Towards a whole plant reconstruction for Austrohamia (Cupressaceae): New fossil wood from the Lower Jurassic of Argentina

Abstract

Early diversification of modern conifer lineages occurred during the Late Triassic and Early Jurassic, and worldwide ecosystems were dominated by conifers throughout the Jurassic. The knowledge about the palaeobiology and palaeoecology of basal representatives of those clades, however, has only recently begun to be developed due to the relative scarcity of complete plant reconstructions for many of these conifer families. In regards to the Cupressaceae sensu lato, some reconstructions have been proposed, although none has linked all plant organs. One of the oldest records of this family is the genus Austrohamia, described from the Lower Jurassic of Argentina and China. The original material consists of impressions of leafy branches, organically attached to ovulate and pollen cones. This conifer has a combination of characters that support its assignation to the Cunninghamioideae subfamily, the most basal member of the Cupressaceae stem group. In this paper, we describe permineralized woods from the same strata where Austrohamia minuta was found in the Cañadón Asfalto Basin, Chubut Province, Argentina. The fossil woods were assigned to the genus Protaxodioxylon, due to homoxylic pycnoxylic secondary xylem, with distinct growth rings, radial tracheid pitting of mixed type, abundant axial parenchyma, taxodioid cross-fields and uniseriate homocellular rays. Consistent with the differences in other Protaxodioxylon species, we propose a new specific taxon for the Patagonian specimens. This genus has often been related to the taxodiaceous Cupressaceae. This linkage, together with the fact that all the conifer impressions from these strata correspond to Austrohamia, reinforces the idea that the wood belongs to the same biological entity as A. minuta. From this interpretation, Austrohamia represents the most complete Mesozoic Cupressaceae to date.