The anatomical and morphological characters in living and fossil cycads: a cladistics view

MARTINEZ, LEANDRO CARLOS ALCIDES; STEVENSON, DENNIS

Buenos Aires

Congreso; The 35th Annual Meeting of the Willi Hennig Society; 2016

The Willi Hennig Society

The cycads are enigmatic seed plants with a long history from Palaeozoic to present. The living Cycadales are defined by five anatomical and biochemical synapomorphies (girldling traces, coralloid roots, cycasin, leaf bases with an omega pattern and primary thickening meristem). Their living genera are mainly classified based on reproductive characters. However, in fossil forms the assignment could be some complicated. The fossil stems usually preserve some synapomorphies (girldling traces, medullary bundles) or they have a set of features proper of cycads (e.g. manoxylic wood, primary and seconday rays, polylxylic wood, and leaf bases armour). However, the leaves have some features in their morphology or some cuticle characters (e.g. epidermal cells, vein pattern, stomate type, trichomes) that resembles or are common to other groups (Pteridosperms, Bennettitales, Magnoliphytas). The leaf is primarily the photosynthetic organ of land plants, and therefore it is always exposed to environmental conditions. For that reason, the leaves are subject to selective pressure of environmental changes, and probably, these are the cause of distinct lineages of lands plants have commonly homoplasious characters in theirs leaves. In this particular case, the absence of autosynapomorphies in the leaves of extant cycads detracts from and restricts the certain assignment of fossils remains to Cycadales. The goal in this communication is to evaluate, by a cladistic analyisis, some anatomical and morphological characters in stems and leaves of extant cycads, to compare with fossil taxa assigned as probably cycadophytes. The assignment of some fossil genera into Cycadales is questioned.