Pisanosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Ornithischia

Family: Pisanosauridae

Genus:Pisanosaurus

Species:Pisanosaurus mertii

Introduction

Considered one of the earliest dinosaurs discovered, Pisanosaurus was a small, herbivorous dinosaur that lived in the Late Triassic period, occupying forests and woodlands in what would become South America.

During the Late Triassic, the earth's landmass was united in a single supercontinent known as Pangaea. Most of Pangaea was an arid desert, with scattered forests and scrub plains. Parts of the supercontinent were beginning to split, due to the movement of tectonic plates and climate changes leading to rising sea levels.

The Late Triassic was the beginning of what was later called the “age of dinosaurs,” a time when dinosaur groups emerged and spread around the world, competing with large reptiles and other creatures for dominance of terrestrial habitats. By the end of the Triassic, the dinosaurs had become the most dominant terrestrial group, largely supplanting the Triassic reptiles and amphibians that had dominated the land for millions of years. Some scientists believe that Pisanosaurus may have been the first member of the Ornithischia.

Classification

Biologists use two primary methods to classify organisms. Linnaean taxonomy creates groups based on overall physical similarity. This system was used to create the basic categories of life, such as the organizing of organisms into kingdoms, phyla, classes, etc.

An alternative system, called “cladistics,” seeks to create groups that better reflect the evolutionary relationships between species. Cladistic analysis focuses on tracing the inheritance of certain key traits, and places organisms into groups called “clades” that contain organisms possessing a certain trait with their common ancestor.

Early examination of Pisanosaurus specimens indicated that it belonged to the order Ornithischia, or “bird-hipped” dinosaurs, a group defined by their backward-facing hip bones. Pisanosaurus was a primitive or basal member of the Ornithischia and was therefore distantly related to many of the most successful herbivorous dinosaur groups in the Mesozoic. Cladistic analysis places Pisanosaurus on a branch that emerged from the dinosaurs, just after the evolution of the ornithischian design. In 2017, however, some scientists suggested that Pisanosaurus was actually part of the Silesauridae family of dinosauriformes, a non-dinosaur relative of dinosaurs. The reclassification would not only remove Pisanosaurus as the oldest Ornithischia, but would suggest that Ornithischia was absent from the Triassic Period entirely.

Anatomy

Pisanosaurus was a small herbivorous or omnivorous dinosaur, ranging in size from 0.9 to 1 meter (3–3.2 ft) and weighing between 3 and 3.2 kilograms (6–7 lbs). It had longer rear legs, with slender feet. Only fragmentary remains have been found, and little is known about its overall appearance. Paleontologists are fairly certain that the animal was bipedal and herbivorous, like many members of the ornithischian group.

Later species from ornithischian groups have been found with skin impressions indicating mosaic scales, and Pisanosaurus likely had similar skin. There is no evidence of coloration, but several larger predatory dinosaurs have been found in the area, suggesting that the small Pisanosaurus may have been cryptically colored for camouflage. The animal had large eyes, relative to its body, indicating good vision.

Intelligence

Paleontologists have yet to calculate the EQ for Pisanosaurus. Later members of the ornithischian order ranged from between 0.5 to 1.0 on the scale, placing them at the low to mid range for dinosaurs as a whole. As Pisanosaurus was a basal dinosaur, it is possible that it had lower intelligence than many later dinosaurs. Pisanosaurus may have displayed intelligence on par with some modern reptiles like the iguana or tortoise.

Reproduction and Population

Pisanosaurus is known from only a few specimens and paleontologists have found little evidence of the animal's reproductive habits, parental behavior, or population size.

Later ornithischians like the ceratopsians, or “horned dinosaurs,” and the hadrosaurs, or “duck-billed” dinosaurs, laid eggs, a characteristic that was also likely inherited from species closely related to Pisanosaurus. Although evidence indicates that some later ornithischians gathered to lay and brood their eggs, Pisanosaurus, as a more primitive representative of the order, might not have possessed any of these reproductive behaviors.

Diet

Paleontologists know little about the diet and lifestyle of Pisanosaurus. Only incomplete fossils have been found, but the animal is believed to have been herbivorous, like most later ornithischians. Paleontologists suggest that Pisanosaurus most likely lived in forests and fed on low lying vegetation. The skull of Pisanosaurus reveals a beak, which paleontologists believe was used to crop leaves and stems from plants.

Behavior

Pisanosaurus is only known from partial skeletons and therefore anatomical clues to its behavior are lacking. It is thought that Pisanosaurus was a relatively agile and small dinosaur that probably lived in the undergrowth, eating plants and perhaps insects. There has been no evidence to suggest cooperative or group behavior or social breeding. Given that the animal lived in an area with a number of larger dinosaurs and predatory creatures, Pisanosaurus probably used its speed and small size to hide from predators as it explored the Triassic scrub and forests for food.

Habitat and Other Life Forms

The Late Triassic was a time of intense change, as the earliest dinosaurs spread across Pangaea and became the dominant terrestrial animals of the new age. Pisanosaurus and other early dinosaurs in South America lived in a world that was still dominated by the large reptiles and amphibians of the Early Triassic.

The climate of Late Triassic South America varied from arid scrub to seasonal forests. The dominant types of vegetation included many varieties of ferns and tree ferns, and a wide variety of gymnosperms, like conifers and cycads. Many small reptiles and amphibians lived in the Late Triassic, many resembling species living today. There were no birds or flying mammals, but the first varieties of flying reptiles called pterosaurs began to emerge during this period.

Pisanosaurus shared its habitat with a variety of large reptiles, including the rhynchosaurs, an extinct order of herbivorous reptiles, and several varieties of cynodonts, a suborder of mammal-like reptiles.

In terms of dinosaurs, Pisanosaurus lived alongside one of the earliest known large theropods, the carnivore Herrerasaurus, which might have hunted small dinosaurs like Pisanosaurus in addition to the large reptiles. There were also several genera of herbivorous dinosaurs living in the area, including some of the prosauropods, a group that would give rise to the largest dinosaurs ever to exist, the herbivorous sauropods of the Jurassic.

Research

Pisanosaurus was discovered in the Ischigualasto Formation of Argentina and was described by Argentine paleontologist Rodolfo Casamiquela in 1967. The species was described from fragments of a single skull and skeleton and no additional specimens have been found.

The primary debate over Pisanosaurus centered on which dinosaur group the animal belonged to. The animal was first assigned to the group ornithopod, in the family Heterodontosauridae. Further examinations convinced paleontologists to reclassify Pisanosaurus as an ancestor species in the ornithischian, not of the ornithopod clade.

Bibliography

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