In Argentina, an almost intact dinosaur fossil was unearthed that is estimated to have weighed less than 1 kilogram in life. It is believed to be the smallest dinosaur species discovered in South America to date. It is a species of the tiny dinosaur Alvarezsaurus, but it shows a different form from known specimens, suggesting that small dinosaurs independently evolved multiple times in different lineages.
A research team led by Peter Makovicky, a professor in the Department of Earth and Environmental Sciences at the University of Minnesota, said on the 26th in the international journal Nature that it had unearthed the complete skeleton of Alnashetri cerropoliciensis, a dinosaur that lived 90 million years ago in the Cretaceous period of the Mesozoic era in northern Patagonia, Argentina.
◇First complete skeleton found in South America
Alnashetri belongs to the Alvarezsaurids, dinosaurs similar to birds. Like birds, these dinosaurs had long snouts and small teeth, and short forelimbs with only a single thumb. So far, most Alvarezsaurus fossils have been discovered in Asia. Some were found in South America, but they were fragmentary, making it difficult to determine their identity.
In 2012, only the hind leg bone of a dinosaur as small as a chicken was found in northern Patagonia. But because it was incomplete, it was hard to determine anything beyond the possibility that it might be an Alvarezsaurus. It was also unclear whether it was a juvenile or an adult. In 2014, the research team excavated an almost perfect fossil in the same area, finding clues to solve the mystery of Alvarezsaurus dinosaurs that lived in South America.
After 10 years of research, the team confirmed that Alnashetri is an adult Alvarezsaurus dinosaur. Its age is estimated at least 4 years or older, but its body weight in life was estimated to be only 700 grams. It is smaller than a chicken. Surprisingly, this dinosaur showed a different appearance from known Alvarezsauruses.
Alnashetri had long, slender hind legs along with long forelimbs, and in particular, three fingers remained on the forelimbs. The teeth were also large. Makovicky said, "With the entire skeleton discovered, we obtained information to understand what Alnashetri shares in common with other species and what sets it apart," adding, "It is like finding the Rosetta Stone of paleontology."
The Rosetta Stone is a stele made in Egypt in the 2nd century B.C. that bears a royal decree written in Egyptian hieroglyphs, Demotic script, and ancient Greek, providing a decisive clue to deciphering hieroglyphs. Comparing this dinosaur fossil with similar small dinosaurs from Asia means researchers can more closely reveal the process by which birds evolved from dinosaurs.
◇Similar to birds but with traits of carnivorous dinosaurs
Alvarezsaurus was once considered an early ancestor of birds. The newly excavated Alnashetri showed that although Alvarezsaurus dinosaurs look birdlike, they were in fact non-avian theropods. Theropods are bipedal carnivorous dinosaurs like Tyrannosaurus or Velociraptor.
Until now, small Alvarezsauruses were thought to have had short, robust forelimbs with a single large thumb. Makovicky said, "Alnashetri does not fit the existing mold," and added, "Although it is on the small side among Alvarezsauruses, its teeth and forelimbs are not reduced."
Previously, scientists thought dinosaurs shrank in size as their diet shifted to ants and termites. In contrast, Alnashetri, though smaller than its later relatives, had the typical features of theropod carnivorous dinosaurs. That means it became small long before adapting to an ant-eating diet.
Makovicky said, "Aside from being small in size, Alnashetri had the typical structure of a theropod dinosaur," adding, "Given its small body, it likely had a more diverse menu of prey along with invertebrates." In other words, it may have preyed on small, mouse-like mammals.
The team also analyzed Alvarezsaurus fossils held in museums in North America and Europe and demonstrated that these dinosaurs appeared when Earth's continents were connected as the supercontinent Pangaea. The explanation is that Alvarezsauruses being unearthed on multiple continents is likely not because they crossed the sea, but because the continents where they originally lived later split apart.
The researchers said they expect more secrets of Alvarezsaurus to be revealed going forward. They have already discovered another fossil at the site where Alnashetri was excavated. Makovicky said, "We have found the next chapter of the Alvarezsaurids and are currently restoring it in the laboratory."
References
Nature (2026), DOI: https://doi.org/10.1038/s41586-026-10194-3