Published in
Interpreter A Journal of Mormon Scripture.
First-level search seems to indicate that the Hatt find includes clearly Pleistocene
E. conversidens bones "found with" bones of species (including
B. taurus) "introduced" by the Spanish. Whether through enthusiasm, incompetence, or disingenuousness, it is seldom reported that the
E. conversidens bones are in a different layer than the
B. taurus bones; the "with" boils down to no more than Pleistocene horse bones and teeth being found in the same cave as post-columbian Spanish cattle.
http://www.mormonthink.com/backup/horses.htm
http://www.mormonmesoamerica.com/horses.html
There is, in fact, an interesting find about 800 miles from the demesne: In 2005, in a
cueva near the
Agua Hedonia lagoon (near Carlsbad, CA), the bones of a domestic horse and a
burro were unearthed. Carbon dating indicated that the horse, at least, died somewhere between 1605 and 1725 C.E., or some 50 years before
el Missión de San Diego de Alcala (the first of the Spanish missions in Ca) was established.
http://www.utsandiego.com/news/2005/jul/17/centuries-old-bones-of-horses-unearthed-in/
http://www.lrgaf.org/origins.htm
http://www.hannah-steenbock.de/did-wild-horses-survive-in-america-after-all-probably-not/
Do notice, however, the horse and
burro were unshod, and appear to have been solitary; that even the earliest possible date is post-colombian (indeed, well after the Diné, for instance, are recorded to have engaged in horse trading); and that
el Missión de San Diego de Alcala is not the first Spanish expedition to the area, only the first established settlement.
Not to mention, merely finding one anomalous horse skeleton does not support the idea of a mighty horse culture "covering the land" for "thousands of years"; unless you would have us believe in some sort of "Smith Family (Jadeite) Robinson" encountering the last remnant of the horse (and the
burro) in isolation.
But thank you for a diverting half-hour of reading.
When do you think that the mighty civilization, "covering the land" for "thousands of years" (as described in the
BoM) is to be supposed to have taken place, and where do you think it is to be supposed to have been located?