Stegosaurus, meaning “roofed lizard,” is instantly recognizable due to the double row of large, kite-shaped plates running along its back and the deadly spikes on its tail, known as the thagomizer. This herbivore roamed the Earth during the Late Jurassic period. The history of this iconic animal in paleontology is rooted in a specific, foundational discovery that first brought these armored reptiles to scientific light.
The Initial Discovery
The first fossil material attributed to Stegosaurus was recovered in 1877 near the small town of Morrison, Colorado, during the intense and competitive period known as the “Bone Wars.” The initial discovery belongs to Arthur Lakes, a geologist and fossil hunter working for Othniel Charles Marsh.
Lakes discovered the fragmented remains at his YPM Quarry 5, situated north of Morrison. The material consisted of several caudal (tail) vertebrae, a single dermal plate, and various other postcranial elements. The sparse, fragmented nature of these bones complicated initial attempts at understanding the creature’s full anatomy.
Naming and Classification of the First Find
Othniel Charles Marsh formally named the new genus in 1877, designating the discovery as the type specimen for Stegosaurus armatus. This name literally means “armored roof lizard.” The original fossil fragments were assigned the catalog number YPM 1850 and became the holotype for the species.
Marsh’s initial interpretation of the anatomy was significantly different from the modern depiction. He hypothesized that the bony plates lay flat across the creature’s back, overlapping like shingles on a roof, which is the direct source of the genus name. Marsh also misidentified some accompanying bones, including the dentary bone of a sauropod dinosaur. Due to its fragmented state and Marsh’s misinterpretations, S. armatus is now considered a dubious type specimen, though it remains the historical starting point for research.
The Broader Geological Context
The discovery of Stegosaurus is tied directly to the vast geological feature known as the Morrison Formation. This rock unit is a sequence of sedimentary layers spanning a huge area across the Western United States, including parts of Colorado, Utah, and Wyoming. The formation dates to the Late Jurassic period, between approximately 155 and 145 million years ago.
During this time, the landscape was a semi-arid basin characterized by floodplains and rivers. The rapid burial of animal remains in these ancient channels created the conditions necessary for fossilization. The Morrison Formation is one of the most fruitful sources of giant dinosaur fossils, yielding Stegosaurus and contemporaries like Allosaurus, Apatosaurus, and Diplodocus.