The Argentine territory appears to us with its own faunal characteristics since the last times of the Mesozoic era; before this time the layers of the earth offer us only hints of marine fauna, more or less similar to those discovered in other countries; the cretaceous formations instead contain the remains of a terrestrial fauna rich above all in reptiles, including dinosaurs of gigantic size such as Argyrosaurus, whose length was approximately 40 meters, of carnivorous species such as Genyodectes, and small crocodiles of the genus Notosuchus. The remains of the Miolania are attributed to the same geological period, huge horned tortoise, similar to others found in more recent strata in Australia. There has also been talk of Cretaceous mammals in Patagonia, but the most modern investigations tend to show that this group did not appear in Argentine soil until the Paleocene, in which there are already various types of highly evolved Mammals, including Ungulates such as Colpodon and Notostylops, some of them large as Pyrotherium ; or also numerous Toothless and Marsupials similar to the current Dasyuriae of Australia, together with and with probable precursors of the Didelphidiae Americans. This fauna, both as a whole and individually in most of its components, cannot be compared to that of any other country, and its sudden appearance in Patagonia suggests that it probably immigrated from some disappeared region, perhaps from a transpacific continental bridge.
According to itypeusa, the bradyseisms that constituted the so-called Patagonian formation were not intense enough to cancel that terrestrial fauna, and its types evolved through the tertiary periods, while the whole retained its own character until the most recent Pliocene, that is when others began to spread. forms coming from the north, such as Carnivores, Ruminants and large Proboscidians. The vital competition that established between these immigrant species led to the extinction of many of the types that could be considered as native: the first to disappear were the carnivorous Marsupials of the Dasiuroidei group, and the enormous birds of the Phororhacus group ., which for their characters were among the waders and birds of prey and for their size exceeded the largest ostriches. The Pampean fauna, characteristic of the Argentine Pleistocene era, was represented by a mixture of forms recently immigrated from the north, such as mastodons, deer, llamas, horses and felines of the Smilodon genus, as well as the last specimens of the previous fauna, including Ungulates such as Macrauchenia or the Toxodon and numerous Toothless, most of them real giants compared to the current species, such as the Megatherium, the Milodon and the different genera of Gliptodonti, the latter covered by an enormous almost spherical armor similar to that of the current armadillos. The skeletons of all these animals, which today occupy almost half of the halls of the museum in La Plata, give an idea of the richness and variety of that fauna, to which only the one currently inhabiting central Africa could be compared. This comparison, of course, refers only to the number and variety of species, since it is more than evident that there was no immediate relationship between the African and South American faunas, nor any direct migration between the two continents.
The geological or climatological event that determined the rapid disappearance of the Pampean fauna is not yet known with certainty, but considering that many of those large mammals (Megaterio, Mastodon, etc.) were organized for an eminently herbivorous regime, we can only affirm that the extinction of the fauna had to follow a profound change in the flora. It also seems to be shown that this change occurred after the appearance of man in America, from which we want to deduce a great antiquity of the human species in Argentina. In reality the facts of which we are aware do not allow us to deduce other than that many pampean species have resisted until more recent times than what is generally assumed,and Onoippidium) in some Patagonian caves. The current fauna of Argentina still preserves representatives of that fauna which consist of types adaptable to all conditions, such as the guanaco and the ñandú, and various species of rodents, animals that for their kind of life and for their smallness find a effective protection against all changes in the environment.