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

Recently discovered Silurian and Devonian coalified mesofossils provide an additional source of data on early embryophytes. Those reviewed in this pap er are considered of some relevance to understanding the early history of b ryophytes while highlighting the difficulties of recognizing bryophytes in often very fragmentary fossils. The first group comprises sporophytes in wh ich terminal sporangia contain permanent dyads and tetrads. Such spores (cr yptospores) are similar to those found dispersed in older Ordovician and Si lurian strata, when they are considered evidence for a land vegetation of e mbryophytes at a bryophyte grade. The phylogenetic significance of plants, where the axes associated with both dyad-and tetrad-containing sporangia ar e branching, a character state not found in extant bryophytes, is discussed . The second group comprises axial fossils, many with occasional stomata, i n which central conducting strands include G-type tracheids and a number of novel types of elongate elements not readily compared with those of any tr acheophyte. 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 compare d with the leptoids and hydroids of mosses, hydroids of liverworts and pres umed water-conducting cells in coeval Lower Devonian plants such as Aglaoph yton. It is concluded that lack of information on the chemistry of their wa lls hampers sensible assessment of their functions and the affinities of th e plants. Finally, a minute fossil, comprising an elongate sporangium in wh ich a central cylindrical cavity containing spores and possible elaters ter minates in a complex poral dehiscence apparatus, is used to exemplify probl ems of identifying early bryophytes. It is concluded that further progress necessitates the discovery of pre-Upper Silurian fossils with well-preserve d anatomy, as well as a re-evaluation of criteria used to assess existing a nd new Devonian fossils for bryophyte affinity.