Paleogeography and Geology of the Silurian:
Huge reef complexes in many parts of the world. In fact, the Silurian and Devonian see the largest volume of metazoan-generated carbonates in Earth's history.
Global oceanic highstand after the big terminal Ordovician regression
In Appalachian Orogen of Laurentia, late flysch and molasse from the Taconic Orogeny, Appalachians erode down to base, resumption of stable margin (carbonate and sandstone) deposits.
In the foreland basin and mid-craton of Laurentia, reefs so big that they allowed evaporation of water in the middle of the basin.
Paleogeography and Geology of the Devonian: Extremely warm conditions worldwide: evaporites common North and South of 30°.
Continued widespread tabulate-stromatoporoid reef complexes around the world.
In eastern Laurentia, the Taconic Appalachians had worn all the way down by the beginning of the Devonian. No longer molasse deposits, but instead once again stable platform deposits (Helderberg Group, Oriskany Sandstone, etc.).
Collision of Laurentia, eastern Avalonia, and Baltica to form Laurussia (sometimes called Euramerica):
Iapetus Ocean closed in the North, called the Caledonian Orogeny (Silurian-Devonian boundary)
Rolls southward, compressing western Avalonia (or Avalonian "peninsula" of Laurentia) between Laurentia and Baltica, called the Acadian Orogeny
(second of the Appalachian Orogenies: Late Devonian)
Molasse of the Caledonian-Acadian Orogeny is called Old Red Sandstone in Europe, and the Catskill Clastic Wedge in North America.
In northern Laurentia, Ellesmere Orogeny (possible collision with one of the Asian blocks)
In western Laurentia, the first of the Cordilleran orogenies:
The Antler Orogeny (Late Devonian - Mississippian)
Collision of the Klamath island arc (now part of Nevada, but previously a separate island system) with Laurentia
Towards end of Devonian, CO2 levels begin to drop:
Phanerozoic maximum during Cambro-Ordovician
Drop actually begins during Silurian, but huge drop starting in Late Devonian
C is being bound up in terrestrial sediment by action of land plants (by soil, by coal deposition, and in plants themselves)
Major mass extinction event in Late Devonian
Evolution of Life in the Silurian and Devonian
Colonization of the land:
Algal mats in intertidal zone back in Proterozoic
Possible short excursion by mollusks and arthropods in Cambrian onward
However, long-term settlement on land doesn't happen until latest Ordovician
Why colonize the land?
For plants, new untapped nutrient resources
For animals, plants & other animals for food
Protection from predators
Ability to survive desiccation as lakes dry up
Land dwellers need to combat:
Gravity: no longer buoyant
Desiccation: no longer surrounded in liquid
Lack of dissolved oxygen/carbon dioxide: need to process gaseous air
Lack of water to aid in reproduction
Lack of dissolved nutrients in surrounding medium: need other ways to get food
For animals: different senses (sound moves slower and more weakly, pressure and electric senses don't work, air has different refractive index than water, etc.)
Different groups of organisms combat these features in different ways.
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 desiccation, 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.
Devonian flora:
Plants continue the "conquest" of land started in the Ordovician.
The group containing insects and their flightless relatives
Oldest definite fossils in the Devonian; may have actually appeared in the Silurian
Once thought to be close to the myriapods, but now recognized as a subgroup of Crustacea
Other early colonists (with no fossil record) probably included fungi and various "worms" (nematodes, earthworms, etc.).
Very simple early terrestrial community in the Silurian with simple plant producers, millipede herbivores, centipede and arachnid carnivores, worm and myriapod detritivores, and fungi decomposers.
Devonian hexapods still wingless. Scorpions common, many still partly aquatic. Spider-like arachnids (but not true spiders) also present.
Marine life of the Devonian:
Tabulate-stromatoporoid reefs continue to flourish, supporting large and diverse
ecosystem
Devonian Nekton Revolution: During the Devonian, huge increase of the diversity of swimming forms, especially ones that could swim well within the water column (and not just above the substrate).
Very common, very diverse, very short-lived species: excellent index fossils
Beaks modified into pumps: probably planktonivores
Larvae were microscopic (0.5 mm diameter) and planktonic; in contrast, larval nautiloids were/are macroscopic (15 mm diameter) and hang out on the sea floor
MAJOR fish radiation: Devonian sometimes called "Age of Fish"
Advanced fish begin with bony exoskeleton and partly bony endoskeleton. Some major groups:
"Ostracoderms":
Armored jawless fish
A grade, not a clade)
Front ends generally heavily mineralized, rears covered with smaller scales
"Lobe-fins": paired fins have long bones down the main axis
Important clade from Devonian onward
Sarcopterygians include:
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)
Footprints showing tetrapods (stegocephalians with toes) present in the Early-Middle Devonian 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 in diversity.
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
