Herrerasaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Herrerasauridae

Genus:Herrerasaurus

Species:Herrerasaurus ischigualastensis

Introduction

Herrerasaurus was an early predatory dinosaur and a relative of the predatory dinosaur group that dominated the terrestrial environment throughout the Mesozoic Era (roughly 251 to about 66 million years ago). Paleontologists now believe that Herrerasaurus was part of one of the earliest radiations of dinosaurs and therefore served as an early example of basic predatory dinosaur design.

In the Triassic period, the continents were joined in a single land mass called Pangaea. Over millions of years, beginning in the Late Triassic, tectonic movement and an encroaching sea, known as the Tethys Seaway, caused Pangaea to split into northern and southern supercontinents known as Laurasia and Gondwana. Geographic isolation led to the development of distinct northern and southern assemblages of dinosaurs.

Herrerasaurus and its relatives evolved before Pangaea split apart, and members of the group were present in both the northern and southern hemispheres. Bipedal predatory dinosaurs were enormously successful in the Jurassic and Cretaceous periods, evolving into numerous species.

Classification

Scientists utilize two primary methods to classify organisms into different categories. Traditional Linnaean taxonomy places organisms in a ranked hierarchy based on overall physical similarity. An alternate system, cladistics, creates groups called “clades” in an effort to reconstruct the evolutionary relationships between species.

Within the Linnaean system, dinosaurs have long been split into two orders, Ornithischia and Saurischia, based on their skeletal structure. (Although some researchers have challenged this basic division, it remains in wide use.) Herrerasaurus is generally considered a saurischian, because of the orientation of its pelvis. From there its classification has long been debated, due to its primitive status and unusual features. However, most researchers place it as a primitive member of the suborder Theropoda, which is comprised of bipedal dinosaurs that were primarily predatory. It is a member of the family Herrerasauridae, one of the earliest dinosaur groups, the members of which were generally bipedal, predatory, and had similar skull and limb features.

Cladistic analysis also uses the clade Theropoda, based on the presence of a forked bone called a furcula, or “wish bone,” and features of the skull. Cladists are uncertain about Herrerasaurus’s position in more specific clades, as it possesses a number of primitive characteristics that seem to separate it from the later theropods. Most scientists consider it a basal theropod, but some paleontologists believe that the species belonged to a more primitive group of dinosaurs that were ancestors of the theropods and other groups of dinosaurs.

Anatomy

Herrerasaurus was a medium-sized predator, reaching lengths of between 2.1 and 4 meters (6.8–13.1 ft) and standing roughly 1.1 meters (3.6 ft) at the hip. The animal was stocky in build, weighing between 200 and 350 kilograms (440–771.6 lbs). Herrerasaurus had strong rear legs and was bipedal, with three-toed, clawed feet. The animal had five fingers on its hands, but only the first two fingers and the thumb ended in claws.

Herrerasaurus had a thin, long skull with rows of sharp, recurved teeth in the jaws. The jaw was hinged with a flexible joint, allowing it to adjust to the movements of its prey. The animal had large nostrils and forward-facing eyes, giving the species binocular vision.

Herrerasaurus had an usual pelvis that differed from those of other theropod dinosaurs in the Triassic period and more closely resembled those of later Jurassic and Cretaceous theropods. The animal had a long and heavy tail, like many later theropods, which was presumably used for balance.

Paleontologists believe that Herrerasaurus was probably covered in fine, mosaic scales, like many later dinosaurs, and may also have been cryptically colored, allowing the animal to blend into its environment.

Intelligence

Scientists have used encephalization quotient (EQ), based on brain and body size, to estimate animal intelligence. While some later theropods achieved relatively high EQs within the range of 5 to 5.8, Herrerasaurus likely had a lower value. The animal may have been similar in intelligence to a large lizard, like the Komodo dragon, the modern world's heaviest lizards. Paleontologists generally believe that most dinosaurs were significantly more intelligent than the prehistoric reptiles that preceded them.

Reproduction and Population

Scientists are uncertain about the reproductive behavior of Herrerasaurus. The species was most likely oviparous, or egg-laying, and nests of fossilized eggs have been found for some later theropod species. Some theropods were also brooding animals, like modern birds, however this behavior may have evolved in a later species. Alternatively, Herrerasaurus may have covered its nest in vegetation for insulation and protection.

Paleontologists are also uncertain if male and female Herrerasaurus were identical in appearance or whether they varied in size or other characteristics. Herrerasaurus is known from only a limited number of specimens, which may indicate that the species was rare in its environment.

Diet

Herrerasaurus was a carnivore, likely subsisting on a variety of prey. In addition to numerous varieties of small amphibians and reptiles in Triassic Argentina, Herrerasaurus might also have fed on early herbivorous dinosaurs. There were a variety of sauropodomorphs in the Late Triassic environment, including the largest terrestrial herbivores of the period. Herrerasaurus was most likely a solitary predator, stalking the forests and open scrub of Argentina for prey.

Behavior

While paleontologists believe that some theropods of the Jurassic and Cretaceous displayed social behavior, some believe that this was a later development in the group and that early theropods were more likely solitary predators. Herrerasaurus lived in an environment that contained both forested zones and drier, arid plains, and may have hunted in both environments.

Although Herrerasaurus was a predator, it was not the largest predatory species in Triassic Argentina and might have also faced danger from larger predators. Several larger dinosaur predators lived in the environment, as well as crocodylomorphs, ancestors of the crocodiles that evolved into powerful predators in the Late Triassic.

Habitat and Other Life Forms

South America was still joined to North America in the Carnian stage of the Triassic period, and much of the central portion of the area was covered by arid deserts and plains. Along the coasts, the climate was more temperate, and a variety of vegetation, including early gymnosperms like conifers and cycads, existed. Plants in the group Sphenopsida, which includes the “horsetail” plants, were common in areas with significant moisture, as were a variety of fern species.

One of the most common groups of terrestrial animals in the Late Triassic was the large dicynodonts, relatives of the reptilian ancestors of modern mammals. One genus, Placerias, dominated in North America. In Argentina, the related species Ischigualastia jenseni also made up a large portion of the vertebrate herbivores.

In addition to Herrerasaurus, other dinosaurs in Triassic Argentina included the medium-sized theropod predators Ischisaurus and Frenguellisaurus (though there is contention in the scientific community that those dinosaurs are actually the same genus as Herrerasaurus). The sauropodomorph group was represented by Coloradisaurus and Riojasaurus. Small ornithischian dinosaurs also lived in the area, represented by primitive species like Pisanosaurus. In addition to terrestrial predators, the flying reptiles, or pterosaurs, evolved and spread in the Triassic, dominating the aerial environment until the evolution of birds in the Cretaceous.

Research

Herrerasaurus was first studied by Argentine paleontologist Osvaldo Reig and was named after the Argentinean rancher who discovered the first Herrerasaurus fossils in 1959. The Ischigualasto Formation in Argentina proved to be one of the most productive sites for uncovering dinosaurs and reptilian fossils from the Late Triassic period. Paleontologists discovered a number of primitive predatory dinosaurs in the area, including species like Eoraptor, which are among the most primitive dinosaurs discovered anywhere in the world.

A team headed by paleontologist Paul Sereno made a major find of a well-preserved Herrerasaurus skull in 1988. Together with other fossils from the Ischigualasto Formation, this allowed a more complete reconstruction of the unusual animal and further insight into early dinosaur evolution. Much subsequent research focused on classification, with most scientists considering Herrerasaurus either a basal saurischian or a basal theropod. Paleontologists continue to investigate the evolutionary relationships of herrerasaurids, which holds implications for broad theories of dinosaur evolution.

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