For the last several weeks several activities have been keeping me busy, but out of the field and the lab. Specifically, I’ve been working on four manuscripts (some with deadlines), a poster for the SVP meeting, several lectures, and our upcoming Ordovician exhibit that’s due to open next year. This has made it difficult to keep up with blog entries; not so much because of time, but because it’s not terribly interesting to say “I spent the last two days tracking down references”. That’s about to change, as I’ll be attending both the GSA and SVP meetings this fall and should have lots to write about. In the meantime, my background work on the Ordovician exhibit does give me some stuff to talk about.
Many of the Ordovician rocks in Virginia are relatively deep-water sediments deposited in the Appalachian Trough, a foreland basin associated with the North American-Taconic collision (see JMU’s Geologic Development of Virginia page for a more thorough description). The fossils in these units include a mix of pelagic animals that lived in the water column and sank to the seafloor upon death, some shallow-water taxa carried into deeper water by turbidity flows, and deep-water animals living on the seafloor. Many of these deep-sea animals were trilobites.
The image at the top of the page is the cephalon (head) of Cryptolithus tesselatus; this particular specimen is from the Martinsburg Formation in Pennsylvania. Cryptolithus is a member of a group of trilobites called the trinucleids, which had large, domed cephalons and small bodies. Fortey and Owens (1999) suggested that these trilobites were specialized filter feeders, stirring up sediment from the seafloor and filtering out food particles under the domed cephalon.
The Appalachian Trough was deep enough that the seafloor was well below the photic zone (the depth to which enough sunlight penetrates for photosynthesis to occur–about 100 m in clear water). It is therefore interesting to note that many of the Appalachian Trough trilobites, including Cryptolithus, had no eyes! This is apparently a derived condition, since older Cambrian trilobites almost all had eyes. In other words, these trilobites secondarily lost their eyes. It is worth noting, however, that at least some eyeless filter-feeding trilobites are found in shallower water, so perhaps a filter feeding lifestyle also has an influence on the loss of eyes in trilobites.