Friday Find: Fossil Horse (Correction appended)

Gavin Sutter had no problem describing "What I Did Over the Summer" when he started 2nd grade. In July, the eight year-old Californian went on a fossil dig in Nevada with a local museum team. His mother found the antler of an early deer, others found rodents, canids, rhinos, turtles and mustelids.

Little Gavin filled a gap in the fossil record. The teeth and bones shown here represent a three-toed horse, two more toes per foot than horses use today, and fewer than the ancestors of all horses had.

The description in news articles doesn't include a genus name or even a clear description of the systematic position of the fossil. It has, however been dated to 15 million years ago, which was a critical time in horse evolution, as the images here illustrate. Around 20 million years ago, three-toed horses did not possess the characteristic teeth of modern horses, but the skull shape was closer to what we think of as horse-shaped.

Update: Dr. Hilton, a paleontologist who has examined the specimen, believes it is from something like Parahippus. That means that this specimen comes from a key point in horse evolution. Teeth are transitional between earlier groups and the modern horse shape, and the foot was transitional between the form necessary to walk on three toes to a shape that allowed the horse to run on only the middle toe. More on that anon.

By ten million years ago, one-toed horses were the dominant group, and other groups were going extinct. In between, the horse radiation seems to have been quite diverse, yielding genera like Merychippus, regarded as true equines and marking the beginning of the speedy plains-dwelling group of horse ancestors, as well as smaller genera like Protohippus.

There was a third clade in existence at the time, referred to as hipparions. This represented at least 16 species in four named genera, and the clade managed to spread from North America into Europe and Asia. Hipparions had three toes, but the outer two did not reach the ground.

I can't find a publication describing the specimen, which would have given us enough context to know where this ancestral horse fits relative to known species. Indeed, it would have clarified whether this specimen represents a new species at all. One news report does describe the specimen as having "helped fill a key gap in the evolution of the horse," but do we trust New Zealand's "Horse Talk" to nail those scientific details?

We do know that Richard Hilton, of Sierra College, was part of the expedition. Hilton is, among other things, the author of Dinosaurs and Other Mesozoic Reptiles of California. I've sent him a request for more information, and hope to hear back soon.

Update: Dr. Hilton writes that Horse Talk got it wrong. The find "confirms what is known but adds a bit in the history of mammalian paleontology and geologic history of NW Nevada."

The find does not "fill a key gap in the evolution of the horse." Dr. Hilton explains that the fossil will be be used as an educational specimen to illustrate some key points about young Gavin's find:

His find is interesting as the fossil bones were picked up in a lahar (volcanogenic mudflow) 15 million years ago and show the violence in which they were transported and finally deposited. His find is special because it was found by such a young boy.

Thanks to Dr. Hilton, chairman of the Sierra College Natural History Museum, for his help.