Abstract: Fossil wood is one of the crucial proxies for understanding terrestrial vegetation composition and development in the Earth's history. The gymnosperm wood taxon Xenoxylon Gothan is a significant member of the Mesozoic flora. To date, more than 20 species of Xenoxylon have been described. However, its botanical affinities have remained enigmatic ever since it was described, over a century ago. Here we perform a phylogenetic analysis of Xenoxylon to understand the systematic relationship with extant conifers. Data come from four nucleotide regions (trnL-F, trnK-matK, rbcL, psbA-trnH), xylological characters, and biomolecular composition of five extant conifer families; for Xenoxylon, no nucleotide data are available. Using maximum parsimony in Tree Analysis using New Technology, Xenoxylon appeared basal to Araucariaceae in the data set combining genes and xylological characters, whereas Xenoxylon is placed next to Podocarpaceae in the data set combining genes and biomolecular characters. To find a reliable systematic placement of Xenoxylon, a combined data set of genes, xylological and biomolecular characters is analyzed. Our results and interpretations indicate that Xenoxylon is closely related to Podocarpaceae. This first phylogenetic analysis of Xenoxylon fills the knowledge gap of the systematic relationship of this taxon and contributes to a better understanding of the evolution of extant Podocarpaceae.

Key words: chemotaxonomy, cladistics, fossil wood taxon, TNT, xylology.