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Acrostichum is considered today an opportunistic fern in disturbed areas, which indicates the first stages of colonisation of such zones. However, in the fossil record, Acrostichum appears related to fluvio-lacustrine environments, freshwater marshes and mangrove deposits. We report here for first time fossil evidence of Acrostichum that reveals a pioneering behaviour of this fern in the colonisation of perturbed communities in Europe, which corroborates previous assumptions about the paleobiology of Acrostichum. Plant remains were collected from the Chattian (late Oligocene) La Val fossil site (Estadilla, Huesca, northeastern Spain) belonging to the Sariñena Formation, which mainly embraces crevasse splays, levees and floodplain deposits. Evidence shows that Acrostichum grew within the levee’s vegetal community or close to/on the river banks as well as on floodplain areas and closer to/on the shores of ephemeral ponds. But most importantly, the observed co-existence of Equisetum and Acrostichum remains in the same beds indicates that such strata represent short-lived inundated terrains, e.g., floodplains where the water table was temporarily stagnant. Evidence shows wetland environments dominated by pioneering taxa, implying a pioneering role for Acrostichum during the late Oligocene in the Iberian Peninsula.

This paper reports a previously unknown leaf-flora from the Upper Oligocene/Lower Miocene of the Ebro Basin, NE Spain, a period with a relatively poor vascular-plant fossil record in Southern Europe. The presence of Acrostichum sp. is also important. This fern is extremely significant from the point of view of palaeoecology and the depositional environment. The macroflora appears to yield sufficient morphological characteristics to be identified at genus level, and sometimes at species level, although cuticles are not preserved. This article presents the first data obtained from the new outcrop at La Val; the following families have been identified: Pteridaceae, Dennstaedtiaceae, Equisetaceae, Pinaceae, Lauraceae, Hamamelidaceae, Betulaceae, Myricaceae and Salicaceae. The fossil plant assemblage is correlated with the Cadibona floristic complex (Mai, Tertiäre Vegetationsgeschichte Europas. Methoden und Ergebnisse, Gustav Fischer, Jena, 691 pp., 1995) and suggests a subtropical-to-warm temperate climate, rainy and wet, with a short dry season. The age of the assemblage is Late Oligocene/Early Miocene.

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