Seagrass foliage from the Maastrichtian type area (Maastrichtian, Danian, NE Belgium, SE Netherlands)

Seagrasses are the only angiosperms that have invaded the marine environment. Fossil seagrasses are rare and received only little attention to date. However, among the scarce plant fossils in the marine deposits of the Maastrichtian type area, remains of seagrasses are relatively common. The present paper provides a detailed description of the morphology and anatomy of the leaves. Maastrichtian and Danian material is considered here as a single species: Thalassotaenia debeyi gen. nov., sp. nov. Campanian material is assigned to Zosterites, but this is not necessarily a seagrass. Thalassotaenia includes the oldest anatomically preserved material of seagrass leaves. Comparison with extant seagrasses shows that there is not an easy match with any of the extant genera or families. Amphibolis and Thalassodendron (Cymodoceaceae) and Posidonia (Posidoniaceae) show the greatest resemblance. Possibly, Thalassotaenia represents the leaves of an ancestor of the Cymodoceaceae–Posidoniaceae (–Ruppiaceae) clade found in several phylogenetic analyses. Because Thalassotaenia also shows some features not found in extant seagrasses, it might also represent an extinct clade of seagrasses.