Abstract Detail

Green Land: Multiple Perspectives on Green Algal Evolution and the Earliest Land Plants

Strother, Paul [1].

The fossil record of land plant origins.

The origin of land plants (embryophytes) represents a fundamental evolutionary stage involving the de novo origin of complex multicellularity within the streptophyte lineage. Phylogenetic thinking obliges us to view the origin of land plants ultimately as a species bifurcation in a lineage tree; indeed, this has to be true, but it masks the enormity of what actually transpired as a series of evolutionary novelties (sensu Wagner) combined with repurposed co-opted algal genes to evolve the developmental program that was expressed in the first plant embryo. Recently published molecular studies now robustly place the origin of embryophytes in the middle Cambrian, but the oldest fossil record of macroscopic plant compressions remains steadfastly dated to the middle Silurian. The explanation for this temporal discrepancy has been written off to the “imperfections” of the sedimentary and fossil record, but my view is that the fossil record does, in fact, provide a valid record of the algae-embryophyte transition. Instead of being viewed as a disjunct record of algal vs. plant spores, cryptospores from Cambrian to Devonian strata can be interpreted to provide a record of the algae-embryophyte transition. But they do so under the assumption that these early spores were produced by charophyte style of reduction division that is not the same as meiosis in plants today (isometric tetrads, which first occur during the Darriwilian, probably track the beginnings of plant-like meiosis). The transition from spores borne in permanent tetrads to free trilete spores near the Ordovician/Silurian boundary, may track the incorporation of the use of callase during sporogenesis. The assignment of Silurian cuticle-like tissues and organic tubes to the fungi is only weakly supported by morphology. An alternative interpretation as “pre-vascular” elements could shed light on the timing of the evolution of the apical cell and axial development in the streptophyte ancestor of the embryophytes. The antithetic origin of the plant sporophyte as espoused by Bower in 1908 remains a useful paradigm for the interpretation of fossil remains. Within the context of that paradigm, I try to show that a timeline of plant developmental origins can be derived from a literal interpretation of fossils that tracked the serial origins of evolutionary novelties in the evolving streptophyte lineage that became the land plants.

1 - Boston College, Weston Observatory, 381 Concord Road, Weston, MA, 02493, United States

spore dyads

Presentation Type: Symposium Presentation
Number: 0004
Abstract ID:154
Candidate for Awards:None

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