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The first tetrapods appeared nearly 400 million years ago. The change from lobe-finned fishes took about 50 million years in the Devonian: Tiktaalik lived about 375 million years ago, about 10 million years before Acanthostega and Ichthyostega, which had four limbs and may have had both gills and lungs.

Amphibians (about 6,770 species: about 5,965 frogs, 620 salamanders and 185 caecilians) still depend on water. Their skin is moist and permeable and is a gas-exchange surface along with the lungs and mouth cavity; most salamanders have no lungs. Larvae have gills and a lateral line, which are lost at metamorphosis.

The amniotic egg, which appeared with the first amniotes about 340 million years ago, freed reptiles from water. The amnion cushions the embryo and keeps it hydrated, the chorion carries out gas exchange, the allantois stores nitrogenous waste and the yolk sac supplies nutrients. Skin with keratin and waxy lipids cuts water loss.

Experiment: Measuring water loss through model skins

1. Soak the three sponges and let them drip for one minute. Measure their dimensions and calculate the exposed surface area of each (top and four sides).
2. Leave sponge A bare ('amphibian skin'). Wrap sponge B in cling film with the bottom open ('scaly skin'). Coat the exposed faces of sponge C with a thin layer of petroleum jelly ('waxy lipids').
3. Weigh each sponge on its dish and record the starting mass.
4. Leave them side by side, away from radiators. Reweigh every 10 minutes for 40 minutes (and again after 24 hours if you can).
5. Calculate the mass lost, the percentage of the starting water lost, and the rate of loss in g per hour per cm² of exposed surface.
6. Plot mass lost against time for all three, and estimate the uncertainty of each reading from the balance resolution and repeat readings.

Time (min)	A bare (g)	B cling film (g)	C lipid coat (g)
0
10
20
30
40

Questions

1. Name the four membranes of the amniotic egg and give the function of each.

2. Use the textbook figures to calculate the percentage of amphibian species that are salamanders and that are caecilians.

3. From your practical, compare the rate of water loss per cm² for the three sponges. Which is fastest, and what are two sources of uncertainty?

4. Explain why warmer beaches could change the sex ratio of a sea turtle population, and why this matters.

Sources for every fact on this sheet: echoesof.science/school/reptiles-amphibians/sources/

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