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

JOSEFINA BODNAR; ALICIA LUTZ; MARÍA LAURA PIPO; MARISOL BELTRÁN; JUAN M. DROVANDI; ARI IGLESIAS; CARINA COLOMBI

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 examples 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 paleotropics during 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 and Umkomasia ovulate organs. At first, plants with Rhexoxylon stems were interpreted as lianas due to their unique vascular anatomy. Afterwards, more and larger specimens were discovered; including several one-meter high trunks in life position, enabling to interpret that they were self-supporting plants with a tree habit. Two kinds of whole-plant concepts have been proposed for arborescent Corystospermaceae: one with a palm-like habit (i.e. unbranched stem with an apical crown of evergreen leaves) and other with a conifer-like habit (i.e. profusely branched stem with deciduous leaves). There has been much discussion about these two contrasting hypotheses, since a plant family showing both types of architecture is not currently known. In this contribution, we present new reconstructions based on fossil plants coming from the Middle Triassic of Western Argentina (Sorocayense Group, San Juan province), and reanalyze the current evidence, in order to enlighten corystosperm habit dilemma. We reconstructed two arborescent Corystospermaceae from Sorocayense Group, one from Barreal Formation (including Tranquiloxylon sp. trunks, Zuberia zuberi leaves, Pteruchus barrealensis pollen organs and Umkomasia speciosa ovulate organs), and other from Cortaderita Formation (comprising Rhexoxylon cortaderitaense trunks, Z. feistmantelii leaves, Pteruchus sp. pollen organs, U. macleani ovulate organs). These reconstructions, along with new observations and reinterpretations, allow us to develop a third hypothesis for the whole-plant concept of corystosperms. This model is characterized as a sympodial tree, with a scarcely and irregularly branched stem, orthotropic first-order branches, bipinnate leaves helicoidally borne in the branches, and adventitious an buttress roots. Although branched, the type of architecture proposed here is very different from the habit of ginkgos and conifers, but it is more similar to that present in some eudicots.