Episode 150 – Stegosaurs

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They are among the most famous and the most recognizably bizarre of all dinosaurs. This episode, we discuss the fossil record, evolutionary history, and many morphological mysteries surrounding Stegosaurs.

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The Plated Dinosaurs

Few dinosaurs are as instantly recognizable as Stegosaurus, with its unique profile, short front arms, small head, and massive back plates. In fact, these features are common to a group called stegosaurs, the plated dinosaurs. These quadrupedal herbivores lived all over the world from the Middle Jurassic to the Early Cretaceous.

Stegosaurs – and their armor – came in a variety of sizes and styles, though they all shared the same generally recognizable stegosaur-y features.
Top left: Tuojiangosaurus by Paleocolour, CC BY-SA 4.0
Top right: The (relatively) small Huayangosaurus by Nobu Tamura, CC BY 3.0
Bottom left: Long-necked Miragaia by Nobu Tamura, CC BY 3.0
Bottom right: The famous Stegosaurus by Fred Wierum, CC BY-SA 4.0
These dino images are not all to scale with each other!

Stegosaurs, like their cousins the ankylosaurs, carried around characteristic body armor. In stegosaurs, this armor includes rows of plates and/or spines along the back, two pairs of spikes at the tip of the tail, and sometimes spikes sticking out of the shoulders.

Left: Stegosaurus and its close relatives had the familiar arrangement of big, broad plates on their backs. Photo by Perry Quan, CC BY-SA 2.0
Right: Other stegosaurs like Kentrosaurus had spikes along their backs and on their shoulders. Photo by LoKiLeCh, CC BY 3.0

A Unique Suit of Armor

For well over a century, paleontologists have puzzled over the function of the back plates of stegosaurs. It might be that they served some form of defense, and it might be that they had some role in regulating body temperature, and it seems very likely that they were display features not unlike the elaborate headgear seen on other animals.

Stegosaur plates are highly modified osteoderms (skin bones), similar to those found in other dinosaurs like ankylosaurs and other reptiles like crocodilians.
Image by Tim Evanson, CC BY-SA 2.0

Less puzzling are the spikes at the end of stegosaur tails. This apparatus – four sideways-pointing spikes on a stiff tail – is called a thagomizer thanks to a Far Side cartoon. Like the tails of ankylosaurs, stegosaur tails were formidable weapons. There are even fossils of predators like Allosaurus with injuries seemingly induced by the mighty swing of a thagomizer.

Stegosaurs wielded one of evolution’s most formidable natural weapons, which paleontologists have come to call a thagomizer.
Top: Thagomizer at the end of Kentrosaurus‘ tail. Image by FunkMonk, CC BY-SA 3.0
Bottom: An isolated thagomizer. Image by Kevmin, CC BY-SA 4.0
There is ample evidence that stegosaurs used their tail weapons for defense against predators like Allosaurus. Image by mrwynd, CC BY 2.0

Learn More

Stegosaurs, an overview (technical, open access)
Stegosaurus, an overview

Stegosaurus Plate Debate (Smithsonian Magazine, 2012)
The Stegosaurus Plate Controversy (Tetrapod Zoology, 2016)

The Double Dinosaur Brain Myth

About Thagomizers:
Watch Out for That Thagomizer! (2011)
Kentrosaurus Had a Formidable Swing (2011)
Allosaurus Died From wStegosaur Spike to the Crotch, Wyoming Fossil Shows (2017)

If you enjoyed this topic and want more like it, check out these related episodes:

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