Stegosaurus

Lore & Background

Stegosaurus, one of the many dinosaurs described in the Bone Wars, was first discovered by Arthur Lakes and H.C. Beckwith, consisting of several caudal vertebrae, a dermal plate, and several additional postcranial elements that were collected north of Morrison, Colorado, at Lakes' YPM Quarry 5. These first, fragmented bones (YPM 1850) became the holotype of Stegosaurus armatus when Yale paleontologist Othniel Charles Marsh described them in 1877. Marsh initially believed the remains were from an aquatic turtle-like animal, and the basis for its scientific name, 'roof(ed) lizard' was due to his early belief that the plates lay flat over the animal's back, overlapping like the shingles on a roof. Though several more complete specimens have been attributed to Stegosaurus armatus, preparation of the bones and analysis has discovered that this type specimen is actually dubious. Because of this, the International Commission on Zoological Nomenclature decided to replace the type species with the more well known species Stegosaurus stenops. Marsh also incorrectly referred several fossils to S. armatus, including the dentary and teeth of the sauropod Diplodocus and putting sauropod limb bones and an Allosaurus tibia under YPM 1850. The first mounted skeleton of a stegosaur was created in 1910 at the Peabody Museum of Natural History by Richard Swann Lull, using the type specimen of S. ungulatus (YPM 1853).

Reader's Guide

Stegosaurus were large, heavily built, herbivorous quadrupeds with rounded backs, short fore limbs, long hind limbs, and tails held high in the air. Due to their distinctive combination of broad, upright plates and tail tipped with spikes, Stegosaurus is one of the most recognizable kinds of dinosaurs. The function of this array of plates and spikes has been the subject of much speculation among scientists. Today, it is generally agreed that their spiked tails were most likely used for defense against predators, while their plates may have been used primarily for display, and secondarily for thermoregulatory functions. Stegosaurus had a relatively low brain-to-body mass ratio. It had a short neck and a small head, meaning it most likely ate low-lying bushes and shrubs. One species, Stegosaurus ungulatus, is one of the largest known of all the stegosaurians, with the largest known specimens measuring about 7.5 m long and weighing over 5 MT.

Did You Know?

• Stegosaurus remains were first identified during the 'Bone Wars' by Othniel Charles Marsh at Dinosaur Ridge National Landmark.
• The first known skeletons were fragmentary and the bones were scattered, and it would be many years before the true appearance of these animals became well understood.
• Mounted skeletons of Stegosaurus did not become a staple of major natural history museums until the mid-20th century.
• The type specimen of Stegosaurus armatus (YPM 1850) was collected north of Morrison, Colorado, not from Como Bluff, Wyoming.

The Bone Wars and a Misidentified Giant

Stegosaurus entered the fossil record during the heated rivalry of the Bone Wars, a period when competing paleontologists raced to name and claim new species. Beckwith from a quarry north of Morrison, Colorado. Marsh, working with these fragmentary remains, initially mistook the animal for something resembling an aquatic turtle. The competitive atmosphere meant fossils were often misassigned; Marsh, for instance, lumped a Diplodocus jaw and an Allosaurus shin bone into his Stegosaurus material. It would take decades of careful preparation before the true posture and plate arrangement of these animals became clear.

A Body Built for Browsing, Not Brute Force

Stegosaurus was a massive, heavily built quadruped whose silhouette was dominated by broad, upright plates running along its rounded back and a pair of formidable spikes guarding the tip of its tail. Despite its imposing size—some specimens of S. ungulatus reached roughly 7.5 metres in length and exceeded five metric tons in weight—the animal carried a surprisingly small head on a short neck, a configuration that strongly suggests it fed on low-lying shrubs and bushes rather than reaching high into the canopy. Its brain-to-body mass ratio was notably low, placing it among the less cognitively complex of its contemporaries. The forelimbs were short while the hind limbs were longer, and the tail was held high in the air. In its Late Jurassic habitat, Stegosaurus shared the landscape with long-necked sauropods such as Apatosaurus, Diplodocus, and Camarasaurus, as well as the predatory Allosaurus, which may have targeted it. Today, scientists broadly agree that the tail spikes served as a defensive weapon against such predators, while the elaborate plates likely functioned first as a visual display and secondarily as a means of regulating body temperature.

A Tangled Taxonomic History

Sorting out exactly which fossils belong to which Stegosaurus species has proven remarkably difficult. Although the genus was named in the 1870s, only three species—S. stenops, S. ungulatus, and S. sulcatus—are universally accepted by the paleontological community today. Complicating matters, the original type species, S. armatus, is now considered dubious because the holotype material is so fragmentary that it cannot be confidently diagnosed. In response, the International Commission on Zoological Nomenclature stepped in to designate S. stenops as the replacement type species, a significant administrative correction for such a well-known genus. Other names Marsh introduced have not survived: S. duplex is now regarded as a synonym of S. ungulatus, and S. 'affinis,' based on a single lost hip bone, was declared a nomen nudum. The question of whether Cope's Hypsirhophus discurus represents a separate genus or merely another Stegosaurus remains debated, with some researchers pointing to vertebral differences.

From Fragment to Icon

Despite being one of the most instantly recognizable dinosaurs in popular culture—appearing in films, on postal stamps, and in countless other media—Stegosaurus took an unusually long time to find its place in museum halls. The earliest known skeletons were so fragmentary and their bones so scattered that the animal's true posture and the arrangement of its plates remained poorly understood for decades. It was not until the mid-twentieth century that mounted Stegosaurus skeletons became a regular feature of major natural history museums. Because complete specimens are rare, many institutions have had no choice but to construct composite mounts by combining bones from multiple individuals. A turning point in understanding came in 1885, when a nearly complete, articulated subadult skeleton was unearthed, preserving previously unknown elements including a full skull, throat ossicles, and plates still in their natural positions. Marshall P. Felch oversaw the collection of this remarkable find. Even so, the sheer number of specimens—over eighty individuals—has not eliminated the need for careful reconstruction, and the genus continues to reward researchers with new anatomical details long after its initial description.

Common Misconceptions (Editorial)

1. Despite its name meaning "roof-lizard," Stegosaurus did not have its plates lying flat over its back like roof shingles; that was an early misinterpretation by Othniel Charles Marsh. 2. The first Stegosaurus fossils were collected near Como Bluff, Wyoming, not from Morrison, Colorado. 3. The original type species, Stegosaurus armatus, is now considered dubious and has been replaced by Stegosaurus stenops as the type species by the International Commission on Zoological Nomenclature.

Why It Matters (Editorial)

Stegosaurus endures as a cultural icon because its bizarre anatomy—those upright kite-shaped plates and a spiked tail called a thagomizer—captures the imagination in a way that more ordinary creatures cannot. Its story also reflects the messy, competitive birth of paleontology during the Bone Wars, reminding us that even our most familiar dinosaurs were once mysterious puzzles that scientists had to painstakingly piece together.

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