The geographic distribution of these Middle Ordovician cryptospores might suggest a Gondwanan origin for land plants ( Rubinstein et al., 2010 Wellman, 2010).
470 Ma), with cryptospore assemblages acquiring embryophyte characteristics indicating establishment of a bryophytic style of meiosis by this time ( Strother & Taylor, 2018). However, a dramatic shift in morphology occurs in the Middle Ordovician (c. An association of cryptospores with bryophytic organisms is suggested by some Lower Devonian bryophyte-like plants that possess in situ cryptospores ( Edwards, Wellman, & Axe, 1999) however, some putative protracheophyte fossils, such as Devonian Cooksonioid-species also produced dyad and tetrad spores, and thus they could be characteristic of multiple early land plant lineages ( Edwards et al., 2014).Ĭambrian and Lower Ordovician cryptospores exhibit morphological affinity with streptophyte algal lineages rather than embryophytes, with sporopollenin in the presumed wall of the zygote that serves as the dispersal or overwintering entity ( Strother & Taylor, 2018). Among extant land plants obligate tetrads occur in liverwort taxa, e.g., Sphaerocarpus terrestris, Haplomitrium gibbsiae, Riccia curtisii ( Austin, 1869 Douin, 1909 McAllister, 1916 Renzaglia et al., 2015 Strasburger, 1909), with ultrastructural and geochemical analyses indicating some Silurian cryptospore walls are chemically similar to those of extant liverwort spores ( Steemans, Lepot, Marshall, Le Herisse, & Javaux, 2010). Obligate tetrads might be advantageous in habitat colonization especially if species are dioecious ( Gray, 1985). Cryptospores occur as monads, dyads and tetrads and are presumably meiotic products, with the dyads and tetrads either being naked or obligate, the latter with the dyads or tetrads enclosed by an envelope ( Edwards et al., 2014). Monolete spores have a single line at the position where they were separated from the spore mother cell whereas trilete spores have three radiating lines indicating derivation from a meiotic tetrad. Cryptospores are a non-phylogenetic assemblage of spores with sporopollenin containing walls distinct from trilete and monolete spores and pollen grains, but resembling land plant spores ( Strother, 1991).
1 Edwards, Duckett, & Richardson, 1995 Edwards et al., 2014 Rubinstein, Gerrienne, de la Puenta, Astini, & Steemans, 2010 Strother & Taylor, 2018 Strother, Traverse, & Vecoli, 2015 Taylor, 1995 Wellman, Osterloff, & Mohiuddin, 2003). Florent, in Current Topics in Developmental Biology, 2019 2.1 Cryptospores and cryptophytesįew identified macrofossils represent the early colonization of land, but cryptospores recovered from terrestrial or near offshore deposits dating from as early as the mid-Cambrian are plentiful ( Fig. The plants that produced the early trilete spores were land plants, possibly from bryophyte-like organisms as well as vascular plants. 420 mya, cryptospores virtually disappeared, possibly because trilete spore-producing plants outcompeted the cryptophytes. A major change in spore floras occured in the Early Silurian when cryptospore diversity decreased and trilete spore diversity increased. For the next 30 million years, from the Middle Ordovician to Early Silurian, the cryptospores were globally distributed, but showed little variation suggesting an evolutionary stasis ( Kenrick et al., 2012 Gensel, 2008 Wellman, 2004). The first cryptospores appeared in the Middle Ordovician (roughly 470 mya (million years ago)) and increased in morphological diversity shortly after. The affinity or affinities of cryptospore-producing plants and their exact morphology are unclear because the fossils are fragmentary nonetheless, they were only several millimeters tall with a unique combination of features that are not found in living taxa. They were thought to be bryophytes ( Kenrick et al., 2012 Wellman, 2004), but more recently, they have been called ‘cryptophytes’ – an extinct assemblage of land plants (a non-monophyletic grade) predating the various modern bryophyte lineages of land plants ( Edwards et al., 2014).
The earliest plant life on land reproduced via cryptospores, but there is uncertainty about the taxonomic affinity of these plants. Nagalingum, in Encyclopedia of Evolutionary Biology, 2016 Bryophytes as the Earliest Land Plants?
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