What scientists still argue about
Stegosaurus is one of the best-known dinosaurs, but some of the biggest questions about it are still open. Here is the evidence on each side, so you can see how palaeontologists weigh it up.
Jump to: The plates | Males and females | Herds | Bite | Species | When | Size | The hip space | The throat
What were the plates for?
Scientists are not exactly sure what the plates were for. Ideas include warning off predators, recognising their own kind, and controlling body temperature. Natural History Museum: Stegosaurus (Dino Directory)
Defence is now out of fashion, because the plates, though large, are thin and fragile. Natural History Museum: A Stegosaurus brought to life An AMNH curator points out the plates were only loosely attached, so may not have stood up to a predator’s bite. American Museum of Natural History: Apex Stegosaurus fossil and “11 Questions About Stegosaurus”
The plates seem to have been full of blood vessels, which could have carried warm blood up to shed heat. Natural History Museum: A Stegosaurus brought to life Wind-tunnel tests in 1976 showed the plates could shed heat, but a 2010 follow-up found no sign they evolved for that. Smithsonian Magazine: Stegosaurus plate debate, 2012
A 2005 study of plate microstructure found no evidence they were used to radiate heat. Smithsonian Magazine: Stegosaurus plate debate, 2012 Stegosaur plates and spikes varied greatly between species, which suggests display drove their evolution. Smithsonian Magazine: Stegosaurus plate debate, 2012
The Natural History Museum now favours one of two jobs: display, or controlling temperature. Natural History Museum: A Stegosaurus brought to life An AMNH curator says display, to mates or rivals, is the most likely function. American Museum of Natural History: Apex Stegosaurus fossil and “11 Questions About Stegosaurus”
A Smithsonian curator says the plates’ purpose “remains a bit of a mystery”. Smithsonian Magazine: Meet the dinos of “Deep Time”, 2019 Smithsonian Magazine wrote in 2024 that the plates’ function remains an enigma. Smithsonian Magazine: Apex on display in New York, 2024 · checked 27 Oct 2026
Where that leaves it: defence has mostly been ruled out, and display is the favourite of most of the sources we read, but the Natural History Museum still keeps temperature control open and the Smithsonian calls it a mystery.
Can we tell males from females?
That study found two plate shapes, wide oval plates and tall narrow ones, and proposed they were males and females. Saitta (2015), Evidence for sexual dimorphism in the plated dinosaur Stegosaurus mjosi, PLOS ONE
A study of Sophie the same year said there are, as yet, no reliable ways to tell male from female Stegosaurus. Maidment, Brassey & Barrett (2015), The postcranial skeleton of an exceptionally complete individual of Stegosaurus stenops, PLOS ONE Sophie’s sex is unknown: the name doesn’t mean it was female. Maidment, Brassey & Barrett (2015), The postcranial skeleton of an exceptionally complete individual of Stegosaurus stenops, PLOS ONE
It is difficult to tell male and female dinosaurs apart: to prove a female you need eggs inside, or a special egg-laying bone tissue. Natural History Museum: Stegosaurus Q&A with Prof Paul Barrett
Where that leaves it: one 2015 study says yes, from plate shape. Another study from the same year says there is no reliable way yet. Both are peer-reviewed. Further work is needed.
Alone or in herds?
The Natural History Museum says Stegosaurus didn’t live in herds: probably alone or in small groups. Natural History Museum: Stegosaurus Q&A with Prof Paul Barrett
An AMNH curator says stegosaur fossils are often single animals, sometimes found with big sauropods, and stegosaur-only herds may have been rare. American Museum of Natural History: Apex Stegosaurus fossil and “11 Questions About Stegosaurus”
A 2015 study described a Montana quarry where at least five plated stegosaurs were buried together, which possibly suggests they were social. Saitta (2015), Evidence for sexual dimorphism in the plated dinosaur Stegosaurus mjosi, PLOS ONE
Where that leaves it: the museums lean towards solitary animals or small groups, while one fossil site of at least five suggests some may have been social. Fossils show where bodies ended up, not always how animals lived.
How hard could it bite?
An earlier 2010 computer model suggested a weak bite, able to cut twigs under half an inch thick. Smithsonian Magazine: The weak bite of Stegosaurus, 2010 · checked 27 Oct 2026
CT scans of Sophie’s skull showed a bite force similar to a cow’s or a sheep’s, and efficient jaws. Natural History Museum: A Stegosaurus brought to life
A 2016 study estimated its bite at 231 to 410 newtons, higher than two other plant-eating dinosaurs it tested. Lautenschlager, Brassey, Button & Barrett (2016), Decoupled form and function in disparate herbivorous dinosaurs, Scientific Reports · checked 27 Oct 2026
Where that leaves it: the newer work, using Sophie’s separate skull bones, finds a bite in the range of living plant eaters. The 2010 estimate was weaker. The studies used different methods and different skulls.
How many species of Stegosaurus?
Marsh’s first species, Stegosaurus armatus (1877), was based on scrappy fossils. Smithsonian Magazine: Playing the stegosaur name game, 2010 In 2013 the official naming commission made Stegosaurus stenops the type species, replacing S. armatus. Maidment, Brassey & Barrett (2015), The postcranial skeleton of an exceptionally complete individual of Stegosaurus stenops, PLOS ONE
The Natural History Museum’s directory still lists the type species as armatus. Natural History Museum: Stegosaurus (Dino Directory) · checked 27 Oct 2026
That study also treats Stegosaurus ungulatus as the same species as S. stenops. Maidment, Brassey & Barrett (2015), The postcranial skeleton of an exceptionally complete individual of Stegosaurus stenops, PLOS ONE The 2015 study of Sophie lists Hesperosaurus and Wuerhosaurus as the same genus as Stegosaurus. Maidment, Brassey & Barrett (2015), The postcranial skeleton of an exceptionally complete individual of Stegosaurus stenops, PLOS ONE
Some scientists count Hesperosaurus as a species of Stegosaurus (Stegosaurus mjosi). Others, including one of the scientists who named it, say it is a separate genus. Smithsonian Magazine: Playing the stegosaur name game, 2010
Whether two fossils are separate species is decided by scientists comparing bones, and they don’t always agree. Smithsonian Magazine: Playing the stegosaur name game, 2010
Where that leaves it: the type species is now Stegosaurus stenops. Whether Hesperosaurus and other plated dinosaurs belong in Stegosaurus is still argued about.
When did it live?
The Natural History Museum dates it to the Late Jurassic, 152 to 145 million years ago. Natural History Museum: Stegosaurus (Dino Directory) · checked 27 Oct 2026
The Smithsonian’s label dates that Stegosaurus to 157 to 153 million years ago. Smithsonian NMNH virtual tour: Stegosaurus spikes vs. Ceratosaurus claws · checked 27 Oct 2026 The Smithsonian’s Deep Time article gives 157 to 148 million years ago for Stegosaurus. Smithsonian Magazine: Meet the dinos of “Deep Time”, 2019 · checked 27 Oct 2026
Sophie is about 150 million years old and the only Stegosaurus in a public collection outside the USA. Natural History Museum press release: World’s most complete Stegosaurus goes on show, 2014 · checked 27 Oct 2026 Apex lived into old age, and the museum is studying a thigh-bone sample to learn how stegosaurs grew. American Museum of Natural History: Apex Stegosaurus fossil and “11 Questions About Stegosaurus” · checked 27 Oct 2026
Where that leaves it: all of the sources put it in the Late Jurassic, around 150 million years ago. The exact span depends on which fossils and which rock dates are used.
How big and how heavy?
The Natural History Museum gives its length as 9 metres. Natural History Museum: Stegosaurus (Dino Directory) · checked 27 Oct 2026 Other Stegosaurus specimens reach up to nine metres long, the Museum says. Natural History Museum: Stegosaurus Q&A with Prof Paul Barrett · checked 27 Oct 2026
Sophie weighed about 1,600 kg in life, roughly the weight of a small rhino. Natural History Museum: Secrets of the Stegosaurus, 2014 · checked 27 Oct 2026 The exact estimate was 1,560 kg, with a likely range of 1,082 to 2,256 kg. Brassey, Maidment & Barrett (2015), Body mass estimates of an exceptionally complete Stegosaurus, Biology Letters · checked 27 Oct 2026
Equations based on leg bones gave 2,355 to 3,751 kg, far more, until the team allowed for Sophie not being fully grown. Brassey, Maidment & Barrett (2015), Body mass estimates of an exceptionally complete Stegosaurus, Biology Letters
An American Museum of Natural History curator describes an adult’s body as about five tons. American Museum of Natural History: Apex Stegosaurus fossil and “11 Questions About Stegosaurus” · checked 27 Oct 2026 The same article says Stegosaurus could weigh almost 14,000 pounds (about 6.4 tonnes) and grow 21 to 30 feet (6.4 to 9.1 m) long. Smithsonian Magazine: “Apex” sells for $44.6 million, 2024 · checked 27 Oct 2026
When it was sold, Apex was reported as 11 feet (3.4 m) tall and 20 feet (6.1 m) long, and more than 30 percent larger than Sophie. Smithsonian Magazine: “Apex” sells for $44.6 million, 2024 · checked 27 Oct 2026 Apex, a big old Stegosaurus, is 11.5 feet (3.5 m) tall and 27 feet (8.2 m) long, about 30 percent larger than others. Smithsonian Magazine: Apex on display in New York, 2024 · checked 27 Oct 2026
Where that leaves it: Sophie’s 1,600 kg is a careful estimate for one young animal. Bigger adults were heavier, but we haven’t found a single agreed adult weight, and the sources range from “five tons” to almost 14,000 pounds.
What was the space in its hips?
No dinosaur had a second brain. Smithsonian Magazine: The double dinosaur brain myth, 2012 Marsh described a space in its hips as “a posterior braincase” in 1881, the start of the second-brain myth. Smithsonian Magazine: The double dinosaur brain myth, 2012
The leading idea is that the hip space held a glycogen body, like birds have, though what it does is still unclear. Smithsonian Magazine: The double dinosaur brain myth, 2012
The Natural History Museum says the hip space was a store of food substances to help survive lean times. Natural History Museum: A Stegosaurus brought to life An AMNH curator calls it an enlarged space for the nerve cord, not a brain. American Museum of Natural History: Apex Stegosaurus fossil and “11 Questions About Stegosaurus”
Where that leaves it: it was not a brain. What filled it is still unknown.
Why do some have throat armour?
Some Stegosaurus specimens have small pebble-like bones in the throat. Sophie doesn’t, and nobody yet knows why. Maidment, Brassey & Barrett (2015), The postcranial skeleton of an exceptionally complete individual of Stegosaurus stenops, PLOS ONE
The Smithsonian’s Stegosaurus has bony nodules guarding its neck. Smithsonian Magazine: Meet the dinos of “Deep Time”, 2019
Where that leaves it: unknown. It could be about age, sex or species, or simply which bones happened to survive.
Despite being so famous, well-preserved Stegosaurus fossils are surprisingly rare: most specimens are less than half a skeleton. Maidment, Brassey & Barrett (2015), The postcranial skeleton of an exceptionally complete individual of Stegosaurus stenops, PLOS ONE With so few good skeletons, one new fossil can change the answer. That is why we mark figures that can change with the date we checked them.