Notes on sporangia and spore masses containing tetrads or monads from the Lower Devonian (Lochkovian) of the Welsh Borderland, U.K.

Abstract

A small number of very fragmentary sporangia containing tetrads are described using light, scanning and transmission electron microscopy as part of a study on diversity and disparity in Lower Devonian (Lochkovian) vegetation in the Welsh Borderland. The dispersed genus for permanent sculptured tetrads, Acontotetras, is recorded in two isolated discoidal sporangia, one of which has a similar sporangial wall construction to that in Cooksonia, the other with a non-cellular covering reminiscent of the recently described Lenticulatheca and Paracooksonia. Conversely, the dispersed spore genus for permanent, enveloped laevigate tetrads, Cheilotetras, is found within two sporangia with valvate construction. Another example of the latter contains unique sculptured monads. Further diversity is exemplified by spore masses containing tetrads belonging to Velatitetras and Cheilotetras, but some evidence of splitting in specimens of the latter genus led to discussion on the state of maturity of the in situ spores, and as to whether or not the dispersed palynomorphs would have been so as permanent tetrads or trilete monads. Such studies add to diversity, although a dearth of well-preserved specimens preclude erection of new taxa, and to disparity in local Early Devonian vegetation that was dominated by plants of very low stature.