Silveirinha Formation

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The Silveirinha Formation is an Early Eocene rock layer (Ypresian, about 55.8 to 48.6 million years ago) in the Mondego Basin, in central-western Portugal (Região Centro, Coimbra District). It is mainly made of sandstone, siltstone and some conglomerates, formed in an alluvial, river-floodplain setting. The clastic rocks include calcitic lenticular conglomerates, laminated and cross-bedded sands, and brownish-red silts.

This formation is rich in fossils for the Early Eocene, including mammals, birds, amphibians, reptiles, mollusks and ostracods, making it one of Europe’s most diverse faunas from that time. Notable fossils include the taeniodont Eurodon silveirinhensis and the ostracod Cypris silveirinhaensis, which are named after the formation.

The unit is named after the Silveirinha clay pit, discovered in 1977 by Rui Pena dos Reis of Coimbra University and defined as a formation in 1981 by Autunes and colleagues. The Silveirinha clay pit outcrops in the western part of the Mondego Basin.

Depositional interpretation: the conglomerates reflect crevasse-splay deposits in an alluvial plain with channels flowing through a river system that drained north or east. The Early Eocene climate was much warmer than today. Freshwater indicators such as ostracods, gastropods, amphibians and pelomedusid turtles are common, with rare bivalves suggesting that brackish water was not far away.

Biogeography: it is suggested that the European species Diacodexis antunesi was more primitive than the North American D. ilicis, supporting a Europe-to-America dispersal for this genus.