Sweetea milowensis gen. et sp. nov., a Middle Mississippian (Viséan) pteridosperm preserved in a coastal marsh setting, Hartselle Sandstone, Alabama

Abstract

The Mississippian (Lower Carboniferous) is a time of terrestrial plant diversification and experimentation of growth architectures. Yet, the majority of taxa in which leaves are known conform to a bipinnate, tripinnate, or quadripinnate leaf architecture. Simple, compound (pinnate) leaves are rare and uncommon. Sweetea milowensis is described as a new genus and species of a small, simple, compound leaf preserved as isolated, dehisced foliage in a Viséan marsh litter dominated by the cormose lycopsid Harsellea dowensis in the Black Warrior Basin, Alabama, USA. It is characterized by a naked petiole, exhibiting a flared, thickened base, and pinnatifid rachis comprised of bilobed and trilobed leaflets in a descending spacing toward its tip. Sweetea gen. nov. is compared with coeval pteridophyte and pteridosperm megaphyll taxa, and it is suggested that the size of the plant's leaf may have been in response to saltwater spray or tidal incursion into the back barrier marsh. Arguments against whether Sweetea could represent a heterophyllous form of a previously described taxon, or the possibility that it is the product of heteroblastic development, are discussed. A review of the 33 commonly reported monopinnate, bipinnate, tripinnate, and quadripinnate Mississippian morphotaxa shows that only five of these possess a monopinnate-leaf architecture. And, amongst these, Sweetea is unique.