of Rochester Shale from Wayne and Monroe Counties. Diacalymene can be recognized by the triangular shape of the cephalon and the longer, narrower overall shape of the trilobite. (Not pictured)

Family Dalmanitidae – Dalmanites limulurus is one of the common Rochester Shale trilobites. It is easily recognizable with a wide semicircular head, long genal spines, and broad, tapering pygidium that terminates with a short spine. Exoskeleton material is best preserved along the edges of a specimen; the central thorax often possesses no exoskeleton. Dalmanites may be small, but most average about 60 mm in length. Dalmanites limulurus have been split into 2 subspecies: the western ( typical specimens seen from Middleport, New York) are Dalmanites limulurus limulurus; the specimens found in eastern Rochester exposures are designated Dalmanites limulurus lunatus . D.l. lunatus have a shorter pygidial spine, the anterior process that extends forward from the cephalon is wider and longer, and the free cheeks are narrower. Another yet unidentified Dalmanites species has been found in Wayne County, New York.

Family Homalonotidae – Trimerus delphinocephalus is a rare and easily recognized trilobite. The cephalon and pygidium are large, long and smooth. The cephalon is triangular, the eyes are small; the pygidium is triangular, ending in a point. Though fragments and partial specimens are most often found, complete specimens are spectacular, some have been found exceeding 150 mm!

Family Brachymetopidae – Radnoria sp. generally measure 20 mm or smaller. The exoskeleton is typically thick, the cephalon exhibiting long genal spines, closely set eyes, and the pygidium is semicircular and broad. Some literature indicates that Radnoria are often associated with the bryozoa layers. Radnoria is presently being described and a species name will soon be assigned.

Family Proetidae – Decoroproetus corycoeus is a very rare and highly sought after trilobite. At first glance it may resemble Radnoria, however Decoroproetus is more elliptical rather than oval, the eyes are much larger and set more forward, and the pygidium is smaller and more triangular.

Acanthopyge sp., Cheirurus sp., Deiphon pisum, Maurotarion sp., Odontopleura sp. and Staurocephalus sp. have been found in the Rochester Shale, but all are known from fragments, no complete specimens have been found.

The question now to be addressed - where can one go to collect these trilobites? The answer is, you will have to do your homework. Most of the exposures are located on private property. The most famous and presently the most active site is a quarry in Middleport, New York. This privately owned quarry’s sole focus is for fossils. Their finely prepared material can be purchased at fossil shows or online at various sites. This location is not open to collectors.

As previously indicated, the Rochester is exposed in creeks and roadcuts, which are typically found on private property. Using topographic maps to narrow in on potential areas, you can then knock on doors and ask for permission to explore the shale. Do not just walk onto somebody’s property – that is trespassing and you may get arrested, plus ruin any chance for anybody to collect on that property in the future!