Sprecher
Beschreibung
Middle Ordovician (Darriwilian) cryptospore assemblages from Saudi Arabia constitute one the world’s oldest record of earliest land plants, thus providing critical information on early evolution of spore-producing plants. In living cryptogams, the tapetum or tapetal-like tissue that lines the sporangial locule produces sporopollenin which is then deposited exogenously onto the developing spore tetrad. The confirmed presence of a tapetum in liverworts, and new information on spore wall ultrastructure in extant hornworts suggest new ways to assess tapetal activity and sporopollenin deposition in an evolutionary context.
Here, we review what can be inferred about spore development based on lower Paleozoic cryptospore ultrastructure and compare that to what we now know about basal bryophyte sporoderm development. Laminar sporoderm is characteristic of Cambrian and Early Ordovician cryptospores, implying a strictly endogenous source for the sporopollenin. By the late Cambrian, likely endogenous pathways had evolved to produce multiple enclosing spores with the final units as dyads (e.g., Agamachates). A marked change first occurs in Cryptotetras from Darriwilian strata of Saudi Arabia, in which a single basal lamina is found underlying a homogenous spore wall. This implies exogenous addition of sporopollenin to a pre-existing endogenously-produced framework of lamellae, occurring inside of a space separated from the external environment (sporangium). The sporopollenin is produced in cells lining the sporangium; i.e., a tapetum. Thus, the fossil record of spore walls records the serial acquisition of developmental pathways over millions of years from various sources that culminated in a canalized scheme seen today in their descendants, the embryophytes.