The South American native ungulates of the Urumaco Formation. Urumaco and Venezuelan Palaeontology

BOND M.; GELFO J. N.

The fossil record of the Northern Neotropics

Indiana University Press

Lugar: Indiana; Año: 2010; p. 256 - 268

The mammalian fauna of Cenozoic South America is known from scientific literature to be mainly composed of a series of different aggregate waves of migrants, which originally were recognized and nominated by Simpson as the First to the Third Strata . The mammal living fauna of South America still retain members of the three strata , but unfortunately during the Cenozoic the orders of endemic ungulates pertaining to the First Stratum became extinct without leaving moderndescendants. Theactual South American ungulates are all northern immigrants from the Third Stratum. The zenith of the South American endemic or native ungulates can be traced back to the Paleogene. In the Casamayoran South American Land Mammal Age (=SALMA) (Eocene) they are represented, conservatively, by 5 orders (if Notopterna is recognised to be distinct from Litopterna the number of orders increases to six) with more than 40 genera. After that, during the Cenozoic there was a significant diminution in the number of taxa and in the diversity, though areduction in frequency was not observed. This was very probably related to the environmental worldwide changes which affected the biota during the end of the Eocene and the early Oligocene. By these times the decrease of the world wide temperature led to the known Icehouse-World . As a consequence the mammalian communities underwent several changes in their lineages. This global climatic depreciation led to several faunistic changes, suchas the Grande Coupure in Europe. In South America the end of the Eocene was associated with several geological events. Among them, the sea level fluctuation and the Andean orogeny with the rise of the AndesMountain, stressed the climatic changes in South America. So during the early Neogene, for example in the Santacrucian SALMA (early Miocene), the endemic ungulates were reduced to only three orders (Astrapotheria, Litopterna and Notoungulata) and approximately 20 genera; nearly half of previous numbers. As stated above, the taxonomic diversity decreased but not its frequency, and holdover lineages became specialized and distinctive, almost stereotyped inside each group. As it is also well known, the evolutionary history of the native ungulates, as well as that of many other South American mammals, is based mainly on the Cenozoic fossil record from Argentina, which means the southern part of South America, with a considerable latitudinal range of fossiliferous sites. Hence this history is biased, when we consider the rest of South America, especially due to the scattered and sometimes scarce fossil record of the most tropical areas. In these outcrops the scientific research has been not so extensive as it was in Argentina, in part due to the difficulties in the field. Considering the Venezuela fossil record, the oldest mammal is supposed to came from the Eocene rocks of the Trujillo Formation. Another important outcrop in Venezuela is the Urumaco Formation,which represents one of the few glimpses in the Tertiary of the northern areas. Its fossil vertebrates, particularly the mammal ones, indicate a probable Huayquerian SALMA (upper Miocene) for this fauna. Only the middle Miocene La Ventan fauna from Colombia is well known as an early counterpart for comparison between tropical areas. There are also some scattered findings of probable middle Miocene La Ventan age in Acre, Braziland a recently described La Ventan fauna for Peru is very similar to that of Colombia. In Argentina, the Chasicoan and Huayquerian SALMAs are chronologically nearer the Urumaco fauna. They represent extrapatagonian faunas; the Colloncuran SALMA is the last, more or less well understood fauna, after the Santacrucian time; reflecting the changing pattern of the mammalian faunas, specially the native ungulates, in the southern area. The poorly known Mayoan SALMA fauna, middle Miocene? probably will be of special interest in the future to compare one of the last patagonian faunas before the shift of the mammalian record to more northern areas in Argentina. But unto this date the Mayoan mammals are not understood well enough, as some other new records of potential middle Miocene faunas in Patagonia, to serveas a useful antecedent to compare with earlier tropical faunas before Urumaco. Therefore, given these limitations in the knowledge of the middle Miocene, the Colloncuran SALMA in Argentina and the La Ventan SALMA of Venezuela and Colombia allow us to consider with some accuracy the differences between a southern fauna, probably with a major predominance of open areas, and a tropical one, that probably habited more forested and closed regions.In this work we examine what we know about the record of the native ungulates representing the three surviving orders on to the middle Miocene: Astrapotheria, Litopterna and Notoungulata, emphasizing the Urumaco Formation ungulates fossil record.