A common question I get from the public is “How do you identify a species?”. It happens that I’ve been trying to make a lot of identifications in preparation for my SEAVP talk, so this seems a good time to give an example.
The skull shown above is the most complete odontocete (toothed whale) skull we’ve ever found at Carmel Church. The views, from top to bottom, are dorsal (top), ventral (bottom), left lateral (side), and right lateral. For many years, this type of long-snouted skull would have been referred to the European genus Eurhinodelphis, and in fact most museum displays will have these skulls labeled as Eurhinodelphis. In 2005, Olivier Lambert showed that some specimens referred to Eurhinodelphis actually had a quite different skull structure from the genotype, and erected the new genus Xiphiacetus for those specimens. As it turned out, most specimens, including all the North American ones, belong to Xiphiacetus, while Eurhinodelphis is only known from the North Sea Basin in Europe.
Xiphiacetus is probably the most common odontocete in the Calvert Formation, and it’s also the most common at Carmel Church, with a minimum of four individuals. But there are two species of Xiphiacetus known from the Calvert, X. bossi and X. cristatus. Which species does this skull represent?
When a new taxon is described, a required part of the description is the diagnosis, which is just a list of characters that make the new taxon unique. Here are the first two (out of six) characters that Lambert (2005) used to distinguish X. cristatus (p. 213):
“Emended diagnosis. This species differs from Xiphiacetus bossi in the following combination of characters: major thickening of the maxilla on the supraorbital process; posterior margin of the maxilla on the cranium notched by a forwards indentation of the frontal and the supraoccipital laterally to the vertex;…”
Both of these characters involve the maxilla, the bone that holds most of the teeth in the upper jaw. In whales, the maxilla also extends onto the top of the head (a condition known as telescoping). Here’s a larger view of our Xiphiacetus skull, in dorsal view:
Lambert’s second character mentions a notch along the back edge of the maxilla, caused by a forward projection of two other bones (the frontal and supraoccipital). Zooming in on the posterior edge, that notch is clearly visible (left side of image):
Using Lambert’s diagnosis, this skull is clearly an example of Xiphiacetus cristatus, rather than X. bossi.
What about our other Xiphiacetus specimens from Carmel Church? We have two other partial skulls, but neither is as well preserved as this one. Both have some thickening of the maxilla, but not as pronounced as in this specimen. The posterior margins of the maxilla are badly damaged in both specimens, and I haven’t yet been able to determine if they are notched. For now, they both have to be considered Xiphiacetus sp.
If you’re in Martinsville, the Xiphiacetus cristatus skull is on exhibit in the “Uncovering Virginia” hall.
Lambert, O., 2005. Review of the Miocene long-snouted dolphin Priscodelphinus cristatus du Bus, 1872 (Cetacea, Odontoceti) and phylogeny among eurhinodelphinids. Bulletin de l’Institut royal des Sciences naturelles de Belgique, Sciences de la Terre 75:211-235.
I’m still making incremental changes to the blog, so you may see some experimentation with various things in the coming months. If you use the archive quick links, you’ll find that I’ve updated the Carmel Church page to include all my pages on the site, including individual specimens as well as excavation updates. (I noticed that just over 1/3 of my pages have been about Carmel Church; I think I need to write about some other things!)