Every fact in the three reptile and amphibian lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the facts are simplified or less certain: species numbers (about 6,770 amphibians, about 9,550 reptiles in the four groups) are textbook figures [1][2] and change as new species are described; the textbook's own lizard and snake numbers do not add up exactly to its squamate total [2]. Birds descend from reptiles, so biologists include them in the reptile group; in these lessons 'reptiles' means the non-bird reptiles [2]. 'Amphibians lay eggs in water' is a simplification: some lay them in damp places on land, and some salamanders fertilise eggs internally [1][3]. The sponges, bottles and cling film in the experiments are models of skin and bodies, not real animals.
All sources
[1] Biology 2e, 29.3 Amphibians
OpenStax (Rice University) · Reliability: High
Type: Peer-reviewed open university textbook
Amphibians (frogs, salamanders and caecilians) are vertebrate tetrapods; the name means 'dual life'. About 6,770 living species: about 620 salamanders (Urodela), 5,965 frogs (Anura) and 185 caecilians (Apoda). Skin is moist and permeable, kept so by mucus glands; most water is absorbed through the skin. Breathing surfaces: skin, lungs and the mouth (buccal) cavity; most salamanders are lungless. Many frogs and salamanders release toxic chemicals from skin glands; more toxic frogs often have bright warning colours. Tadpoles have gills, a lateral line and a finned tail but no limbs; at metamorphosis gills, tail and lateral line disappear, four limbs grow and lungs and an eardrum form. The Mexican axolotl stays in the larval stage. Frog eggs are fertilised externally, protected by jelly, and would dry out quickly, so they are laid in moist places. The first tetrapods date from nearly 400 million years ago; the change from lobe-finned fishes to tetrapods took about 50 million years in the Devonian; Tiktaalik lived about 375 million years ago, about 10 million years before Acanthostega and Ichthyostega.
The amniotic egg is the key characteristic of amniotes. Its four membranes: the yolk sac (nutrients via blood vessels), amnion (cushions the embryo and keeps it hydrated), chorion (gas exchange) and allantois (stores nitrogenous wastes). First amniotes about 340 million years ago; reptiles originated about 300 million years ago. Most non-bird reptile eggs, such as turtles', have leathery shells. Reptile skin contains keratin and waxy lipids that reduce water loss; amniotes breathe with lungs. Reptiles grow throughout life and shed skin; snakes shed the whole skin at once. Most reptiles are ectotherms whose main source of body heat is the environment; they bask in the sun or seek shade or go underground to cool. Reptiles can survive on about 10 percent of the calories needed by a similar-sized endotherm. Groups: Crocodilia (25 species), Sphenodontia (2 species, the tuataras of New Zealand), Squamata (about 9,200 species: about 6,000 lizards, 3,600 snakes), Testudines (about 325 turtles and tortoises). Birds are descended from theropod dinosaurs.
[3] Reptiles and amphibians: introduction, distribution and life history
U.S. National Park Service · Reliability: High
Type: National government parks agency
Amphibians and reptiles are ectotherms: they warm up or cool off by behaviour such as basking or seeking shade. Amphibians have permeable, glandular skin, which makes them sensitive to environmental change; they breathe through the skin and the lining of mouth and throat and may have lungs or gills. Amphibians have a larval and an adult stage that often look very different (tadpole and frog); most frogs fertilise their eggs externally in water. Reptiles have scales and at least one lung, always fertilise internally and have amniotic eggs that can be laid on land; some bear live young. Reptiles hatch as smaller versions of the adults, with no larval stage. Examples: frogs, toads and salamanders; snakes, lizards, turtles and tortoises.
[4] Rising temperatures turning major sea turtle population female
NOAA Fisheries · Reliability: High
Type: National fisheries agency (news release on a peer-reviewed study)
Sand temperatures determine the sex of turtle hatchlings, with warmer temperatures producing more females. On the warmer northern beaches of the Great Barrier Reef, 86.8 percent of adult turtles and 99.8 percent of sub-adult turtles in the study were female.
Type: National geological and biological survey (fact sheet)
Bsal is a chytrid fungus that affects amphibian skin; it is a close relative of Bd, which many scientists consider the greatest disease threat to amphibian biodiversity and which can drive species and populations to extinction. An estimated 42.5 percent of the world's amphibians are threatened at some level (IUCN).