Two-seeded cones of probable gnetalean affinity from the Morrison Formation (Late Jurassic) of Utah and Colorado, USA

Abstract

Three dimensional calcitic casts of a two-seeded compound cone are described from the Upper Jurassic Brushy Basin Member of the Morrison Formation based on specimens from Colorado and Utah. Cones of Bassitheca hoodiorum gen. et sp. nov. are broadly obovate in face view, ~3.5 mm high, ~2.6 mm wide, and viewed from above have two planes of symmetry. Micro-CT scanning of numerous specimens shows that each cone has two well-developed orthotropous seeds positioned opposite each other in the major plane and separated by a median longitudinal septum in the minor plane. At a higher level in the cone, a median apical septum in the major plane separates two depressions arranged opposite each other. The two well-developed seeds in the major plane are enclosed by bracts, but numerous incomplete cones, together with cones showing lines of dehiscence, indicate that each seed was shed by the abscission of a lateral valve. One specimen indicates that three vascular bundles entered the base of each well-developed seed and these are interpreted as the vascular supply to the envelope of a chlamydospermous ovule. The opposite and decussate arrangement of bracts at the base of the cone, the paired arrangement of the two well-developed seeds perpendicular to the two apical depressions, combined with the inferred seed envelope that was apparently shed with the seed, indicates a probable relationship to Gnetales and specifically to extant Ephedra. Along with Dayvaultia tetragona Manchester et Crane, also known from the Brushy Basin Member of the Morrison Formation, Bassitheca hoodiorum provides further evidence that the Gnetales were more significant in the Morrison vegetation than has been recognized previously.