How every fact on this site was checked
136 facts. Each one is in our own words, each one has a source, and each one was checked against that source on 13 November 2026.
How we check
We read each source and wrote each fact in our own words. Our build stores the text of every source page, and for each fact it checks that the key words and figures, the “checks” in the table, really appear on the source page. If any check fails, the site does not build. Where good sources disagree, we show each version in a “Sources differ” box rather than picking one. Anything that is our own reading, rather than a sourced fact, is labelled Our opinion. The drawings are our own and simplified; the size chart is to scale using the figures quoted.
Known disagreements
- What the name refers to. The NHM explains ‘three lobes’ and describes both the head-to-tail and side-to-side splits; the British Geological Survey says the name refers to the side-to-side split only.
- When they first appeared. About 520 million years ago (NHM) or 521 million years ago (Britannica).
- How long they lasted. More than 250 million years (NHM feature) or a fossil record spanning 275 million years (NHM news story).
- When they died out. Just before the end-Permian Great Dying (NHM), at the end of the Permian about 250 million years ago (BGS), or about 245 million years ago (UC Museum of Paleontology).
- Spriggina. Possibly ancestral to trilobites (Britannica) or unlikely to be related (NHM).
- What they ate. Predators, scavengers, mud-grazers, filter feeders and plankton-eaters (NHM, Britannica); Dudley Council’s page says they probably grazed on plants.
- Horseshoe crabs. Perhaps the closest living relatives (NHM feature), and only distantly related (NHM news story).
- The age of Wren’s Nest. 427.7 to 429 million years (Black Country Geopark), about 420 million years (Dudley Council), 428 million years for the Ripple Beds (Dudley Council), or about 430 million years (Scientific American blog).
- The size of Wren’s Nest. About 101 acres, 45 hectares (Geopark), or 100 acres (Dudley Council).
- The biggest trilobite’s name. The NHM page spells it “Isoteus rex”; the usual spelling is Isotelus rex.
What we left out
- Some pages we wanted to use would not load for us to check, including museum pages about Isotelus rex. Nothing from them is used.
- The Scientific American piece is a blog post, so we use it only for the coat of arms and the colour of the fossils.
- The trilobite-o-meter numbers are our own arithmetic from the sourced sizes, not quoted facts.
The log
| # | Fact | Source | Checks |
|---|---|---|---|
| 1 | Trilobites were sea animals and arthropods, the group that includes insects, spiders and crustaceans. | Natural History Museum: How trilobites conquered prehistoric oceans | 2 passed |
| 2 | Geologists know they lived in the sea from the rocks they are found in and the other fossils found with them. | British Geological Survey: Trilobites | 1 passed |
| 3 | Everything we know about trilobites comes from fossils. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 4 | The museum’s trilobite expert Richard Fortey says there were trilobites as spiky as porcupines and as smooth as boiled eggs, bigger than lobsters and smaller than gnats. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 5 | Despite all that variety, they kept to one very particular body plan. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 6 | Among extinct animals, trilobites are probably second only to dinosaurs in fame. | UC Museum of Paleontology, Berkeley: Introduction to the Trilobita | 1 passed |
| 7 | After the Cambrian explosion they became one of the main parts of ocean life; the Early Palaeozoic has been called ‘the age of trilobites’. | Natural History Museum: How trilobites conquered prehistoric oceans | 2 passed |
| 8 | More than 20,000 species of trilobite have been described. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 9 | Britannica describes them as extinct fossil arthropods, easily recognised by their three-lobed, three-part form, and only ever living in the sea. | Encyclopaedia Britannica: Trilobite | 2 passed |
| 10 | The word trilobite means ‘three lobes’. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 11 | Every trilobite had a head called the cephalon, a tail called the pygidium and a thorax in between. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 12 | They were also split into three from side to side: a central axis with a lobe on each side, called the pleurae. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 13 | The British Geological Survey says the name refers to that side-to-side split, not to head, thorax and tail. | British Geological Survey: Trilobites | 1 passed |
| 14 | The middle of the head is the glabella, with fixed cheeks and free cheeks on each side. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 15 | Many had spiky back corners to the head, called genal spines. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 16 | The thorax segments could bend, so a trilobite could curl into a ball, a bit like a woodlouse. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 17 | Rolling up protected its softer underside when danger came. | British Geological Survey: Trilobites | 1 passed |
| 18 | The segments of the head and tail were fused, but those of the thorax were not. | British Geological Survey: Trilobites | 1 passed |
| 19 | They were armoured on top; fossils of their soft undersides are much rarer. | Natural History Museum: How trilobites conquered prehistoric oceans | 2 passed |
| 20 | Usually only the back of the exoskeleton became a fossil; the underside may have been covered by soft material that could not fossilise. | British Geological Survey: Trilobites | 1 passed |
| 21 | In life they had antennae on the head, used to sense the world chemically. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 22 | They walked on jointed legs: three pairs under the head and one pair under each segment of the thorax and tail. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 23 | Each leg had two branches: a lower one for walking and an upper one with many fine filaments, perhaps for breathing. | British Geological Survey: Trilobites | 1 passed |
| 24 | Soft parts are known for only a few species, from special deposits called Lagerstätten. | British Geological Survey: Trilobites | 2 passed |
| 25 | Britannica says their exoskeleton was made of chitinous material. | Encyclopaedia Britannica: Trilobite | 1 passed |
| 26 | The NHM notes that, unlike the exoskeleton, their legs were not strengthened with calcium carbonate, so they rarely fossilised well. | Natural History Museum news: Trilobite fossil reveals how ancient arthropods mated (2022) | 2 passed |
| 27 | To grow, a trilobite had to shed its exoskeleton. | Encyclopaedia Britannica: Trilobite | 1 passed |
| 28 | Shed exoskeletons, or parts of them, are relatively common fossils. | Encyclopaedia Britannica: Trilobite | 1 passed |
| 29 | A facial suture across the cheeks helped the animal moult. | British Geological Survey: Trilobites | 1 passed |
| 30 | A cast-off exoskeleton often fell to pieces, which is why trilobite fossils are usually found as fragments. | British Geological Survey: Trilobites | 1 passed |
| 31 | Trilobites began life as tiny larvae called protaspides and moulted regularly as they grew. | Natural History Museum: How trilobites conquered prehistoric oceans | 2 passed |
| 32 | Young trilobites added thorax segments one at a time until they had the adult number, then simply grew larger. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 33 | Fortey thinks the earliest growth stages of many trilobites were part of the plankton. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 34 | Trilobite eyes were made of calcite, a crystal: the NHM says no other animal is known to have had that before or since. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 35 | Fortey says other arthropods have soft eyes, made of cuticle like the rest of the body. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 36 | Most were stacks of calcite prisms pointing in slightly different directions; more prisms probably meant better eyesight. | Natural History Museum: How trilobites conquered prehistoric oceans | 2 passed |
| 37 | Species had anything from one to several thousand lenses. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 38 | Most had compound eyes made of many separate lenses, though blind forms are known. | British Geological Survey: Trilobites | 2 passed |
| 39 | Some big, curved eyes gave a view forwards, backwards, sideways, up and even down. | British Geological Survey: Trilobites | 1 passed |
| 40 | Phacops had more complex lenses, and may have been able to see an object clearly and judge how far away it was. | British Geological Survey: Trilobites | 1 passed |
| 41 | Phacops eyes had drop-shaped lenses rather than hexagonal ones. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 42 | Erbenochile had tall, tower-like eyes that could probably scan for prey at a distance. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 43 | Asaphus kowalewskii had eyes on stalks. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 44 | One species of Cyclopyge had eyes fused into one large ‘super eye’. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 45 | Others had no eyes at all, and may have burrowed in mud or lived deep in dark water. | Natural History Museum: How trilobites conquered prehistoric oceans | 2 passed |
| 46 | Some with giant bulging eyes were probably open-ocean swimmers watching for prey. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 47 | Some were predators and others scavengers, perhaps eating worm-like creatures and other small animals on the sea floor. | Natural History Museum: How trilobites conquered prehistoric oceans | 2 passed |
| 48 | In 2023 scientists described a Bohemolichas incola with its last meal still in its gut: tiny crustaceans called ostracods, bivalves and a starfish relative. | Natural History Museum: How trilobites conquered prehistoric oceans | 3 passed |
| 49 | Swimmers may have harvested plankton, many probably grazed mud, and some filtered food from the water. | Natural History Museum: How trilobites conquered prehistoric oceans | 3 passed |
| 50 | Some may even have farmed sulphur-eating bacteria, perhaps in their gills, as some clams do. | Natural History Museum: How trilobites conquered prehistoric oceans | 2 passed |
| 51 | Britannica adds that some probably ate plankton. | Encyclopaedia Britannica: Trilobite | 1 passed |
| 52 | Dudley Council’s Wren’s Nest page says trilobites probably grazed on plants. | Dudley Council: Geology of Wren’s Nest National Nature Reserve | 1 passed |
| 53 | They were probably hunted by large Nautilus relatives, and later by jawed fishes. | Natural History Museum: How trilobites conquered prehistoric oceans | 2 passed |
| 54 | Anomalocaris was long suspected, but recent research suggests it may have preferred soft-bodied prey. | Natural History Museum: How trilobites conquered prehistoric oceans | 2 passed |
| 55 | Spines may have evolved to put off predators such as early jawed fishes. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 56 | Some trilobites lived at particular depths, so they help geologists work out ancient environments. | British Geological Survey: Trilobites | 1 passed |
| 57 | Trails such as Cruziana show that some trilobites moved about on the sea floor. | British Geological Survey: Trilobites | 1 passed |
| 58 | Cyclopyge could see downwards as well as in other directions, so geologists think it swam or floated above the floor. | British Geological Survey: Trilobites | 2 passed |
| 59 | A Burgess Shale fossil of Olenoides serratus, described in 2022, shows two pairs of legs interpreted as claspers. | Natural History Museum news: Trilobite fossil reveals how ancient arthropods mated (2022) | 2 passed |
| 60 | So males may have gripped females while mating, as horseshoe crabs do today. | Natural History Museum news: Trilobite fossil reveals how ancient arthropods mated (2022) | 1 passed |
| 61 | The NHM’s Greg Edgecombe says it is the first time trilobite males and females were told apart by their legs rather than their shells or size. | Natural History Museum news: Trilobite fossil reveals how ancient arthropods mated (2022) | 1 passed |
| 62 | Fossil trilobite eggs found in 2016 suggested a brood pouch. | Natural History Museum news: Trilobite fossil reveals how ancient arthropods mated (2022) | 1 passed |
| 63 | Walliserops had a large trident on its head, perhaps for contests over mates. | Natural History Museum: How trilobites conquered prehistoric oceans | 2 passed |
| 64 | The smallest known is Acanthopleurella stipulae, 1.5 mm long. | Natural History Museum: How trilobites conquered prehistoric oceans | 2 passed |
| 65 | That is narrower than a strand of spaghetti. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 66 | The largest is Isotelus rex, 72 cm long and 40 cm wide (the NHM page spells it Isoteus). | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 67 | A typical trilobite, Calymene, is about two centimetres long. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 68 | Many trilobites were 3 to 6 cm long. | British Geological Survey: Trilobites | 1 passed |
| 69 | Paradoxides was a giant, up to 60 cm or more. | British Geological Survey: Trilobites | 1 passed |
| 70 | Tiny blind agnostids were only a few millimetres long. | British Geological Survey: Trilobites | 1 passed |
| 71 | Some scientists have questioned whether agnostids are really trilobites at all. | British Geological Survey: Trilobites | 1 passed |
| 72 | Britannica says Paradoxides harlani, found near Boston, grew to more than 45 cm and may have weighed as much as 4.5 kg. | Encyclopaedia Britannica: Trilobite | 2 passed |
| 73 | Trilobites appeared in the Cambrian Period, 538 to 485 million years ago. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 74 | The first are known from around 520 million years ago. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 75 | Britannica puts their first appearance at 521 million years ago. | Encyclopaedia Britannica: Trilobite | 1 passed |
| 76 | That makes them about 100 million years older than the first shark and 290 million years older than the first dinosaur. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 77 | They lasted more than 250 million years, through six geological periods. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 78 | An NHM news story says their fossil record spans 275 million years. | Natural History Museum news: Trilobite fossil reveals how ancient arthropods mated (2022) | 1 passed |
| 79 | Our own species has been around for only 0.1% as long as trilobites lasted. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 80 | They appear suddenly in Cambrian rocks, and how they evolved is still a mystery. | Natural History Museum: How trilobites conquered prehistoric oceans | 2 passed |
| 81 | Spriggina, a segmented animal from 550-million-year-old rocks in Australia, looks a bit like a trilobite, but scientists now think the two are unlikely to be related. | Natural History Museum: How trilobites conquered prehistoric oceans | 2 passed |
| 82 | Britannica says Spriggina may be an ancestor of the trilobites and other arthropods. | Encyclopaedia Britannica: Trilobite | 1 passed |
| 83 | Because there were so many and new species evolved fast, they are excellent index fossils for dating rocks. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 84 | Paradoxides is found in England, Wales, Newfoundland, Sweden, Spain and Siberia, showing those rocks are all middle Cambrian. | British Geological Survey: Trilobites | 1 passed |
| 85 | Different trilobites on each side help show that Scotland was once near North America, across an ocean called Iapetus. | British Geological Survey: Trilobites | 1 passed |
| 86 | Extinctions at the end of the Cambrian wiped out many early groups. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 87 | An ice age at the end of the Ordovician, 440 million years ago, hit them again. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 88 | The survivors bounced back in the Silurian, splitting into as many species as before. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 89 | At the end of the Devonian, oxygen-poor water killed off coral reefs, an important trilobite home. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 90 | After that only one group, the Proetida, continued into the Carboniferous and Permian. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 91 | They vanish from the fossil record just before the Great Dying at the end of the Permian, when 90% of animals were wiped out. | Natural History Museum: How trilobites conquered prehistoric oceans | 2 passed |
| 92 | The British Geological Survey says they finally died out at the end of the Permian, about 250 million years ago. | British Geological Survey: Trilobites | 1 passed |
| 93 | The UC Museum of Paleontology says the last went extinct about 245 million years ago. | UC Museum of Paleontology, Berkeley: Introduction to the Trilobita | 1 passed |
| 94 | None survive: there is no sign of hidden living trilobites in the deep ocean. | Natural History Museum: How trilobites conquered prehistoric oceans | 2 passed |
| 95 | Chitons, woodlice and sea isopods look a bit like trilobites, but none is closely related. | Natural History Museum: How trilobites conquered prehistoric oceans | 2 passed |
| 96 | Their closest living relatives might be horseshoe crabs. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 97 | The NHM’s news story calls trilobites and horseshoe crabs distantly related. | Natural History Museum news: Trilobite fossil reveals how ancient arthropods mated (2022) | 1 passed |
| 98 | The first record of a trilobite is from Britain: in 1698 the Reverend Edward Lhwyd published a drawing of one found in Carmarthen. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 99 | At the time it was mistaken for the skeleton of a fish. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 100 | Almost 100 years later, the German naturalist Johann Walch officially named the group trilobites. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 101 | Around Carmarthen, trilobite tails were known as ‘petrified butterflies’ and linked with legends of Merlin. | British Geological Survey: Trilobites | 1 passed |
| 102 | A trilobite from the area, Merlinia, is named after him. | British Geological Survey: Trilobites | 1 passed |
| 103 | The NHM explains the ‘butterflies’ as Merlinia tails that broke away and look like wings. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 104 | In Sweden and China, trilobite tails were called butterfly stones, and in China also bat stones. | British Geological Survey: Trilobites | 2 passed |
| 105 | At Arcy-sur-Cure in France, a 15,000-year-old settlement, archaeologists found a trilobite drilled to be worn as an amulet. | UC Museum of Paleontology, Berkeley: Introduction to the Trilobita | 2 passed |
| 106 | By the mid-19th century people wore Dudley trilobites as jewellery. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 107 | Calymene blumenbachii was first found in the 18th century at Wren’s Nest in Dudley. | Natural History Museum: How trilobites conquered prehistoric oceans | 1 passed |
| 108 | Local miners called it the ‘Dudley locust’, and it ended up on the Dudley County Borough Council coat of arms. | Natural History Museum: How trilobites conquered prehistoric oceans | 2 passed |
| 109 | The BGS lists three nicknames: Dudley Locust, Dudley Bug and Dudley Insect. | British Geological Survey: Trilobites | 1 passed |
| 110 | Calymene sits at the centre of Dudley’s coat of arms. | British Geological Survey: Trilobites | 1 passed |
| 111 | Dudley Council says it was so common in 19th-century quarries that it became a symbol of the limestone mining industry. | Dudley Council: Geology of Wren’s Nest National Nature Reserve | 2 passed |
| 112 | On the crest it sits between an anchor for the sea and a miner’s Davy lamp. | Scientific American blog: The Dudley Bug (Glendon Mellow, 2011) | 1 passed |
| 113 | The Scientific American blog says the design was adopted in the 1950s, though trilobites are even in the walls of 11th-century Dudley Castle. | Scientific American blog: The Dudley Bug (Glendon Mellow, 2011) | 2 passed |
| 114 | The real fossils are not red; they came out of yellowish rock. | Scientific American blog: The Dudley Bug (Glendon Mellow, 2011) | 1 passed |
| 115 | The blog calls them about 430 million years old. | Scientific American blog: The Dudley Bug (Glendon Mellow, 2011) | 1 passed |
| 116 | Another common Wren’s Nest trilobite is Dalmanites. | Dudley Council: Geology of Wren’s Nest National Nature Reserve | 1 passed |
| 117 | Trilobites are probably the most famous fossils at Wren’s Nest. | Dudley Council: Geology of Wren’s Nest National Nature Reserve | 1 passed |
| 118 | Wren’s Nest became England’s first national nature reserve set up for its geology, in 1956. | Black Country Global Geopark: Wren’s Nest National Nature Reserve | 1 passed |
| 119 | The Geopark dates its rocks to between 427.7 and 429 million years old. | Black Country Global Geopark: Wren’s Nest National Nature Reserve | 1 passed |
| 120 | Dudley Council’s geology page says approximately 420 million years. | Dudley Council: Geology of Wren’s Nest National Nature Reserve | 1 passed |
| 121 | The same page dates the Ripple Beds sea to 428 million years ago. | Dudley Council: Geology of Wren’s Nest National Nature Reserve | 1 passed |
| 122 | Over 700 types of fossil are found there: 186 were first described there and 86 are found nowhere else on Earth. | Black Country Global Geopark: Wren’s Nest National Nature Reserve | 1 passed |
| 123 | The Geopark gives the reserve as about 101 acres (45 hectares). | Black Country Global Geopark: Wren’s Nest National Nature Reserve | 1 passed |
| 124 | Dudley Council calls it the 100-acre site. | Dudley Council: Geology of Wren’s Nest National Nature Reserve | 1 passed |
| 125 | The area was quarried and mined for limestone until 1925. | Dudley Council: Geology of Wren’s Nest National Nature Reserve | 1 passed |
| 126 | Sir Roderick Murchison’s 1839 book The Silurian System drew heavily on Dudley: about 65% of the Wenlock fossils it illustrated came from there. | Dudley Council: Geology of Wren’s Nest National Nature Reserve | 1 passed |
| 127 | Over 400 million years ago, Dudley was covered by coral reefs and tropical seas. | Dudley Council: Wren’s Nest National Nature Reserve | 1 passed |
| 128 | The Ripple Beds are ripple marks from the floor of the Silurian sea. | Dudley Council: Geology of Wren’s Nest National Nature Reserve | 1 passed |
| 129 | Collecting is allowed from the loose scree at the foot of the rock faces, but not from the faces themselves. | Black Country Global Geopark: Wren’s Nest National Nature Reserve | 1 passed |
| 130 | Dudley Council asks visitors not to use hammers or other tools on the rock faces, and to take only a few fossils at a time. | Dudley Council: Geology of Wren’s Nest National Nature Reserve | 2 passed |
| 131 | It also asks you to record that your fossils came from Wren’s Nest. | Dudley Council: Geology of Wren’s Nest National Nature Reserve | 1 passed |
| 132 | The rock faces and mining features are protected by law, as a Site of Special Scientific Interest and a Scheduled Ancient Monument. | Black Country Global Geopark: Wren’s Nest National Nature Reserve | 1 passed |
| 133 | There is no visitor centre, café or toilets. | Black Country Global Geopark: Wren’s Nest National Nature Reserve | 1 passed |
| 134 | There are four way-marked walks and geoart sculptures. | Black Country Global Geopark: Wren’s Nest National Nature Reserve | 1 passed |
| 135 | Wardens run ‘Hunt the Dudley Bug’ guided walks. | Dudley Council: Wren’s Nest National Nature Reserve | 1 passed |
| 136 | In Britain, trilobites are found in Cambrian, Ordovician and Silurian rocks in Wales and the Welsh Borderland, in Devonian rocks of south-west England and in Carboniferous rocks in places such as Lancashire. | British Geological Survey: Trilobites | 2 passed |