The fossils from near Santa Fe included the horse tooth which had puzzled Darwin as it had been previously thought that horses had only come to the Americas in the 16th century, close to a "Toxodon" tooth and a tooth of "Mastodon andium" (now "Cuvieronius hyodon"). Owen confirmed that the horse tooth was of an extinct South American species which he named "Equus curvidens", and its age was confirmed by a corroded horse tooth among the Punta Alta fossils. This discovery was later explained as part of the evolution of the horse.