Every fact in the three dinosaur 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 are less certain or simplified: sources give slightly different dates. We use 252 to 66 million years ago for the Mesozoic Era (National Park Service, Australian Museum); Britannica's summary gives 251 to 65.5 million years and Imperial College's 2010 news story says about 65 million years. The first dinosaurs are dated to about 230 million years ago by the Australian Museum and about 245 million years ago by Britannica; we use the Australian Museum figure. Sources also differ on whether Richard Owen named the dinosaurs in 1841 or 1842, so the lessons do not give the year. The asteroid impact is the explanation favoured by the international panel reported by Imperial College, but other causes such as volcanic eruptions have been discussed. The speed equation for ages 15โ16 gives only rough estimates: a study with living birds walking in mud found that none of the equations predicted speed accurately. The rule 'hip height โ 4 ร footprint length' is a rule of thumb, and children's bodies may not follow it closely.
All sources
[1] What Makes a Dinosaur a Dinosaur
US National Park Service (NPS) ยท Reliability: High
Type: National park service science page
Dinosaurs are extinct animals with upright limbs that lived on land during the Mesozoic Era, 252 to 66 million years ago (general definition); Dimetrodon, marine reptiles, flying pterosaurs and crocodiles are often wrongly called dinosaurs: they lived during the Age of Dinosaurs but were not dinosaurs; birds evolved from small carnivorous dinosaurs during the Jurassic, so not all dinosaurs are extinct, and many palaeontologists say 'non-avian dinosaurs' to exclude birds; Dinosauria can be defined as the group descended from the most recent common ancestor of Diplodocus carnegii, Triceratops horridus and the house sparrow; Sir Richard Owen described Dinosauria from Iguanodon, Megalosaurus and Hylaeosaurus.
The Mesozoic Era ran from 252 to 66 million years ago; dinosaurs first appeared in the Middle Triassic, about 230 million years ago, and dominated the land for roughly another 140 million years; all dinosaurs had limbs held directly under the body, and their ankles bent in one plane like a hinge; crocodiles and pterosaurs are not dinosaurs; living birds are specialised theropod dinosaurs; early birds had sharp teeth, long bony tails and claws on their hands; 'dinosaur' comes from Greek and means 'terrible lizard'; the non-avian dinosaurs ended at the close of the Mesozoic, 66 million years ago.
Dinosaurs are divided into ornithischians and saurischians on the basis of the structure of the hip (pelvic girdle); most had a long tail held straight out, probably for balance; the shape of the teeth shows whether a dinosaur ate meat or plants; most, if not all, laid eggs; some were probably warm-blooded; dinosaur fossils have been found on every continent, including Antarctica; explanations for the extinction include volcanism and climate change, and one or more asteroid impacts whose dust blocked sunlight for several years.
[4] Asteroid killed off the dinosaurs, says international scientific panel
Imperial College London (news, March 2010) ยท Reliability: High
Type: University news report on a peer-reviewed review
A panel of 41 international experts reviewed 20 years of research and concluded that an asteroid around 15 km wide hitting Chicxulub in Mexico caused the mass extinction that wiped out more than half of all species on Earth, including the dinosaurs; material thrown up blocked sunlight and caused a global winter; the impact also caused large fires, huge earthquakes and landslides that set off tsunamis; the Deccan Traps volcanic eruptions in India, which lasted about 1.5 million years and released about 1,100,000 kmยณ of basalt lava, were judged not to be the primary cause.
Natural History Museum, London ยท Reliability: High
Type: Natural history museum article
There is a direct link between modern birds and theropod dinosaurs, which were two-legged meat-eaters such as Deinonychus; Deinonychus was found to be feathered; the first feathered dinosaur was announced in 1996 and many more have been found since; Archaeopteryx, from the Late Jurassic, is sometimes called the earliest bird; the gradual change from ground-living theropods to flying birds probably began about 160 million years ago.
Royal Belgian Institute of Natural Sciences (Museum of Natural Sciences, Brussels) ยท Reliability: High
Type: National natural history museum page
Not all dinosaurs are extinct: birds are descended from theropods; very well-preserved fossils found in south-west China since 1996 show traces of plumage; Sinosauropteryx had simple downy hairs that kept it warm but could not fly; Caudipteryx could not fly and used long feathers on its arms and tail for display; Archaeopteryx, found in Germany, had wings with asymmetrical feathers and could glide from branch to branch.
Royal Belgian Institute of Natural Sciences (Museum of Natural Sciences, Brussels) ยท Reliability: High
Type: National natural history museum page
In 1878 miners in the Sainte-Barbe coal mine at Bernissart, Belgium, found bones 322 m underground; about thirty relatively complete Iguanodon skeletons were dug out; Iguanodon bernissartensis skeletons are 6.29 to 7.30 m long; Iguanodon was a plant-eater; its thumb was a large cone-shaped spike, possibly a weapon against predators; the skeletons were first mounted in 1882 standing upright like kangaroos, but in 1980 David Norman concluded that the back was held roughly horizontal and that Iguanodon walked on all fours; the skeletons are displayed in a large glass case at the museum in Brussels.
The Open University (OpenLearn) ยท Reliability: Medium-high
Type: University open-learning article
The speed of a dinosaur can be estimated from trackways when two things are known: the stride length (the distance between two prints of the same foot) and the animal's size, measured as hip height; hip height is typically about four times the length of the footprint; Alexander's formula gives an estimate of speed and assumes the leg is kept straight; other evidence such as muscle size suggests Tyrannosaurus rex probably could not run.
[10] Speed from fossil trackways: calculations not validated by extant birds on compliant substrates
Liverpool John Moores University research repository (peer-reviewed article) ยท Reliability: High
Type: Peer-reviewed research article
Gives Alexander's (1976) equation u = 0.25 g^0.5 ฮป^1.67 h^โ1.17, with u the speed in m/s, g = 9.81 m/sยฒ, ฮป the stride length in m and h the hip height in m (or 4 ร track length); in tests with guineafowl walking in mud, none of the equations accurately predicted speed, so trackway speeds may be substantially wrong or carry major uncertainty.