Every fact in the three fossil 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/sยฒ 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 lessons simplify: there is no single age at which remains become a fossil: the British Geological Survey says fossils are typically more than 10,000 years old, while the US National Park Service says fossils do not have to be old; the lessons say 'usually very old'. The play-dough and plaster moulds are models of moulds and casts; real fossils form over thousands to millions of years. The coin model for ages 15โ16 shows the idea of a half-life; real radiometric dating needs careful laboratory measurements of very small amounts of isotopes, and is applied to rock or mineral specimens. Mary Anning's story is taken from the Natural History Museum; the lessons give only the parts the museum states.
All sources
[1] How are fossils formed?
Natural History Museum, London ยท Reliability: High
Type: Natural history museum education article
Very few things become fossils: most things that die rot away completely; petrification steps: the animal dies, soft parts such as skin and muscle rot away, the remains are buried by sediment (usually mud, sand or silt), many more layers build up and turn the lower layers into sedimentary rock, and water seeps into bones and teeth, turning them to stone as it leaves minerals behind; this can take thousands or even millions of years; body fossils are remains such as bones, teeth and shells; trace fossils are signs of activity such as footprints, imprints and coprolites (fossilised dung); a mould is a hole left when buried bone or shell dissolves, and a cast forms when minerals or sediment fill a mould; about 99% of fossils found are of marine animals; fossils of sea animals are found on Mount Everest because of tectonic uplift.
Natural History Museum, London ยท Reliability: High
Type: Natural history museum article
Mary Anning (1799โ1847) was a pioneering palaeontologist from Lyme Regis on England's Jurassic Coast; when she was 12 her older brother Joseph found an enormous skull, and about a year later Mary uncovered the rest of the skeleton, an ichthyosaur (Temnodontosaurus platyodon); she found the first complete, articulated skeleton of a plesiosaur ever discovered, and a pterosaur; her fossils remain important for understanding Jurassic marine life.
British Geological Survey (BGS) ยท Reliability: High
Type: National geological survey education page
Fossils are the preserved remains of plants and animals whose bodies were buried in sediments, and traces of life, typically more than 10,000 years old; body fossils are preserved parts of organisms, trace fossils are evidence such as footprints, burrows and roots; rapid burial by sediment is required; hard parts (bones, shells, teeth) can be replaced by minerals and literally turned into stone; a dissolved shell can leave a mould, and a cast is a positive impression formed against the mould; preserved remains such as insects in amber are the rarest form; fossils are typically found in sedimentary rocks; ammonites make excellent guide fossils for the relative age of rock layers.
US National Park Service (NPS) ยท Reliability: High
Type: Government agency education page
Burial by sediment soon after death prevents decomposition, weathering and scavenging; hard parts such as shells, bones, teeth and wood are more durable than soft tissue; fossils form more often where sediment is being laid down, such as in the sea and on river floodplains; the chance of any individual organism becoming a fossil is very small and the fossil record is incomplete; insects and birds are fairly rare as fossils, and upland organisms less common than marine or floodplain ones; preservation types include permineralisation and replacement, moulds and casts, compressions and preserved remains (drying, freezing, amber, asphalt).
US National Park Service (NPS) ยท Reliability: High
Type: Government agency education page
A fossil is evidence of life preserved in a geologic context; fossils do not have to be old, although most are; body fossils are remains of animals, plants and microorganisms (bones, teeth, shells, wood, leaves); trace fossils are evidence of activity such as footprints, burrows and trails, and reveal behaviour that would not otherwise be known; examples include dinosaur bones, petrified wood, insects in amber, shark teeth and mammoth tusks; fossils provide the history of life on Earth.
Most fossils form when an organism dies and is quickly buried by sediment; soft tissue usually decays; permineralisation, where minerals carried by groundwater fill spaces inside cells, is the most common form of preservation (teeth, bones, shells, wood); trace fossils such as nests, burrows, footprints and coprolites record activity; amber can preserve organisms up to 100 million years old; the baby mammoth Lyuba was preserved in ice about 40,000 years ago; most land organisms never get the chance to become fossils.
In a sequence of rock layers the bottom layer was deposited first and is the oldest, each higher layer being younger (principle formally proposed by James Hutton); because fossils record the slow but progressive development of life, scientists use them to identify rocks of the same age throughout the world; William Smith used fossils as markers to identify a layer wherever it was exposed; the precise time a layer represents is usually unknown.
Index fossils are forms of life which existed during limited periods of geologic time and are used as guides to the age of the rocks in which they are preserved.
[9] Geologic age: using radioactive decay to determine geologic age
US Geological Survey (USGS) ยท Reliability: High
Type: Government science agency education resource
Radioactive decay is a spontaneous process in which a parent isotope loses particles from its nucleus and forms a daughter product; the half-life is the time it takes for one half of a radioactive isotope in a sample to decay; slowly decaying isotopes are used as geologic clocks: uranium-238 to lead-206 (half-life 4.5 billion years), uranium-235 to lead-207 (704 million years), potassium-40 to argon-40 (1.25 billion years); age t = (1/ฮป) ln(1 + D/P), with D daughter atoms, P parent atoms and ฮป the decay constant; simple in theory, but the laboratory work is complex.