by Outreach & Communications Coordinator Brittney Elizabeth Stoneburg
Last week the Western Science Center released a new paper about Micoene horses from Southern California in UC Berkeley Museum of Paleontology’s open access journal PaleoBios. This research has given us a clearer picture of what the area around San Bernardino National Forest looked like millions of years ago, and all it took was a few small horse teeth.
Excavations have been going on in the Cajon Valley Formation for years, but in 2018 Western Science Center researchers were granted a permit by the U.S. Forest Service. It dates back to the Miocene Epoch, which spanned 23.03 to 5.333 million years ago, and the Cajon Valley Formation fossils in the WSC collections are approximately 16.5 to 14 million years old. Horses living during this time period were much smaller than modern horses (a newspaper article on paleontologist John Merriam’s excavations nicknamed them “doll ponies” in 1913) and had three toes on each foot, unlike modern horses, which only have one toe on each foot.
The field site in 2018, with specimen positions marked. Lead author Brittney Elizabeth Stoneburg for scale.
We eventually identified three species of horses from the WSC sample - Archaeohippus mourningi, Scaphohippus sumani, and Parahippus brevidens (or as I tend to refer to their respective sizes in my head, tall, grande, and venti!). Each of them helped us gain a clearer understanding of prehistoric California, and what the environment there was like millions of years ago.
Teeth from the WSC Cajon Valley Formation sample. A. Parahippus brevidens. B. Archaeohippus mourningi. C. Scaphohippus sumani.
While it wasn’t particularly surprising to have found several Ar. mourningi specimens, as these tiny horses had been reported before in the formation, Scaphohippus and Parahippus ended up being a more complicated story.
Scaphohippus, for example, was originally separated into two species - S. sumani and S. intermontanus. As mentioned, you can identify many different species of horse based primarily on their teeth, and Scaphohippus is no different. S. intermontanus was said to have a less complex tooth surface than S. sumani, and was mostly known from the nearby Barstow Formation. But as we visited museum collections and tried to determine which species of Scaphohippus we had in our sample, we grew increasingly confused. These horse teeth looked so similar that it was incredibly difficult to tell the two species apart! Finally, instead of comparing individual teeth, we decided to look at whole tooth rows, and that sparked a question.
We had just assumed that the idea of there being two species of Scaphohippus was correct, but what if it wasn’t? What if the reason the teeth looked so similar is because they came from the same species?
How the realization felt to the authors.
Many of the fine details that help us identify horse teeth appear with a certain amount of wear, or in a particular tooth position. In Scaphohippus, the defining features are called “plications”, small enamel folds in the center of the tooth that increase in number as the horse would wear the tooth down with use. The number of plications in a particular tooth was part of what differentiated S. sumani from S. intermontanus, but when we looked at the tooth rows, we realized this difference wasn’t consistent enough to base an entire species on. And so our question was answered - our specimens belonged to S. sumani, because we no longer considered S. intermontanus a separate species! (S. sumani was named first, and so that name had what’s called “taxonomic priority”).
After that, it was time to turn our attention to a particularly strange little tooth that didn’t fit either Archaeohippus or Scaphohippus. Shaped like an ear lobe, this molar was markedly different from all the other teeth we had collected.
We searched high and low for an identification, but no previously published specimen from the Cajon Valley Formation matched. Then, at last, we stumbled on Parahippus brevidens, a Miocene horse species that had first been identified in the Mascall Formation in Oregon. Our tooth matched the holotype (the fossil a species’ description is based on) almost exactly.
Parahippus brevidens had been mentioned here and there in databases with Cajon Valley Formation specimens, but this paper is the first research confirming that these horses did indeed live in Southern California. This has extended the known range for these animals by about 400km and means they appeared up and down the west coast, at least from Oregon to Southern California! We learned so much new information about this horse species and its range, all from a single tooth.
So we figured out what three species of our horse we had in our sample - great! But what does that mean?
A faunal comparison between the Cajon Valley Formation and the Barstow Formation. Adapted from Figure 7 of Stoneburg et al (2021).
It means that this research is another piece in the puzzle of figuring out what California's ancient past looked like. Knowing now that we have three species of horses in the same place gives us a clearer picture of the Cajon Valley Formation's ecological profile. For example, we can now more accurately compare and contrast the formation’s fauna with the well-studied Barstow Formation, which is similar in age to the Cajon Valley Formation and is only around 65 miles away. Yet we see these consistent differences in the fauna found in each formation - for example, while Parahippus brevidens is now confirmed to have been found in the Cajon Valley Formation, it has yet to appear in the Barstow. Larger horses, like Hypohippus affinis and Megahippus mckennai, appear in the Barstow Formation, but haven’t appeared in the Cajon Valley Formation. What caused these faunal differences? Was there some sort of geological barrier impeding movement? Were the environments markedly different despite the proximity? We don't know quite yet, but these are the types of questions we can start asking.
There’s still so much we don’t know about the Cajon Valley Formation, and California during the Miocene as a whole, so every bit of research helps expand our understanding of a long gone environment. You can definitely expect more research about the “doll ponies of Southern California” to come!
