The role of Mid-Palaeozoic mesofossils in the detection of early bryophytes

Abstract

Recently discovered Silurian and Devonian coali¢ed mesofossils provide an additional source of data on early embryophytes. Those reviewed in this paper are considered of some relevance to understanding the early history of bryophytes while highlighting the di¤culties of recognizing bryophytes in often very fragmentary fossils. The ¢rst group comprises sporophytes in which terminal sporangia contain permanent dyads and tetrads. Such spores (cryptospores) are similar to those found dispersed in older Ordovician and Silurian strata, when they are considered evidence for a land vegetation of embryophytes at a bryophyte grade. The phylogenetic signi¢cance of plants, where the axes associated with both dyadand tetrad-containing sporangia are branching, a character state not found in extant bryophytes, is discussed. The second group comprises axial fossils, many with occasional stomata, in which central conducting strands include G-type tracheids and a number of novel types of elongate elements not readily compared with those of any tracheophyte. They include smooth-walled, evenly thickened elongate elements as well as those with numerous branching § anastomosing projections into the lumen. Some of the latter bear an additional microporate layer, but the homogenized lateral walls between adjacent cells are never perforate. Such cells, which occur in various combinations in central strands, are compared with the leptoids and hydroids of mosses, hydroids of liverworts and presumed water-conducting cells in coeval Lower Devonian plants such as Aglaophyton. It is concluded that lack of information on the chemistry of their walls hampers sensible assessment of their functions and the a¤nities of the plants. Finally, a minute fossil, comprising an elongate sporangium in which a central cylindrical cavity containing spores and possible elaters terminates in a complex poral dehiscence apparatus, is used to exemplify problems of identifying early bryophytes. It is concluded that further progress necessitates the discovery of pre-Upper Silurian fossils with well-preserved anatomy, as well as a re-evaluation of criteria used to assess existing and new Devonian fossils for bryophyte a¤nity.

Novel conducting tissues in Lower Devonian plants

“…The tracheid itself is amongst the widest recorded in the strand, occurs towards the centre and is flanked by those with larger perforations, in some cases so wide and closely spaced that the intervening wall has fragmented. Such variation may provide evidence in support of a hypothesis suggesting that uniform thickening of the intervening secondary wall was the ancestral tracheid state, and that pitting of lateral walls evolved in response to extension growth and increased requirements for lateral movement of water in plants of greater stature (Edwards, 1993(Edwards, , 1999(Edwards, , 2000Cook & Friedman, 1998). It is of some relevance here that Kobiyama & Crandall-Stotler (1 999) postulated that the G-type tracheid pitting evolved via aggregations of plasmodesmata into primary pit fields, and that this character state was regarded as a synapomorphy of the zosterophylls, lycopods and other tracheophytes, when plasmodesmata-linked characters were plotted on to cladograms involving algae, bryophytes and tracheophytes developed by Kenrick & Crane (1997).…”

“…connecting annular secondary thickenings (Figs 34, 35) (Edwards, 2000), although individual tracheids have pits of similar size. Figure 34 shows an example where the pits are smallest (200-400 nm) and thus approaching the size of plasmodesmata.…”

Early land plant spore assemblages from the Late Silurian of Skåne, Sweden

“…However, few plant remains with in situ cryptospores have been reported (Wellman et al, 2003;Hagström & Mehlqvist 2012). Trilete spores have more often been found in situ in meso-or macrofossils of vascular plants (Edwards 2000;Edwards & Wellman, 2001;Edwards & Richardson 2004). In some cases, several spore morphotypes have been found in the sporangia of the same plant species.…”

A timeline for terrestrialization: consequences for the carbon cycle in the Palaeozoic

“…The fossil record of bryophytes is meagre in comparison and certainly not representative of the likely true diversity. Tantalizing evidence from minute fragments of charcoal preserved in some early sites (figure 2n) testifies to a much greater diversity at the bryophyte grade [40][41][42], but the precise affinities of these fragmentary fossils and their relationships to major extant clades remain elusive.…”