The vegetative structure of a Lower Cretaceous conifer from Arkansas: further implications for morphospecies concepts in the Cheirolepidiaceae

A reconsideration of the vegetative structure of a cheirolepidiaceous conifer with Pseudofrenelopsis parceramosa foliage is presented based on new collections from the Lower Cretaceous Holly Creek Formation (Aptian/Albian) of Arkansas. Analysis of these fossils contributes new data to an emerging whole plant concept, and allows for comparison with a previous reconstruction of a P. parceramosa plant from the English Wealden. The Arkansas vegetative shoots differ in lacking any open-sheathed leaves, the presence of papillae on all subsidiary cells, and the absence of a hypodermis. Furthermore, the Arkansas conifer exhibited helically arranged branches throughout, in contrast to the whorled branching of the Wealden plant reconstruction. The woods from Arkansas and England have identical thickened bars between the tracheid pits, and a similar arrangement of crossfield pits. However, the Arkansas wood has much higher rays, and lacks mixed pitting. These findings, along with previous accounts of significant differences in pollen cone structure, indicate that the Arkansas and English Wealden conifers represent different biological species with generally similar leafy shoots. Therefore, concepts regarding the biogeography, paleoecology, and phylogeny of P. parceramosa, most of which assume that this morphotaxon is a proxy for a single species of reconstructed plant, must be reconsidered.