bryophytes (including mosses liverworts and hornworts) first appeared in the fossil record around the middle ordovician.
bryophytes (including mosses liverworts and hornworts) first appeared in the fossil record around the middle ordovician.
when do bryophytes (including mosses liverworts and hornworts) first appear in the fossil record?
around the middle ordovician
the evidence of plant evolution changes dramatically in the ordovician with the first extensive appearance of spores in the fossil record (cambrian spores have been found, also). the first terrestrial plants were probably in the form of tiny plants resembling liverworts when, around the middle ordovician, evidence for the beginning of the terrestrialization of the land is found in the form of tetrads of spores with resistant polymers in their outer walls. these early plants did not have conducting tissues, severely limiting their size. they were, in effect, tied to wet terrestrial environments by their inability to conduct water, like extant liverworts, hornworts, and mosses, although they reproduced with spores, important dispersal units that have hard protective outer coatings, allowing for their preservation in the fossil record, in addition to protecting the future offspring against the desiccating environment of life on land. with spores, plants on land could have sent out large numbers of spores that could grow into an adult plant when sufficient environmental moisture was present.
timeline of plant evolution
bryophytes are non-vascular plants encompassing mosses, liverworts, and hornworts, which most often form symbioses with members from the cyanobacterial genus "nostoc". depending on the host, the cyanobiont can be inside (endophytic) or outside the host (epiphytic). in mosses, cyanobacteria are major nitrogen fixers and grow mostly epiphytically, aside from two species of "sphagnum" which protect the cyanobiont from an acidic-bog environment. in terrestrial arctic environments, cyanobionts are the primary supplier of nitrogen to the ecosystem whether free-living or epiphytic with mosses. cyanobacterial associations with liverworts are rare, with only four of 340 genera of liverworts harbouring symbionts. two of the genera, "marchantia" and "porella," are epiphytic, while the genera "blasia" and "cavicularia" are endophytic. in hornworts however, endophytic cyanobionts have been described in more than triple the number of genera relative to liverworts. bryophytes and their cyanobacterial symbionts possess different structures depending on the nature of the symbiosis. for instance, colonies of cyanobacterial symbionts in the liverwort "blasia" spp. are present as auricles (small dots) between the inner and outer papillae near the ventral surface of the liverworts; whereas, cyanobionts in the hornworts "anthoceros" and "phaeoceros" are present within the thallus', in specialized slime cavities. however, cyanobacteria first must locate and physically interact with their host in order to form a symbiotic relationship. members of the cyanobacterial genus "nostoc" can become motile through the use of hormogonia, while the host plant excretes chemicals to guide the cyanobacteria via chemotaxis. for instance, liverworts in the genus "blasia" can secrete hif, a strong chemo-attractant for nitrogen-starved and symbiotic cyanobacteria. cells of "nostoc punctiforme", which have been shown to possess genes encoding proteins that complement chemotaxis-related proteins within flowering plants belonging to the genus "gunnera".
cyanobiont