Colonization of the land:

• Algal mats in intertidal zone back in Proterozoic
• Possible short excursion by mollusks and arthopods 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.

Plants (Plantae):

• Terrestrial viridiphytes (green "algae")
• 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.

Devonian flora:
Plants continue the "conquest" of land started in the Ordovician.

Rhynia:

• Spore-bearing vascular plant with spore organs at tops of upright stalks
• Common in Early Devonian
• Named after the Rhynie Chert, which contains the remains of many early terrestrial plants and arthropods
• Development of various tougher vascular tissue for support: wood or wood-analogues.

Other Devonian plant developments:

• Development of leaves:
  • Increase surface area for more photosynthesizing, allowing for more energy (i.e., faster growth)
  • Increase surface area for stomata, allowing for more "breathing" (good for faster growth and in response to decreased CO₂ in atmosphere
• Plant activity helps hold soil together
• Plant activity tends to bind carbon (in soils, coals, and the plants themselves), contributing to decrease in global CO₂ levels

New groups of plants in Devonian:

• Lycopods:
  • Club mosses and their relatives
  • Grow by shoots just underneath soil
  • Become trees during Mississippian; important trees of the Coal Swamps
• Ferns
• Sphenopsids (horsetails and their relatives, recently discovered to be a type of fern)
• Archaeopteris:
  • A Late Devonian spore plant with wood = a "progymnosperm"
    • True wood is otherwise only present in seed plants
  • Up to 20 m tall, one of the oldest known trees
• Seed plants
  • Internal fertilization of plant, so do not need external moisture to reproduce
  • Pollen lands on sex organ of plant, is bundled into a seed that can fall off and sprout when conditions are favorable
  • Early seed plants include so-called seed ferns, so-called because grossly resembled ferns

Arthropods (Arthropoda):

• Permanently invaded terrestrial realm at least three times (chelicerates, uniramians, isopod crustatceans)
• Already had many features making them good prospects for life on land:
  • Chitinous skeleton to support mass
  • Jointed limbs to move them around
  • Complex mouthparts to feed on plants or each other
  • Box gills (and later box lungs &/or trachea) to breath air
• By the Late Silurian, several groups had already invaded land:
  • Arachnids
    • A clade of chelicerates, close relatives of the eurypterids
    • Silurian arachnids included scorpions, which were still partially aquatic and still had gills, and most likely mites and other small forms
    • Spiders evolved later in Paleozoic
  • Myriapods
    • A group containing millipedes (herbivores and detritivores) and centipedes (carnivores)
    • Present in the Silurian onward
    • Contraversial trace fossils in the Late Ordovician
  • Hexapods
    • 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).

Eurypterids begin to decline, but still present

Ammonoids:

• Descendants of nautiloid-grade cephalopods
• Also shelled, and mostly coiled
• More complex sutures
• Very common, very diverse, very short-lived species: excellent index fossils
• Beaks modified into pumps: probably planktonivores

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
  • Mostly sediment feeders
• "Placoderms":
  • Jawed fish
  • Typically did not have teeth, but instead a ridge of bone for cutting (a few have some teeth)
  • Some bottom feeders, others major predators, still others planktonivorous filter feeders
    • By separating feeding jaws and breathing gills, could evolve larger body size
  • Fronts heavily armored; body skeleton less mineralized
  • At least some retained young in body
  • Long thought to be a clade, but now recognized to be a paraphyletic "grade"
• "Acanthodians"
  • Once considered a clade, but now seen to be a grade: the primitive members of the toothed fish groups
    • Some are the basal members of the chondrichthyians, other the basal memembers of the actinopterygians
  • Most primitive known fully-toothed fish
  • Probably actively pursued prey
• Chondrichthyians:
  • "Cartilaginous fish"
  • Sharks and their ancestors
  • Bony tooth-like scales, but cartiligenous skeletons
  • Diverse in Devonian, although none resembling the modern mako/great white/etc. type
  • Included various predators, shellfish eaters, etc.
• Osteichthyians
  • Bony fish: internal skeleton of bone (as well as bony scales in more primitive forms)
  • Have a lung: outpouching of gut to absorb oxygen from air (in some later groups becomes exapted as swim bladder)
  • Until recently thought to be a Devonian-onward group, but new discoveries show early osteichthyians in the Silurian
  • Two main subdivisions:
    • Actinopterygians
      • "Ray-finned fish"
      • Clade including most modern fish
      • Rare in Paleozoic, become more common in Mesozoic
    • Sarcopterygians
      • "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)

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 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

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Last modified: 14 January 2011