Search the Community

L.S., Thought it would be fun to share this "chance encounter" I had at a mineral show. The photograph below shows a slab of petrified wood from the Triassic of the Isalo II Fm. of Madagascar. When material from this locality is offered for sale (which happens often and in large quantities), it is usually labelled as "Araucarioxylon" or simply as "petrified wood" (where the latter may actually be better). While most of the wood indeed has an "Araucaria-like" anatomy (see Rössler et al. 2014 for a recent discussion on the nomenclature), I recently was lucky enough to "find" something else. While the left-most photograph may not directly show it, the anatomy of this particular slab is quite different from the common Araucaria-like specimens. I tried to clarify the anatomy by contouring the main structures seen, which hopefully makes visible how this wood consists of multiple rings of perimedular bundles and wedge-shaped structures, showing both centripetal and centrifugal xylem (inward and outward growing regions, per as provisionally indicated by the blue and red arrows). This curious growth form (by modern wood standards, at least) is characteristic for the stems of some groups of Mesozoic seed ferns, such as those from the Umkomasiales order. The best-known genus with this type of anatomy is probably Rhexoxylon (see Archangelsky and Brett 1961), but there are more similar genera, making it difficult to arrive at a more specific identification.