Maiaspora: a new miospore genus with enigmatic sculpture from the late Visean of European Russia

We describe a new miospore genus Maiaspora gen. nov. from the upper Visean (middle Mississippian) of the southern wing of Moscow Syneclise (European Russia). A combination of methods (optical light microscopy (LM), scanning-electron microscopy (SEM), confocal laser microscopy (CLSM) and image-analysis) to study the same individual miospore specimens revealed a remarkable sculptural type for which we propose the new term ‘metareticuloid’. Metareticuloid sculpture is defined as a complex arrangement of muri with apiculate elements, polygonal lumina and circular scrobiculae at the base of the lumina. The discovery of fossil spores with metareticuloid sculpture allows the recognition of clear morphological differences between microreticulate and foveolate sculptures which palynologists have historically had difficulty in separating. Virtual cross-sections provided by CLSM show that miospores with metareticuloid sculpture display perfectly-preserved, funnel-shaped tunnels (like a gallery of cave stalactites) in the deep exospore. In contrast, miospores with microreticulate or reticulate and partly-vermiculate sculpture lack such vertical tunnels (termed scrobiculae). The morphology of Maiaspora gen. nov. is considered in comparison with Microreticulatisporites (Knox non sensu Potonié et Kremp) Bhardwaj emended Oshurkova, Pseudoreticulatispora Bharadwaj et Srivastava emended Price et Foster, Vadaszisporites (Deák et Combaz) emended Juhasz, Foveosporites Balme, Foveotriletes van der Hammen ex Potonié, Garotriletes Singh et Singh, Assamiasporites Mehrota et Sah, Margotriletes Mehrota et Sah. The morphological conceptions of Microreticulatisporites and Foveotriletes are reinterpreted by numerical typification. The adaptative significance of metareticuloid sculpture is discussed in relation to modern and fossil spores, and in terms of harmomegathic mechanisms of angiosperm pollen.