Ecological interactions in conifers (Agathoxylon and Protocupressinoxylon) from the Punta del Barco Formation (Baqueró Group, upper Aptian), Patagonia, Argentina

Abstract

The Punta del Barco Formation is the uppermost unit of the upper Aptian continental deposits of the Baqueró Group, cropping out in Argentinian Patagonia. These units have yielded highly diverse and abundant megafloras that contain some of the oldest evidence of angiosperms in the region. The parent vegetation grew in lacustrine to fluvial environments, with progressively increased volcanic activity that certainly affected the dynamics of the palaeocommunities. In this work, new fossil woods from the Punta del Barco Formation are described and referred to Agathoxylon Hartig 1848 and Protocupressinoxylon Eckhold 1923, probably allied to Araucariaceae and Cheirolepidiaceae respectively, and constituting two of the three conifer families already reported from the unit. Rootlets traversing some of the studied specimens are interpreted as evidence of seedlings growing in fragments of wood acting as nurse-logs, an ecological strategy recorded at least since the late Palaeozoic, in this particular case probably reflecting a response to the volcanic activity recorded in the unit. Two styles of fungal activity are identified in the fossil woods: Group 1 comprises microsclerotia and associate hyphae, found in close association with rootlets, interpreted as dark septate endophytes; and Group 2, consisting of hyphae traversing wood tracheids, along with evidence of degraded wood, interpreted as wood decay fungal activity.