Cavilignum pratchettii gen. et sp. nov., a novel type of fossil endocarp with open locules from the Neogene Gray Fossil Site, Tennessee, USA

Abstract

A suite of fossil endocarps representing a new taxon from the early Pliocene Gray Fossil Site, Tennessee, USA, is described as Cavilignum pratchettii gen. et sp. nov. Cavilignum is represented by circular to oblong endocarps that have a truncate apex, a mucronate base, and a smooth outer surface. The endocarp wall has three layers. The outermost layer is of uncertain composition. The inner wall structure is fibrous, with an outer layer of radially oriented fibers and an inner layer of circumferentially oriented fibers. The endocarps typically have four chambers (locules) divided by thick septa; two narrow canals penetrate the septa from the apex to the base of each endocarp. The locules are open apically. Because no evidence of structures that may have sealed the chambers was found, Cavilignum is interpreted as having open germination pores. Apically oriented, open germination pores most obviously suggest an affinity with the angiosperm family Symplocaceae, but endocarps of Symplocaceae differ from Cavilignum in several critical characteristics (e.g., wall histology, presence of a basal pit). While several other groups (e.g., Anacardiaceae, Cornales) produce drupaceous fruits with endocarps comparable in some characteristics to those of Cavilignum, none is structurally congruent enough to suggest an affinity with the new fossil genus. Thus, Cavilignum cannot be conclusively assigned to a known extinct or extant angiosperm genus or family. Cavilignum is the first extinct plant genus described from the Gray Fossil Site macroflora.