Banner Season for Rare Penn Dixie Trilobite

Three Partial Pseudodechenella rowi Specimens Uncovered

By Jay Wollin, Educator

Anyone who has ever visited Penn Dixie knows first-hand that trilobites, or at least “trilo-bits”, are a common occurrence at the site. Visitors flock from around the globe to try their luck at uncovering one of the site’s treasured complete trilobites. The most common of which by far is the Eldredgeops rana (Green, 1832) which is often still referred to, albeit erroneously, as Phacops rana (1). In addition to the abundant E. rana specimens, visitors are often fortunate enough to stumble across examples of the less common, and undescribed Greenops sp. It is even possible to find the even more uncommon Bellacartwrightia sp. (Lieberman & Kloc, 1997), which is frequently mistaken for a Greenops sp. based on similar features and overall appearance.

Pseudodechenella rowi
Pseudodechenella rowi, a rare Middle Devonian Trilobite found July 2017 by an unknown visitor in the Smoke Creek Trilobite bed.

This season has marked the beginning of a banner year with the uncovering of not just one, and not just two, but three of one of the site’s rarest and most sought after species. The Pseudodechenella rowi (Green, 1838) was first discovered in 1837 in Otsego county, New York by George L. Le Row, and is considered a rare trilobite in all but one locality where it is found (2).

Pseudodechenella_Jessica Laton
P. rowi found by Jessica Laton Hesske in June, 2017 in the Smoke Creek Trilobite bed.

Unlike the other trilobites at Penn Dixie, which are members of the order Phacopida, the P. rowi is a member of the order Proetida. From a cursory examination, it may be difficult to recognize any significant difference between the Phacopids and the Proetids, in fact they share many similarities.

Pseudodechenna_May visitor
P. rowi found by an unnamed visitor in May, 2017 in the Bayview Coral bed.

Trilobites in general were some of the first animals on Earth to develop eyes. The trilobites of the Cambrian period all exhibited advanced compound eye structures known as schizochroal eyes. In these trilobite eyes, up to 700 individual eye lenses with individual corneas separated by sclera are grouped together in rows and files creating compound eye surfaces. Later in the evolutionary timeline, many trilobites developed even more complex holochroal eyes. In these eye structures as many as 15,000 tiny lenses are combined without sclera under one cornea to create the eye surface.

eyeholochroal  eyeschizochroal
An example of holochroal eyes. (Clarkson, 1975) An example of schizochroal eyes. (Levi-Setti, 1993)

It is with these features that we can most easily differentiate the Phacopids from the Proetids. All members of the the order Proetida feature the more common holochroal eyes, whereas the Phacopida—which translates literally to “lens face”—all retained the more primitive schizochroal eyes (3).

While the Phacopids did not survive the end of the Devonian period, the Proetids managed to persist through the Carboniferous and Permian periods as the last remaining order of trilobites, finally going extinct during the great Permian extinction.

Like the Phacopid Greenops sp. and B. whitelyi, the cephalon, or head, features extended and tapered genal spines on either side. However, the P. rowi can be easily distinguished from these other species by its large, smooth, and laterally elongated glabella, or nose.

greenops bellacartwrightia-calliteles-2_pinboarditemoverlay-e1499374418902.jpg pseudodechenella-rowi.jpg
Greenops sp. (American Museum of Natural History) Bellacartwrightia whitelyi (American Museum of Natural History) Pseudodechenella rowi (American Museum of Natural History)

To easily distinguish between the P. rowi and the E. rana trilobites, the glabella can be compared when visible. The E. rana features a broad, stout glabella which is covered in small bumps, whereas the glabella of the P. rowi is elongated and very smooth. In the event that the cephalon is missing, damaged, or simply obscured from view, the pygidium can also be used for identification. The pygidium of the E. rana has pleural furrows that extend to the edge, while the P. rowi has a distinct margin or border between the ends of the pleural furrows and the edge.

57cdf1226778c_20160905_175903(Large).jpg.6f805b2f9abf4d7eabb2aeef7e6ade00-057  pseudodechenella-rowi
Eldredgeops rana (Jay Wollin) Pseudodechenella rowi (American Museum of Natural History)

The increasing number of P. rowi finds at the site has us hopeful that there are many more yet to be uncovered. If you happen to be among the fortunate few to find one of these rarities, we invite you to submit to us your photographs and information so that we can share your fantastic finds with our friends and trilobite fans around the world!

As always, happy hunting!

— Jay


Author’s notes: Special thanks to Gerald Kloc and Karl Wilson for their assistance! Karl Wilson’s book — Field Guide to Devonian Fossils of New York — includes a detailed listing of these trilobites and many other Penn Dixie fossils. The book is available for $18 through the Paleontological Research Institution and is in Penn Dixie’s gift shop.


References:

(1) The Phacops rana classification was changed by Struve in 1990 after a morphologic study of differences between African and North American/Northern European Phacops examples. For further information see this article posted by trilobite expert Gerald Kloc.

(2) Hall, James. Paleontology of New York 7 (1888): 119-122.

(3) Additional information about trilobite eyes can be found here.

Bellacartwrightia: A Singular Specimen

Bellacartwrightia enlarged
Bellacartwrightia sp. trilobite uncovered by Alasdair Gilfillin at Penn Dixie in 2016.

Every so often, one of our visitors uncovers a truly spectacular fossil. The preservation might be perfect, the assemblage of different fossils might be unique, or the type of fossil might be very uncommon. In this case, we present a beautifully preserved and uncommon trilobite called Bellacartwrightia.

Bellacartwrightia side view
Sideview of Bellacartwrightia. Trilobite is approximately 1.5 inches long.

Penn Dixie member Alasdair Gilfillan discovered this trilobite at our park on October 3, 2016. Our dig season was coming to a close and Alasdair decided to spend a weekend visiting us from New Jersey. Alasdair dug into the infamous Smoke Creek trilobite bed of the Windom Shale and unearthed what he thought was a Greenops — an uncommon trilobite that seems to represent one or two of every 100 or so trilobites that are found. Instead, Alasdair found something much rarer. He writes:

You may remember that I found a nice (though at the time partially covered) trilobite which I thought was a Greenops that day. I managed to get it prepped and it turns out that it was a Bellacartwrightia, a much rarer form. The prep guy did a really nice job and it turned out to be a really fantastic specimen. Please find enclosed the photographs. The trilobite is ~ 1.5 inches long.

Alasdair adds that the prep work was done by Bob Miles — a former Penn Dixie board member who also took the photographs. We thank Alasdair for sharing his images and for his donation of many fossil specimens that were used in our school programs.

Bellcartwrightia front view
The Bellacartwrightia cephalon (head) resembles that of Greenops, but the two genera are not closely related.

Bellacartwrightia is uniquely found in the Devonian rocks of the Hamilton Group in New York State. This fossil was first described by Lieberman and Kloc in 1997; the original paper can be downloaded here. Bellacartwrightia was named after the wife of paleontologist Bruce Lieberman, who at the time was a postdoctoral fellow at the American Museum of Natural History. Dr. Lieberman is now at the University of Kansas. The paper explains how Bellacartwrightia is different from Greenops, another trilobite with a somewhat similar appearance. From page 29:

In addition, the members of this genus are phylogenetically distant from species assigned to true Greenops…These two Middle Devonian genera have not shared acommon ancestor since, at latest, the Siegenian [approx 411 million years ago], based on an analysis of ghost lineages. To treat these species as members of a genus Greenops would necessitate placing all of the asteropyginines within the genus Greenops.

There you have it — a new genus of trilobites first documented in 1997 and one of our members finds an excellent specimen 20 years after the discovery!

Bellacartwrightia
Bellacartwrightia in the host rock — Windom Shale.

Alasdair was kind enough to share additional photos of the Bellacartwrightia as well as some of his other treasures from Penn Dixie. Our visitors are welcome to keep any fossils that they find, but we do appreciate photos of particularly cool fossils for use on our website.

Phacops rana double plate
A plate of Phacops rana trilobites found in 2015.
Phacops rana single plate
A single Phacops from 2016.
Phacops rana enlarged
Phacops trilobite. Prep work by Bob Miles.

For further reading, here are some links:

Evolutionary and biogeographic patterns in the Asteropyginae (Trilobita, Devonian) Delo, 1935 on AMNH

Bellacartwrightia whiteleyi on AMNH

Textbook Bellacartwrightia on Trilobites.com

Bellacartwrightia on fossilmuseum.net