Further insights into trilete spore producers from the Early Devonian (Lochkovian) of the Welsh Borderland, U.K.

Abstract

New data on trilete spore producers are presented from a Lower Lochkovian locality in the Welsh Borderland, U. K. Discoidal sporangia assigned to Cooksonia pertoni subsp. apiculispora illustrate further variation within the Streelispora–Aneurospora morphon. Laevigate crassitate spores belonging to Ambitisporites warringtonii were found in sporangia assigned to C. pertoni subsp. pertoni which possesses notably different sporangial wall anatomy from C. pertoni subsp. apiculispora. A separate group of discoidal sporangia is similar to C. pertoni subsp. reticulispora based on gross morphology and reticulate in situ spores, but the presence of a thick cuticular covering sets these specimens apart. After close inspection of their ultrastructure, the spores were assigned to Synorisporites, despite their resemblance to Chelinospora. New data on Cooksonia banksii include the recognition of an additional layer encasing the spore mass, which is associated with globules interpreted as Ubisch bodies. Their in situ spores are unusual, possessing distal polar thickenings, features that are rarely seen in the coeval dispersed spore record. Sections reveal that the spores possess spongy areas that coincide with these thickenings. Therefore, it is considered that C. banksii does not belong to Cooksonia and is assigned to a new genus, Concavatheca. Two spheroidal specimens contain spores with proximal ‘emphanoid’ muri and apiculate distal surfaces and are assigned to Iberoespora, based on their additional thickenings on the proximal surfaces. Spores similar to Emphanisporites sp. A were found within a discoidal spore mass, which is contradictory to previous studies where Emphanisporites is found within elongate sporangia. It is suggested that ‘emphanoid’ muri represent a convergent character. The in situ spores in Tortilicaulis are assigned to Apiculiretusispora, although they are unusual because they possess minute sculpture on both the distal and proximal surfaces, and sectioning reveals a tri-layered ultrastructure.