GEOCHEMICAL SUPPORT FOR A CLIMBING HABIT WITHIN THE PALEOZOIC SEED PLANT GENUS MEDULLOSA

It is difficult to determine what fossil plants looked like when they were alive. With rare exceptions, many fossil plants disarticulate upon death, and physical damage can obscure the original morphology of plant organs. Structural properties of plant tissues are a consequence of cell wall biochemistry, and stable carbon isotope analysis provides a new perspective on this classic problem: tissues that provide structural support to plants are enriched in the biopolymer lignin, which can be 2 to 7‰ depleted in 13C relative to cellulose-rich tissues in the same plant. This isotopic difference allows the biomechanical properties of fossil plant tissues to be identified in the fossil record.

In order to resolve the growth form of an ambiguous fossil plant, we analyzed the stable carbon isotope composition of tissues from a stem of the Pennsylvanian Period seed plant Medullosa anglica that was permineralized in calcium carbonate. Absolute δ13C values can vary between plants for ecological or environmental reasons, so we compared tissues from the same specimen to determine relative amounts of lignin. We sampled the vascular tissue, cortical tissue, pith, and coal ball matrix; for context, we also sampled cortical tissues from medullosan petioles found in similar coal balls.

Results show that vascular tissue is depleted in 13C relative to cortical tissue by 0.5 to 0.8‰. Because vascular tissue cannot function without the presence of lignin, we interpret these results to show that that cortical tissues were not lignified and, at least in this medullosan type, did not contribute significant structural support to the plant. This supports hypotheses that medullosans with this anatomy had lax stems and leaned or climbed on neighboring plants, a common mode of growth in Carboniferous Period plants, including the stem group seed plants Lyginopteris and Callistophyton. It is noteworthy that a large fraction of the evolutionary radiation of early seed plants took place in the forest understory, much like the Cretaceous Period radiation of flowering plants.