Stegosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Stegosauridae

Genus: Stegosaurus

Species: Stegosaurus armatus

Introduction

With spikes on its tail and distinctive rows of alternating triangular plates running down its back, Stegosaurus is one of the most iconic species of dinosaur. There is debate among paleontologists in regards to the number of Stegosaurus species, but three species—S. armatus, S. stenops, and S. ungulatus —are recognized as members of the genus. Although S. armatus is the type species, S. stenops is perhaps the most recognized.

Stegosaurus lived during the Late Jurassic, long after the break-up of Pangaea and the subsequent division of Gondwanaland and Laurasia. It was a period generally characterized by high atmospheric carbon dioxide concentrations and monsoonal rain. Climatic conditions varied on the different continents, however, with current geologic evidence suggesting that a seasonally dry climate existed in the western interior areas of North America where Stegosaurus was commonly found. These areas resembled a complex mosaic of savannah-like areas and floodplains, with riparian areas capable of supporting multi-story forest canopies and herbaceous ground vegetation, providing adequate food and refuge to support large herbivores during periods of drought.

Classification

Current classification of dinosaur species relies predominately on two systems, the Linnaean system and cladistics. Carl Linnaeus developed the Linnaean system in the mid-1700s, in an attempt to categorize similar organisms together by placing them into hierarchical groupings—that is, kingdom, phylum, class, order, family, genus, and species.

Cladistics is the second classification system used and is favored by most modern paleontologists. First introduced in the 1950s, cladistics aims to highlight relationships between organisms in regards to kinship and ancestry, grouping species with common traits and common ancestors into “clades.” In dinosaur cladistics, the major dinosaur groupings are Theropoda, Sauropodomorpha, Ornithopoda, Thyreophora, and Marginocephalia, all of which are comprised of several different clades. In the late 2010s, Matthew Baron and his colleagues proposed a new clade, Ornithoscelida, grouping the ornithischians, such as Stegasaurus, together with theropods, such as Allosaurus.

Paleontologist Harry Seeley concluded that dinosaurs can be divided into one of two orders: Ornithischia (or bird-hipped dinosaurs) and Saurischia (or lizard-hipped dinosaurs). All the armored, back-plated Thyreophora dinosaurs belong to the Ornithischia order. Stegosaurus belongs to the suborder Stegosauria, which it shares with other close relatives such as Kentrosaurus, Lexovisaurus, and Tuojiangosaurus.

Anatomy

The most distinctive physical features of the Stegosaurus were the spikes on its tail and distinctive rows of alternating triangular plates running down its back. Stegosaurus had 17 or 18 of these dermal plates (highly modified osteoderms) running in two rows on either side of the backbone. Although fossil evidence indicates that they were not physically attached to the skeleton, they were made from bone and contained many tubular shaped channels. Paleontologists suggest these grooves were filled with blood vessels. One hypothesis is that the spines were perhaps as part of a thermoregulation system in addition to their possible use as protection. Others think they were for species recognition or for display in attracting mates.

Tail spikes (thagomizers) are a common trait of such stegosaurids as Stegosaurus, Kentrosaurus, and Tuojiangosaurus, though the number differs for each species. Stegosaurus was historically thought to have eight such spikes; however, current evidence indicates that only four spikes were situated at the end of their tails, which measured 60 to 90 centimeters (2 to 3 feet) long. It is believed that the tail spikes were most likely used for defense against predators; the discovery of an Allosaurus gore wound consistent with stegostaur tail spike supports that assessment.

With its toothless beak-like mouth and small cheek-teeth, Stegosaurus was a large herbivorous dinosaur, measuring 9 meters (30 feet) long and 2.5 to 3 meters (8 to 10 feet) high. It was initially thought that Stegosaurus was a bipedal species due to the fact its hind limbs are almost twice as long as its sprawling forelimbs. Further examination revealed, however, that it was actually a quadrupedal dinosaur. Its front and hind hoof-like feet also differed in the number of toes, with five wide toes on its forefeet and only three on its hindfeet.

Intelligence

The most common method used to measure animal intelligence is the encephalization quotient (EQ). Stegosaurus had a very small sized brain with an EQ value of between 0.52 to 0.56, placing it at the lower end of the dinosaur intelligence scale (Humans have an EQ of roughly 7.44 and dolphins measure approximately 5.31). Given the necessity of a large brain to perform complex behavior, researchers believe that the least intelligent dinosaur species would possess limited cognitive capability and behavioral flexibility. This also supports the theory that most herbivore dinosaurs were less intelligent than were their carnivorous cousins.

One of the most intriguing aspects of Stegosaurus's anatomy is its supposed “second-brain.” Although current scientific opinion states that the large oval space located at the base of the Stegosaurus spinal cord is similar in form and function to a “glycogen body” (found in extant bird species and containing glycogen), originally it was hypothesized that this structure was a “second-brain” used to control its tail and rear-body movements.

Reproduction and Population

The process of fossilization is very rare, and the vast majority of plants and animals will die without preservation. Although fossils that do exist can provide clues, it is very difficult for paleontologists to measure past population numbers of prehistoric animals or behavior accurately.

Evidence suggests that Stegosaurus was an oviparous species, laying up to several eggs at a time. Nesting sites in areas favored by Stegosaurus have been found, suggesting some possible form of parental care following hatching. Although female dinosaurs may have possessed a wider, more robust pelvic anatomy to allow for the passage of eggs, precise determinants of sex are usually destroyed during the fossilization process, making it difficult to measure sexual dimorphism.

Diet

Stegosaurus species were large terrestrial herbivores, living during the Late Jurassic period about 150 million years ago. The relatively low to the ground position of the Stegosaurus head suggests that this species was restricted to feeding on low-lying gymnosperms and vegetation such as ferns, horsetails, cycads, and mosses. Given their large and long hind limbs, however, some researchers believe that Stegosaurus could have stood on its back legs to increase its reach when feeding. If this was the case, then Stegosaurus may have also consumed tree species such as ginkgoes and conifers. Its toothless beak-like mouth and small cheek-teeth would have been ideally suited to its herbaceous diet, plucking and pulling up vegetation and grinding it with its rear cheek-teeth. Based on 3-D computer modeling, its bite force may have been comparable to that of a modern sheep or cow.

Behavior

While herding behavior is common among extant herbivores and the benefits are strongly related to defense and protection, the reasons for herding behavior in dinosaurs is less clear. Prehistoric animal behavior is difficult to determine, and researchers must infer much from fossilized remains or from behavior seen in animals today.

Paleontological evidence from bone beds and trackways has suggested that certain herbivore species traveled in herds. Evidence for Stegosaurus herd behavior is quite strong, with trackways discovered in Colorado, indicating that these species lived in non-segregated herds consisting of both juveniles and adults. Supersized adults had little to fear in regards to predators, but herd living would have offered a measure of protection to their offspring. This differs from the sauropod herbivores, however, which appeared to live in age-segregated herds.

Habitat and Other Life Forms

Stegosaurus species roamed the earth during the Late Jurassic, some 150 million years ago. It likely shared habitat, although perhaps not food resources, with some of the largest sauropod herbivores that have ever existed, such as Camarasaurus, Apatosaurus, Diplodocus, and Brachiosaurus. Although the Stegosaurus was large, it was likely vulnerable to predation by the large carnivore species of western North America, including Allosaurus, Torvosaurus, Marshosaurus, and Ceratosaurus.

Though the boundary between the Jurassic and Cretaceous periods was marked by a period of cooling, most of the Late Jurassic was characterized by a warm and moist climate. Some arid areas were still present, including drier conditions in the western interior of North America, but wetter areas were covered in lush green vegetation including plants such as bennettitaleans, cycads, ferns, ginkgoes, and conifers. Even today, close relatives of these plants still exist in the form of Chinese ginkgo, redwoods, cypresses, pines, and yews.

Research

The Great Dinosaur Rush, also known as the infamous Bone Wars between Othniel Charles Marsh and Edward Drinker Cope from the 1870s to 1890s, was responsible for the discovery of hundreds of dinosaur species in the western North American regions such as Colorado and Montana. This included the first Stegosaurus species, S. armatus, unearthed by Marsh in 1877 in the Morrison Formation, Colorado. Subsequent examination of the site over the next several years uncovered many Stegosaurus remains and led to the description of three different species: S. armatus, S. stenops, and S. ungulatus. There is still much debate, however, with new research presented in 2008 suggesting that significant modification of Stegosaurus classification may be required.

Research continues, with new finds and new technology affording paleontologists the ability to examine specimens in greater detail. For instance, “uncontroversial” evidence has indicated a member of Stegosaurus resided in Europe (Portugal). Until this time, Stegosaurus was not known to have existed outside of the North American region, and it adds weight to the theory of intermittent fauna exchange between these two landmasses during the Jurassic period. In 2017 footprints belonging to stegostaurs were also discovered on the Isle of Skye in Scotland.

In a 2015 analysis, Evan Thomas Saitta found a potential indication of sexual dimorphism in Stegosaurus: differences in the width, shape, and thickness of the spines. The most probable explanation appeared to be that full-grown males had wider, thinner spines than adult females did, rather than variation among individuals or species.

Adratiklit boulahfa, the oldest stegosaur discovered to date and the first to be found in North Africa, was located in the Atlas Mountains of Morocco in 2019. It was dated to about 168 million years ago in the Middle Jurassic in what was then Gondwana. The discovery suggests that stegosaurs are older and were more widely distributed than was previously believed.

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