Herrerasaurus

Herrerasaurus is a likely genus of saurischian dinosaur that lived during the Late Triassic. Reaching about 6 meters (20 feet) in length and 350 kilograms (770 pounds), it ranks among the earliest dinosaurs known from fossils. Its name honors rancher Victorino Herrera, who spotted the first remains in 1958 in South America. All fossils of this meat-eater come from the Ischigualasto Formation in northwestern Argentina, which dates to the Carnian stage around 231.4 million years ago. The only species, *Herrerasaurus ischigualastensis*, was named by Osvaldo Reig in 1963. The names *Ischisaurus* and *Frenguellisaurus* are now considered synonyms.

For a long time, Herrerasaurus’s classification was uncertain because only scattered fragments were known. Scientists suggested it might be a basal theropod, a basal sauropodomorph, a basal saurischian, or even a non-dinosaurian archosaur. That changed in 1988, when a nearly complete skeleton and skull were uncovered. Since then, most studies on dinosaur origins place Herrerasaurus as an early saurischian. It belongs to the Herrerasauridae, a family of similar early dinosaurs that appeared during the first major dinosaur radiation.

**Discovery**

Paleontologist Osvaldo Reig named Herrerasaurus after Victorino Herrera, an Andean goatherd who noticed its fossils near San Juan, Argentina, in 1959. Those same rocks later yielded *Eoraptor* and are part of the Ischigualasto Formation, from the late Carnian stage. In the same 1963 paper, Reig also named *Ischisaurus cattoi*, now considered a juvenile synonym of Herrerasaurus. Reig originally thought Herrerasaurus was an early carnosaur, but debate raged for the next three decades. In 1970, Steel classified it as a prosauropod; in 1972, Peter Galton said it could only be placed within Saurischia. Later cladistic analyses put Herrerasaurus and *Staurikosaurus* at the base of the dinosaur tree, before ornithischians and saurischians split. Some researchers even argued the remains were not dinosaurian.

Two more partial skeletons with skulls were named *Frenguellisaurus ischigualastensis* by Fernando Novas in 1986, but this too is now a synonym. That specimen was found in 1975 and described by Novas as a primitive saurischian, possibly a theropod. In 1992, Novas and Paul Sereno both concluded that *Frenguellisaurus* and *Ischisaurus* belonged to Herrerasaurus. A complete Herrerasaurus skull was recovered in 1988 by a team led by Sereno. Based on these new fossils, Thomas Holtz and José Bonaparte placed Herrerasaurus at the base of the saurischian tree, before theropods and prosauropods diverged. Sereno, however, favored classifying it as a primitive theropod. Both views persist: Rauhut (2003) and Bittencourt and Kellner (2004) support the early theropod idea, while Max Langer (2004), Langer and Benton (2006), and Randall Irmis and colleagues (2007) back the basal saurischian hypothesis. If Herrerasaurus were a theropod, it would mean theropods, sauropodomorphs, and ornithischians split even earlier, before the middle Carnian, and that each lineage independently evolved certain dinosaur features like a more advanced ankle or open hip socket. This is supported by large three-toed footprints from the early Carnian Los Rastros Formation in Argentina, which predate Herrerasaurus by millions of years.

Studying early dinosaurs like Herrerasaurus and *Eoraptor* has major implications for whether dinosaurs form a single, monophyletic group. That idea was explicitly proposed in the 1970s by Galton and Robert T. Bakker, who listed shared skull and body traits inherited from a common ancestor. Later authors added more. A 1992 study of Herrerasaurus by Sereno suggested that only one skull feature and seven body features were truly inherited from a common ancestor; the rest likely came from convergent evolution. Sereno’s analysis also led him to propose several new shared dinosaur traits.

**Description**

Herrerasaurus was a lightly built, two-legged meat-eater with a long tail and a fairly small head. Adults had skulls up to 56 centimeters (22 inches) long, reached 6 meters (20 feet) in total length, and weighed about 350 kilograms (770 pounds). Smaller individuals were around 4.5 meters (15 feet) long and weighed roughly 200 kilograms (440 pounds). It walked fully upright on strong hind legs with short thighs and long feet, suggesting it was a fast runner. Each foot had five toes, but only the middle three (digits II, III, and IV) bore weight. The outer toes (I and V) were small, and the first toe had a tiny claw. The tail, partly stiffened by overlapping spine projections, helped balance the body and also aided speed. The forelimbs were less than half the length of the hind limbs.