Oldest recorded in situ tracheids

Abstract

RECENT critical reviews¹ have suggested that evidence from microfossils² (that is spores with triradiate marks, sheets of “cells” or tubes with tracheid-like thickenings) should not be considered sufficient to demonstrate the existence of vascular plants in Silurian times; the only acceptable evidence being megafossils with tracheids in situ. Such was the evidence presented by Lang³ when he described smooth axes from the Downton Series (Pridolian εβ₂) of the Welsh Borderland. These vascularised axes were found in association with two species of Cooksonia, very simple plants with smooth forking axes terminating in globose sporangia which contained spores bearing trilete marks. Megafossils morphologically similar to Cooksonia have been described from strata of similar age throughout the world, but the Lang specimens are the only ones with unequivocal tracheids and thus have hitherto been considered the earliest vascular plants. We report here the occurrence of a vascular plant in the slightly older Whitcliffian strata (Ludlow Series) of South Wales.