. Oldest macrofossil record of Agathis (Araucariaceae), early Paleocene of Patagonia, Argentina, and its evolutionary significance

ESCAPA I.; ARI IGLESIAS; WILF PETER; CÚNEO RUBEN

NEW ORLEANS

Congreso; Botanical Society of America Annual Meeting; 2013

Botanical Society of America

The Salamanca Formation (ca. 61.7 Ma) in central Patagonia bears one of the best preserved and diverse Paleocene macrofloras in Gondwana and documents a robust recovery of Patagonian floras following the end-Cretaceous extinction event. Angiosperms are by far the most abundant and diverse group in the compression floras, but conifers of the families Podocarpaceae and Araucariaceae are more prevalent in the petrified woods. The abundant Agathis compression macrofossils reported here are remarkable, especially considering that until very recently, all previous fossil evidence for Agathis came from Australia and New Zealand, starting from the late Paleocene; however, Agathis macrofossils from the Eocene of Patagonia are now being described separately. Clues about the presence of Agathis in the Salamanca Formation initially came from leaves morphologically similar to (but narrower than) modern Agathis in shape, venation, organization, and random stomatal orientation. However, the finding of ovuliferous scales and associated winged seeds that preserve sufficient apomorphic characters assure that the genus was present. Associated pollen cones are probably related to the same taxon. The features that confirm the inclusion of the fossils in Agathis include obtriangular ovuliferous complexes with completely fused bracts and scales, distally thickened margins, and elongated seed scars. Interestingly, particular character states of the ovuliferous complexes and seeds in the Patagonian specimens (e.g., lack of a proximal scallop, seeds with two equally developed wings) suggest its basal position in the genus, as part of a putative early Paleocene stem group. Recent phylogenetic analyses have suggested that the evolutionary histories of Araucaria and Agathis were markedly different in terms of their timing of morphological differentiation. The fossil record shows that the Araucaria clade achieved its modern morphology by the Middle Jurassic or earlier, but the evidence here presented for an early Paleocene, basal member of the Agathis clade, combined with Eocene evidence of the crown genus, suggests that the acquisition of derived traits seen in extant species of the agathioid clade occurred much later and was completed during the early Cenozoic. This explains the absence of clear Mesozoic Agathis vs. abundant and diverse Mesozoic Araucaria. Overall, this report represents not just one of the most complete Agathis macrofossil records but also the oldest one worldwide, showing that Agathis was a major, trans-Antarctic forest component for many millions of years.