Systematics of a palaeoecologically significant boreal Mesozoic fossil wood genus, Xenoxylon Gothan

The genus Xenoxylon was established for gymnosperm-like fossil wood with xenoxylean radial pitting and large oopores in its cross-fields. It is restricted to the Mesozoic and to the Northern Hemisphere, where it is commonly found. As an indicator of cool-wet boreal climates, as suggested by its distribution, Xenoxylon is of special interest to palaeoclimatology. Its systematics, however, is poorly known with several species needing re-evaluation. Xenoxylon nomenclature is also still unresolved, with various synonyms used for the same taxa even in recent literature. Having reviewed several types and a large number of specimens we performed a taxonomical and nomenclatural reappraisal of all species known to date. The results of this review are presented here, together with a taxonomical key. For most taxa stratigraphical and geographical distributions have also been re-evaluated. ► 31 species of Xenoxylon, a widespread Mesozoic fossil wood, are reappraised. ► Most of the available types were revisited and 285 specimens studied. ► Synonymic rate was found to be high. ► A taxonomical key is proposed.