Cryptospores and cryptophytes reveal hidden diversity in early land floras

Abstract

50I.50II.52III.53IV.66V.71VI.72VII.7475References75 Summary Cryptospores, recovered from Ordovician through Devonian rocks, differ from trilete spores in possessing distinctive configurations (i.e. hilate monads, dyads, and permanent tetrads). Their affinities are contentious, but knowledge of their relationships is essential to understanding the nature of the earliest land flora. This review brings together evidence about the source plants, mostly obtained from spores extracted from minute, fragmented, yet exceptionally anatomically preserved fossils. We coin the term ‘cryptophytes’ for plants that produced the cryptospores and show them to have been simple terrestrial organisms of short stature (i.e. millimetres high). Two lineages are currently recognized. Partitatheca shows a combination of characters (e.g. spo‐rophyte bifurcation, stomata, and dyads) unknown in plants today. Lenticulatheca encompasses discoidal sporangia containing monads formed from dyads with ultrastructure closer to that of higher plants, as exemplified by Cooksonia. Other emerging groupings are less well characterized, and their precise affinities to living clades remain unclear. Some may be stem group embryophytes or tracheophytes. Others are more closely related to the bryophytes, but they are not bryophytes as defined by extant representatives. Cryptophytes encompass a pool of diversity from which modern bryophytes and vascular plants emerged, but were competitively replaced by early tracheophytes. Sporogenesis always produced either dyads or tetrads, indicating strict genetic control. The long‐held consensus that tetrads were the archetypal condition in land plants is challenged.

Extant diversity of bryophytes emerged from successive post-Mesozoic diversification bursts

“…This phylogeny, together with the one previously produced for mosses 15 , was time-calibrated by relaxed molecular clock analyses, modelling the uncertainty of fossil ages used for calibration using three increasingly conservative prior distributions (see Methods). The dated phylogenies were then employed to determine net diversification rates (sensu 16 ) at different taxonomic levels, and also the tempo and amplitude of shifts in diversification rates through time. liverworts, within the range reported for ferns and are much more recent than those observed in gymnosperms (Fig.…”

“…In analysis I, the lower and upper bounds of the lognormal distribution associated to each fossil were set so as to encompass the timespan of the geological era attributed to the fossil. The lower bound for the root node was set at 475 Ma, which corresponds to the oldest nonambiguous land plant fossil 16 . In analysis II, the lognormal distribution was centred on the fossil age estimate and the upper bound reached, for each constrained calibration node, 475 Ma.…”

“…10 for review). One hypothesis for the comparatively lower species richness of bryophytes as compared with angiosperms is that, in line with the Sequential Replacement Hypothesis, and similar to the Cryptophytes that encompass a pool of diversity from which the ancestor of modern bryophytes and vascular plants emerged 16 , bryophytes were competitively replaced by early tracheophytes. Diversification analyses fuelled by the ever-increasing availability of molecular phylogenies and new analytical tools challenged this view but returned contrasting scenarios of bryophyte evolution.…”

Earliest land plants created modern levels of atmospheric oxygen

“…The evolution of land plants is the obvious candidate, with the first nonvascular plants (ancestors of extant mosses, liverworts, and hornworts) colonizing the land in the Middle to Late Ordovician (∼470-445 Ma), followed by the first vascular plants in the Silurian (∼445-420 Ma) and Early Devonian (∼420-390 Ma; Fig. 1) (18,19).…”

Late Ordovician (Katian) spores in Sweden: oldest land plant remains from Baltica

“…The palynological assemblages are dominated by marine microfossils but we have here for the first time also identified spores from land plants represented by cryptophytes (sensu Edwards et al 2014). The presence of a sparse but wellpreserved cryptospore assemblage including Tetrahedraletes S c a n i a n GFF 136 (2014) Fig.…”