At a Glance

Attribute	Detail
Scientific name	Stegosaurus stenops
Pronunciation	STEG-oh-SORE-us
Name meaning	“Roof lizard” — from Greek stegos (roof) + sauros (lizard)
Classification	Ornithischia > Thyreophora > Stegosauria > Stegosauridae > Stegosaurus
Period	Late Jurassic, approximately 155–150 Ma
Found in	Morrison Formation, western North America; close stegosaur relatives and possible referred material have been reported from Portugal and North Africa
Diet	Herbivore — ferns, cycads, horsetails, low-growing conifers
Length	Approximately 9 m (30 ft)
Weight	Approximately 3,000–5,000 kg (6,600–11,000 lb)
Status	Extinct

Quick Answer: Stegosaurus facts cover one of the most recognisable dinosaurs ever to have lived — a large, slow-moving herbivore of the Late Jurassic, approximately 155 to 150 million years ago, defined by a double row of upright bony plates along its spine and four tail spikes used in active defence. It lived in the floodplains of what is now western North America, disappeared roughly 84 million years before the Chicxulub impact at the end of the Cretaceous, and has been misunderstood in at least one significant way — the two-brains myth — ever since it was first described in 1877.

At roughly 9 metres (30 ft) long, with body mass estimates ranging widely depending on specimen and reconstruction, Stegosaurus was not the largest dinosaur of its time and place. The Morrison Formation housed sauropods that dwarfed it entirely — but it was among the most architecturally distinctive.

The double row of plates running from neck to tail, the four-spiked thagomizer, the small skull, the rump-high posture: no other animal in the fossil record looks quite like it. What those plates were actually for remains one of palaeontology’s more genuinely open questions, and the answer is almost certainly more complex than any single hypothesis suggests.

How Big Was Stegosaurus?

Field	Detail
Length	Approximately 9 m (30 ft)
Height at hips	Approximately 2.7 m (9 ft)
Weight	Approximately 3,000–5,000 kg (6,600–11,000 lb)
Plate height	Up to 60 cm (24 in) on the largest individuals
Size vs human	Roughly 3× the length of an average adult human

Body mass estimates for Stegosaurus vary considerably across published studies, and no single figure should be treated as settled.

For more details, see our full Stegosaurus size post.

What Did Stegosaurus Eat?

Field	Detail
Diet type	Herbivore
Primary food sources	Ferns, cycads, horsetails, low-growing conifers
Feeding height	Low-browsing — skull positioned close to ground level
Tooth type	Small, leaf-shaped teeth with limited occlusal surface
Feeding evidence	Tooth morphology; feeding posture inferred from skeletal geometry

Stegosaurus could not chew in the way mammals do — its teeth were simple, and its jaw mechanics were limited. It likely cropped vegetation and relied on gut fermentation to process plant material.

For more details, see our full Stegosaurus diet post.

Why Did Stegosaurus Have Plates?

Field	Detail
Number of plates	Approximately 17 in S. stenops
Plate arrangement	Double alternating row along the spine
Plate composition	Bone — not keratin-covered like horns
Primary hypotheses	Thermoregulation; sexual display; species recognition; defence
Current consensus	No single function confirmed — multiple roles likely

The plate function debate remains genuinely open. Vascular channels documented in plate bone tissue are consistent with thermoregulation, but display and species recognition are equally well supported by the evidence available.

For more details, see our full Stegosaurus plates post.

Stegosaurus Tail Spikes: The Thagomizer

(the four-spiked tail weapon known informally as the thagomizer)

Field	Detail
Number of spikes	Four — two pairs
Spike length	Up to approximately 90 cm (35 in)
Term origin	Gary Larson’s The Far Side, 1982; adopted by Smithsonian palaeontologists
Fossil evidence of use	Allosaurus caudal vertebra with spike-shaped puncture wound documented
Function	Active defensive weapon — direct fossil evidence confirmed

The thagomizer is the one feature of Stegosaurus anatomy whose function is directly supported by fossil evidence rather than inference alone.

For more details, see our full Stegosaurus tail spikes post.

Stegosaurus vs Allosaurus

Field	Detail
Did they coexist?	Yes — both documented in the Morrison Formation
Fossil evidence of interaction	Allosaurus vertebra with thagomizer puncture wound; Stegosaurus material with theropod bite marks
Allosaurus size	Approximately 8.5–12 m (28–39 ft); up to ~2,000 kg (4,400 lb)
Speed advantage	Allosaurus — considerably faster by biomechanical estimates
Stegosaurus defence	Thagomizer; bulk; possibly plate display as deterrent

Direct fossil evidence of predator–prey interaction between these two species is among the most compelling in the Morrison Formation record.

For more details, see our full Stegosaurus vs Allosaurus post.

Discovery and Fossil History

Field	Detail
First described by	Othniel Charles Marsh, 1877
First specimen	Morrison Formation, Colorado
Context	Bone Wars — competitive fossil-hunting rivalry between Marsh and Cope
Most complete specimen	Sophie — Natural History Museum, London; approximately 85% complete
Key institutions	Natural History Museum London; Smithsonian Institution; Denver Museum of Nature and Science

The discovery of Stegosaurus occurred during one of the most competitive and consequential periods in the history of vertebrate palaeontology.

For more details, see our full Stegosaurus discovery post.

Why Did Stegosaurus Go Extinct?

Field	Detail
Extinction timing	Approximately 150 Ma — end of the Late Jurassic
Extinction type	Late Jurassic faunal turnover — not a mass extinction
Time before K-Pg event	Approximately 84 million years
Confirmed cause	None established in published literature
Ecological successors	Ankylosaurs; later ceratopsians

Stegosaurus was not killed by the Chicxulub asteroid. It had been extinct for 84 million years before that event occurred.

For more details, see our full Stegosaurus extinction post.

Stegosaurus Habitat: The Morrison Formation

Field	Detail
Formation	Morrison Formation, western North America
States represented	Colorado, Utah, Wyoming, Montana, South Dakota, Oklahoma, New Mexico
Climate	Warm, seasonally semi-arid
Vegetation	Ferns, cycads, horsetails, conifers — no grasses or flowering plants
Key contemporaries	Allosaurus, Diplodocus, Brachiosaurus, Camarasaurus, Camptosaurus

The Morrison Formation is one of the most intensively studied dinosaur-bearing geological units in the world, and its fauna represents a snapshot of Late Jurassic North America in exceptional detail.

For more details, see our full Stegosaurus habitat post.

How Fast Was Stegosaurus?

Field	Detail
Typical walking speed	Approximately 6–7 km/h (3.7–4.3 mph)
Maximum speed estimate	Approximately 6–8 km/h (4–5 mph)
Locomotion type	Quadrupedal; graviportal
Speed evidence basis	Biomechanical modelling — most studies date from early 2010s
Primary defence strategy	Thagomizer — not flight

Speed estimates for Stegosaurus are model-dependent and should be treated as indicative ranges. Updated biomechanical analyses using newer methods have not yet been widely published for this species.

For more details, see our full Stegosaurus speed post.

When Did Stegosaurus Live?

Field	Detail
Period	Late Jurassic
Epochs	Kimmeridgian to Tithonian
Date range	Approximately 155–150 Ma
Continental configuration	Laurasia and Gondwana partially separated; North America and Europe still connected
Time before T. rex	Approximately 82 million years

The gap between Stegosaurus and Tyrannosaurus rex is greater than the gap between T. rex and the present day — one of the most striking illustrations of deep time in palaeontology.

For more details, see our full Stegosaurus time period post.

Did Stegosaurus Have Two Brains?

Field	Detail
Did it have two brains?	No
Myth origin	Enlarged sacral cavity described by early researchers as a possible nerve centre
Modern interpretation	Glycogen body or enlarged sacral nerve ganglion — not a brain
Brain volume	Approximately 70 cm³
Encephalisation	Among the lowest documented for any non-avian dinosaur

The two-brains myth entered popular culture before science caught up and has proven resistant to correction ever since — despite being debunked for decades.

For more details, see our full Stegosaurus brain myth post.

How to Pronounce Stegosaurus

Field	Detail
Pronunciation	STEG-oh-SORE-us
Syllable stress	Primary stress on first syllable — STEG
IPA	/ˌstɛɡ.ə.ˈsɔːr.əs/
Most common error	Placing equal stress on SORE rather than STEG
Syllable count	Four

For more details, see our full Stegosaurus pronunciation post.

Allosaurus fragilis was the apex predator of the Morrison Formation and the primary large theropod directly contemporaneous with Stegosaurus — fossil evidence documents predator–prey interaction between the two species.
Diplodocus carnegii and Brachiosaurus altithorax shared the same formation but browsed at heights far beyond Stegosaurus’s reach, reducing direct dietary competition.
Kentrosaurus aethiopicus, from the broadly contemporaneous Tendaguru Formation of Tanzania, is the closest ecological and taxonomic parallel to Stegosaurus outside North America.
Huayangosaurus taibaii, from the Middle Jurassic of China, represents the earlier stegosaurid lineage from which Stegosaurus descended.

Common Questions About Stegosaurus

What are 3 interesting facts about Stegosaurus?

Stegosaurus had a very small brain relative to body size, often popularly compared to the size of a walnut — one of the lowest brain-to-body ratios of any known dinosaur. Its tail spikes, known as the thagomizer, left puncture wounds in Allosaurus bones that are preserved in the fossil record. And Stegosaurus disappeared roughly 84 million years before the asteroid that killed T. rex — meaning it is a more ancient relative to T. rex than T. rex is relative to us.

Was Stegosaurus a friendly dinosaur?

“Friendly” is not a scientific behavioural category. Stegosaurus was a large herbivore with no dietary interest in attacking other animals, but fossil evidence of thagomizer injuries on Allosaurus bones confirms it was capable of delivering serious wounds when threatened. Its behaviour toward members of its own species is not documented in the fossil record.

How big is a Stegosaurus compared to a human?

At approximately 9 m (30 ft) in length and around 2.7 m (9 ft) at the hips, Stegosaurus was roughly three times the length of an average adult human. Its body mass estimate of approximately 3,000–5,000 kg (6,600–11,000 lb) represents many times the mass of an average adult human.

Did Stegosaurus live at the same time as T. rex?

No. Stegosaurus and Tyrannosaurus rex are separated by approximately 82 million years. Stegosaurus went extinct around 150 Ma; T. rex did not appear until approximately 68 Ma. They never coexisted, never shared a habitat, and never interacted in any form.

What killed Stegosaurus?

No single confirmed cause has been established. Stegosaurus disappeared at the end of the Late Jurassic as part of a broader faunal turnover — not a mass extinction event. Changing vegetation, shifting climates, and competition from emerging herbivore lineages have all been proposed as contributing factors, but the fossil evidence does not yet support a definitive explanation.

Stegosaurus remains one of the most studied and most misunderstood dinosaurs in the fossil record — a genuinely strange animal whose plates still divide opinion, whose brain has been mythologised for over a century, and whose place in deep time is more remote than most people realise. The spokes linked throughout this page go deeper on every topic covered here.

References

A. Primary Taxonomic and Institutional Sources

Marsh, O. C. (1877). A new order of extinct Reptilia (Stegosauria) from the Jurassic of the Rocky Mountains. American Journal of Science and Arts, Series 3, 14, 513–514.

Kirkland Museum of Fine & Decorative Art (historical archival references to O.C. Marsh collections and Bone Wars context).

Natural History Museum. Sophie the Stegosaurus — specimen information and mounted skeleton archive.

Smithsonian Institution. National Museum of Natural History — Stegosaurus collections and vertebrate paleontology references.

Denver Museum of Nature & Science. Morrison Formation and Jurassic dinosaur specimen references.

B. Major Peer-Reviewed Literature

Galton, P. M., & Upchurch, P. (2004). Stegosauria. In D. B. Weishampel, P. Dodson, & H. Osmólska (Eds.), The Dinosauria (2nd ed., pp. 343–362). University of California Press.

Maidment, S. C. R., Norman, D. B., Barrett, P. M., & Upchurch, P. (2008). Systematics and phylogeny of Stegosauria (Dinosauria: Ornithischia). Journal of Systematic Palaeontology, 6(4), 367–407.

Carpenter, K., Sanders, F., McWhinney, L. A., & Wood, L. (2005). Evidence for predator-prey relationships: Examples for Allosaurus and Stegosaurus. In K. Carpenter (Ed.), The Carnivorous Dinosaurs (pp. 325–350). Indiana University Press.

Fastovsky, D. E., & Weishampel, D. B. (2005). The Evolution and Extinction of the Dinosaurs (2nd ed.). Cambridge University Press.

Foster, J. R. (2003). Paleoecological Analysis of the Vertebrate Fauna of the Morrison Formation (Upper Jurassic), Rocky Mountain Region, USA. New Mexico Museum of Natural History and Science Bulletin 23.

Lucas, S. G. (Ed.). (2014). The Morrison Formation: Extinct Ecosystems of the Jurassic. New Mexico Museum of Natural History and Science Bulletin.

C. Functional Anatomy and Behaviour

Christiansen, P., & Tschopp, E. (2010). Exceptional stegosaur integument impressions from the Upper Jurassic Morrison Formation of Wyoming. Swiss Journal of Geosciences, 103, 163–171.

Main, R. P., de Ricqlès, A., Horner, J. R., & Padian, K. (2005). The evolution and function of thyreophoran dinosaur scutes: Implications for plate vascularization and thermoregulation hypotheses. Paleobiology, 31(2), 291–314.

Buffrénil, V. de, Farlow, J. O., & de Ricqlès, A. (1986). Growth and function of Stegosaurus plates: Histological evidence and interpretation. Historical Biology.

D. Public Education and Terminology References

Larson, G. (1982). The Far Side — origin of the term “thagomizer.”

Benton, M. J. (2015). Vertebrate Paleontology (4th ed.). Wiley-Blackwell.

Naish, D., & Barrett, P. (2016). Dinosaurs: How They Lived and Evolved. Smithsonian Books.

E. Geological and Habitat References

Turner, C. E., & Peterson, F. (2004). Reconstruction of the Upper Jurassic Morrison Formation extinct ecosystem—A synthesis. Sedimentary Geology, 167(3–4), 309–355.

Foster, J. (2007). Jurassic West: The Dinosaurs of the Morrison Formation and Their World. Indiana University Press.