Phylogenetic analysis of the Equisetaleans: preliminary results

ESCAPA, IGNACIO; CÚNEO, RUBÉN

Bariloche, Argentina

Conferencia; VII International Organization of Paleobotany Conference; 2004

Museo Paleontológico Egidio Feruglio

Equisetum is the only modern representative of a group of plants that during the late Paleozoic and early Mesozoic showed a much greater diversity- There is also some discussion about the origin of this genus as well as on the evolutionary pattern of the entire group. The great quantity of fossil natural taxa and morphotaxa that somehow have been incorporated into the Equisetaleans have been subjected to several taxonomic classifications at the suprageneric level (Boureau, 1964; Good,1975; Meyen, 1987; Stewart and Rothwell, 1993; Taylor and Taylor, 1993). As a result, one genus can be found as belonging to more than one family according to different authors. ln order to clarify these basic taxonomic problems, a cladistics analysis has been performed using as terminal taxa thirteen genera normally assigned to the order Equisetales. However, the use of the cladistic method in this particular case shows some difficulties as follows: 1) The order Equisetales includes a vast majority of extinguished taxa so that many characters can not be found as modern counterparts, 2 ) Most of the fossil genera assigned to Equisetaleans are preserved as compression-impressions. Only in a few cases (e.g. Calamostachys) anatomical characters can be observed. These characters are not be included in the matrix since they are not normally observed in most of the taxa considered. 3) Among fossil Equisetaleans it is very common that evolutionary parallelism occurs. As a result, taxa with morphologically similar vegetative structures can be found associated or connected to different reproductive organs. On these grounds, eight characters have been selected for the analysis; seven of them belonging to reproductive parts. The ingroup is composed of 13 genera with known reproductive organs.