Stegosaurus

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

These dinosaurs were large, heavily built quadrupeds that ate plants. They had rounded backs, short front legs, long back legs, and tails held high in the air. Because of their unusual mix of broad, upright plates and a spiked tail, Stegosaurus is one of the most recognizable kinds of dinosaurs. Scientists have long discussed what the plates and spikes were used for. Today, it is generally agreed that the spiked tails were most likely used for defense against predators, while the plates may have been used mainly for display and, to a lesser degree, for thermoregulatory functions. Stegosaurus had a short neck and a small head, so it most likely ate low-lying bushes and shrubs. One species, Stegosaurus ungulatus, is one of the largest known of all the stegosaurians. The largest known specimens measured about 9 metres (30 ft) long and weighed over 6.9 metric tons (7.6 short tons).

Stegosaurus remains were first identified during the "Bone Wars" by 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 the arrangement of their plates. Even though Stegosaurus was popular in books and films, mounted skeletons did not become a standard feature of major natural history museums until the mid-20th century. Many museums had to build composite 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 postal stamps, and in many other kinds of media.

History and naming

Stegosaurus was one of many dinosaurs described during the Bone Wars, a time of rivalry between scientists. Its first bones were found by Arthur Lakes and H.C. Beckwith north of Morrison, Colorado. In 1877, Yale paleontologist Othniel Charles Marsh named the first species, Stegosaurus armatus, which became the holotype. Marsh first thought the plates lay flat like shingles (tiles) on a roof, which 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, a site from the Upper Jurassic of the Morrison Formation. Lakes and William Harlow Reed collected the type specimen of Stegosaurus ungulatus there in 1879. 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, and 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. Scientists also argued for many years about how the plates were arranged, and 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 a double row of kite-shaped plates rising up its back and two pairs of long spikes near the end of its tail. S. stenops reached 6.5 m (21.3 ft) in length and 3.5 metric tons (3.9 short tons) in body mass. S. ungulatus reached 7 m (23.0 ft) in length and 3.8 metric tons (4.2 short tons). Its long, narrow skull was small for its body and had a small antorbital fenestra, a hole between the nose and eye that 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. When Othniel Charles Marsh studied a cast of the brain cavity in the 1880s, it was the smallest of all known dinosaur endocasts for its size. An animal weighing over 4.5 metric tons (5 short tons) had a brain of no more than 80 g (2.8 oz), which helped support the old idea that dinosaurs were not smart. 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 fused to the atlas bone. The neural spines on the tail got smaller toward the tip and disappeared near the 35th tail bone. The shoulder blade, or scapula, was sub-rectangular, and the coracoid was sub-circular. Each hind foot had three short toes, and each fore foot had five toes, but only the inner two toes had a blunt hoof. The front legs were much shorter than the stocky 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. There were between 17 and 22 plates and flat spines. These were osteoderms, which are bony-cored scales, and 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, and that the plates in Hesperosaurus were covered by keratinous sheaths.

Classification and species

Stegosaurs are related to ankylosaurs, and 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 and 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, and 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, and 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 in the first mounted stegosaur skeleton at the Peabody Museum of Natural History. Stegosaurus sulcatus, named in 1887, is known from a partial skeleton and is mainly known for its large, furrowed spikes. Other names are doubtful or have been moved to other groups, including 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, but by 1891 he had changed his mind, and today it is considered to have walked on four. Bakker suggested it could rear up on its hind legs to reach high leaves, while Carpenter opposed this idea. 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 of mixed ages. Juveniles had not yet fused their scapula and coracoid or their lower hind limbs, and their pelvic region was like that of young Kentrosaurus. Its small, peg-shaped teeth were very different from those of other plant-eating ornithischian dinosaurs, and it likely 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 once thought. Its hip space, once called a "second brain," more likely served other purposes, such as holding 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 disputed this. 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, and comparisons with the 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, and bone studies of histological structure linked the blood vessels to fast plate growth. The plates may have helped display to others, making them blush or appear taller, and they 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. Metabolism was likely cold-blooded, possibly ectothermic or gigantothermic, based on spectroscopy of signals linked to oxidative phosphorylation, and its ancestors may have been endothermic.

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Paleoecology

The Morrison Formation is thought to have been a semiarid place with clear wet and dry seasons and flat floodplains. Plants included forests of conifers, tree ferns, and ferns along rivers, and fern grasslands with 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 bivalves, snails, ray-finned fishes, frogs, salamanders, turtles like Glyptops, sphenodonts, lizards, and crocodile relatives such as Hoplosuchus. They also include several species of pterosaurs such as Harpactognathus and Mesadactylus, many dinosaur species, and early mammals such as docodonts (like Docodon), multituberculates, symmetrodonts, and triconodonts.

Dinosaurs that lived alongside Stegosaurus included meat-eating theropods Allosaurus, Saurophaganax, Torvosaurus, Ceratosaurus, Marshosaurus, Stokesosaurus, Ornitholestes, Coelurus, and Tanycolagreus. Long-necked sauropods were the most common, including 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, and 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 before accurate pictures of the dinosaur could be made. 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 elsewhere. Jobin showed the dinosaur standing on two legs, with a long neck, plates along its tail, and spikes covering 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 reconstruction 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, based 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, which was used in a 1927 painting for the Field Museum of Natural History. Rudolph F. Zallinger painted it 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.