Abstract Detail

Paleobotany

Strother, Paul [1].

Cryptospores record the canalization of meiosis in the evolving sporophyte.

Studies in classical morphology by Bower and in modern bryophyte sporogenesis by Brown & Lemon both indicate that the evolution of the plant sporophyte began with spores and that the vegetative plant body evolved later. These observations are consistent with the direct reading of the fossil record of cryptospores which precede the first occurrence of sporangiate plant axes by tens of millions of years. Cryptospores comprise the miospores of early land plants which do not posses a trilete mark and are typically dispersed as dyads and tetrads. But they also include the meiotic products (diaspores) of algae that were evolving in response to selection in subaerial habitats during Early Paleozoic time. The recently described Cambrian cryptospore, Adinosporus, possesses laminated sporoderm which appears homologous to the primitive condition in crown group embryophytes. Adinosporus does not form regular meiotic tetrads. Instead, it is derived from a sporocyte in which karyokinesis and cytokinesis are decoupled to produce irregular combinations of enclosed dyads. Adinosporus joins Agamachates and other newly-described Cambrian genera, Spissuspora and Vidalgea in their common lack of geometrically regular configurations of attached spore-bodies. In fact, consistently regular combinations of meiotic dyads and tetrads are not found in the fossil record in strata older than Darriwilian (mid-Ordovician). Therefore, cryptospores s.l. appear to record a shift in sporogenesis in which karyokinesis and cytokinesis become more tightly coupled, resulting in the production of pairs of isomorphic dyads (Dyadospora and Didymospora) and tetrads (Cryptotetras and Tetrahedraletes). Cryptospores of Darriwilian age have long been considered to be the spores of the earliest embryophytes, and, consequently, to record of the origin of land plants. However, the stratigraphic first occurrences of these taxa is really a record of the canalization of meiosis in diploid sporocytes. There is some evidence from spore wall architecture to indicate that these sporocytes were derived from a unilocular sporangium which possessed a tapetum, but even this stage in sporophyte evolution could have occurred very early on in the evolutionary path to embryonic development in plants. A literal reading of the fossil record in the context of the Bower model would indicate, therefore, that spores began in the Cambrian, by mid-Ordovician time, spores were produced in a sporangium comprised of diploid tissue, but that the remaining structural features of the plant sporophyte (including the embryo, itself) were yet to evolve.

1 - Boston College, Paleobotany Laboratory, Weston Observatory, 381 Concord Road, Weston, MA, 02493, USA

Keywords:
land colonization
Cambrian
Palynology
origin of land plants
meiosis.

Presentation Type: Oral Paper
Session: 23, Paleozoic and Mesozoic Paleobotany
Location: 102/Savannah International Trade and Convention Center
Date: Tuesday, August 2nd, 2016
Time: 8:15 AM
Number: 23001
Abstract ID:438
Candidate for Awards:None