Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 34.4 Tetrapods Are Gnathostomes That Have Limbs
Tetrapods are jawed vertebrates that have evolved four limbs with digits, enabling life on land. This transition from aquatic lobe-finned fishes to land-dwelling vertebrates was one of the most significant evolutionary events in vertebrate history, paving the way for amphibians, reptiles, birds, and mammals.
Derived Characters of Tetrapods
- Four limbs with digits:
- Evolved from paired fins; enabled movement on land and body support
- Digits transmit muscle forces to the ground
- Neck:
- Early tetrapods had a neck with one vertebra for vertical head motion
- A second vertebra enabled side-to-side movement
- Fused pelvic girdle:
- Hind limbs attached to the backbone, improving force transmission
- Loss of gills in adults:
- Pharyngeal clefts became ears, glands, or other structures
- Some species (e.g., axolotls) retain gills into adulthood
The Origin of Tetrapods: Tiktaalik as a “Fishapod”
- Tiktaalik (~375 MYA): fossil intermediate between fish and tetrapods
- Fish-like features: fins, scales, gills, lungs
- Tetrapod-like features:
- Full set of ribs
- Neck
- Flat skull with eyes on top
- Limb bones (humerus, radius, ulna, wrist)
- Large, strong pelvis and rear fins
- Likely capable of “walking” in shallow water
- Fossils show stepwise limb evolution over ~60 million years:
- Some early forms had gills and weak limbs; others had strong limbs and lost gills
Amphibians: First Major Tetrapod Group
- ~6,000 species in three clades:
- Urodela: salamanders (tailed)
- Anura: frogs (tailless)
- Apoda: caecilians (legless, burrowing)
Key Features and Life Cycle:
- Many show “dual life”: aquatic larvae → terrestrial adults
- Frogs: from tadpoles to carnivorous legged adults
- Some species are fully aquatic or terrestrial
Ecology and Behavior:
- Live in moist habitats; rely on skin for gas exchange
- Lay jelly-coated eggs in water; require moisture
- Mostly external fertilization; some show parental care:
- Eggs may be carried in backs, mouths, or stomachs
Threats to Amphibians:
- Population declines due to:
- Chytrid fungus
- Habitat loss
- Pollution
- Climate change
- Amphibians may be early indicators of environmental problems
In a Nutshell
Tetrapods, with limbs evolved from lobe-finned ancestors, were the first vertebrates to colonize land. Fossils like Tiktaalik show how traits like necks and wrists emerged gradually. Amphibians represent the earliest living tetrapods, showcasing a range of life strategies and development—but many are now under threat, highlighting their ecological sensitivity and evolutionary importance.