Richard Owen had expertise in comparative anatomy, and his professional judgements revealed a succession of similar species in the same locality, giving Darwin insights which he would later recall as being central to his new views. Owen met Darwin on 29 October 1836 and quickly took on the task of describing these new fossils. At that time the only fully described fossil mammals from South America were three species of "Mastodon" and the gigantic "Megatherium". On 9 November, Darwin wrote to his sister that "Some of them are turning out great treasures." The near-complete skeleton from Punta Alta was apparently very closely allied to anteaters, but of the extraordinary size of a small horse. The rhinoceros-sized head bought for two shillings near the city of Mercedes was not a megatherium, but "as far as they can guess, must have been a gnawing animal. Conceive a Rat or a Hare of such a size— What famous Cats they ought to have had in those days!"