Les plus anciens mammifères terrestres du Miocène marin de Lisbonne - le gisement du Km 10

Abstract

Key-words: Mammals-Other fossils-Upper Aquitanian -Lisbon-Tagus Basin. For the first time, small mammals were found at the earliest marine level in the northeastern part of the lower Tagus basin, to the NE of Lisbon. At this new locality, at the 10 kilometer of the Lisbon-Oporto A1-IP1 highway, conglomerates yielded, along with marine fossils, more or less abraded teeth and bones from insectivores,lagomorphs, rodents and small artiodactyls (sec Tableau 1). Age may he ascribed to the lower Miocene, MN 2b Neogene mammal unit (about 22 My), but an early MN 3 age cannot be entirely excluded. That corresponds to latest Aquitanian (or less probably earliest Burdigalian) (sec Tableau 2). This is the first hitherto found locality with small mammals of this age as far as Portugal is concerned, as weIl as the oldest locality so far known in the Tagus basin. Km 10 is somewhat older than the localities of Universidade Católica and A venida do Uruguay in Lisbon (ANTUNES & MEIN, 1986). Hence we can rather accurately date the age of the first marine transgression in the northeastern part of the lower Tagus basin. This shows that in this region there are nomarine equivalents of the "Venus ribeiroi beds" (Aquitanian, Division 1 of the Lisbon Miocene series). Correlation between this unit and the uppermost levels of the essentially paleogene "Complexo de Benfica" may he possible. Fossils at km 10 point out to shallow, coastal, highenergy marine environments. Sedimentological features are compatible with this model. Dry land and swamps with brackish (or ev en fresh) waters werepresentnearby. From those areas came remains of mammals, crocodylians, as weIl as oysters and charophytes that were later transported to the sea. Sea was warmer than the ex tant Atlantic at the same latitudes, even if conditions were not strictly tropical then. These conditions surely intluenced climate in the nearby reglons. Ecological data concerning mammalian faunas distinctly pointout to nearby forest-ri ch environments, much more so than for Universidade Cat6lica and Avenida do Uruguay localities, from where drier, even steppe environment forms largely prevail.