Staurikosaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Herrerasauridae

Genus:Staurikosaurus

Species:Staurikosaurus pricei

Introduction

Staurikosaurus pricei earned renown as one of the first dinosaur finds of the Southern Hemisphere—and as one of the earliest dinosaurs known. Like its close relatives Herrerasaurus and Eoraptor, Staurikosaurus brought paleontologists deeper into prehistory and expanded the record of the dinosaur age. The species helped usher in the age of the dinosaurs in the Late Triassic, a time period in which Staurikosaurus and its relatives were just emerging and did not yet dominate the landscape. They were basal, or primitive, dinosaurs who would give rise to the much larger and more populous species of the Jurassic and Cretaceous periods.

Classification

Paleontologists traditionally used the hierarchical Linnaean classification system, developed by Carolus Linnaeus in 1735, to establish dinosaur taxonomy. This system comprises different tiers of classification (such as class, order, family, genus, and species) based on shared characteristics within each grouping. Species, the lowest tier, is based on the most specific shared traits. The biologist Ernst Haeckel added the phylum tier, which linked animals according to common ancestry. Additional sub-categories were added later.

Staurikosaurus and its nearest relatives have led to much debate among the paleontological community regarding classification. Under the Linnaean system, Staurikosaurus has typically been classified in the order Saurischia, one of two broad dinosaur orders based on key anatomical differences. The saurischians comprise the more lizard-like dinosaurs whose pelvic, or hip, structure pointed down and forward. Dinosaurs belonging to the other order, Ornithischia, had a bird-like pelvis that pointed down and back toward the tail. (However, in the twenty-first century the longstanding division between saurischia and ornithischia was challenged by some researchers.)

For some time, scientists debated classification of Staurikosaurus beyond its place in the saurischian order. Saurischia traditionally splits into two suborders—theropods and sauropodomorphs. Some paleontologists believe that Staurikosaurus and its close relations might predate the split between these two suborders, but others conclude that Staurikosaurus (as well as Herrerasaurus and Eoraptor) belongs to the suborder of bipedal, carnivorous theropods.

Staurikosaurus is not usually assigned to any theropodian infraorder; however, its familial classification has also caused some scientific strife. Most paleontologists considered Staurikosaurus a close enough relation to Herrerasaurus that they classified the two under one family, Herrerasauridae. Most references continue to cite this familial classification today.

The genus Staurikosaurus was named for the constellation of the Southern Cross, which appears in the skies over Brazil where the fossil was unearthed. The species taxon, pricei, was assigned in honor of the fossil's discoverer, Brazilian paleontologist Llewellyn Ivor Price.

Scientists increasingly use another classification system, called phylogenetic or cladistic classification. This system classifies animals in family trees, called cladograms, grouped by common ancestry. Each split in the tree creates a new group based on shared physical traits. According to cladistics, Staurikosaurus belongs to the Theropoda clade, or branch, of the dinosaur family tree. Theropoda splits further into numerous subclades, including the Tetanurae to which most later, more famous carnosaurs belong. Staurikosaurus belongs instead to Herrerasauridae, one of several smaller, non-tetanuran subclades, along with the genera Herrerasaurus and Chindesaurus.

Anatomy

An early specimen of dinosaur, Staurikosaurus was small compared to the giants of the Jurassic and Cretaceous periods. Up to 2 meters (6 ft) in length and 1 meter (3 ft) in height, Staurikosaurus nevertheless resembled later theropods because of its two-legged (bipedal) gait. Its hind legs were long and slim, designed for running, and the dinosaur had long feet and much shorter arms. Fossil evidence is insufficient to reconstructs its hands and feet thoroughly, but paleontologists have presumed to include most depictions with five fingers and five clawed toes.

The torso of Staurikosaurus was slender and extended back horizontally into a thin tail of nearly the same length. Fossils indicate a stiff tail that would have been held at hip level and was likely used to balance Staurikosaurus when in motion. Fossils remains of the skull are incomplete, but they provide sufficient evidence to show that Staurikosaurus had a long head on a short neck, with a thin, tapered snout that concealed sharply curved and serrated teeth for consuming meat.

Dinosaurs have generally been regarded as cold-blooded animals, like modern lizards, though the term itself is deceptive. Modern lizards are ectothermic, meaning that they rely on the sun and other environmental factors to warm their bodies. Though science has traditionally held dinosaurs, specifically non-avian dinosaurs, to be ectothermic, some evidence suggests that some might have been endothermic, meaning that they could heat their bodies from within by consuming food. Scientists continue to debate whether Staurikosaurus and other dinosaurs might have been ectotherms, endotherms or something in between.

Intelligence

Scientists have used encephalization quotient (EQ), based on brain and body size, to estimate animal intelligence. Fast-moving carnivores tend to rank higher on the EQ scale than their slower, herbivorous counterparts. As a small, swift, carnivorous dinosaur, Staurikosaurus likely ranks between 1.0 and 2.0 on the EQ scale as the later carnosaurs would. Staurikosaurus, however, is one of the most primitive dinosaurs known, and its fossil record is incomplete. These circumstances make it difficult to draw firm conclusions regarding its probable intelligence.

Reproduction and Population

The earliest dinosaurs, including Staurikosaurus, have yielded limited fossil evidence. Paleontologists believe that the first group of genuine dinosaurs, which included Staurikosaurus, made up a small part of their habitat's population—no more than 10 percent.

Most paleontologists believe that dinosaurs were oviparous, meaning that they reproduced by laying eggs, like modern lizards and birds. Insufficient fossil evidence exists to conclude for sure that Staurikosaurus was an egg layer or to draw any further conclusions regarding its mating, nesting, and parenting behavior. Fossils found for Jurassic and Cretaceous theropods reveal that at least some theropods were oviparous, but paleontologists are uncertain whether they tended their eggs or their hatchlings. Evidence for some dinosaurs indicates that many herbivores laid eggs in communal nests and that parents—or other adults of the species—remained with the eggs and tended the hatchlings for a time.

Diet

Staurikosaurus and its closest relatives were carnivores who likely fed on smaller dinosaurs and other small animals, especially early reptiles, such as rhynchosaurs, and cynodonts, the ancestors of mammals. In general, they would have hunted prey smaller than themselves and would themselves been prey for larger carnivores. Though some flesh-eating dinosaurs are known to have been omnivores that also consumed vegetation when convenient, the dental structure of Staurikosaurus suggests that it was strictly carnivorous.

Behavior

As only incomplete fossils exist for Staurikosaurus and similarly limited evidence has been unearthed for its closest relations, scientists have not been able to draw many conclusions regarding the behavior of these early theropods. Most conclusions have been derived based on evidence for later theropods. Though carnivores are generally considered more solitary, less gregarious dinosaurs than their herbivorous counterparts, it is believed that some carnivorous species did travel in small groups. More fossils would need to be found, however, to draw such a conclusion for Staurikosaurus. Thus far, it is known that Staurikosaurus was bipedal, fast, and likely spent much of its time hunting or scavenging for faunal food and avoiding predation.

Habitat and Other Life Forms

Staurikosaurus lived during the Late Triassic period (roughly 235–201 million years ago) and was among the first of the true dinosaurs to emerge on the landscape. At the time, the earth's surface looked very different than it does today. Modern continents instead formed one giant landmass, or supercontinent, known as Pangaea. This supercontinent is believed to have been located around the equator, giving the land a largely tropical and sub-tropical climate. With only small inland bodies of water and no inland seas, the climate stayed fairly constant, with little if any seasonal variation. Dry conditions prevailed in many areas, giving rise to great deserts, while other regions varied from cooler and wetter forestland to steamy jungles and wetlands. The Late Triassic is also believed to have been a period of much volcanic activity, which also would have made the landscape and climate challenging.

The Late Triassic was also a time of extreme plant and animal diversity. Horsetails, ferns, and gymnosperms, such as conifers and cycads, covered much of the landscape. Fauna ranged from amphibians, reptiles, cynodonts, insects, and fish to the newly emerging dinosaurs. Crocodiles, turtles, and frogs could be found, as could small mammals and many varieties of sea creature. Because the land formed one congealed mass, terrestrial animals could roam and intersperse freely.

Other Late Triassic dinosaurs included carnivorous Coelophysis, Eoraptor, Herrerasaurus, and Liliensternus, as well as herbivorous Mussaurus, Placerias, Plateosaurus, and Riojasaurus. Staurikosaurus is believed to have lived in a primarily forested region.

Research

Staurikosaurus was a unique and remarkable find both for its geographic location and its age. Brazilian paleontologist Llewellyn Ivor Price first discovered the fossil—a nearly complete skeleton and partial skull—in the Upper Santa Maria Formation of Brazil in 1970. The specimen was later named and described by Edwin H. Colbert of the American Museum of Natural History. At the time, fossil finds in South America were a rare occurrence, and the early age of Staurikosaurus made it one of the oldest known dinosaurs to date. Herrerasaurus, the closest known relation to Staurikosaurus, was unearthed in the Ischigualasto Formation of Argentina nearly a decade earlier in 1963.

From the 1970s forward, numerous other early fossils were uncovered in South America, as well as on other continents. In 1993, Eoraptor was discovered in the same formation as Herrerasaurus. In 1999, Guaibasaurus, a slightly later dinosaur, was found in the Caturrita Formation of Brazil. Several fossils for Chindesaurus, the next closest relation to Staurikosaurus, were unearthed in the Petrified Forest Formation of Arizona. In 2019 a group of researchers published a paper suggesting that a species controversially known as Teyuwasu barberenai, discovered in the same area as Staurikosaurus, may in fact be another example of the latter.

Bibliography

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