Psittacosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Psittacosauridae

Genus:Psittacosaurus

Species:Psittacosaurus mongoliensis

Introduction

Psittacosaurus is the most documented dinosaur ever discovered, with fossils found from more than 400 individuals ranging from hatchlings to adults. This has made it highly important to scientists. Fossil records indicate that Psittacosaurus was abundant in Asia during the Early Cretaceous period. It is also the most genus-poor and species-rich group of dinosaurs. More than ten species of Psittacosaurus have been named, but not all have been validated.

Despite its well-studied status, mystery still surrounds this dinosaur. Originally classified alongside primitive ornithopods, Psittacosaurus's characteristic parrot-like beak led to its reclassification with ceratopsians. Ceratopsian dinosaurs are renowned for their elaborate horns and frills. Psittacosaurus lacked those features and was a fraction of their size, but its bristled tail and social nature provide connections with its horned descendants. Fossil remains reveal that Psittacosaurus was a social animal that lived in extended families and cared for its young.

Classification

Scientists classify dinosaurs in different ways. The Linnaean system is a hierarchical classification system that narrows species into order, genera, and species. In the late twentieth and early twenty-first century, paleontologists moved towards cladistic or phylogenetic classification systems to classify dinosaurs according to their common ancestry and shared physical characteristics. A Linnaean analysis places Psittacosaurs as a basal (primitive) member of the larger Ceratopsia infraorder. The ceratopsians are grouped together because they share common features of the skull, most notably a rostral bone that gives these animals a parrot-like beak. The more evolved ceratopsians walked on four legs and were characterized by horned faces and elaborate neck frills. Triceratops is the most famous genus of the group, weighing up to 8 tons (17,600 lbs) and ornamented with three facial horns and an impressive neck frill. Psittacosaurus, however, was a primitive member of the ceratopsian infraorder. It had no horns or frill, it was likely bipedal (walked primarily on its rear legs), and it weighed only 24 to 26 kilograms (53–57.5 lbs).

Cladistic analysis is based on more specific physical characteristics; for example the shape of the skull, features of bone, and the arrangement of the skeleton. Researchers divide the ceratopsian clade into further groups, and although opinions on exact divisions and relationships differ, many include a split between Neoceratopsia (containing more highly evolved ceratopisans) and Psittacosauridae (containing some of the primitive forms). The physical features that characterize the Psittacosauridae clade are so unique that it consists of a single genus, Psittacosaurus. It is the most species-rich genus of dinosaurs ever discovered. Psittacosaurus mongoloiensis is the type species in the genus.

The Psittacosauridae clade is characterized by a short, flat skull, pointed cheek bones, small nares (nostril channels) that sit high on the skull, and only four fingers on the front hands. Until the discovery of the rostral (beak) bone that connects Psittacosaurus with the neoceratopsians, Psittacosaurus was classified as a primitive species of ornithopod. It is important to note that cladistic analyses can change over time as new specimens are discovered and as science advances.

Anatomy

Psittacosaurus was approximately 2 meters (6.5 ft) long, making it among the smallest of the ceratopsians. It supported its light frame on strong, agile rear legs that were 60 percent longer than its front limbs. Its agile form implies that it could probably run quickly when required. Although Psittacosaurus is classified as bipedal (walked on two legs), some paleontologists believe that it may have also used its front limbs for walking, as well as grasping foliage. There is also evidence that its locomotion may have changed over its lifespan, with the young favoring all fours and adults walking bipedally. Further, some scientists suggest that at least some Psittacosaurus species may have been strong swimmers or even lived a semi-aquatic lifestyle.

Psittacosaurus's parrot-like beak equipped the animal for selective grazing, and its leaf-shaped teeth at the rear of its mouth were designed for shearing vegetation. The shape of the cheek bones produced a sharp, pointed appearance to the face, and the skull was flat and long. Fossils of Psittacosaurus hatchlings include specimens that measure only 11 to 13 centimeters (4–5 in) long. Older Psittacosaurus specimens, estimated to have reached about eleven years in age, are estimated to have weighed approximately 20 kilograms (44 lbs).

Psittacosaurus had thick skin that was covered in an irregular pattern of large and small scales. Along the midline of its tail, Psittacosaurus is believed to have sported a row of vertical bristles that numbered more than 100 and were approximately 15 centimeters (6 in) long. There is some conjecture among paleontologists as to whether Psittacosaurus's bristles are an early example of feathers or a fossilized extension of skin. As the debate continues, artistic renditions of Psittacosaurus may show the animal with or without bristles. If the bristle theory is true, the purpose of the bristles is unlikely to be associated with thermoregulation because they cover only a small portion of the animal's body. Scientists believe, therefore, that the bristles probably served a display function for use in territorial defense or mating. Some scientists speculate that the bristles were brightly colored.

Intelligence

Scientists measure animals’ intelligence using the encephalization quotient (EQ), which is determined through the ratio of the animal's brain weight to body weight. Endothermic (warm-blooded) animals generally have larger brains than ectothermic (cold-blooded) animals. Assuming that a higher EQ equals higher intelligence, scientists can predict the intelligence of animals, living and extinct. Paleontologist James A. Hopson estimated the brain to body weight ratio of dinosaurs by rating them in comparison to the brain to body weight ratio of a crocodile. Hopson developed a scale that showed if a dinosaur's EQ was smaller or larger than expected.

The EQ score for ceratopsians such as Psittacosaurus ranges from 0.7 to 0.9 (Hopson). This compares with 0.2 for sauropods, 0.5 for stegosaurs, 1.0 for crocodiles, up to 2.0 for theropods, and 5.8 for dromaeosaurids. Therefore, Psittacosaurus, a ceratopsian, likely falls in the mid-range of dinosaur intelligence.

Reproduction and Population

Fossilized remains of Psittacosaurus hatchlings arranged in nest configurations suggest that these were oviparous (egg-laying) animals. Scientists also know that Psittacosaurus produced large clutches of young. At one site, thirty-four pigeon-sized Psittacosaurus juveniles were found with one adult in a nest no larger than 0.5 square meters (5.4 sq ft). Scientists conclude from the finding that Psittacosaurus brooded (provided parental care) over large clutches of eggs, and continued to care for their young for some duration after hatching.

In 2007, paleontologists unearthed the fossilized remains of a herd of six Psittacosaurus juveniles that had been killed suddenly by a mud slide. Examination revealed that the specimens were aged from 18 months to 3 years, demonstrating that they came from more than one clutch of eggs. This led paleontologists to speculate that Psittacosaurus lived in family or social units in which younger animals may have been cared for by adults.

The Psittacosaurus population is unknown, but fossil records indicate that it was abundant during the Early Cretaceous period in the area known today as Asia. The distribution of Psittacosaurus species ranged from Siberia in the north to Thailand in the south. Reconstruction of the life stages of these animals indicates that the Psittacosaurus population consisted of mainly babies and adults. The low number of juveniles suggests that Psittacosaurus was vulnerable to heavy predation. Heavy predation may also explain why Psittacosaurus produced such large numbers of offspring.

Diet

The shape of Psittacosaurus's teeth and jaw indicate that it was herbivorous (plant-eating). Its pointed beak was designed for cropping vegetation, and its powerful jaw was designed for shearing leaves and branches. Psittacosaurus's digestion was also aided by the presence of gastroliths (stomach stones) in the animal's stomach. These small stones, which were swallowed by the animal, helped to grind up food as it passed through the digestive system. This adaptation is unique to Psittacosaurus among ceratopsians, and it has not been observed in any other ornithischians (a traditional broad classification of dinosaurs with bird-like hips).

Given its short stature, it is likely that Psittacosaurus grazed on mainly low-lying plants, such as ferns, cycads, and primitive angiosperms (flowering plants). Psittacosaurus's bipedal posture and four-fingered hand would have also enabled it to reach low branches of trees. The animal's powerful beak and gastroliths (stomach stones) likely enabled it to eat nuts and seeds from gymnosperms (seed-bearing plants), such as Ginkgo.

Behavior

The discovery of clutches and small groups of Psittacosaurus at fossil sites indicate that these animals lived in herds and very likely exhibited social behaviors. Evidence of brooding (caring for eggs) and parental care in the genus indicates that Psittacosaurus was predisposed for social behaviors. Given that Psittacosaurus's size made it vulnerable to predation, travelling in numbers would have provided protection to the group, especially the young. Thick skin and agility also afforded Psittacosaurus some protection from predators. Scientists speculate that Psittacosaurus's primary defensive mechanism was running.

Other evidence of social behavior is found on the tail of Psittacosaurus—a row of vertical bristles that scientists propose were used in mating and defensive displays. Evidence of social behavior in Psittacosaurus is important for paleontologists because it further connects Psittacosaurus with its horned descendants. Later ceratopsians, such as Triceratops and Centrosaurus, are believed to have practiced complex social behaviors, including ritual battle and mating displays. Evidence of social behavior in Psittacosaurus has fueled speculation that ceratopsians developed horns and frills after the advent of group living.

Habitat and Other Life Forms

Psittacosaurus lived in the geographic area we know today as Asia. The majority of Psittacosaurus discoveries have occurred in Mongolia, China, and Russia. The Gobi Desert, an arid region that straddles the national border of Mongolia and the People's Republic of China, has been an especially abundant source of Psittacosaurus fossils. The area looked much different in the Cretaceous period than it does today. Asia was still part of Laurasia, a super-continent formed by Asia, Europe, and North America.

During the Early Cretaceous period, the Gobi Desert consisted of rolling sand dunes that ranged from 100 to 250 meters (300–800 ft) high. It was a harsh landscape that experienced sporadic droughts and drenching rains. The dunes were sparsely vegetated, with fresh water ponds forming between some dunes. This provided a place where vegetation, such as gymnosperms (non-flowering, seed-bearing plants) and angiosperms (flowering plants), flourished. Scientists agree that herbivorous animals such as Psittacosaurus helped to spur the rapid rise of angiosperms. Along with insects, herbivorous dinosaurs assisted by spreading pollen between flora.

Psittacosaurus shared its environment with a diverse range of dinosaur species. This included the primitive iguanodont, Penelopognathus; early carnivorous tyrannosaurids, such as Xiongguanlong and Dilong; and primitive forms of hadrosaurs, ankylosaurs and sauropods. Psittacosaurus also coexisted with some of the earliest known mammals, such as Repenomamus. In 2005, an excavation in China discovered a fossil of Repenomamus that had the undigested remains of a juvenile Psittacosaurus in its stomach. Repenomamus was only the size of a large domestic cat, yet it was capable of swallowing a Psittacosaurus juvenile. The discovery proved that primitive mammals competed with carnivorous dinosaurs for food, and that Psittacosaurus was a prime target for predators.

Research

The Gobi Desert is an important site for paleontological discoveries. Thousands of specimens have been excavated from the area, which has led scientists to question how animals came to be fossilized in such large numbers and so completely. Scientists had initially speculated that animals had become covered in sand during violent dust storms. Today, however, scientists believe that it is more likely that animals were trapped in mud slides. They propose that the geology of the Gobi Desert during the Cretaceous period consisted of high, sparsely vegetated sand dunes that sat on bedrock. During heavy rains, water would seep through the sand but would not be able to penetrate the bedrock. Accumulating water would liquefy the sand resulting in sudden, dramatic mud slides. The speed and weight of these mud slides would be capable of killing animals suddenly and quickly, and the sediments would produce detailed fossils.

The relative abundance of Psittacosaurus fossils, including several complete specimens, makes the species important to general research on dinosaurs. One specimen found in China included remarkably preserved soft tissues, including skin pigments that showed evidence of countershading (a form of camouflage seen in many forest-dwelling species). This was used as the basis for a reconstruction by noted paleoartist Bob Nicholls, unveiled in 2016 to widespread media coverage as the most accurate reconstruction of a dinosaur ever to that point.

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