Stegosaurus

Stegosaurus (meaning "roof-lizard") is a group of extinct, plant-eating, four-legged armored dinosaurs that lived in the Late Jurassic. It is known for its unusual kite-shaped upright plates along its back and spikes on its tail. Fossils of this dinosaur have been found in the western United States and in Portugal. The rocks they come from are from the Kimmeridgian to Tithonian ages, which date to between 155 and 145 million years ago. Only three kinds from the upper Morrison Formation in the western US are accepted by all scientists: S. stenops, S. ungulatus, and S. sulcatus. Remains of over 80 individual animals of this type have been found. Stegosaurus lived alongside dinosaurs such as Apatosaurus, Diplodocus, Camarasaurus, and Allosaurus. Allosaurus may have hunted it.

These dinosaurs were large, heavy quadrupeds that ate plants. They had rounded backs, short front legs, long back legs, and tails held high in the air. Their mix of broad, upright plates and a spiked tail makes Stegosaurus one of the most recognizable dinosaurs. Scientists have long wondered what the plates and spikes were used for. Today, most scientists agree that the spiked tails were most likely used to defend against predators. The plates may have been used mainly to display, and to a lesser degree, to help control body temperature (thermoregulatory functions). Stegosaurus had a short neck and a small head, so it most likely ate low bushes and shrubs. One kind, Stegosaurus ungulatus, is one of the largest known of all the stegosaurians. The largest known specimens were about 9 metres (30 ft) long and weighed over 6.9 metric tons (7.6 short tons).

Scientists first identified Stegosaurus remains during the "Bone Wars" with help from Othniel Charles Marsh at the Dinosaur Ridge National Landmark. The first known skeletons were incomplete, and their bones were scattered. It would be many years before scientists fully understood how these animals really looked, including their posture and how their plates were arranged. Stegosaurus was popular in books and films, but mounted skeletons did not become a standard feature of major natural history museums until the mid-20th century. Many museums had to build displays from several different specimens because complete skeletons were lacking.

Today, Stegosaurus is one of the better-known dinosaurs. It has appeared in films, on postage stamps, and in many other kinds of media.

History and naming

Stegosaurus was one of many dinosaurs described during the Bone Wars, a time when scientists were rivals. Arthur Lakes and H.C. Beckwith found its first bones north of Morrison, Colorado. In 1877, Yale paleontologist Othniel Charles Marsh named the first species, Stegosaurus armatus. Its holotype is the main sample used to define the species. Marsh first thought the plates lay flat, like shingles or tiles on a roof. This is how the name "roof lizard" came about. Later, because these first bones were doubtful, the International Commission on Zoological Nomenclature replaced the type species with the better known Stegosaurus stenops. Marsh also wrongly linked some fossils to this species, including teeth from Diplodocus and a leg bone from Allosaurus. On the other side of the rivalry, Edward Drinker Cope named Hypsirhophus discurus in 1878 near Garden Park, Colorado. Many researchers now think it is a synonym of Stegosaurus. Fossils from the American Museum of Natural History also became part of the genus's history.

The most important finds came from Como Bluff in Albany County, Wyoming. This site dates from the Upper Jurassic and belongs to the Morrison Formation. In 1879, Lakes and William Harlow Reed collected the type specimen of Stegosaurus ungulatus there. The greatest discovery came in 1885, when Marshall P. Felch collected a nearly complete skeleton near Cañon City, Colorado. Marsh named it Stegosaurus stenops in 1887. It helped him make the first reconstructed skeleton, published in 1891. The skeleton is now displayed at the Smithsonian's National Museum of Natural History in Washington D.C. Later, the Peabody Museum of Natural History built the first mounted skeleton of a stegosaur in 1910, led by Richard Swann Lull. For many years, scientists argued about how the plates were arranged. By the early 1960s, most agreed on two rows of alternating plates. Later mounts appeared at the American Museum of Natural History, the Carnegie Museum of Natural History, and the Denver Museum of Nature and Science. In 2024, a skeleton nicknamed "Apex" sold for a record amount for a fossil.

Description

The four-legged Stegosaurus is one of the easiest dinosaurs to recognize. It had two rows of kite-shaped plates along its back and two pairs of long spikes near the end of its tail. S. stenops grew to about 6.5 m (21.3 ft) long and weighed about 3.5 metric tons (3.9 short tons). S. ungulatus grew to about 7 m (23.0 ft) long and weighed about 3.8 metric tons (4.2 short tons).

Its long, narrow skull was small for its body. It had a small antorbital fenestra, which is a hole between the nose and eye. This hole is found in most archosaurs, including modern birds. It had no front teeth, so it probably had a horny beak called a rhamphotheca instead.

Its brain was very small. In the 1880s, Othniel Charles Marsh studied a cast of the space where its brain sat. For its size, it was the smallest of all known dinosaur casts. An animal weighing over 4.5 metric tons (5 short tons) had a brain of no more than 80 g (2.8 oz). This helped support the old idea that dinosaurs were not smart, but that idea is now mostly rejected.

Its vertebral column had about 27 bones in front of the sacrum and about 46 tail bones. The first neck bone, the axis bone, was often joined to the atlas bone. The neural spines on the tail became smaller toward the tip and disappeared near the 35th tail bone. The shoulder blade, or scapula, was roughly rectangular. The coracoid was roughly circular.

Each hind foot had three short toes. Each front foot had five toes, but only the inner two had a blunt hoof. The front legs were much shorter than the sturdy back legs, which gave it an unusual posture. Its head was probably no higher than 1 m (3.3 ft) above the ground.

Its most recognizable features were its plates. It had between 17 and 22 plates and flat spines. These were osteoderms, which are bony-cored scales. They were not attached to the skeleton but grew from the skin. The largest plates, over the hips, could be more than 60 cm (24 in) wide and 100 cm (39 in) tall. Scientists have suggested that the plates may have been arranged differently in each species. They have also suggested that the plates in Hesperosaurus were covered by keratinous sheaths.

Classification and species

Stegosaurs are related to ankylosaurs. Their bony plates and spines grew from low bony ridges, like those of early armored dinosaurs. In 2019, Galton studied armor plates from the Lower Jurassic of India. He thought they came from an ankylosaur, a member of Ankylosauria. This suggests that ankylosaurs and their sister group, the stegosaurs, may have started early in the Early Jurassic. Most known stegosaurs belong to the Stegosauridae, which lived in the later Jurassic and the Early Cretaceous. Paul Sereno defined this group. Its members have been found across the Northern Hemisphere, in Africa, and possibly in South America.

Many species once named for Stegosaurus are now considered invalid or the same as earlier species, so only two are well known. Stegosaurus stenops, named in 1887, is the best known because it includes at least one complete skeleton. Its holotype was collected in 1886 near Cañon City, Colorado. It was shorter than other species, at 6.5 m (21 ft). Stegosaurus ungulatus, named in 1879 from remains at Como Bluff, was the largest species, at 7–9 m (23.0–29.5 ft). A fragmentary specimen from Portugal, from the Kimmeridgian–Tithonian stage, has been tentatively assigned to it. In 1910, Richard Swann Lull used its type specimen for the first mounted stegosaur skeleton at the Peabody Museum of Natural History. Stegosaurus sulcatus, named in 1887, is known from a partial skeleton. It is mainly known for its large, furrowed spikes. Other names are doubtful or have been moved to other groups. These include Stegosaurus armatus, which was replaced as type species by S. stenops in a 2013 ruling of the ICZN. Some species were placed in other genera, such as Hoplitosaurus, Lexovisaurus, and Hesperosaurus mjosi, found in Johnson County, Wyoming. A species from Madagascar is known only from teeth.

Paleobiology

Scientists have studied how Stegosaurus moved, grew, and ate. Marsh first thought it walked on two legs. By 1891 he had changed his mind, and today it is thought to have walked on four. Bakker suggested it could stand up on its back legs to reach high leaves, but Carpenter disagreed. Its lower legs, the tibia and fibula, were short compared with the femur, so it probably could not run fast. Tracks show that it lived in herds with animals of different ages. Young ones had not yet fully joined their scapula and coracoid or their lower back legs, and their pelvic region looked like that of young Kentrosaurus. Its small, peg-shaped teeth were very different from those of other plant-eating ornithischian dinosaurs. It probably did not swallow gastroliths to help grind its food. A 2010 study measured its bite force in newtons and found it was less than half that of a Labrador Retriever. A 2016 study of a specimen called Sophie, compared with Erlikosaurus and Plateosaurus, suggested its bite was stronger than people once thought. Its hip space, once called a "second brain," more likely held a glycogen body, which may help supply the nervous system.

Scientists have long debated what the plates were for. Marsh thought they were armor, but Davitashvili disagreed. Studies of Hesperosaurus suggested the plates were covered in horn-like material. Like the sails of the pelycosaurs Dimetrodon and Edaphosaurus, the plates may have helped control body temperature. Bony plates of Alligator support this idea, though others have questioned it. The plates may also have helped the animal absorb heat from the sun, which fits with a surface area argument. Bone studies of histological structure linked blood vessels to fast plate growth. The plates may also have helped show off to others, making them blush or look taller. They also display features that differed in shape between males and females. The tail spikes may have been used in combat, as shown by a process with damage from a spike. It was likely cold-blooded, possibly ectothermic or gigantothermic. This idea is based on spectroscopy of signals linked to oxidative phosphorylation. Its ancestors may have been endothermic.

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Paleoecology

The Morrison Formation was probably a dry place with clear wet and dry seasons and flat floodplains. Plants included forests of conifers, tree ferns, and ferns along rivers. Fern grasslands had a few trees, such as Araucaria-like conifer Brachyphyllum. Fossils show green algae, fungi, mosses, horsetails, ferns, cycads, ginkgoes, and several kinds of conifers. Animal fossils include clams, snails, fish, frogs, salamanders, turtles like Glyptops, sphenodonts, lizards, and crocodile relatives such as Hoplosuchus. There were also several kinds of pterosaurs, such as Harpactognathus and Mesadactylus, many kinds of dinosaurs, and early mammals. These included docodonts (like Docodon), multituberculates, symmetrodonts, and triconodonts.

Dinosaurs that lived alongside Stegosaurus included meat-eating theropods such as Allosaurus, Saurophaganax, Torvosaurus, Ceratosaurus, Marshosaurus, Stokesosaurus, Ornitholestes, Coelurus, and Tanycolagreus. The long-necked sauropods were the most common. They included Brontosaurus, Brachiosaurus, Apatosaurus, Diplodocus, Camarasaurus, and Barosaurus. Other ornithischians included Camptosaurus, Gargoyleosaurus, Dryosaurus, and Nanosaurus. Stegosaurus is often found at the same sites as Allosaurus, Apatosaurus, Camarasaurus, and Diplodocus. It may have preferred drier places than these other dinosaurs.

Cultural significance

Stegosaurus is one of the most recognizable dinosaurs. It has appeared in films, cartoons, comics, and children's toys. Most early Stegosaurus fossils were incomplete, so it took many years to make accurate pictures of the dinosaur. The earliest popular picture was an engraving by the French science illustrator Auguste-Michel Jobin. It appeared in the November 1884 issue of Scientific American and in other places. Jobin showed the dinosaur standing on two legs, with a long neck, plates along its tail, and spikes on its back. The spikes may have come from a mistake about the teeth. Marsh had noticed that the teeth were oddly shaped and scattered, so he thought they might be small spines.

Marsh published a more accurate skeleton drawing of Stegosaurus in 1891. Within ten years, it had become one of the most illustrated kinds of dinosaur. Artist Charles R. Knight first drew Stegosaurus ungulatus in a November 1897 issue of The Century Magazine. He based the drawing on Marsh's skeleton. That drawing later became the model for the stop-motion puppet in the 1933 film King Kong. Knight later showed the plates in a staggered double row. He used this in a 1927 painting for the Field Museum of Natural History. Rudolph F. Zallinger painted the plates the same way in his "Age of Reptiles" mural in 1947. In 1904, a paper mache model of Stegosaurus was shown at the Louisiana Purchase Exposition in St. Louis. It was later moved to the Smithsonian's National Museum of Natural History in Washington, D.C. In the 2010s, after renovations, it was moved again to the Museum of the Earth in Ithaca, New York.