A taxonomic revision of the late Paleozoic lyginopterid Sphenopteridium germanicum and description of its globose-stem growth habit

Abstract

A specimen of the lyginopterid pteridosperm Sphenopteridium germanicum (Weiss) Kerp et DiMichele comb. nov. was discovered at the Late Pennsylvanian (Missourian/Kasimovian) Kinney Quarry, New Mexico, complete from roots to crown of attached leaves. Taxonomic review, based on vegetative and associated reproductive organs, supports Mamay's assignment to Sphenopteridium Schimper. However, the species, Sphenopteridium manzanitanum Mamay is vegetatively identical to Sphenopteris germanica Weiss. We review Sphenopteris germanica, concluding that it should be assigned to Sphenopteridium, and that S. manzanitanum should be considered a later homonym, and placed in synonymy of S. germanicum. Arnophyton kuesii is obviously a juvenile version of the same species. The principal study specimen has a growth habit consisting of a short, subterranean or surficial, mound-like stem bearing a tuft of ascendant leaves and a vertically disposed taproot, so it was a low-growing, upright form. The study specimen is of sufficient size to suggest that this is the mature growth configuration, but it does not represent the maximum size the species could attain, based on excavated leaves much larger than those found in attachment. Inferred from its sedimentary context, the specimen was likely growing along a river or estuary bank and was incorporated whole as the bank eroded and collapsed, resulting in transport into the embayment in which the Kinney flora and fauna were preserved. Sphenopteridium germanicum plants were, thus, relatively small and low growing; there is no evidence of laterally initiated stems or plantlets indicative of a clonal growth form. The growth habit reported here is new, enlarging the range of growth architectures known in late Paleozoic plants, thus spotlighting the morphological richness that had evolved by that time, despite significantly lower diversity than that of the modern world.