A new genus of Late Cretaceous cycad stem from Argentina, with reappraisal of known forms

Abstract

An anatomically preserved mature cycadalean stem from Allen Formation (Late Cretaceous) at the Bajo de Santa Rosa locality, Río Negro Province, Argentina, is described and assigned to Chamberlainia pteridospermoidea gen. nov. et sp. nov. The new taxon is characterized by centrifugal and centripetal polyxyly, medullary vascular bundles, mucilage canals and concentric extraxylary bundles. Amongthe Encephalartoideae, the Tribe Encephalarteae is characterized by the présence of centripetal polyxyly, medullary vascular bundles and extrafascicular vascular bundles. Fossil stems of the Encephalarteae are: Fascivarioxylon, Chamberlainia, Worsdellia, and Menucoa. Lyssoxylon and Brunoa characterize the Tribe Diooeae with a lack of centripetal xylem and absence of medullary vascular bundles; the tribe appeared simultaneously in the Triassic of western Laurasia (Lyssoxylon) and southwestern Gondwana (Micheliilloa). In South America, the groupdisappeared after the Cenozoic (Bororoa), whilein North and Central America it remains as an endemie (Dioon). The Encephalarteae emerged in the Triassic of western Laurasia (Charmorgia). In Gondwana, the Tribeis foundin the Jurassic of India (Fascivarioxylon), the Cretaceous of Antarctica (Centricycas) and Argentina (Chamberlainia and Worsdellia), and the Tertiary (Menucoa) of Argentina. The Tribe remains as an extant endémie groupin southern Africa (Encephalartos) and Australia (Lepidozamia and Macrozamia).