Stegosaurus

Credit: Matt Dempsey.

*Stegosaurus* is a genus of herbivorous Stegosaur that lived in Wyoming and Colorado during the Late Jurassic.

## History

*Stegosaurus* was named by Othniel Charles Marsh (1877) during the bone wars. These first remains were assigned to S. armatus, believing them to be of an aquatic turtle-like animal, naming it the "roofed lizard" due to his interpretation that they lay flat over the back, somewhat like shingles. Much more material was found in the next years, from which Marsh continued to study until 1897. Edward Drinker Cope (1878) named *Hypsirhopus discurus*, a fragmentary stegosaur from Cope's Nipple site, Cope's Quarry 3, Garden Park, Colorado. Many later assigned these to Stegosaurus, though vertebral differences highlighted by Peter Galton (2010) suggests it may be distinct. *S. ungulatus*, named by Marsh (1879) gave a detailed description of Stegosaurus collected the following year. *S. "affinis"* was named by Marsh (1881) from a pelvis, but it is thought to have an insufficient description, so it is a nomen nudum. Later, however, this specimen was lost.

Marsh continued to collect and study new specimens, naming *S. stenops*, *duplex* and *sulcatus* in 1887. Though not fully prepared at the time, the near-articulate *S. stenops* allowed him to fully reconstruct the entire animal. The first *S. ungulatus* paleoillustration filled missing elements with *S. stenops*, and published by him in 1891. However, Richard Lydekker (1893) accidentally published the skeletal under *Hypsirhopus*.

*S. marshi* was named by Frederick Lucas (1901), which he later merged with *Hoplitosaurus*, re-examining the plate formation, which he found to form double rows above each rib base. He commissioned Charles R. Knight to create S. ungulatus paleoart. Later the following year, Lucars re-reexamined the plates, finding they likely sat in staggered rows. Richard Swann Lull (1910) the staggered pattern was due to shifting of the skeleton after death, mounting the first skeletal mount at the Peabody Museum of Natural History, which had paired plates. Charles Gilmore (1914) debated this, noting several *S. stenops* (now including the fully-prepared holotype) preserved alternating plates near the back's peak, having no evidence of shifting after death. Gilmore and Lucas' interpretations are, generally, accepted now, with Lul's mount changed in 1924. Sophie the Stegosaurus is the most complete known specimen. Sophie is a young adult of indeterminate sex, who reached 5.6 meters (18 feet) long and 2.9 meters (9.5 feet) tall. They are 85% intact and discovered in Wyoming, with 360 bones. In December 2014, it was put on display at the Natural History Museum.

## Description

*Stegosaurus* was a quadrupedal dinosaur with a distinct double row of kite-shaped plates arranged vertically and 4 long spikes on the end of its tail, being one of the easiest dinosaurs to identify. They could grow up to 9 meters (29.5 feet) long, but were dwarfed by contemporaneous sauropods. It likely needed armour, since it lived with large theropods. Most Stegosaurus are mature, but recent juveniles have been found. One subadult is 4.6 meters (15.1 feet) long and 2 meters (6.6 feet) tall.

Stegosaurus has a long and narrow skull that was proportionally small. The antorbital fenestra is small. The position of the skull suggests it may have browsed low vegetation, suggested by the lack of front teeth and their probable replacement by a rhamphotheca. The mandible has flat upward extensions that hid the teeth from the side, which probably helped support a turtle-like beak, The presence of the latter probably insinuate the existence of cheeks. The beak was likely unique to *Stegosaurus* and some advanced Stegosauridae. Others interpret these ridges as a modified from other ornithischians, who may have supported fleshy cheeks rather than beaks. Their teeth are small, triangular and flat, Wear facets suggest food grinding took place.

Despite their size, their braincase was very small. Marsh obtained a braincase in the 1880s, which showed the brain was very small (the smallest proportionally in any dinosaur), being no more than 80 grams (2.8 ounces). This contributed to the, now obsolete, idea that dinosaurs were unintelligent. The actual brain anatomy is poorly known, but the size fits for a slow herbivore with limited behavioural complexity.

*S. stenops* has 27 bones in the vertebral column anterior to the sacrum, with the sacrum having 4 in most subadults and ~46 caudals. There are ~10 cervicals and ~17 dorsals, having a greater total than *Hesperosaurus*, 2 greater than *Huayangosaurus* and probably less than Miragaia. The cervicals become larger posteriorly. The centra become elongate past the first few dorsals, front-to-back, with the transverse processes becoming more dorsally elevated. ne of the dorsals incorporates into the sacrals, with some including a caudosacral. *S. stenops* has 46 caudals, and up to 49, with the neural spines shrinking along both centra series, where they disappear at caudal 35. The mid-tail has bifurcated neural spines.

Multiple well-preserved *S. stenops* show all regions of the body. The scapula is sub-rectangular with a robust blade. The acromion ridge is slightly larger than *Kentrosaurus*, though not always fully preserved. The blade is somewhat straight, curving nearer to the posterior. A small bump adorns the back of the blade, which would have anchored the triceps. The coracoid is sub-circular and articulates with the scapula.. The hindlegs have 3 digits and 5 on the forelegs. All limbs were supported by pads behind the toes. The inner two toes on the forefeet bear a blunt hoof. The phalangeal formula is 2-2-2-2-1. The forelimbs were much shorter, which results in a strange posture. The tail is held clear if the ground with the head positioned low down, likely no higher than 1 meter (3.3 feet) high.

It had 17 and 22 separate plates, which were highly-modified osteoderms. They were not directly attached, arising from the skin, The largest plate were over the hips, measuring over 60 centimeters (24 inches) wide and 60 centimeters (24 inches) tall. Galton (2010) suggested these may have varied between species, and that their appearance in profile may have identified species, The type *S. ungulatus* has flattened spine-like plates in the tail that are near-identical in shape and size, being mirrored, suggesting they were arranged in pairs. Most plates are manifestly chiral, with no two being the same in a single individual, but they have instead been correlated between individuals. Integument in *Hesperosaurus* plates have a smooth surface with long, parallel, shallow grooves, indicating a keratinous sheath.

Tyler Greenfield proposes *"Stegosaurus" sulcatus* is a dacentrurine due to similar dermal spines. Thus, the species "*sulcatus*" would make the Morrison dacentrurine *Alcovasaurus*/*Miragaia sulcatus*.

## Paleobiology

### Plates

Several interpretations of *Stegosaurus* plates have been offered. In terms of dinosaur reconstruction, it was often debated, with four major theories:

1. They lay flat on the back, as Marsh originally reconstructed.
2. Marsh (1981)'s single plate row along the back.
3. Two plate rows arranged to match, as illustrated by Knight and proposed by Lull.
4. Two rows of plates, arranged in a staggered pattern, seen as valid today.

*Stegosaurus*' plates have a debated function. Marsh suggested they were armour, though Davitashvili (1961) disputed this, claiming they were fragile and ill-placed to suit this. Still, others have defended this theory. Bakker (1986) suggests they were keratinous, having bony cores. Christainsen and Tschopp (2010), studying *Hesperosaurus* integument, concluded they were coated in a sheath of keratin, strengthening the plate and providing a sharp cutting edge. Bakker states it could flip its osteoderms from side-to-side to present an opponent many sharp spines, having them impeding successful attack. He contends, stating they had an insufficient width for them to easily erect without continuous muscular stress. Plate mobility has also been contended.

Another possible function is thermoregulation, since they sit in such a way similar to *Dimetrodon*, the ears of rabbits and elephants and *Edaphosaurus*. The plates had blood vessels that ran through grooves, with air flowing around the plates cooling the blood. Buffrénil, et al. (1986) found "extreme vascularization" in the outermost bone layer, which suggests they were thermoregulatory in use. In 2010, structural comparisons of Stegosaurus plates to alligator osteoderms supports the potential thermoregulation, so this role likely exists.

However, the latter hypothesis has been seriously questioned, since stegosaurus like *Kentrosaurus* have lesser surface area, suggesting that cooling was not important enough to evolve such an adaptation. It has also been suggested they could absorb sun heat, since a cooling trend towards the Later Jurassic, so a large ectothermic reptile may have increased surface area to absorb solar radiation. Christainsen and Tschopp (2010) state a smooth keratinous covering would have hampered thermoregulation, but it can not be ruled out since cattle and ducks use beaks to expel heat despite their keratinous sheaths. Histological surveys of plate microstructure attribute vascularization for nutrient transportation to match rapid plate growth.

The plates are also thought to have blushed a bright colour when blood pumped into the plates, However, Christainsen and Tschopp (2010) consider this unlikely, since they were made of keratin and not skin. Their size also suggests they could have made the animal taller to intimidate enemies or impress other *Stegosaurus*. A 2015 study on the shapes and sizes of *Hesperosaurus* plates suggest they were sexually dimorphic, with wide plates in males and taller plates in females. Christainsen and Tschopp (2010) propose the display function would have reinforced the keratinous sheath, increasing the visible surface, with most horn structures being brightly coloured. Some also suggest they were used to identify the same species, but the use of exaggerated structures in dinosaurs, as a whole, for species identification has been questioned, since nothing like this exists in modern species.

### Predation

*Allosaurus* and Stegosaurus where known to have a predator/prey relationship. One *Allosaurus* had a hole in its crotch that fit the shape of a thagomizer, and the *Allosaurus* surviving to tell the tale. *Allosaurus* was a common predator of stegosaurs, including *Stegosaurus*.

### Brain

*Stegosaurus*' brain was small, about the size of that of a walnut. But that didn't necessarily make *Stegosaurus* dumb. Though Stegosaurus was low on the intelligence scale, was far from stupid, and a formidable opponent if provoked.

Early paleontologists thought that *Stegosaurus* had a brain in its hips, and thought that the brain controlled the hind parts, and wagged the tail. Later, however, scientists re-evaluated the socket that once was thought to hold another brain. They concluded that the socket was for controlling the rear, but was rather a collection of nerves, a glycogen body, helped balance, or even helped supply other compounds to the body.

### Growth

Juveniles show probable growth stages, Both known juveniles are small, one being 1.5 meters (4.9 feet) long and the other being 2.6 meters (8.5 feet) long. They are immature due to unfused scapulas and coracoids. The pelvic region is similar to juvenile *Kentrosaurus*. One 2009 study shows that the plates and thagomizer had delayed histological growth when compared to the skeleton, with the osteoderms increasing when maturity was reached. A 2013 study concluded that *Kentrosaurus* has quicker growth, contradicting the general rule that large dinosaurs grew faster than small ones, based on rapid deposition of highly-vascularized fibrolamellar bone.

### Thagomizer

There is debate if the thagomizers were for display, argued by Gilmore (1914), or a a weapon. Robert Bakker notes the tail was likely more flexible than other dinosaurs since it lacks ossified tendons, leading to it as a weapon. However, as Carpenter notes, the plates overlap many tail vertebrae, so movement was limited. Bakker also finds its rear could have been maneuvered easier by keeping the hindlegs stable and pushing with its forelimbs, where it could swivel to attack. McWhinney et al. showed a high trauma-related damage incidence, which also leads towards the thagomizer as a defense mechanism. They found 9.8% of Stegosaurus as having injuries on these mechanisms. This is supported by a punctured tail vertebrae in *Allosaurus*, where a Stegosaurus tail spike fits perfectly. *S. stenops* has four spikes, each ~60-90 centimeters (2-3 feet) long. Articulated stegosaur armour show that, in some species, the thagomizer protrudes horizontally, and not vertically as often depicted. Marsh first described *S. ungulatus* a having 8, but later research corrected this, finding it had 4.

### Diet

*Stegosauru*s was herbivorous, but had teeth and jaws very different from herbivorous ornithischians, suggesting it employed a different feeding strategy (which is not yet well understood). Others have teeth that can grind food in planes other than orthal. Stegosaurus and other stegosaurs have small, peg-shaped teeth that likely ground tough food due to wear facets. They were not pressed together very tightly in a block, which was efficient for grinding. No evidence of gastroliths are known, so how Stegosaurus processed such food is poorly understood. They distributed widely through the Late Jurassic, probably eating mosses, ferns, horsetails, cycads, conifers or fruits. One theory states they were low browsers, foraging (at most) ~1 meter from the ground. It may have also raised onto its hindfeet to form a sort of bipedalism, suggested by Bakker, to browse for high fruits and vegetation. Adults, if capable, could forage up to 6 meters (20 feet) from the ground.

A detailed computer biomechanic study in 2010 used 3D models of Stegosaurus teeth in realistic physics and properties to test feeding. They found a bite force of 140.1, 183.7 and 275 newtons for the anterior, middle and posterior teeth respectively, being less than half of a Labrador retreiver's. It could have bitten easily through small green branches, but would have had difficulty with anything over 12 millimeters in diameter. Therefore, it probably fed upon smaller twigs and foliage, and would have been unable to consume larger matter. A 2016 study indicates a much stronger bite force, however, comparing Sophie, *Erlikosaurus* and *Plateosaurus*. Based on their results, it revealed Stegosaurus has a bite similar to herbivorous mammals, such as cattle and sheep. Based on this, S*tegosaurus* likely ate woodier, tougher plants, such as cycads, and was a possible spreader of their seeds.

## Classification

Like the spikes and shields in ankylosaurs, stegosaurian plates evolved from low-keeled osteoderms in basal thyreophorans, such as *Scelidosaurus*. The latter is proposed to be morphologically close to the eurypoda last common ancestor. Galton (2019) interpreted plates from an armoured dinosaur from the Lower Kota Formation, India to be ankylosaurian, arguing this indicates an Early Jurassic origin for ankylosaurians and stegosaurians. The majority of stegosaurs recovered are stegosaurids, defined by Paul Sereno as any stegosaurians close to Stegosaurus than to *Huayangosaurus*. This is widespread group, where members spanned the Northern Hemisphere, Africa and possibly South America. Mateus (2009) concludes:

## Paleoecology

*Stegosaurus* lived in the Morrison Formation during the Late Jurassic. Stegosaurus was likely a grazer, or a forager, eating bushes and other shrubbery. *Stegosaurus* was attacked and eaten by *Allosaurus*, *Saurophaganax* and other carnivores of the Morrison Formation.

Studies say that *Stegosaurus* likely preferred drier settings than other dinosaurs it lived with, though the environment was semiarid. The Morrison Formation had wet seasons, with spanning floodplains. *Stegosaurus* likely lived in the forests made out of *Brachyphyllum* trees, and many other spanning plants.

## Synonyms

* *Hypsirhophus*
* *Diracodon*
* *Alcovasaurus longicollum*

## Species

### Nomen Dubium and Nomen Nudum

* *S. armatus* (replaced)
* *S. "affinis"*
* *S. duplex* (=*S. undulatus*)

### Reassigned Species

* *S. marshi*
* *S. priscus*
* *S. longispinus*
* *S. madagascariensis*

## Gallery

### Reconstructions

**Credit: Mette Aumala on Twitter.**Speculative *Stegosaurus* wearing a "hat" of foliage like modern deer.
**Credit: @ruadhribrennan on Twitter.****Credit: @ruadhribrennan on Twitter.**
### Mounts

*Stegosaurus* at the Royal Tyrrell incorrectly postured and labelled *"S. armatus"*.Juvenile mount.
## References

1. ↑ https://www.nhm.ac.uk/discover/dino-directory/stegosaurus.html
2. ↑ https://www.britannica.com/animal/Stegosaurus
3. ↑ https://www.sciencefocus.com/nature/stegosaurus/
4. ↑ https://eastern.usu.edu/museum/exhibits/stegosaurus
5. ↑ https://twitter.com/Bhmllr/status/1477830368942301184
6. ↑ https://www.theguardian.com/science/2014/dec/04/sophie-stegosaurus-london-natural-history-museum
7. ↑ https://www.nhm.ac.uk/discover/stegosaurus-brought-to-life.html
8. ↑ https://www.nhm.ac.uk/press-office/press-releases/world\_s-most-complete-stegosaurus-goes-on-show.html
9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 https://en.wikipedia.org/wiki/Stegosaurus
10. ↑ https://blogs.scientificamerican.com/tetrapod-zoology/the-stegosaurus-plate-controversy/
11. ↑ https://twitter.com/TylerGreenfieId/status/1427333182966226944