Morphology and wall ultrastructure of a new and highly distinctive megaspore from the Middle Jurassic of Yorkshire, UK

•We describe a new megaspore form the Middle Jurassic deposits of Yorkshire, England.•The megaspore has novel and highly distinctive ornament.•Evidence from morphology and wall ultrastructure suggest that the megaspore is most likely lycopsid.

We describe a new and rather unusual megaspore recovered from Middle Jurassic terrestrial deposits of Yorkshire, England that we name Reticuspinosporites whytei gen. et sp. nov. The contact area is non-trilete and is formed where an outer sculptured layer has not developed. This outer layer covers the remainder of the megaspore and bears novel sculpture consisting of a highly irregular reticulum with areas ‘infilled’ to form plateaus that bear long spines. Analysis of wall ultrastructure reveals a four-layered wall comprising from inside to outside: (i) innermost, separated lamina; (ii) inner homogeneous layer; (iii) central spongy layer; and (iv) outermost homogeneous layer that forms the sculpture. Wall ultrastructure is not entirely diagnostic but is most suggestive of lycopsid affinities. Unusually the megaspores commonly occur in pairs. They are not attached at their contact faces but by their equatorial or distal surfaces through entanglement of their spines. We interpret this feature as possibly an adaptation for floating and transport by water.