From the collections room (Calippus)

I recently received an email requesting information about fossil horses from Virginia. To my amazement, I discovered that I had never done a blog post on the Carmel Church horse; a truly remarkable achievement considering how much I talk about Carmel Church!

Our first horse at the quarry was discovered by Brett during our July 2001 excavation. I only have one photo of the specimen in the ground, and it doesn’t look like much:

The arrows are pointing to what appeared to be two teeth. These could have easily been overlooked, but Brett had fortunately spent time working in the paleontology lab at LSU where she became familiar with Miocene horses.

We had never before found a land mammal in place at Carmel Church (we did have a single tooth fragment found as float), so we jacketed a large area around the two visible teeth. We ended up getting the material shown at the top of the page, partial maxillae with portions of 10 teeth preserved. This may not look like much, but it’s the most complete fossil horse known from Virginia.

Identification of Miocene horses is a bit tricky for someone who isn’t a horse expert. During the middle Miocene horses diversified like crazy, probably associated with the spread of grasslands at around the same time. Given the myriad horse species known from the middle Miocene, which one did we have? After a lot of reading, several visits to other museums, and consulting with Richard Hulbert at the Florida Museum of Natural History, I eventually reached the conclusion that the Carmel Church horse was from the genus Calippus. There are two species of Calippus known from that part of the Miocene and that are close to the size of the Carmel Church horse, C. regulus and C. proplacidus. The proportions and morphology of the teeth in the Carmel Church specimen are closer to C. regulus, but the Calvert Formation is closer to the age one would expect for C. proplacidus. Given the morphology, I think C. regulus is a little more likely. We could be more certain of the identification if I ground one of the molars down to see the enamel pattern in cross section, but I’m not willing to do that!

Calippus is a fascinating horse. The genus first appears in the middle Miocene, and is most common in the midwest, with a number of specimens known from Nebraska and Texas, although it never appears to be the dominant horse in any deposits. Several teeth are also known from Florida, but outside of those specimens the Carmel Church skull is the only record of Calippus from the east coast.

Calippus had high-crowned teeth, which is an adaptation often attributed to a grass diet (although in fact high-crowned horses seem to have eaten whatever they wanted). But while nearly all the horses were evolving toward much larger body sizes in the Miocene, Calippus reversed that trend. It’s a really tiny horse, dwarfed by its Miocene cousins.

Calippus also has a rather unusual skull morphology. The most remarkable feature is the tip of the muzzle (not preserved in our specimen) which is wide laterally and squared off at the tip (specimen below from the University of Nebraska State Museum):

Examination of the Carmel Church specimen also revealed that the horse was not fully grown. The premolars (the two leftmost teeth on each side of the mouth below, plus the partial tooth in the upper left) are actually deciduous teeth. The first permanent molar (second from the right) was just starting to wear, and the second molar (rightmost tooth) had not erupted. The third molar had apparently not even begun to form. Assuming that Calippus grew at about the same rate as modern horses, that would make this specimen about one year old when it died.

In 2003 Bryce Harrison found a second horse specimen at Carmel Church. This time it was an isolated right third metatarsal (foot or cannon bone, seen below in anterior, posterior, medial, and lateral views).

The postcranial anatomy of Calippus hasn’t been described, and I was unable to locate any specimens of metatarsals definitely associated with Calippus skulls, so I’m not sure what species of horse this represents.

There is one final irony surrounding the discovery of Calippus at Carmel Church. This tiny horse, one of the oldest and most complete fossil horses ever found in Virginia, was found only five miles from the birthplace of the racehorse Secretariat, arguably the most famous of all horses. Secretariat’s weight of 1,200 pounds forms a striking contrast to the estimated 90 pound body weight of an adult Calippus, and emphasizes how much horses have changed over the last 14 million years. Secretariat and Calippus are both featured in the Carmel Church exhibit at the Caroline County visitor center.

Reference:

Dooley, A. C. Jr., 2007. Barstovian (middle Miocene) land mammals from the Carmel Church Quarry, Caroline County, Virginia. Jeffersoniana 18:1-17.