Amphibians and the
Vertebrate Transition to Land

Chondrichthyes

• Large, active predators
• Fairly diverse
• Not highly maneuverable
• No swim bladder
• Hetercercal tail
• Pectoral fins low, not mobile

Actinopterygii

• Highly successful, diverse
• Trend towards high maneuverability
• Homocercal tail
• Dorsally positioned pectoral fins
• Deep body
• Swim bladder
• Broad diversity of feeding styles
• Ram vs. suction feeding
• Jaw protrusion

Chondrichthyes, Actinopterygii, and Land

• No "Land Sharks"
• Several Actinopterygians can briefly survive on land
• Mudskippers, eels, walking catfish
• No fully successful invasions

Tetrapods - Adaptations to Land:
Locomotion and Support

• Reduce weight
• Lighter skull, reduced scales
• Strengthen vertebral column
• Shorten vertebral column
• Zygapophyses
• Axial to appendicular
• Lift body from ground
• Efficient limb recovery

Tetrapods - Adaptations to Land:
Locomotion and Support

• Reduce weight
• Lighter skull, reduced scales
• Strengthen vertebral column
• Shorten vertebral column
• Zygapophyses
• Axial to appendicular
• Lift body from ground
• Efficient limb recovery

Tetrapods - Adaptations to Land:
Feeding

• Feeding
• Suction no longer effective
• Increase grasping, inertial feeding

Tetrapods - Adaptations to Land:
Respiration/Circulation

• Switch from gills to lungs
• Avoids clumping
• Reduces water loss
• Air rich in O₂
• Air is light, easy to move
• Change from unidirectional to bidirectional flow
• Facilitates high metabolic rate ...

Tetrapods - Adaptations to Land:
Respiration and Circulation

• Increased separation of high vs. low O2 blood
• More efficient

Tetrapods - Adaptations to Land:
Sensory Systems

• Water and land are very different sensory media, e.g.,
• Vision
• Refractive index of water, air very different
• Hearing
• Sound transmitted much more effectively in water than air
• Impedance problem

Tetrapods - Adaptations to Land:
Sensory Systems

• Water/land are very different sensory media, e.g.
• Vision
• Refractive index of water, air very different
• Hearing
• Sound transmitted much more effectively in water than air
• Impedance problem

Tetrapods - Adaptations to Land:
Sensory Systems

• Hearing
• Sound transmitted much more effectively in water than air
• Impedance problem

Amphibians

• Some terrestrial specializations
• Still fish-like in many ways, e.g.,
• Lateral undulation
• Inefficient kidney
• Sensory systems
• Aquatic reproduction
• Gills (in addition to lungs)

Invading Land - Why bother?

• Escape drought
• Avoid competition
• Avoid predators
• Exploit new resources

Tetrapod Origins - "Sarcopterygii"

• "Sarcopterygii"
• Have fleshy fins, with rays
• Abundant and diverse in Devonian (400-360 mya)
• Hard-hit by Permian extinction (250 mya)
• Major Groups
• Coelacanths
• Lungfishes
• Rhipidistia (extinct)

Sarcopterygians - Coelacanths

• Were both freshwater and marine
• Could be large (4m)
• Good fossil record, but thought to be extinct for 65 million years
• Discovered alive off the Afirican coast in 1938!
• And again, in 1998, in Indonesia

Sarcopterygians - Coelacanths

• Originally thought to be a remnant of tetrapod ancestors
• Distinctly lobed, fleshy fins
• Internal skeletal fin supports
• Now considered "close" to tetrapods

Sarcopterygians - Lungfishes

• As with coelacanth
• Formerly more diverse
• Fleshy fin
• Skeletal supports
• Discovered in 1830’s
• Originally linked with amphibians

Sarcopterygians - Lungfishes

• Two Families
• Lepidosirenidae
  • African, South American
• Neoceratidae
  • Australian

Lungfishes - Airbreathing

• Obligate in African and South American
• Beginning of divided circulation
• Heart partly divided
• High O₂ blood from lung-> heart-> body
• Low O₂ blood from body -> heart -> lung

Lungfishes - Airbreathing

• Obligate in African and South American
• Beginning of divided circulation
• African lungfishes can aestivate up to 4 years!

Sarcopterygians - Rhipidistians

• Sister-group to tetrapods, based on
• Similarity in skull
• Internal nostrils
• Very close similarity in limb skeleton!

Sarcopterygians - Rhipidistians

• Sister-group to tetrapods, based on
• Similarity in skull
• Internal nostrils
• Very close similarity in limb skeleton!

Sarcopterygians - Rhipidistians

• Sister-group to tetrapods
• Similarity in skull
• Internal nostrils
• Very close similarity in limb skeleton!

Sarcopterygians - Rhipidistians

• Eusthenopteron
• Some well-preserved fossils
• Limbs contain clear
  • Humerus, radius, ulna
  • Femur, tibia, fibula

Early Amphibians

• Could be quite large (5m)
• Very similar to rhipidistians
• Primarily aquatic
• Retained skull similarities
• Retained swimming tail, fin rays
• Retained part of opercular series
• Retained bony scales
• Fish-like in many other respects

Early Amphibians

• Could be quite large
• Very similar to rhipidistians
• But, developed
• Neck
  • Increased head mobility
• Pelvic girdle attachment
  • Increased support
• Digits (up to eight per limb)

Eustheopteron (fish) vs.

Ichthyostega (amphibian)

Amphibians - Two Major Groups

• Temnospondyls
• Apparently close to modern amphibians (Lissamphibia)
• Broad skull
• Anthracosaurs
• Close to reptiles
• Deep skull
• Main point
• Early division of Amphibians
• Modern Amphibians and Amniotes (Reptiles, Birds, Mammals) are far apart.

Modern Amphibians - The Lissamphibia

• About 4,000 species
• Generally restricted to

  moist habitats

• Low metabolic rate
• Positive pressure respiration
• Also use skin
• Partly divided heart
• Separation of high, low O₂
• Return to water for reproduction (usually)

Modern Amphibians - The Lissamphibia

• About 4,000 species
• Generally restricted to

  moist habitats

• Low metabolic rate
• Positive pressure respiration
• Partly divided heart
• Return to water for reproduction (usually)
• Tympanum, columella (= stapes)

Modern Amphibians - The Lissamphibia

• Three Orders
• Anura - Frogs
• Caudata - Salamanders
• Gymnophiona - Caecilians
• Fossil record poor

Anura (frogs, toads)

• Fossils to about 250 mya
• Appear pretty modern
• Highly specialized for jumping
• Elongation of hind limbs
• Loss of tail
• Fusion of bones
• Usually aquatic larval stge; occasionally direct development with no free larval stage

Anura

• Fossils to about 250 mya
• Highly specialized for jumping
• Aquatic larval stage, occasionally direct development
• Usually rely heavily on skin for respiration
• E.g., during hibernation
• Feed via tongue projection

Caudata (salamanders)

• Fossils to about 250 mya
• Least specialized Lissamphibia
• E.g., lateral undulation, four limbs
• Can grow to 1.5 m
• Use skin, gills, and/or lungs
• Some lack lungs (Plethodontidae)
• Feed via tongue projection (terrestrial) or suction (aquatic)

Caudata

• Paedomorphosis common
• Many aquatic species can be induced to "metamorphose"
  • Environmental changes
  • Hormonal treatment

Gymnophiona (caecilians)

• Highly modified for burrowing
• Solid skull
• Loss of limbs (like snakes)
• Typically have live birth