Every fact

136 facts, every one sourced

Every fact on this site in one list, each linked to its source. Our build stores the source text and checks that the key words and figures in each fact really appear in it; see the sources page.

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