ORIGIN AND EVOLUTION OF AMPHIBIA

Devonian 408 - 360 mya, 
Tropical: Equator passed through CA, UT, WY, SD, ND
Transition from Epeiric seas to emergent continents
     increase in amount of freshwater sediments
Gondwana assembled, other plates still converging 
Oceanic plates and arcs converging on NA (Fig. 19.8)
caused warping of the North American craton,

in eastern North America:  Acadian Orogeny 
  convergence advancing with African plate
  Appalachians prominent sediments shed westward
  Catskill clastic wedge and Old Red sandstone thick

Insects and spiders become more abundant on land

Fishes, Ostracoderms, Placoderms, chondrichthyes, Osteichthyes,
Actinopterygii, Sarcopterygii
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Devonian Ichthyostega of Greenland and similar forms in Russia
and Australia ca. 360 m.y.a

From lobefin fishes (rhipidistians) , which had the ability to
breath atmospheric air and the ability to move about on land

Differences in water and land habitats
water                                   land
-----------------------------           -------------------------
buoyancy                                gravity
O2 <10ppm (less in swamps)              O2=19%
density = 1000 X air                    no drag
predators abundant                      predators = rare
food abundant                           rare at first
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Terrestrial plants originated in mid Silurian
[Chlorophytes colonized freshwater first, then moist land
habitats]

[First vascular plants were small Psilophytes in mid Silurian
     = seedless (spore bearing) vascular land plants]
Diverse forests by Devonian; "lycopsid trees" to 7 m (club
mosses)   competition for light drove height increase
Horse tail mosses and ferns (spore-bearing) in Carboniferous
     intermediate stage = Seed Ferns, e.g. Glossopteris

 Why venture out on land?
drying up of habitats hypothesis?
  (but modern ecology provides examples only in humid habitats)
O2 for eggs in shallowest waters
no predators in shallowest waters
Amphibians today breed in shallowest waters


Principle: behavioral changes precede morphological evolution
          especially behavior related to feeding or reproduction
Ichthyostega characteristics
= a mixture of fish and amphibian characteristics
1 meter long
loss of fins except part of caudal
lateral line canals present on head
arm:  humerus, radius, ulna, wrist bones, fingers; shoulder back
leg:  femur, tibia, fibula, ankle bones, 7 toes; hip bones
vertebrae with arches for support in gravity
loss of gill structures in adult

Adaptations to new conditions on land:
  atmospheric oxygen: lungs (gills lost)
  gravity: supportive vertebrae, walking limbs (fins lost)
  reproduction still limited to aquatic habitat

Animals make behavioral choices under pressure:

Predator threat caused animals to seek refuge on land

Atmospheric oxygen favored use of lungs
  gills are expensive to grow; loss was a benefit

Locomotion on land aided survival 
  land locomotion increased predator escape-->survival
  land locomotion increased food capture--> growth

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Diversity of Carboniferous amphibians 360-280 m.y.a.
swamp and forest dwelling amphibians
some with large heads and tiny legs
some long and slender with small or no legs  microsaurs
many broad flat forms see exhibits
some small flat forms with gills possibly larvae of others
fish eaters and arthropod eaters

Carboniferous world
[Mississippian, Pennsylvanian coal measures huge volume of
uneaten, unoxidized plant matter buried underwater]
southern continent with glacial evidence (Gondwana)
North America-Greenland-Europe together on equator (Laurasia)
forests with tall club mosses to 40 m
          horsetails to 15 m
seed ferns and conifers in swamps, 
millipedes, spiders, insects, some huge, wings already developed

What kind of selection on amphibians?