Teacher sheet · Ages 15–16 · about 45–50 minutes
Out of the water: tetrapods, the amniotic egg and ectothermy
Lesson page: echoesof.science/school/reptiles-amphibians/ages-15-16/ · Pupil worksheet: echoesof.science/school/reptiles-amphibians/ages-15-16/print/
Learning aims
- Outline the water-to-land transition and its timescale.
- Describe the amniotic egg and link its membranes to their functions.
- Explain ectothermy and its energy advantage using data.
- Measure and compare rates of water loss with appropriate units and uncertainties.
- Interpret data on temperature-dependent sex and amphibian declines.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: what problems does an animal face when it moves from water to land? |
| 10 min | Read the text; draw and label an amniotic egg. |
| 25 min | Water-loss practical with readings every 10 minutes; calculations. |
| 5 min | Amphibian, reptile or both? game. |
| 5 min | Discuss the turtle and chytrid data: what other evidence would you want? |
Materials for the experiment
- Three identical sponges or blocks of agar (about 4 × 4 × 2 cm)
- Cling film, and a thin layer of petroleum jelly as a 'waxy lipid' coat
- Top-pan balance (0.01 g)
- Ruler
- Petri dishes or watch glasses
- Stopwatch or clock
Safety: Petroleum jelly is slippery: wipe benches and hands. Do not put the balance near water. Check for latex or skin allergies before handling coatings, and wash hands afterwards.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Amphibians evolved into reptiles that then stopped needing water at once. | The change took tens of millions of years; amniotes appeared about 340 million years ago, long after the first tetrapods. |
| Ectotherms cannot control their temperature. | They regulate it by behaviour, such as basking and seeking shade. |
| Turtle sex is set by genes, as in mammals. | In sea turtles the incubation temperature of the nest sets the sex. |
Worksheet answers
- Amnion: cushions the embryo and keeps it hydrated. Chorion: gas exchange. Allantois: stores nitrogenous waste. Yolk sac: supplies nutrients via blood vessels.
- Salamanders 620 ÷ 6,770 ≈ 9%; caecilians 185 ÷ 6,770 ≈ 3%.
- Expected: bare sponge fastest, then the cling film and lipid-coated sponges much slower. Uncertainties: balance resolution, draughts and temperature changes, uneven coating, water lost through the open base, surface area measurement.
- Warmer sand produces more female hatchlings, so warming can make a population almost all female (99.8% of sub-adults in the northern Great Barrier Reef study); a population needs enough males to keep breeding in the future.
Quiz answers
- Which membrane of the amniotic egg stores nitrogenous waste? The allantois. The amnion cushions, the chorion exchanges gases, the yolk sac feeds.[2]
- Frogs are about 5,965 of the 6,770 amphibian species. What percentage is that? About 88%. 5,965 ÷ 6,770 ≈ 0.88.[1]
- A mammal needs 2,000 kJ a day. Using the textbook estimate, a reptile of the same size needs about… 200 kJ a day. About 10% of the energy of an endotherm.[2]
- In a group of 500 sub-adult turtles with 99.8% females, how many are male? About 1. 0.2% of 500 = 1.[4]
- Why are amphibians especially sensitive to changes in their environment? Their skin is permeable. Permeable skin lets water and gases through.[3]
- In which geological period did tetrapods evolve from lobe-finned fishes? The Devonian. Over about 50 million years, ending about 360 million years ago.[1]
Going further
Design an experiment to test whether the incubation temperature of eggs can be modelled with data loggers buried at different depths in sand in sun and shade. What would you measure, and what would you keep the same?
Sources used in this lesson
- [1] Biology 2e, 29.3 Amphibians (OpenStax (Rice University)): reliability High
- [2] Biology 2e, 29.4 Reptiles (OpenStax (Rice University)): reliability High
- [3] Reptiles and amphibians: introduction, distribution and life history (U.S. National Park Service): reliability High
- [4] Rising temperatures turning major sea turtle population female (NOAA Fisheries): reliability High
- [5] Batrachochytrium salamandrivorans (Bsal): fact sheet (U.S. Geological Survey): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/reptiles-amphibians/sources/
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