Araucariaceous fossil woods from the Late Triassic Ischigualasto Formation (San Juan Province, Argentina): paleofloristic and paleoclimatic implications

BODNAR, JOSEFINA; SAGASTI, ANA JULIA; CORREA, GUSTAVO; MIRANDA, VICTORIA; MEDINA, FLORENCIA

JOURNAL OF PALEONTOLOGY

PALEONTOLOGICAL SOC INC

Lugar: Lawrence; Año: 2022

0022-3360

In this contribution, fossil woods from the Valle de La Luna Member of the Upper Triassic Ischigualasto Formation, at Ischigualasto Provincial Park, San Juan Province, Argentina are described. The specimens are preserved as silica permineralization in tuffs intercalated with carbonaceous mudstones beds, interpreted as distal floodplain facies. The fossil woods were assigned to the new species Agathoxylon argentinum, since their anatomy differs from the known Mesozoic Gondwanan species of the genus Agathoxylon. The combination of characters present in the new taxon indicates an affiliation with the conifer family Araucariaceae. Signals of fungal-mediated wood decay were observed, comparable with the activity of basidiomycetes. Spherical structures attached to the walls of the tracheids were recognized, which are interpreted as holocarpic chytrid fungi. The growth rings were quantitatively analyzed. Low values of percentage diminution, percentage latewood, and Ring Markedness Index, and a mean percentage skew of +11.5, were obtained, suggesting that the new species was an evergreen gymnosperm. The stratigraphic distribution and taxonomic composition of the Ischigualasto Formation fossil plant-bearing levels were studied. A vegetation change is recorded in the fossil level bearing Agathoxylon argentinum n. sp., marked by the replacement of the corystosperm genera and a diminution of arboreal corystosperms. This floristic change, in addition to other evidence, indicates humid paleoclimatic conditions for the uppermost part of the Valle de La Luna Member of the Ischigualasto Formation.