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ON THE TRACK OF LATE-SURVIVING HORSES IN THE NEW WORLD

A family of North American stilt-legged horses (Haringtonhippus francisci), one of the last New World horse species.

 

Many readers of this newsletter are already aware of the fact that the horse family, Equidae, arose on this continent around 60 million years ago, and that different relatives of the modern horse Equus caballus lived here through all subsequent epochs—until about 11–12,000 years ago, when they apparently died out, along with mammoths, sabretooth cats, ground sloths and many other species in the so-called End-Pleistocene Extinctions (EPEs). That’s the conventional story, but let’s ask a couple of critical questions. First, how do we know when the EPEs occurred? Second, what is the evidence that all New World horses died out at this time?


When the extinctions occurred is mostly based on the radiocarbon (RC) record, which is in turn based on dated macrofossils (bones and teeth). In large-scale RC dating studies, if the dates obtained show a clear break point—i.e., nothing younger than “x” years—it is assumed that the result must be meaningful, presumably because the animals in question had disappeared. That kind of logic is fine as far as it goes, but how do you know that you have sampled enough to be sure of your conclusion, and how would you test for that?


Here is another item to ponder, which concerns the meaning of a given RC date. It is not “absolute”, but is instead a statistical expression that provides a topline estimate of age in years before present (yr BP), which by convention is measured from the year 1950. The topline is properly accompanied by another estimate called the error (all statistical expressions have associated errors). (In length, radiocarbon years are close to, but not the same, as calendar years; for simplicity, in this blog we will assume they are identical.) For example, an age of 15,400 ± 40 yr BP on a sample means that the sample’s true age most probably falls within the range 15,360 to 15,440 yr BP. To increase the likelihood that the “true” age been captured, scientists often double or triple the error estimate. (In the example, using 3x the error would yield a spread of 280 years, 15,520 to 15,280 yr BP.)


This long explanation was necessary because it bears on the second question, how do we know when New World horses finally disappeared? What if some populations have been missed by scientists, and how would their inclusion affect the estimate of the extinction date? The obvious way to increase confidence is to look for younger macrofossils, but that’s easier said than done. Horse bones in paleontological and archeological sites don’t come with convenient labels telling you when their owners passed away. What you have to do is to look for fossils coming from contexts that you have good reason to believe were closer to the present than the EPE itself, and get busy sampling and dating.


In a new research initiative, RAN is going to be doing exactly that. But rather than blindly look for new, unexploited sites, we want to begin with horse fossils that already exist in university and museum collections. A lot of this kind of material already exists in these repositories, but 99% of it has never been dated. This is partly because of cost but also partly because “the problem is already solved!” in the minds of most paleontologists. How wrong that notion may be is clear from a 2021 study by Tyler Murchie, Hendrik Poinar and their colleagues (including me), in which DNA of horses and mammoths was detected in permafrost sediment cores at a dated level that in one case corresponded to roughly 5,700 years ago. That’s half a millennium later than the point at which all New World horses were supposed to have disappeared. It’s an exciting discovery, but as DNA cannot be directly dated, we can only say an equid of some sort lived in Yukon in the middle of our present geological epoch (the Holocene).


Petroglyphs of the Jaguar Cave, Idaho


Another and much more critical problem with the late survival scenario is that there is no supporting macrofossil evidence for so late a date for native horse persistance. As I noted, we start running out of radiocarbon dates for successively younger horse fossils around the 11–12,000-year mark. If horses existed in North America for another 5,000 years, why haven’t their bones been found? The fact that no physical bones or teeth of similar age have been identified effectively means that the Yukon study is an isolate, a possible anomaly. What can be done to resolve this issue?


This is where those university and museum collections I mentioned come in. Here I want to discuss fossils from one particular site. Jaguar Cave is a famous archeological site situated in the southern Bitterroot Mountains of Idaho—serendipitously, right across Birch Creek Valley from some of RAN’s grazing allotments. The cave was occupied several times prehistorically. This is indicated by the presence of artifacts and other signs of human presence, such as hearths and associated plant and animal remains, the latter including bones and teeth of horses and other large mammals. Radiocarbon analyses, obtained on charcoal sampled from two of the hearths in the early 1960s, yielded ages of 11,580 ± 250 and 10,370 ± 350 radiocarbon years before present (roughly equivalent to 9,600 BCE and 8,410 BCE). For a while, the second date was the youngest ever recorded for a site containing evidence of horses and other extinct megafauna anywhere in North America.


Obviously, the Jaguar Cave charcoal dates are not close to the Yukon date indicating late survival, and the older date of the two dates overlaps the EPE. But there is a more fundamental problem to identify: it was charcoal that was dated, not the horse fossils themselves. (It was not possible to reliably date bone in the 1960s.) On top of this, the errors are so large that, at 3x, both of the Jaguar Cave age determinations would completely overlap the EPE and therefore provide no evidence for late survival.


Entrance of the Jaguar Cave


A new study of the Jaguar Cave horse material is therefore needed. Here is our plan. The fossils are part of the collections of the State University of Idaho’s Natural History Museum in Pocatello, Idaho. We are collaborating with the museum’s curators to date as many of the Jaguar Cave horse fossils as we can, as it is a deeply stratified site. The results will tell us whether the benchmark extinction date for horses in this part of the country may need to be modified. If that turns out to be the case, and the fossils are in good condition, then we may be able to go one novel step further, by extracting DNA and identifying which species (or better, species group) the Jaguar Cave horses belonged to.


At present all of this is quite theoretical, but we wanted to give our readers a preview. Whatever the outcome in this particular case, there are a number of other fossils in other collections that are tantalizingly similar in supposed age to the younger Jaguar Cave date, and we will attempt to date (or re-date) as many of them as possible. Younger ages would mean that the extinctions took place over a much longer interval. The last of the native American horses may not be in the direct line of descent of Equus caballus, but the more we know about them the better the science will be.


Stay tuned!



Ross MacPhee PhD, RAN Director of Science



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