Structure of corystosperm fossil forest from the Late Triassic of Argentina.

ARTABE, A.E., L.A. SPALLETTI, M. BREA, A. IGLESIAS, E.M. MOREL Y D.G. GANUZA

Mendoza

Workshop; GONDWANA 12 - "Geological and Biological Heritage of Gondwana":; 2005

The Triassic fossil record of Gondwanaland shows that the forested landscape was dominated by corystosperms and by coniferophytes. A palaeoecological study of a standing Late Triassic forest containing 150 silicified trunks in the Río Blanco Formation of Mendoza province, Argentina, is reported. A mapped portion of the forest floor provides quantitative data (tree density, mean separation of trees and basal area per stump) which, integrated with the taxonomic and sedimentological information, allowed the reconstruction of a plant community that grew along river banks and within proximal floodplain environments. Analysis of architectural and phenological data from this monotypic forest indicates an evergreen community composed of a new corystosperm genus with a canopy height of c. 10-19 m. The new corystosperm taxon, like Rhexoxylon, has polyxylic axes with centripetal secondary xylem, but does not generate perimedullar bundles. The centrifugal secondary xylem produces an undivided solid pycnoxylic cylinder with restricted anomalous activity (cessation and reactivation of small patches of cambium along the centrifugal secondary xylem). The vegetation analysis shows that the forest had a clustered distribution pattern. The trees are grouped into 10 first-order groups which, in turn, are grouped into two second-order groups. There are five first-order groups in each second-order group. The two second-order clusters outline the better preserved regions and discounts those trees that appear to have been more isolated, included in geographical areas affected by recent erosion. The forest density ranges between 727 and 1504 trees/ha, but there are first-order clusters with a mean nearest-neighbour distance of 1.85m. The configuration of the of class-diameter histogram can be interpreted to show early colonization by a few older pioneers (the largest ones) followed by establishment of a large younger cohort of coeval trees. Additionally, the very high density of some groups suggests that these trees were aggressive colonizers which invaded disturbed areas, forming coeval populations. The heights of the new corystosperm trees were predicted from basal diameters using the equations of Niklas (1993, 1994). Corystosperm wood fossils have a mean stem diameter (D) of 24 cm (5–58 cm), with a mean estimated height (Hest.) of 15 m (8–22 m), and a mean critical height (Hcrit.) of 37 m (13–67 m). Quantitative growth ring anatomy analysis (Falcon-Lang, 2000) leads to the conclusion that the trees were evergreen gymnosperms. The cumulative algebraic sum of each cell’s deviation from the mean was calculated for each growth ring increment and plotted as a zero-trending curve (CSDM curve): the new corystosperm genus exhibits right-skewed CSDM curves typical of evergreen conifers. The closely spaced distribution of trees should have restricted foliage development, suggesting a palm-like general appearance of trees, with a crown of leaves at the top of the stem. Based on 9 series and 139 rings, the mean ring width and mean sensitivity (MS) were 3.4697 mm and 0.3140, respectively. The high MS values and the presence of false rings may indicate that the forest community colonized stressed ecosystems.