34.4: Tetrapods are Gnathostomes that have Limbs

Caecilians

- 170 species of apodans - legless and nearly blind, resemble earthworms - inhabit tropical habitats, burrow in moist forest soil

Tetrapods

- 365 million years ago, fins of lobe-fishes evolved into the limbs and feet of tetrapods, after colonization of land they evolved to many new forms

Frogs

- 5,420 species - use their powerful hind legs to hop along the terrain - toads have leathery skin - Nabs insects by flicking its long sticky tongue attached to the front of its mouth - Skin glands secrete distasteful or even poisonous mucus - certain colors to associate with danger to others

Salamanders

- 550 known species of Urodela - some are entirely aquatic, others live on land as adults or their entire life - Live on land: walk with side to side bending of the body - Paedomorphosis- common among salamanders ex. axolotl retains larval features even when sexually mature

Amphibians

- 6,150 species in three clades: Urodela "tailed ones" (salamanders) Anura "tailless ones" (frogs) Apoda "legless ones" (caecilians)

Lifestyle and Ecology of Amphibians

- Amphibians means "both ways of life" - Larval stage of frog: tadpole (aquatic herbivore w/ gills that has a lateral line system and a long finned tail, lacks legs & swims by undulating its tail) - Metamorphosis- develops lungs, legs, external eardrums, and a digestive system evolved for a carnivorous diet, gills disappear and lateral line system - Crawls onto shore and becomes a hunter - Most amphibians are found in damp habitats such as swamps and rain forests - When in dry habitat, they spend time in burrows or under moist leaves where humidity is high - Rely heavily on their moist skin for gas exchange Fertilization is external in most amphibian species - As female sheds them, the male spills his sperm on them - Lay their eggs in water/moist environment bc the eggs lack a shell and will dehydrate quickly w/o water source - lay lots of eggs bc egg mortality is high - other species lay a bit, but have more parental care - males may house eggs on back, mouth or stomach in some cases - Tropical tree frogs stir egg masses into moist foamy nests that resist drying - Sometimes eggs are retained in the female reproductive tract where they won't dry out Diverse social behaviors during breeding season - Some quiet, others vocalize to defend territory or sttract females; migration to certain sites involve vocal communication, chemical signaling, and celestial navigation Rapid decline in amphibian populations - disease causing chytrid fungus, habitat loss, climate change, and pollution - sometimes declines turned to extinctions - 9 species extinct since 1980, other 100 unknown

Origin of Tetrapods

- The tetrapod body plan was a modification of a preexisting body plan, aka the lobe-fin one Tiktaalik provided evidence as to how this was done: - Had fins, gills, and lungs; body covered in scales but had a full set of ribs to help it breathe and support its body - Had a neck and shoulders (can move head) - Its front fin had same basic pattern of limbed animals: one bone followed by two bones and small bones that comprise a wrist - Couldn't walk on land but it can prop itself on the water with its fins - Indicates key tetrapod traits like wrist, ribs, and a neck - Allowed us to see how fins progressively became limb-like over time - Great diversity and some kept gills but had weak limbs, others lost gills and got stronger limbs --> walking on land

Derived Characters of Tetrapods

- meaning "four feet" in Greek - No pectoral/pelvic fins, tetrapods have limbs with digits (limbs to support weight on land and digits to transmit muscle generated forces to the ground to walk) - Head separated from the body by the neck (originally had one vertebra on the neck to move up and down and then there was the second vertebra, which moved it side to side) - the bones of the pelvic gridle to which the hind legs are attached, are fused to the backbone - the adults of living tetrapods do not have gills - during embryonic development the pharyngeal slits develop into parts of the ears, certain glands and other structures