Quadriplatanus georgianus Gen. et Sp. Nov.: Staminate and Pistillate Platanaceous Flowers from the Late Cretaceous (Coniacian-Santonian) of Georgia, U.S.A.

Abstract

The Platanaceae are one of the basal-most branches of the main evolutionary line of the eudicot clade, and one of the earliest eudicot lineages to be recognized in the macrofossil record. Because of its phylogenetic position, and the documentation of intralineage temporal morphological change provided by an extensive fossil record, the Platanaceae provide crucial information for understanding phylogenetic relationships at the base of the eudicots. Staminate and pistillate inflorescences and flowers from Coniacian-Santonian strata of Georgia, U.S.A., are an important addition to the known fossil record of Platanaceae. Staminate inflorescences are sessile heads, 0.9-2.5 mm in diameter, formed by ca. 40 closely spaced flowers. Staminate flowers have a whorled, tetramerous, unicyclic perianth, and a tetramerous androecium. Each stamen is opposite a perianth member, and all stamens are inserted on a short androecial ring. Anthers are elongate, the connective is abaxially displaced and is expanded at the apex into a protruding, wedge-shaped extension. Four short androecial appendages are placed on the androecial ring and alternate with the stamen bases. Pistillate inflorescences are sessile heads, 1.1-2.4 mm in diameter, formed by ca. 40 flowers. Pistillate flowers have two weakly differentiated perianth cycles: the outer cycle is connate into a tube, and the inner cycle is also connate, but with four free apical lobes. The apocarpous gynoecium has eight carpels, with two carpels opposite each of the free apical lobes of the inner perianth. Carpels are conduplicate, wedge-shaped, and lack a style. Like other Cretaceous platanaceous taxa, the flowers described from the Coniacian-Santonian of Georgia have a well-developed perianth and a fixed floral merosity, but their tetramerous floral plan differs from the pentamerous pattern of previously described Cretaceous and early Tertiary platanaceous flowers.