Seagrass stems with attached roots from the type area of the Maastrichtian Stage (NE Belgium, SE Netherlands): Morphology, anatomy, and ecological aspects

Seagrasses are the only seed plants to have invaded marine environments successfully. Seagrass fossils are both rare and have received scant attention so far. However, among the limited number of plant fossils from marine strata in the Maastrichtian type area, remains of seagrasses are relatively common. The present paper provides a detailed description of the morphology and anatomy of the stems and attached roots of Thalassocharis bosquetii Debey ex Miquel, 1853, mainly on the basis of silicified material. This has enabled an interpretation of features of the abundant imprints (external moulds) among the specimens studied, inclusive of the type material. Contrary to the outcome of earlier studies, the internodes have been found to constitute leaf scars. T. bosquetii is considered to have been a seagrass, but has a complex stem anatomy, including a substantial central stele with numerous sheathed vascular bundles, an inner cortex with many small intercellular cavities, a middle cortex with distinct fibre strands, and an outer cortex with conspicuously thickened vascular bundles, each of which constricts into a thin vascular strand just below a furrow/pit on the leaf scar. The last-named feature underlies the characteristic stem surface pattern seen in Thalassocharis. The stem anatomy of T. bosquetii is clearly more complex than that of extant seagrasses, which, due to prolonged adaptation to their aquatic habitat, show a more or less reduced anatomy. As far as the complexity of the stele is concerned, T. bosquetii reminds more of some non-marine Alismatales, such as the pondweed family Potamogetonaceae and the monotypical peat bog family Scheuchzeriaceae.