Stegosaurus

• 1. Introduction

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  On the 17th of July 2024, a nearly complete Stegosaurus skeleton nicknamed Apex sold for 44.6 million dollars at a Sotheby's auction in New York City. It was the most ever paid for a fossil. The 27-foot animal had been dug from private land in Colorado in 2022, and because it sat on private ground, it could pass to a private buyer. That sale captures a strange truth about this Late Jurassic herbivore. It is one of the most recognizable kinds of dinosaur on Earth, yet for most of scientific history nobody knew quite what it looked like. Its name means roof lizard, a name given because the man who described it first thought its bony plates lay flat across its back like shingles. He also briefly thought it was an aquatic turtle-like animal. How did a creature whose every plate sparked decades of argument become a fixture of film, stamps, and toys? Why did its discoverer get its posture, its plates, and even its body plan wrong? And what was the purpose of those kite-shaped plates and that spiked tail in the first place?

• 2. Bones Of A Feud

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  Othniel Charles Marsh of Yale described the first Stegosaurus in 1877, from fragmented bones collected north of Morrison, Colorado, by Arthur Lakes and H.C. Beckwith. Those scattered remains, several tail vertebrae and a single dermal plate among them, became the holotype of Stegosaurus armatus. The find came during the bitter rivalry known as the Bone Wars. On the opposing side, Edward Drinker Cope named Hypsirhophus discurus in 1878 from fragments at Garden Park, Colorado, a creature many later treated as the same animal as Stegosaurus. Marsh moved fast and made mistakes. He wrongly assigned to Stegosaurus armatus the teeth of the sauropod Diplodocus, along with sauropod limb bones and an Allosaurus tibia. Lakes returned to the field at Como Bluff in Albany County, Wyoming, in August 1879, pulling large fossils from Quarry 13. Among them was the type specimen of Stegosaurus ungulatus, the hoofed roof lizard, named for its preserved foot. Marsh named species at a rapid clip. Stegosaurus affinis in 1881 rested on a single hip bone that has since been lost, leaving the name a nomen nudum. The original type species, Stegosaurus armatus, proved too fragmentary to compare with anything, and in 2013 the International Commission on Zoological Nomenclature replaced it with the better-known Stegosaurus stenops.

• 3. The Narrow-Faced Lizard

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  Marshall P. Felch unearthed the find that changed everything across 1885 and 1886, at his quarry in Garden Park near Cañon City, Colorado. It was a nearly complete, articulated skeleton of a subadult, with a complete skull, throat ossicles, and plates still in position. Felch divided the skeleton into labeled blocks and prepared them separately before shipping it to Marsh in 1887. Marsh named it Stegosaurus stenops, the narrow-faced roof lizard, that same year. This specimen let him attempt the first reconstructed Stegosaurus skeleton, published in 1891, with missing parts filled in from Stegosaurus ungulatus. Stegosaurus stenops remains the best-known species, drawn from at least 50 partial skeletons of adults and juveniles, one complete skull, and four partial skulls. It reached 6.5 meters in length, shorter than Stegosaurus ungulatus at 7 meters. A third species, Stegosaurus sulcatus, the furrowed roof lizard, stands apart for its unusually large, furrowed spikes. One spike found with its type specimen has such a wide base that researchers suspect it came from the shoulder or hip rather than the tail.

• 4. The Plate Puzzle

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  Four different arrangements of the plates have been proposed since the genus was named, and the argument runs through the whole history of dinosaur reconstruction. Marsh first imagined the plates lying flat like roof shingles, the idea behind the name. As more complete plates surfaced, their shape showed they stood on edge. By 1891 Marsh drew a single row of plates, an idea revived in modified form in the 1980s by Stephen Czerkas using iguana dorsal spines as a guide. Frederick Lucas argued in 1901 for paired plates in two rows, the look seen in Charles R. Knight's reconstruction and in the 1933 film King Kong, though within a year Lucas leaned toward staggered rows instead. Charles Gilmore settled the matter in 1914. He noted that several specimens of Stegosaurus stenops, including the fully prepared holotype, preserved the plates in alternating rows near the peak of the back, with no sign they had shifted after death. By the early 1960s the staggered double row had become the prevailing view, and it remains so. The plates themselves are striking. There were between 17 and 22 separate plates and flat spines, highly modified osteoderms that arose from the skin rather than attaching to the skeleton. The largest, over the hips, could measure more than 60 centimeters wide and 60 centimeters tall. In 2010 Peter Galton suggested the pattern in profile may have helped animals recognize their own species.

• 5. Why The Plates

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  Davitashvili argued in 1961 that the plates made poor armor, too fragile and badly placed, leaving the animal's flanks exposed. That objection opened a debate that has never fully closed. One leading idea is temperature control. The plates carried blood vessels running through grooves, and Buffrénil and colleagues in 1986 found extreme vascularization of the outer bone layer, reading it as evidence the plates worked as thermoregulatory devices. Comparisons in 2010 with Alligator osteoderms supported a similar role. Yet the picture is messy. Kentrosaurus, a related stegosaur, had more low-surface-area spikes than plates, suggesting cooling did not demand such elaborate structures. Christiansen and Tschopp in 2010 studied a Hesperosaurus specimen with skin impressions and concluded the plates were sheathed in keratin, which would have hampered cooling while giving the plates sharp edges. Display offers another explanation. The plates' large size could have made the animal look taller, intimidating rivals or attracting mates. A 2015 study of Hesperosaurus plates proposed they were sexually dimorphic, with wide plates on males and taller plates on females. One vivid theory holds that Stegosaurus pumped blood into the plates to make them blush a warning red, though the horny covering complicates that picture.

• 6. The Thagomizer

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  Gilmore proposed in 1914 that the four tail spikes were only for display. The fossils tell a more violent story. A study by McWhinney and colleagues found that 9.8 percent of examined Stegosaurus specimens carried trauma-related damage to their tail spikes. The clinching evidence is an Allosaurus tail vertebra punctured by a wound into which a Stegosaurus spike fits perfectly. The spike struck from below, displaced a piece of bone upward, and the underside later showed remodeled bone, the mark of an infection that set in afterward. The spikes of Stegosaurus stenops measured about 60 to 90 centimeters long. Articulated armor shows that in at least some species the spikes jutted horizontally from the tail, not vertically as artists often drew them. Robert Bakker noted the tail lacked ossified tendons, so it was likely far more flexible than other dinosaur tails, well suited as a weapon. Marsh originally described Stegosaurus ungulatus with eight spikes, but recent work concluded it too had four, the extra pair likely belonging to another individual.

• 7. A Small Brain And A Second One

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  The braincase of Stegosaurus was no larger than that of a dog, despite an animal that weighed over 4.5 metric tons. In the 1880s Marsh took a cast of the brain cavity and found the brain proportionally the smallest of any dinosaur known at the time. The notion that an animal this heavy carried a brain of no more than 80 grams fed the old popular idea that all dinosaurs were stupid, an idea now largely rejected. Marsh also noticed a large canal in the hip region of the spinal cord, big enough for a structure up to 20 times larger than the brain. From this grew the influential myth of a second brain in the tail, supposedly steering the rear of the body and giving the animal a boost under attack. Barnum Brown disputed that misconception in his article on the American Museum mount. The space more likely held a glycogen body, a structure also found in birds, whose function is not certain but may help supply glycogen to the nervous system. The head sat low, probably no higher than 1 meter above the ground, paired with peg-shaped teeth and jaws likely limited to simple up-and-down movement. How such an animal gathered and processed the plant material to sustain its bulk remains, in the words of researchers, poorly understood.

• 8. From Bone Halls To Auction Block

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  Richard Swann Lull built the first Stegosaurus skeletal mount in the world at the Peabody Museum of Natural History in 1910, using the type specimen of Stegosaurus ungulatus. He set the plates in pairs at first, then remounted it in 1924 with two staggered rows along the midline. Mounted Stegosaurus skeletons stayed rare through the first half of the 20th century, because most fossils were disarticulated, and museums often had to build composites from several individuals. The Carnegie Museum drew on dozens of specimens from Dinosaur National Monument for a composite mount in 1940. In 1982 Stegosaurus became the official state fossil of Colorado, after a two-year campaign begun by a class of fourth graders and their teacher Ruth Sawdo at McElwain Elementary School in Thornton, Colorado. The most complete specimen of all is Sophie, 85 percent intact with 360 bones, found by Bob Simon in 2003 near Shell, Wyoming. The Natural History Museum in London bought it with help from private donors, chief among them Jeremy Herrmann, and put it on display in December 2014. The Apex sale to a private owner revived an old worry. Many researchers insist that important fossils belong in formal institutions, permanently curated, so that anyone studying the past can reach them.