Revisiting R.H. Tschudy’s fern-spore spike concept 40 years later: connecting Tschudy’s ‘disaster taxon’ concept with paleopolyploidization in Stenochlaena J. Sm

Abstract

The Cretaceous/Paleogene (K/Pg) boundary fern-spore spike concept was first introduced by R.H. Tschudy 40 years ago and established the precept that ferns are so-called ‘disaster taxa’ that flourish after natural disasters because of the high colonization potential of their wind-blown spores. Among the least understood topics at the crux of contemporary K/Pg boundary fern-spike studies is whether Stenochlaena-like or other stem blechnaceous ferns were among those that flourished after the K/Pg event, contributing to the Laevigatosporites-dominated phase of the fern-spore spike. For instance, dispersed Stenochlaena-like spores – e.g. Polypodiisporites usmensis (van der Hammen) Khan & Martin – first appear in the upper Eocene stratigraphic record of South America, whereas megafossils of Stenochlaena-like ferns are first known from the upper Paleocene strata of this same region. Beyond this traditional focus on taphonomic and taxonomic uncertainty regarding the identity of dispersed spore producers, however, there is further discordance between contemporary paleobotanical and molecular phylogenetic perspectives on the timing of diversification of stem lineages of blechnaceous ferns in relation to the K/Pg boundary. This investigation reconciles these two perspectives by constraining molecular clock divergence time estimates using contemporary fossil data. If this reconciliation is correct, then paleopolyploidization (whole genome duplication or WGD) associated with the origin of the genus Stenochlaena J. Sm. predated evolution of polocytic stomata, a heavily ornamented exospore, and a hemi-epiphytic to epiphytic habit observed in the crown lineage. Critically, this novel perspective elicits a merger of the K/Pg boundary fern spike and K/Pg boundary-WGD concepts, suggesting a link between polyploidy and the ‘disaster taxon’ concept.