Evolution of Amphibians

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Evolution of Amphibians

• How do we define the Amphibia

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Amphibians

• They have a 2-phase life history.
• There is usually a free-living aquatic
  developmental stage.
• There is usually a terrestrial juvenile/adult
  stage.
• NOTE this represents a transition from water to
  land just as in the evolution of the group.

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Amphibians

• They have cutaneous respiration. (Think of SA/V
  ratios metabolic rates and dependence on
  water).
• Terrestrial adults have lungs.
• Aquatic larvae and neotenic forms have gills.
• Lungs employ force pumps - they have no diaphragm.

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Amphibians

• They have a pair of sensory papillae in the inner
  ear and double transmission channels.
• Caecilians urodeles and some anurans lack a
  tympanum and middle ear cavity but have a stapes
  which is attached to the shoulder girdle or
  skin!! (This is good for low frequency sound
  waves)

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Amphibians

• They have a 2nd ossicle the operculum which
  with a small muscle joins the pectoral girdle to
  the oval window.
• The stapes conducts high frequency sounds while
  the operculum conducts low frequency sounds.

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Amphibians

• Like other tetrapods amphibians have Basilar
  Papilla but they also have a larger 2nd
  Amphibian Papilla.
• Basilar Papilla 1200 - 1600 Hz
• Amphibian Papilla 200 - 800 Hz.

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Amphibian Vision

• They have specialized visual cells in the retina.
• Salamanders and frogs have green rods in the
  retina (absent in caecilians and all other
  vertebrates).
• What is the function of visual rods What is the
  function of green rods

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Amphibian Skin

• They have 2 types of skin glands
• Mucous glands maintain moist skin for
  respiration.
• Granular glands produce toxic secretions.
• Epidermis is minimal.

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Major Groups of Vertebrates
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Monophyletic Clades

• A monophyletic clade is one in which all the
  daughter groups are members of the clade.
• This is true for amphibians fish and mammals.
• Note that the Aves are a daughter clade of the
  Reptilia. Aves are paraphyletic. Aves are
  covered in Herpetology II (Ornithology)

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Amphibian Teeth

• They have pedicillate Teeth.
• The tooth has a crown and a pedicel. As the
  crown wears it breaks off and is replaced by a
  new one.

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Terrestrialization

• Why make the transition to an aquatic ecosystem
• Terrestrialization may be a secondary event.
• Water bodies may have had low oxygen content and
  thus may have favored air gulpers.
• abundant arthropod and plant resources may have
  been close to shore.

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Terrestrialization

• The old hypothesis was that they wanted to move
  to new bodies of water. Does this make sense

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Terrestrialization

• What problems exist for a fish design on land
• Integumentary water loss.
• Thick dermis but thin epidermis.
• Presence of 2 types of glands (mucous and poison)
• Temperature control
• Respiration
• Bucal pump skin lungs. (Lungs were already
  present in the osteichthys. Lungs actually
  preceed swim bladders.

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Terrestrialization

• Skeletal support
• organization of myomeres in fishes
• water acts as an external skeleton in fish.
• Locomotion
• Hearing

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Terrestrialization

• Feeding
• Once the pectoral girdle is free from the skull
  greater mobility is possible in the skull
  permitting greater feeding. This is enhanced by
  the architecture of the 2 occipital condyls and
  the presence of the atlas. The snout and jaws
  become elongated (seen as a relative shortening
  of otic-occipital region). There is improved
  articulation of the jaw and expansion of the
  primary palate.

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Terrestrialization

• Smell
• Reproduction
• Circulation with multiple respiratory structures.
• Skeletal solutions to terrestrialization.
• Pectoral girdleis divorced from back of skull
  (implications for sound conduction)

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Terrestrialization

• Skeletal Soln. Cont.
• undulatory locomotion
• suspension of vertebral column
• regionalization of vertebral column
• organization of amphibian vertebrae.
• Suspension of internal organs.
• Reorganization of the skull.

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Terrestrialization

• Reorganization of the skull cont.
• reduction of the otic-optic portion of brain.
• evolutin of the dermocranium
• evolution of the splanchnocranium
• evolution of the neurocranium.
• evolution of the mandibular suspensorium.

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One more time What are amphibians

• From last time
• 2-phase life history.
• cutaneous respiration.
• pair of sensory papillae in inner ear.
• Specialized visual cells in the retina (green
  rods).
• Pedicillate teeth.
• 2 types of skin glands.

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These features dont really help us in the fossil
record.

• We would like to understand the evolutionary
  history of amphibians and reptiles. In terms of
  fossils how can we define them
• Usually Tetrapod (excluding ichthyostegalia)
  morph of centra zygapopheses arches ribs
  girdles and presence of 2 occipital condyles and
  1 atlas. Also dual auditory pathways skull
  bones.

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What is our best guess as to the history of the
amphibia

• Middle to late Devonian
• Plants predominantly wetland and watercourse
  edges.
• Plant ht less than 2m with no stratification.
• Single species patches.
• Low diversity but evidence of seasonality of
  seasonality and Gondwana glaciation.
• Some evidence for a Devonian extinction.

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What is our best guess as to the history of the
amphibia

• Middle to late Devonian cont.
• Trend to warm and drier with increased diversity
  of plants.
• By the end of the Devonian there is increased
  structural diversity of plants and wetlands
  appear to be refugia for plants.
• By the Carboniferous all taxonomic groups of
  plants have evolved. There is high level
  taxonomic radiation and architectural
  diversication. Structurally modern plants.

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Animals in late Devonian to early Carboniferous.

• Fossil amphibian trackways in Australia
• Footprints in Brazil
• First fossils from Greenland and Russia include
  Ichthyostegalids and Anthracosaurs.
• Ichthyostegalids are from the Devonian of
  Greenland. They are about 1m in length.

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Animals in late Devonian to early Carboniferous.

• Anthracosaurs are from the Devonian of Russia
  Iowa Scotland and W. Virginia and range in
  size from .5m to 1.5m in length (why so big)
• They have dorsoventrally expanded tails they
  lacked well ossified carpals and were probably
  insect predators.

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A word (or 2) about monophyly paraphyly and
tetrapods

• Tetrapods are called tetrapods because they have
  4 feet. So anything with 4 feet is a tetrapod
  except of course Ichthyostegalia. Also some
  tetrapods dont have any feet (snakes and
  caecilians).
• Major vert. groups may not be mono-phyletic
  including amphibians.

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A word (or 2) about monophyly paraphyly and
tetrapods

• The problem is many amphibian features are tied
  up with the transition from water to land. These
  features may not represent unique morphological
  inventions. Lots of groups have done exactly the
  same thing.

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A word (or 2) about diversity.

• Species diversity over time.
• Architectural diversity over time.
• Mass extinctions.

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Anthracosaurs

• We now consider the Anthracosaurs to include the
  lineage which leads to reptiles and mammals
  (Seymouriamorpha Cotylosauria Batrachosauria
  and Anthracosauroideae).
• It no longer includes the loxommatoids and
  lepospondyls. If it did the Amphibia would be
  paraphyletic.

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Lepospondyls

• This is an unnatural group (Aistopoda
  Microsauria Nectridia) which shares
  morphological features associated with small size
  and aquatic life history.
• Temnospondyls are amphibians and represent the
  first real amphibian radiation. The lissamphibia
  are the 2nd radiation.

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Labyrinthodonts

• This is an unnatural group including
  temnospondyls anthracosaurs and
  ichthyostegalians. The group is defined on the
  basis of shared primitive characters and is not
  monophyletic.

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