Recent Paleo

There is indirect evidence of an extensive land biota starting ~800 -850 Ma. This includes:
1. The clay example I started class with—implies the fixation of enough organic carbon to form humic acids; these weather feldspars and other minerals into clay.
a. But this claim has recently been challenged by the argument that the particular kind of clays could also be generated by abiotic processes; not humic acids.

2. δ13C in nearshore organic carbon suggesting fixation of carbon by land plants;
a. However the isotope evidence is also consistent with freshwater algae or intertidal/esturine algae; not necessarily land plants, per se.
3. Still another argument concerns meandering rivers—here the claim is that you cannot make meandering rivers until there is enough plant cover to hold the soil in the floodplain effectively;
a. the counter argument is that there are meandering systems in abiotic places, like Mars and in the deep sea (like the La Jolla Canyon); also it is not clear that algae, without roots, really could hold soil enough to make meandering river systems.

According to this molecular analysis, vascular plants (those with a means of transporting water within their tissues) appear ~700±45 Ma.
Sound familiar? About the same time frame for diversification of animals and rise in atmospheric O2

1. Reanalysis of the molecular data with a larger number of proteins and longer sequences suggests divergence closer to ~490-425 Ma, and fossils (lower Ordovician ~467 Ma)
2. Hence, the land plant diversification may be much younger than that of animals.

1. Molecular dates tend to place stem group terrestrial spiders & millipedes in the Ediacaran or Cambrian ~550-475 Ma. These dates are similar to the younger land plant molecular divergence dates.
2. Have tracefossils of arthropods by Cambrian-early Ordovocian in dune sandstones (MacNaughton 2002) possibly as old as ~500 Ma.
3. Upper Ordovician trace fossils of back-filled burrows in paleosols—suggestive of millipeds (Retallack 1987)
4. First millipedes and scorpions in Silurian ~438 Ma (possibly Ordovician), spiders in Devonian ~410 Ma (Milulic 1990)

1. Modern plants associated with Arbuscular mycorrhizal (AM) fungi that contribute to nutrient uptake from soils and decaying organic matter.
2. earliest known AM fungi from Marine dolomite in Wisconsin (Guttenberg Formation; mid-Ordovician—460-455Ma)—spores associated with hyphae like modern Glomalean Fungi
3. This record is ~50-60 my older than the Rhynie Chert finds.
4. Use of this fossil find as a tie point in molecular phylogenies suggests major fungal groups diverged about 600-620 Ma (That date again!)
5. Suggests AM associations first established with bryophytes well before rise of vascular plants.

1. By ~475 Ma have spores--typically divided into fours (tetrads-bound by a membrane to each other) or twos as well as non-diagnostic cuticles and vascular bundles that may or may not have terrestrial affinities.
2. Suggests land plants because tetrads imply haploid cells and the sporopollenin coat is decay-resistance.
3. Suggestive of liverworts—evidence from dyads/tetrads, spore-wall structure and cuticles
4. Fossil evidence of bryophyte (liverwort)-like plants associated with spores from the Ordovician in Oman (Wellman 2003).
5. 450-420 Ma have δ13C evidence of mosses and bryophytes in Appalacian rocks suggesting ‘stem-group’ land plants (Tomescu et al. 2009, Palaeo3).
6. In mid-late Early Silurian have trilete spores that undergo a major radiation in shape of the trilete mark.
7. By Late Silurian ~396±12 Ma, have rhyniophyte flora a. In Rhynie Chert (Scotland)—a hot-spring associated freshwater pool assemblage with preservation of complete organisms including sperm and flagella (Taylor 2005) b. with simple, possibly non-vascular stick-like rootless and leafless stems (axes) with terminal sporangia—includes Cooksonia in Euramerican deposits; “Cooksonia” includes some pre-vascular plants and probable vascular plants (Kenrick 1997) c. in Australia, plant remains in marine deposits (dated by graptolites) include Baragwanathia—a lycopsid with true roots and leaves (DiMichele 1992); 423-398 Ma, Late Silurian-E. Devonian. d. If dating is correct, suggests plant assemblages much more advanced in Australia than other places
8. Rhyniophytes probably formed turfs of single plants (with sprawling rhizomes and short vertical axes); lack of roots restricts such plants to wet environments.

In the Devonian (~410 Ma) have the evolution of lobed-fin fishes (Maisey 1996; Coates 2001);
1. the tetrapod limb develops by reduction in fin rays within the ‘lobe fins
2. A genetic homology extends only to upper limb bones whereas the lower limb bones and digits are independently derived from structures in fish proper;
3. first tetrapods have as many as 8 digits which are later standardized at 5 digits (Maisey 1996).

Lungs and feet evolve in fully aquatic taxa; first fully terrestrial tetrapods by ~327 Ma.

Early seed plants include naked seed plants—Gymnosperms-- conifers, cycads and Pteridosperms, which diversified as land dried out in the Permian ~306 Ma

First appear in late Carboniferous ~310 Ma as Pelycosaurs—“Sail-backed Synapsids”—most are predators, but first herbivores by ~300 Ma