Scutellosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Basal

Genus:Scutellosaurus

Species:Scutellosaurus lawleri

Introduction

Scutellosaurus was a small, herbivorous dinosaur that lived in Early Jurassic North America. A basal ornithischian, Scutellosaurus was one of the earliest representatives of a large group of primitive dinosaurs that dominated during the Jurassic period (201–145 million years ago) and produced a variety of large dinosaur species.

In the Triassic period, the continents of the earth were united in a single land mass known as Pangaea. The dinosaurs evolved during this period and spread across the globe. In the Jurassic and Cretaceous periods, Pangaea split completely into two hemispheres, permitting the southern and northern dinosaur groups to further diverge.

Scutellosaurus evolved in the Early Jurassic and represented one of the major dinosaur groups in the northern half of Pangaea. Ancestors of Scutellosaurus are found in the northern and southern continents, indicating that the ancestral species arrived in the area before the supercontinents split from Pangaea.

Classification

Scientists use two systems to organize animals into groups. Linnaean taxonomy, or “systematics,” groups organisms according to overall physical similarity, while an alternate system called “cladistics” creates groups called “clades” in an effort to reconstruct the evolutionary relationships between species.

Systematic analysis places Scutellosaurus into the order Ornithischia based on the “bird-like” orientation of its pelvis. From there, the species is classified in the suborder Thyreophora, which represents “armored” dinosaurs such as the Stegosauria (which includes Stegosaurus) and the ankylosaurs, a large group of herbivorous dinosaurs characterized by bony dermal plates, wide bodies, and angular skulls. Scutellosaurus is often considered the oldest known armored dinosaur, or earliest thyreophoran. Further classification of Scutellosaurus is still open for debate. For example, Scutellosaurus is sometimes placed into the family Fabrosauridae, a group of primitive ornithischian dinosaurs defined largely by their small size and features of the skull. (There are sometimes considered the basal-most group, or family, of ornithischian dinosaurs.) However, cladistic analysis indicates that the Fabrosauridae may not be a natural group and may be abandoned in future analyses. Also, the dinosaur's infraorder can be classified as incertae sedis, which is Latin for “of uncertain placement” and a term used to describe a taxonomic group whose broader relationships cannot be defined. Indeed, there is considerable debate over dinosaur taxonomy in general, including the differentiation between Ornithischians and Saurischians.

Cladistic analysis places Scutellosaurus in the clade Thyreophora, which is divided from the other Ornithischia by the presence of bony formations in the skin called osteoderms. Scutellosaurus divided from the Thyreophora early in the formation of the group and had primitive characteristics. Later representatives of the Thyreophora had more substantial armor plating and became far larger in the Jurassic and Cretaceous. Scutellosaurus is also debatably classified as a more advanced member of the Scelidosauridae clade, a sister group to the Ankylosauria and Stegosauria, the members of which were less well-developed.

Anatomy

Scutellosaurus was a small dinosaur, measuring approximately 1.2 meters (3.9 ft) long and reaching about 50 centimeters (20 inches) at the hip. Paleontologists estimate that the dinosaur weighed about 10 kilograms (22 lbs). Scutellosaurus is unusual for the Thyreophora because it was partially bipedal, while later members of the group were generally obligate quadrupeds. Scutellosaurus had relatively strong front legs, which might have occasionally been used for walking.

Scutellosaurus had a relatively small, triangular-shaped head and unusual dentition. The animal had short cutting teeth in the front of its jaws, with sharper, pointed teeth on the sides, helping to slice through vegetation. The animal had a muscular tail that might have been useful for balance when Scutellosaurus reared onto its hind legs.

Scutellosaurus was covered in a rudimentary bony armor formed from numerous osteoderms, called scutes, especially thick along the animal's dorsal surface and around the skull. Along the animal's back were taller, triangular scutes that paleontologists have interpreted as an evolutionary stage towards the evolution of the large crests on Jurassic dinosaurs like Stegosaurus.

Intelligence

Research indicates that most dinosaurs were probably less intelligent than modern mammals living similar lifestyles. Scutellosaurus was similar in size to a small dog but was likely far less intelligent. Members of the Thyreophora that evolved in the Jurassic and Cretaceous had low encephalization quotient (EQ) values—a measurement of estimated intelligence based on relative brain and body size—and are considered among the least intelligent dinosaurs.

Reproduction and Population

Paleontologists have not yet discovered eggs or juvenile specimens from Scutellosaurus, and the reproductive lifestyle of the animal remains largely a mystery. Eggs have been discovered for some later members of the Thyreophora and paleontologists believe that dinosaurs like Scutellosaurus were also egg-laying animals.

Scutellosaurus is known from only a few partial specimens, and paleontologists are therefore unable to estimate population size and density. It is also uncertain whether males and females of the species were identical or whether they displayed variation in size or appearance.

Diet

The dentition of Scutellosaurus establishes the species as an herbivore, with short cutting teeth similar to those found in modern herbivorous reptiles such as iguanas and tegu lizards. Vegetation in the Triassic included dozens of species of “pteridophytes,” or ferns, which grew close to the ground and would have made appropriate food for the relatively small Scutellosaurus.

There were also primitive members of the gymnosperm group, including a variety of simple conifers and members of the Gingko group. Another major group of gymnosperms, the cycads, also began to spread during this period, many growing low to the ground and within the reach of smaller herbivores.

Behavior

The scutes of Scutellosaurus probably helped deter predators but might have evolved primarily to protect the animal's skin from rough vegetation as the small dinosaur foraged in the undergrowth. Scutellosaurus would have been vulnerable to a variety of predatory reptiles and dinosaurs. There is no evidence to suggest social behavior in Scutellosaurus; many later members of the Thyreophora are thought to have lived as solitary herbivores or in small family groups.

Habitat and Other Life Forms

Early Jurassic North America saw the rise of the Mesophyta, as the climate across the Northern Hemisphere became more temperate. A variety of ferns, conifers, cycads, and gingkos combined to create lush forests, rich tropical zones, and dry upland scrub plains. The pterosaurs had emerged by this time and became the dominant flying reptiles throughout the Mesozoic Era. Species like Carniadactylus and Peteinosaurus were early representatives of the group in the Northern Hemisphere. Herbivorous dinosaurs in the Early Jurassic included relatives of the later sauropods, like Massospondylus, which were the largest terrestrial animals in the Early Jurassic. The early sauropods were especially common and prolific animals, evolving into the most successful herbivores during the Late Jurassic.

Predatory species that may have threatened Scutellosaurus included early theropods like Syntarsus and primitive crested predators like Dilophosaurus. Other predators present included some of the earliest crocodile relatives, like Protosuchus.

Research

The type specimen of Scutellosaurus lawleri was discovered in the Kayenta Formation of Arizona and later described by paleontologist Edwin Colbert in 1981. An additional specimen was uncovered in Arizona in 1970. Most research has been based on these fossils.

Notable research into thyreophoran cladistics was undertaken by paleontologist Richard Butler, who published a review of the basal ornithischians, including species like Scutellosaurus, in 2007. Studies continue to probe the evolution of armored dinosaurs, with Scutellosaurus remaining an important early example.

Bibliography

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