Cheirolepidiaceae from the Early Jurassic of Patagonia: Reconciling the Impression and Permineralization Record of Conifer Cones.

LESLIE, ANDREW; ESCAPA, IGNACIO

Edmonton

Congreso; BOTANY 2015; 2015

Permineralized fossils offer spectacularly detailed information about the anatomy and morphology of ancient plants, but they are comparatively rare and are often localized to specific geographic and taphonomic settings. Impression and compression fossils record less detailed information, but they are much more widespread in the fossil record. Reconciling these two records represents a critical goal of paleobotany, so that we can better understand not only the anatomy and morphology of fossil plants, but also their full temporal, geographic, and ecological contexts. Here we describe new conifer remains from the Lower Jurassic of Chubut Province, Patagonia, which highlight the importance of synthesizing information from both impressions and permineralizations. The fossils in this study were found in the Taquetren locality from the base of the Lonco Trapial Formation, which has been radiometrically dated to the Lower Jurassic. Specimens include compressions of seed cones and small wedge-shaped cone scales that would be assigned to the taxon Araucarites (usually thought to represent Araucariaceae) based on their shape and apparent morphology. Closer inspection of three-dimensionally preserved impression fossils, however, shows that these isolated scales have three small lobes similar to the ovuliferous scales of permineralized Pararaucaria in the Cheirolepidiaceae. Other features of the seed cone further suggest placement in Cheirolepidiaceae rather than the Araucariaceae, including a proportionally small central cone axis, a relatively small number of ovuliferous scale complexes, well-developed bracts that are separated from the ovuliferous scales for most of their length, and seeds borne distally on the ovuliferous scales rather than proximally. A comparison of the impression fossils from Taquetren with silicified Pararaucaria delfueyoi and P. patagonica cones, also from the Jurassic of Patagonia, shows they are remarkably similar in their overall appearance, including details of their phyllotaxy, the rhomboidal external surfaces of the ovuliferous complexes, and the placement of the bracts. The isolated scales found in Taquetren therefore likely represent shed ovuliferous scales of an early Pararaucaria-like cheiroplepidaceous conifer rather than ovuliferous scale complexes of an Araucariaceae. These fossils suggest that at least some cone scales placed in the common and geographically widespread Mesozoic genus Araucarites may actually represent members of Cheirolepidaceae, because the difference between isolated cone scales of the two families can be very difficult to determine in compressed specimens.