So on to the solution to the other skull. No scale was necessary, though the mosses nearby show that it was a small skull, only a couple centimeters at maximum, which discounts elephant as a possibility. First, are the teeth differentiated? Yep, so it is not a fish, reptile or amphibian - as in those groups all the teeth look the same. Ok, so let's count the teeth (click the picture to blow up). To determine dental formula, you count all teeth on one side of the mouth in each type (incisors, canines, premolars, molars). I see 5 incisors, 1 canine, 3 premolars and 4 molars in this beast. Compare to yourself: 2 incisors, 1 canine, 2 premolars and 3 molars (2 if you have had your wisdom teeth out). Dental formula is important in determining relationships and taxonomic groups.
Now look at the openings on the roof of the mouth (palatal fenestration - literally palate windows). Do you have those? Didn't think so. Now we can conclude that the beast in question is nothing like you - the teeth are different, the skull is structured differently, so what could it be? As I said earlier, it is a mammal, but it is not a group of mammals that most people in the US deal with on a regular basis. There are three main groups of mammals: placentals, marsupials and monotremes. Placental mammals have "normal" pregnancies and comprise most mammalian diversity on earth: us, sheep, cows, mice, capybaras, elk, shrews, hippos, elephants, horses, wolves, whales, etc. Marsupials have pouches, to which very undeveloped babies climb and suckle for months, these include Kangaroos, opossums (the lone marsupial in the USA) and koalas. Monotremes are the whacky mammals, the platypus and the echidna, which lay eggs.
In addition to reproductive strategy, skeletal and dental structure differentiate the groups: placentals have only three incisors, marsupials have up to five, and monotremes have wacky (or no) teeth. Additionally, marsupials have openings in the palate, while placentals do not. Therefore, this photo shows all you need to deduce that the beast in question is a marsupial. A quick google search reveals only two marsupials in Patagonia: Dromiciops (Monito del Monte) and a shrew opossum (Rhyncholestes raphanurus). The opossum has only 4/3 incisors (four upper, three lower), thus this skull must be from Dromiciops).
And what should you know of Dromiciops? Like the Coelocanth, the Tuatara and several other taxa, it is what we call a "living fossil". The last surviving member of a once diverse and widespread (in Australia and Antarctica, back when it was not at the south pole) group, the microbiotheria, it survives only in Chilean and Argentine forests. Not only is it the only remaining species of its genus and family, it is the only one left of its order (compare: our order is the primates, all prosimians [lemurs, et al.] monkeys and apes - thus it would be like just one monkey species is left). Its closest relatives are known from fossils in Australia and Antarctica.
It is a nocturnal omnivore, eating fruit as well as insects and small vertebrates, one each of the wren and rayadito nests here were eaten by Dromiciops, and I occasionally happen upon them in the boxes sleeping for the day. However, they scamper away very quickly, and thus I have not gotten a picture yet, but for good ones, see here: http://www.discoverlife.org/mp/20q?search=Dromiciops+gliroides.