Virginia has a remarkably complete rock record, at least for the Phanerozoic Eon (the last 541 million years or so). With the exception of the Permian Period, every Phanerozoic time period is represented in Virginia by at least a few fossiliferous rocks. However, different groups of organisms are not evenly distributed through Virginia’s rocks. For example, almost all of Virginia’s Carboniferous fossils are terrestrial plants. Almost all of Virginia’s vertebrate fossils come from the Triassic, Paleogene, and Neogene periods. But there are a few exceptions, including the one shown above.
This small fossil is the holotype specimen of Bothriolepis virginiensis, first described by Weems et al. in 1981. Bothriolepis is a placoderm, a group of armored fish found only in Silurian and Devonian deposits. This specimen (USNM 265220) was collected in the Upper Devonian Fourknobs Formation (Chemung Formation in older literature) from a roadcut near Winchester, Virginia, along with 7 other specimens referred to the same species, all of which are housed at the U. S. National Museum. At roughly 380 million years old, these are the oldest known jawed vertebrates, or gnathostomes, from Virginia. (There are conodont elements, which are almost certainly from jawless vertebrates, going back to at least the Ordovician in Virginia. That’s a post for another day!)
In most placoderms the head was encased in bony armor plates, and it’s these plates that are usually preserved as fossils. Here’s a reconstruction of Bothriolepis from the American Museum of Natural History:
Here’s the same reconstruction with the approximate preserved part of B. virginiensis outlined in red:
There are a bunch of species of Bothriolepis, and as with many species-rich genera there is some question about the validity of many of these species. Thomson and Thomas (2001) suggested that B. virginiensis was actually the same species as B. nitida, but in a follow-up paper Weems (2004) reexamined B. virginiensis and concluded that it was a distinct species from B. nitida, based mostly on the proportions of the headshields.
Last year the Shenandoah Valley Discovery Museum transferred their paleontology collections to VMNH. Most of these were Cretaceous fossils from Montana, but there was a small collection of additional fossils including several trays of Devonian specimens from the Winchester area. One of these turned out to be part of the cranium of Bothriolepis, collected by John Happ in 1997 (which is now cataloged as VMNH 120143):
The preservation of this specimen is interesting. This is about half of the headshield, specifically the right half. It’s flipped over, so your actually looking at the inside of the top of the head. The front of the head is toward the top, and the oval opening in the center is the orbital foramen for the eyes (both eyes share the same skull opening in Bothriolepis). In some places around the edges the bone has been lost, but the dimpled surface has left an impression in the sediment that is still visible.
Given that the VMNH specimen is from Winchester, there’s a very good chance that it’s B. virginiensis. However, since there are so many different Bothriolepis species it would be nice to have some additional confirmation of this identification. Fortunately, Weems (2004) largely used headshield measurements to identify B. virginiensis, and that’s what we have preserved in the VMNH specimen. The three most significant measurements are shown on the VMNH specimen below:
The yellow line is the overall length of the headshield. The blue line is the combined length of the nuchal and postpineal plates on the midline. The red line is half the maximum width of the headshield; Weems actually used the total width of the headshield, but since ours was incomplete we measured the half-width and multiplied by 2.
Using these measurements, Weems graphed the nuchal+postpineal length against the headshield width, with both measurements divided by the headshield width (essentially, this normalizes the measurements to adjust for different overall body sizes). Using these measurements he showed that B. virginiensis and B. nitida fall on different, non-overlapping parts of the graph:
Modified from Weems, 2004.
Notice the blue dot that I’ve added to the graph. That’s the VMNH specimen, which falls out in the B. virginiensis part of the graph, just as we suspected it would. Given all this, I feel pretty confident in referring this specimen to Bothriolepis virginiensis.
Weems, R. E. 2004. Bothriolepis virginiensis, a valid species of placoderm fish separable from Bothriolepis nitida. Journal of Vertebrate Paleontology 24:245-250.
Weems, R. E., K. A. Beem, and T. A. Miller. 1981. A new species of Bothriolepis (Placodermi: Bothriolepidae) from the Upper Devonian of Virginia (USA). Proceedings of the Biological Society of Washington 94:984-1004.
Thomson, K. S. and B. Thomas. 2001. On the status of species of Bothriolepis (Placodermi, Antiarchi) in North America. Journal of Vertebrate Paleontology 21:679-686.