Amphibia: Origin of Tetrapods | Zoology Optional Notes for UPSC PDF Download

Tetrapods and Their Fossil Ancestors

Evolutionary Milestones:

1. Tetrapod Limb Evolution:

• Transformation: A limb evolved to support the body on land.
• Weight Distribution: Unlike fishes, where the body is buoyed by water, tetrapods bear their entire body weight on their four limbs.

2. Respiratory Changes:

• Gill Loss: As lungs became more efficient, gills diminished in the adult stage.
• Cutaneous Respiration: Loss of scales allowed cutaneous respiration as an alternative to gills.

3. Auditory Adaptations:

• Hyomandibular Transformation: The hyomandibular bone transformed into a columella, transmitting sound vibrations from air to the inner ear.

4. Skeletal Modifications:

• Vertebral Changes: The vertebral column transformed to support body weight on land.
• Skull Evolution: Skull developed two occipital condyles for better locomotion and visibility.

5. Adaptations for Land Living:

• Lateral Line System Disappearance: This aquatic feature vanished in the terrestrial environment.
• Adipose Tissue Development: Adipose tissue evolved to prevent water loss through evaporation on land.

Fossil Ancestors of Tetrapods

1. Osteolepis:

   • Lobed-finned fish with elements homologous to tetrapod limbs.
   • Lived approximately 375 million years ago, utilizing both gills and lungs.

2. Eusthanopteron:

   • Crossopterygian fish with lobed fins resembling tetrapod limbs.
   • Fully aquatic during the Devonian period.

3. Tiktaalik roseae:

   • Labyrinthodont fossils discovered in 2006.
   • Featured limb bones with a tetrapod arrangement but wrapped in fish-like fins, enabling swimming and shallow water walking.

4. Acanthostega:

   • Intermediate between fish and tetrapods.
   • Forelimbs paddle-like with eight digits, hind limbs with six digits.

5. Ichthyostega:

   • Earliest fossils from the late Devonian period.
   • Possessed a fish-like skull, scales, and lateral line system, but relied entirely on lungs for respiration.

Terrestrial Giants - Eryops, Cacops & Amphibamous

These stem animals, evolving from stegocephalians, became giants and dominant terrestrial carnivores during the Carboniferous and Permian periods.

• Massive bodies with strong limbs and dermal scales for protection.
• Some reached up to five feet in length.

Aquatic Wonders - Buettneria

Buettneria, the largest amphibians of their time, displayed a secondary simplification of the vertebral column.

• Reached lengths of up to 6 feet, possibly aquatic but briefly ventured onto land.

Reptilian Precursors - Eogyrinus

Small lizard-like labyrinthodonts with five digits in each limb, serving as ancestors to Anthracosauria, which later evolved into reptiles.

Origin of Amphibians

Key Factors Leading to Amphibian Evolution:

• Specialized lobe-finned fishes equipped with lungs.
• Marshy conditions fostering anatomical adaptations.
• Lungs and lobed fins transforming into tetrapod limbs.
• Invasion of land by forests, attracting amphibian prey and predators.

In the Carboniferous and Permian periods, primitive labyrinthodonts mastered terrestrial living, becoming dominant predators. However, the Permian mass extinction wiped out large amphibians, marking a significant chapter in the evolutionary history of tetrapods. Over 70% of tetrapod families and almost 90% of species vanished, leaving a profound impact on the course of vertebrate evolution.