A Devonian Seed Megaspore

Abstract

THE earliest record of fossil structures securely identified as seeds is from the Lower Carboniferous (Mississippian). From Britain, in particular, a considerable diversity of seeds, some of surprising complexity, have been described from rocks of this age. One type of Pre-Carboniferous stem, Callixylon, previously believed on the basis of its anatomy to be a gymnosperm, has recently¹ been shown to be at a heterosporous pteridophytic level of organization. This has automatically cast doubt on the assignation of any fossil stems to the gymnosperms on the basis of anatomy alone, before the appearance of actual seeds in the fossil record. Maslov's² supposed reproductive body from the Russian Devonian, which he has compared with Carboniferous seed-like structures, has not been generally accepted^3,4 as a record of a Devonian seed. Arnold's statement⁵ that “no seeds of any kind have yet been found in the Devonian” still represents the generally held view of palæobotanists. We are left with a sequence consisting of the earliest appearance of vascular plants, of an apparently purely homosporous type, in the Upper Silurian and early Devonian, followed by the attainment of heterospory in at least two groups (lycopsids and macrophyllous plants) by the later Devonian and culminating in the formation of true seeds in the Carboniferous. The purpose of this communication is to record the occurrence of a fossil which we regard as the first direct evidence of the attainment of a seed plant level of organization in the Devonian.