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

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.

A small number of very fragmentary sporangia containing tetrads or monads, from Lower Devonian (Lochkovian) strata of the Welsh Borderland, are described using light, scanning and transmission electron microscopy. Although a dearth of well-preserved specimens preclude erection of new taxa, this study adds to diversity and disparity in local Early Devonian vegetation that was dominated by plants of very low stature.Figure optionsDownload as PowerPoint slideHighlights
► Recognition of a division within a diverse group of permanent tetrad producers
► Discoidal forms bearing Acontotetras are related to Paracooksonia–Cooksonia lineage.
► Valvate forms bearing Cheilotetras–Tetrahedraletes are more similar to Partitatheca.
► Division here mirrors that seen in the permanent and separating dyad producers.
► Further diversity illustrated, including sculptured monads and masses of Velatitetras