oh man so much confusion, and everyone's grifting, and I kind of suspect that the paper itself relies on ignorance to stir up a little controversy.

Iberian horses moved from south to north. English horses from east to west. This should be obvious why. Spaniards landed in Mexico, with the landmass northward, which is also the direction of their exploration. Anglos landed on the east coast and explored westward, the whole Oregon trail business. There's a point sometime in 1700s when English riding anglos were interacting, trading and warring with Iberian riding indigenous peoples.

What this paper focuses on is the details of south to north Iberian movement. The specific scientific arguments that it's making, supported by phylogeography and archeology is that the adoption of the Iberians by the indigenous tribes with some probabilities happened sometime between 1516 and 1599, and that with some probability dates of samples in specific states fall in some period between 1600 to 1700.

Next there are two simplifications that exist in general lore, and arguably in historic lore. The first one is the idea that horses were introduced by the Anglos on their westward expansion of the American frontier. The second is that Iberian horses were introduced to indigenous tribes predominantly during 1680 pueblo revolt (in future new Mexico), when a whole bunch of Spanish horses were stolen.

The first simplification exists because there's a lot of cowboy movies, and it's just a pop understanding. Nobody really cares about horses anymore. But equestrians know that there's at least vaquero tradition, and Baja California is one of the last surviving places, where men ride Iberian horses in Spanish style. vaquero ranching was dominant on the west coast, it was never "erased", but it was forgotten along with cowboy ranching in general. little known fact, but "buckaroo" is English riff on "vaquero".

The second simplification is the strawman that this paper for some reason decides to construct. It references a 2017 book, where the 1680 pueblo revolt is presented as a kind of turning point, where downtrodden Indian slaves, who were all trained by Spanish, stole some Spanish horses and rode into various tribes. The problems is that 1910 and 1930 articles that this paper references, and the first person accounts that the articles reference themself, despite being written by colonists and cowboys, were still written by serious equestrians so they don't even remotely support this comedic take. Possibly the Pueblo revolt narrative became dominant recently as an attempt to give agency to indigenous peoples.

So now one is ready to understand the political point that this paper makes: between 1600 and 1700 Indians have developed their own sacred equestrian tradition, and that this happened before and independent of Pueblo revolt and definitely by the time the anglos crossed the continent. The problem is that any serious equestrian (I dunno about anthropologists though) already agrees with this conclusion. Existing historic papers don't disagree with this conclusion either, and support it at least in terms of "yeah we went there and they already had horses".

From this perspective Charles c mann is just straight up grifting, trying to imply that indigenous equestrian tradition existed even before the Spanish, and there's a lot of people in his comments that decided it's a done conclusion.

And the fact that some North American horses didn't come east to west is also a well established fact, that unfortunately very few people know, but it's not a novel discovery this paper made.