Stegosaurus and the mystery of the plates
A famous silhouette whose purpose is still debated
Quick facts
- When
- Late Jurassic, about 152 to 145 million years ago
- Size
- About 9 m long
- Defense
- A powerful spiked tail
- Open question
- The plates' purpose: defense, display, or temperature regulation (unresolved)
What happened
Stegosaurus lived in what is now the United States in the Late Jurassic, about 152 to 145 million years ago, growing to roughly 9 metres long, defended by a powerful spiked tail it could swing at predators such as Allosaurus. Its signature row of bony plates was embedded in its skin rather than fused to its skeleton, which is why fossil plates are usually found separated from the body. The Natural History Museum states plainly that scientists are not certain what the plates were for: possibilities include warning off predators, helping members of the species recognize each other, or regulating body temperature, an idea supported by tiny grooves in the plate surfaces that may have held blood vessels. When the paleontologist O. C. Marsh first described a Stegosaurus fossil in 1877 he thought the plates lay flat along the back; only after finding a specimen preserved in mud that had held the plates in place did he realize they stood upright, alternating along the spine.
Why it matters
Stegosaurus shows how much can still be genuinely unknown about an animal known from good fossils and a famous silhouette. The plates are the clearest open question in this timeline so far, and the story of Marsh revising his own reconstruction after better evidence is a small, honest example of how paleontology corrects itself.
How we know
The Natural History Museum's record for Stegosaurus gives its Late Jurassic age, size, tail defense, and the skin-embedded nature of its plates, states outright that their purpose is not certain and lists the competing theories including thermoregulation, and describes Marsh's 1877 naming and his later correction of the plates' orientation.
Sources
- Natural History Museum, London. Stegosaurus (Natural History Museum Dino Directory) (2024) · Reputable sourcenhm.ac.uk · The domain "nhm.ac.uk" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
- U.S. National Park Service. Stegosaurus (Dinosaur National Monument, U.S. National Park Service) · Reputable sourcenps.gov · The domain "nps.gov" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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Part of a timelineAge of Dinosaurs21 events · The age of the dinosaurs across the Mesozoic Era, from the Great Dying that cleared the way to the asteroid that ended their reign.View all →