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 whitelyi (Lieberman & Kloc, 1997), which is frequently mistaken for a Greenops sp. based on similar features and overall appearance.
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).
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.
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.
|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 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.
|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!
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.
(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.