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

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

San Slavador de Bahia

Congreso; International Organization of Paleobotany Conference; 2016

Universidad de Bahia

Reassemblingthe parts of a fossil plant is a key process to understanding plant evolution,particularly into totally extinct groups. Seed ferns comprise many examples of complexreconstructions, since they are a heterogeneous assemblage lacking extant closerelatives. Among them, the family Corystospermaceae has been subject of differentwhole-plant hypotheses. Corystosperms probably originated in the paleotropicsduring the Permian, and became the dominant elements of Triassic Gondwanancommunities. The family was restored based on Dicroidium bifurcating leaves, Rhexoxylontrunks, Pteruchus pollen organs and Umkomasia ovulate organs. At first,plants with Rhexoxylon stems were interpretedas lianas due to their unique vascular anatomy. Afterwards, more and largerspecimens were discovered; including several one-meter high trunks in lifeposition, allowing interpreting that they were self-supporting plants with atree habit. Two kinds of whole-plant concepts have been proposed forarborescent Corystospermaceae: one with a palm-like habit (i.e. unbranched stemwith an apical crown of evergreen leaves) and other with a conifer-like habit(i.e. profusely branched stem with deciduous leaves). There has been muchdiscussion about these two contrasting hypotheses, since it is not currentlyknown a plant lineage showing both types of architecture at the same time. Inthis contribution we proposed new reconstructions based on fossil plants comingfrom the Middle Triassic of Western Argentina (Sorocayense Group, San Juanprovince), and reanalyzed the current evidence, in order to enlightencorystosperm habit dilemma We suggested two reconstructions for Sorocayense corystospermtrees, one from Barreal Formation (including Tranquiloxylon sp. trunks, Zuberiazuberi leaves, Pteruchus barrealensispollen organs and Umkomasia speciosa ovulateorgans), and other from Cortaderita Formation (comprising Rhexoxylon cortaderitaense trunks, Z. feistmantelii leaves, Pteruchussp. pollen organs, U. macleani ovulateorgans). These reconstructions, along with new observations andreinterpretations, allow us to propose a third hypothesis for the whole-plant conceptof corystosperms, characterized as an evergreen tree, monopodial, with scaresand irregular two order branching, first order branches orthotropic, bipinnateleaves helicoidally borne in the branches, and buttress together with stiltroots. Although branched, the type of architecture presented here, is verydifferent from the habit of ginkgos and conifers, but it is more similar tothat of cycads, palms and some eudicots.