Then and Now

STEVENSON, DENNIS; MARTINEZ, LEANDRO CARLOS ALCIDES

Anchorage

Conferencia; Botany Conference 2022; 2022

For fossil cycads, it appears that some features of extant leaf, leaflet anatomy and morphology could be diagnostic if present or at least looked for in fragments. Among these characters are: articulate and decurrent leaflets, midribs from dichotomies, a callus region at the leaflet base, anticlinal wall pegs, perforated epidermal cells, anastomosing of vein dichotomies, etc. Similarly, there are characters in stems such as girdling leaf traces, cone domes, polyxyly, inverse polyxyly, cortical steles, etc. as well as seed and strobilius characters. Some of these are synapomorphies for cycads, e.g., girdling leaf traces and omega pattern of petiole vasculature. Others are synapomorphies for groups within cycads; e.g., the Encephalarteae with adaxially thickened megasporophylls, megasporophylls with lateral lobes, and the presence of medullary bundles or the Zamioideae with articualted leaflets. Genera are also well defined with characters that should be observable in fossils. Examples include truncated leaves in Microcycas, basal callus region at leaflet base in Macrozamia, or the unique sprophyll shape of each genus. This allows for the development of robust phylogenetic data matrices for analyses, placement of cycad fossils on stem lineages within the Cycadales, minimum ages for both taxa and character states to explain the evolutionary radiations in this group of plants. This in turn sets the stage for evo/devo studies on the development and functional significance of the cycad apomorphies.