First evidence of possible terrestrial plants from vascular tissue and spores of the
Ordovician: some spores in Cambrian (but might be from green algae)
Early vascular plants of the Silurian: use tubes in body transport nutrients
& water upwards and manufactured food throughout body
Spores to transmit genetic material:
In primitive plants, spores land and develop into sexual stages,
which transmit sex cells via surface water
Therefore, still need wet surfaces to breed
Plants have a waxy surface to prevent dessication, and openings (stomata)
through the tissue to allow them to breath
A famous early land plant is
Cooksonia, which has spore-bearing organs, vascular tissue, waxy surface,
stomata, but NO leaves
Largest known Silurian plant is 30 cm tall
Baragwanathia, a lycopod (clubmoss), which has very small leaves (increasing
surface area for getting sunlight and for breathing)
Presence of plants modified the surface of the Earth (at least around lakes and streams),
because ground cover would retard erosion. Retention of sediment on land, and incorporation
of decaying plant matter in that sediment: development of first biological soils. Represents a vast new
carbon sink: carbon dioxide levels begin to drop.
Plants continue the "conquest" of land started in the Ordovician.
"Lobe-fins": paired fins have long bones down the main axis
Important clade from Devonian onward
Coelacanths: become more common in later Paleozoic and Mesozoic
Lungfish: once very widespread in both marine and terrestrial environment, adapted for
surviving periods of ponds drying up
Stegocephalians: terrestrial vertebrates and their ancestors (more about them below)
During Devonian, one branch of the sarcopterygians (lobe-fins) develop first wrists, then
digits (fingers and toes). These represent the first
stegocephalians ("terrestrial" vertebrates):
Relative to their kin, stegocephalians (tetrapods and their extinct relatives) are distinguished by:
A reduction in the number of skull bones
A neck (i.e., separation of the skull from the pectoral girdle)
Fin rays replaced by (recent evidence shows "modified into") segmented
bony digits (aka fingers and toes)
Breaking News! (January 6, 2009): Footprints showing tetrapods (stegocephalians with toes) present
in the Early-Middle Deveonian
boundary, about 25 million years older than the oldest body fossil of such animals.
Throughout the Devonian, stegocephalians remain predominantly aquatic.
So by the later part of the Devonian Period there
were vertebrates which had a bony skeleton to support their bodies; bony limbs with wrists, ankles,
and digits to push along on land or on the lakebed; lungs to breath air (but still had gills
to breath in the water).
Note that these are all exaptations: they evolved in some other context, but allowed the stegocephalians
to move around on land.
Many of these Late Devonian stegocephalians still lived their lives essentially only
in the water (such as Acanthostega),
and thus were essentially fish-with-fingers. Others (such as Ichthyostega may have gotten most of their food from land. It was from these latter sort that the
more fully terrestrial vertebrates--the Tetrapoda ("four footed ones"), would evolve.
Still had internal gills, so were mostly aquatic
Used limbs to scull in plant-choked water, and to pull themselves around on land
(if pond dried up, for example)
Not true Amphibia, but "amphibian" in sense of needing water to breed
May have been able to live in salt water (unlike true Amphibia)
With expansion of swimming predators (eurypterids in Silurian, ammonoids and
jawed fish in Devonian), major shift in prey species: trilobites and jawless fish decline
Late Devonian mass extinction:
Traditionally thought to be not at end of Period, but between last two Ages: however, new evidence suggests that
it might actually be at the end of the Period
More than 40 percent of marine genera die out
Associated with pulses of anoxic water onto epeiric seas. Once thought to have been glacially driven. More recent
evidence suggest that diversification and
expansion of land plants led to increased run-off of nutrients into shallow seas, producing eutrophication, producing
anoxic "dead zones" (just as today, where overly-fertilized water does the same).
Victims include acritarchs, "ostracoderms", "placoderms", most remaining trilobites, most groups of ammonoids,
tabulate-stromatoporoid reef complexes