DIFFERENTIATION, AFFINITIES AND PALEOENVIRONMENTAL SIGNIFICANCE OF THE MEGASPORES ARCELLITES AND BOHEMISPORITES IN WEALDEN AND OTHER CRETACEOUS SUCCESSIONS

The ultrastructure of the exine (protective wall) of the Wealden (Lowe r Cretaceous) megaspore species Arcellites hexapartitus is very simila r to that of two other species, A. disciformis and A. santacrucensis, first recovered from mid Cretaceous formations in Greenland and Argent ina respectively. The wall of Wealden A. vectis differs in that the ou ter exoexinal layer is thinner, but it is otherwise similar. The morph ology of these and several other taxa from Barremian and younger Creta ceous deposits coupled with, in some cases, an association with the mi crospore Crybelosporites, suggests derivation from a single, now extin ct, group of plants within the water fern order Marsileales. A. medusu s differs somewhat from this group of species in the construction of b oth the neck around the triradiate suture and the wall. The oldest Wea lden specimens are Valanginian. The possibility that diminutive exampl es might be its microspores suggests a different evolutionary lineage, perhaps linked to Aptian and younger Ariadnaesporites. Megaspores hit herto considered to belong to the same group as Arcellites, and theref ore usually identified as A. pyriformis, first appear in Berriasian de posits in the Wealden succession. The construction of the neck of this spore has, however, led to the suggestion that the species should per haps be referred to a different genus. Bohemisporites was erected just over 10 years ago to accommodate apparently identical Czech specimens , but has been little used since then. The structure of the wall of bo th British and Czech material, which includes a well-ordered exoexinal layer, confirms that assignment to this genus is appropriate. Althoug h the spores may be derived from plants that grew in or near water, re ference to the water fern group is now questioned. Links with the lyco psid order Selaginellales are possible. Regardless of the origins of t he megaspores discussed, their palynological associations and recovery in abundance from sediments laid down in quiet freshwater bodies (pon ds, lakes, disused river channels) render all of them useful indicator s of environmental conditions and composition of local vegetation, and might also support inferences from other data for a seasonal climate during much of the period of deposition of the Wealden succession. (C) 1996 Academic Press Limited