Recent advances into whole-plant reconstruction of Corystospermaceae: new evidences from Middles Triassic of Argentina.

LUTZ, A.I.; DROVANDI J.M.; PIPO M.L.; IGLESIAS A.; BODNAR, J.; BELTRAN M.; COLOMBI C.E.

Salvador de Bahia

Congreso; XIV International Palynological Congress, X International Organisation of Palaeobotany Conference; 2016

Reassembling the parts of a fossil plant is a key process to understanding plant evolution, particularly into totally extinct groups. Seed ferns comprise many exampl es of complex reconstructions, since they are a heterogeneous assemblage lacking extant close relatives. Among them, the family Corystospermaceae has been subject of different whole-plant hypotheses. Corystosperms probably originated in the paleotropicsduring the Permian, and became the dominant elements of Triassic Gondwanan communities. The family was restored on the basis of Dicroidium bifurcating leaves, Rhexoxylon trunks, Pteruchus pollen organs andUmkomasia ovulate organs. At first, plants with Rhexoxylon stems were interpreted as lianas due to their unique vascular anatomy. Afterwards, more and larger sp ecimens were discovered; including several one-meter high trunks in life position, enabling to interpret that they were self-supporting plants with a tree hab it. Two kinds of whole-plant concepts have been proposed for arborescent Corystospermaceae: one with a palm-like habit [i.e, unbranched stem with an ap ical crown of evergreen leaves) an d other with a coniferlike hab it [i.e. profusely branched stem with de ciduous leaves). There has be en much disc uss ion aboutthese two contrasting hypotheses, since a plant family showing both types of archite cture is not currently known. In th is contribution, we present new reconstructions ba se d on fossil plants coming from the Middle Triassic of Western Argen tina (Sorocayense Group, San Juan province), and rea nalyze the cur rent evidence, in order to enlighten corystosperm habit dile mma. We reconstructed two arborescent Corystospermaceae from Sorocayen se Group, one from Barreal Formation (including Tranquiloxyl on sp. trunks, Zuberia zuberi leaves, Pteruchus barrealensis polle n organs and Umkomasia speciosa ovulate organs), and other from Corta derita Formation (comprising Rhexoxy lon cortaderitaense trunks, Z. feistmantelii leaves, Pteruchus sp. polle n organs, U. macleani ovulate organs). These reconstructions, along with new observations and reinterpreta tio ns, allow us to develop a th ird hypot hesis for the whole-plant concept of corystosperms. This model is characterized as a sympodial tree, with a scarcely and irregularly branched stem, orthotropic first- ord er bra nches, bip inn ate leaves helicoidally borne in the branches, and adventitious an buttress roots .Although bran ched, the type of architecture proposed he re is very different from the habit of ginkgos and conifers, but it is more similar to that present in some eudicots.