Herrerasaurus: one of the oldest well-studied dinosaurs

Herrerasaurus ischigualastensis was a large, two-legged predator of Late Triassic Argentina, about 231–229 million years ago. It is one of the oldest well-known dinosaurs and is represented by several skeletons, including a remarkably informative skull and neck. Its mixture of early and specialised features has made its position on the dinosaur family tree a long-running question.

The animal is not securely the first dinosaur, nor is it simply an early version of a tyrannosaur. Most treatments recognise one species in the genus and place it among the herrerasaurids. Some analyses identify it as an early theropod; others place the group near the base of Saurischia but outside the branch that includes true theropods and sauropodomorphs.

This page is part of the dinosaur catalogue. The fossils establish a carnivorous biped from the Ischigualasto Formation, while exact relationships, colour and behaviour require careful qualification.

Quick facts

Name and discovery

Herrerasaurus means “Herrera's lizard”. The genus honours Victorino Herrera, a local resident who found the first remains in San Juan Province. The species name ischigualastensis refers to the Ischigualasto Formation, where the fossils were recovered.

Argentine palaeontologist Osvaldo Reig named the genus and species in 1963 from the incomplete skeleton PVL 2566. He also named Ischisaurus cattoi from the same broad fossil region. The fragmentary early material made it difficult to determine the full body plan, and the animal was variously classified as a primitive dinosaur, an early carnosaur, a sauropodomorph or even a reptile outside Dinosauria.

Fieldwork in 1988 produced specimen PVSJ 407, a largely articulated skeleton with a well-preserved skull and neck. Its anatomy clarified that Herrerasaurus was a dinosaur with a combination of early traits and specialisations. The find made the genus a central comparison point for studying the first phases of dinosaur evolution.

Classification and evolutionary relationships

Most modern studies recognise Herrerasauridae as a family of early predatory dinosaurs. It includes South American genera such as Staurikosaurus, Sanjuansaurus and Gnathovorax. The broader content of Herrerasauria varies among analyses, because relationships depend on which fossils and anatomical characters are included.

One influential placement puts herrerasaurids near the base of Saurischia but outside Eusaurischia, the branch that contains Theropoda and Sauropodomorpha. Under that hypothesis, Herrerasaurus is a saurischian dinosaur but not a theropod in the strict sense. Other analyses recover herrerasaurids as the earliest theropods. Some older or less common results place them outside Saurischia or close to the base of Dinosauria.

The disagreement is not simply due to poor fossils. Herrerasaurus is better known than many dinosaurs of similar age, but it combines anatomical states that can be interpreted in more than one way. Character selection, coding and the choice of comparison groups influence the resulting tree. The classification of dinosaurs is therefore best shown with the competing placements stated, not hidden behind a false certainty.

Species and historical names

One species is generally accepted: Herrerasaurus ischigualastensis. The names Ischisaurus cattoi and Frenguellisaurus ischigualastensis are commonly treated as junior synonyms. In each case, later comparison found that the differences did not justify maintaining separate genera or species.

Synonymy does not mean that the fossils are identical in every detail. It means that the observed differences are not considered sufficient to establish separate valid names. Size, age, preservation and individual variation can all change the appearance of a partial skeleton.

Some other remains from Ischigualasto were historically assigned to Herrerasaurus or related animals without enough diagnostic anatomy. Reassessment has narrowed those referrals. The secure record remains associated with the Ischigualasto Formation in northwestern Argentina rather than a broad, speculative range across Pangaea.

Fossil sample and collections

The holotype PVL 2566 is incomplete, but additional specimens preserve much of the skull, spine, shoulder and pelvic girdles, arms, legs and feet. Important individuals include PVSJ 407, PVSJ 373 and PVSJ 53, held in Argentine and other museum collections. Together they document far more anatomy than the first named specimen alone.

PVSJ 407 preserves an articulated skeleton and a skull about 30 centimetres long. Because the bones remained in anatomical connection, researchers can observe how many elements met in life rather than infer every relationship from separate fossils. That quality is unusually valuable for a dinosaur from the early part of the Late Triassic.

The sample is not a perfect life-history series. Not every individual is equally complete, and some bones are damaged, deformed or absent. There are limits to how confidently specimens can be assigned an age or compared for maturity. Even a relatively rich assemblage does not answer every question about variation or maximum size.

Size and proportions

Published reconstructions range from about 3 to 6 metres long. The upper value represents the larger known individuals and depends on restoring missing sections of the tail and spine. It should not be presented as the standard size of every adult. Smaller skeletons reflect both the range of individuals and uncertainty in how they grew.

Larger specimens have been estimated at roughly 200–350 kilograms, but these are model-based values. A fossil bone does not preserve the body's complete volume or muscle distribution. Changing the assumed width of the torso, the amount of soft tissue or the proportions of missing parts changes the estimate.

Herrerasaurus was lightly built for its size, with a long tail balancing the forward trunk and a bipedal stance. The central toes carried most of the weight, while the outer toes were reduced. This basic arrangement is visible in the skeleton, whereas the exact stance and muscle outline in an illustration remain reconstructed.

Skull, jaws and teeth

The skull was long, low and narrow. Its teeth curved backward and had serrated cutting edges, a clear indication of a carnivorous diet. The skull also preserves openings and attachment regions for jaw muscles, allowing anatomical comparisons with other early dinosaurs.

A joint within the lower jaw allowed some movement between its parts. Researchers have suggested that this mobility may have helped the animal hold struggling prey, but the precise function is inferred. The joint does not preserve a hunting event or demonstrate one particular technique.

The teeth and jaws support a general diet of animal tissue. Small and medium-sized vertebrates were plausible prey in the Ischigualasto ecosystem. No secure stomach contents or unmistakable bite traces identify a preferred species, so a specific menu cannot be stated as fact. Scavenging may have supplemented hunting, but its importance is unknown.

Arms, hands and legs

The forelimbs were shorter than the hind limbs but were not useless. The elongated hand had three large clawed fingers and two strongly reduced outer digits. Joint surfaces and phalanges suggest the hand could grasp and pull an object toward the body. That anatomy is compatible with holding food, but the exact prey-handling motion remains a functional reconstruction.

The hind limbs supported the animal in a bipedal stance. Three central toes formed the principal contact with the ground, while the outer toes were smaller. A long tail counterbalanced the body as it moved. These features establish a terrestrial biped more reliably than they reveal its top running speed.

Claims of exceptional speed are based on biomechanical models, which require assumptions about muscle, body mass and stride. No securely attributed trackway independently records the pace of Herrerasaurus. Its limb proportions indicate active movement, but not a precise speed in kilometres per hour.

Ischigualasto environment and diet

The Ischigualasto Formation records river channels and floodplains under a generally seasonal, relatively dry climate. Volcanic ash within the succession helped geologists date its layers. The sediments and plant remains provide the environmental context, while the dinosaur skeletons record only a portion of the animal community.

Herrerasaurus lived alongside rhynchosaurs, plant-eating and omnivorous synapsids, large predatory archosaurs and other early dinosaurs. It was one of the more common dinosaurs in parts of the formation, but dinosaurs had not yet taken over every large terrestrial niche. Several non-dinosaur reptiles were important members of the ecosystem.

The teeth establish carnivory. Small and medium vertebrates were likely among its potential prey, but no particular genus can be named as a regular target from the current direct evidence. A carcass could also have provided food. The Triassic setting helps explain what resources were available without turning ecological possibility into a witnessed event.

Common myths and reconstructions

Herrerasaurus is sometimes called “the first dinosaur”. A more accurate description is one of the oldest well-studied dinosaurs. The early fossil record is incomplete, and other dinosauriforms and dinosaurs of comparable age are known. The oldest discovered specimen is not necessarily the first animal that evolved.

It is also misleading to describe Herrerasaurus as a miniature tyrannosaur or a direct ancestor of Tyrannosaurus. Both belonged to predatory dinosaur lineages, but tyrannosaurids appeared much later and occupied a different branch. A shared bipedal body plan is not evidence of direct descent.

Modern reconstructions should not present one disputed classification, a precise colour pattern, pack hunting or a record-breaking speed as established fact. The animal's skeleton is informative, but it does not preserve skin, voice, nests or social behaviour. Illustrations necessarily restore missing anatomy and setting.

Why Herrerasaurus matters

Herrerasaurus is important because its age, relatively good skeletons and mixed anatomy place it close to the early radiation of dinosaurs. Its remains show that an animal could combine clear dinosaur features with anatomical states unlike those of later theropods. That makes it useful for testing where the major saurischian branches emerged.

The fossil record supports a firm core of conclusions: one recognised species, a Late Triassic home in Argentina, a bipedal body and a meat-eating jaw. The exact relationship to Theropoda remains debated. Keeping both the direct evidence and that debate visible gives a more accurate account than calling it either a settled “first” or a miniature version of a later predator.