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

摘要

An anatomically preserved mature cycadalean stem from Allen Formation (Late Cretaceous) at the Bajo de Santa Rosa locality, Rio 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. Among the Encephalartoideae, the Tribe Encephalarteae is characterized by the presence 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 group disappeared after the Cenozoic (Bororoa), while in North and Central America it remains as an endemic (Dioon). The Encephalarteae emerged in the Triassic of western Laurasia (Charmorgia). In Gondwana, the Tribe is found in 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 endemic group in southern Africa (Encephalartos) and Australia (Lepidozamia and Macrozamia).