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
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
Angiosperms have covered seeds and flowers; Angiosperm radiation begins in the early Cretaceous
Extinguished nearly all therapsids leaving door open to dinosaurs
1. Remaining mammals were all small-bodied animals (about the size of a shrew)
2. Clue to extinction comes from rift lakes in NE United States: Reports of a “Fern Spike” suggesting a catastrophic event, but link to specific cause (or even of a link between the fern spike and the extinction is uncertain.
3. Also a claim for an impact event—based upon dating of Manicouagan crater in Northern Quebec; but impact date may be too old for the extinction.
4. Whatever the cause of the extinction the result was:
Near complete elimination of large synapsids
Leaving ecological door open for diapsids