Researchers have identified 90 million-year-old fossils that help solve a long-standing mystery about a strange group of prehistoric animals. The discovery was led by University of Minnesota Twin Cities scientist Peter Makowiecki and Argentine paleontologist Sebastian Apesteguia.
Their findings, published in the peer-reviewed journal Nature, describe an almost complete skeleton. Arunachetri seropolisiensis. This dinosaur belonged to a special group of theropod-like birds called Alvarezsaurus. These animals are known for their small teeth and unusually short arms that end in one giant thumb claw.
For decades, scientists struggled to understand this group, as most of the well-preserved fossils were found in Asia. Fossils from South America were often incomplete, leaving large gaps in the evolutionary story.
Patagonia Discovery provides important specimens
almost completed Arunashetri The fossil was discovered in 2014 in northern Patagonia, Argentina, in a site known for its extremely well-preserved fossils of Cretaceous period animals. The species was originally named several years ago based on fragmentary fossils, but the new skeleton provides a clearer picture of the animal’s unusual body structure.
Specimen preparation was a slow and careful process. Over the past decade, researchers have meticulously cleaned and assembled the delicate bones to prevent damage to the small, fragile skeleton.
“Creating a nearly complete, articulated animal from a fragmented skeleton that is difficult to interpret is like finding the Rosetta Stone of paleontology,” said Peter Makowiecki, lead author of the study and a professor in the University of Minnesota’s Department of Earth and Environmental Sciences. “We now have a reference point that allows us to pinpoint the cruder finds and map out evolutionary changes in anatomy and body size.”
The fossil provides scientists with valuable insight into how this dinosaur lineage evolved, became smaller, and spread across ancient continents.
Insights into the evolution of small dinosaurs
skeleton reveals it Arunashetri It differed from later relatives in several ways. This indicates that some alvaresaurids, which had longer arms and larger teeth, had already evolved to very small body sizes before developing specialized features that later species used for what scientists believe was an “ant-eating” diet.
Microscopic examination of the bones revealed that the animal was fully grown and at least 4 years old. These dinosaurs were among the smallest known non-avian dinosaurs and remained small throughout their lives. Even the largest members of this group are only about the size of an average human, which is small compared to most dinosaurs. Arunashetri It weighed less than 2 pounds on its own, making it one of the smallest dinosaurs discovered in South America.
By studying additional Alvarezsaurus fossils preserved in museum collections in North America and Europe, the research team also found evidence that these animals appeared much earlier than scientists previously thought. Their widespread distribution is thought to have occurred when the continents were still connected as the supercontinent Pangea. The subsequent breakup of Earth’s landmass explains how animals became scattered around the world rather than migrating across the oceans.
Fossil remains continue to reveal ancient life
This well-preserved skeleton was excavated from the La Buitrera fossil site, which has yielded many scientifically important discoveries. Previous discoveries from the site include early snakes and small saber-toothed mammals.
“After more than 20 years of research, the La Buitrera fossil site has given us unique insights into small dinosaurs and other vertebrates not found anywhere else in South America,” said Apesteguia, a researcher at Maimonides University in Buenos Aires, Argentina.
Scientists are still actively studying fossils from the same area, and more discoveries may soon add to the story of these rare dinosaurs.
“We’ve already discovered the next chapter of the alvarezsaurid story there, and it’s currently being prepared in the lab,” Makowiecki added.
International research team and support
The research involved an international collaboration of scientists. In addition to Makowiecki and Apestegia, the team also included Jonathan S. Mitchell of Coe University in Iowa. Mr. Jorge G. Meso and Mr. Ignacio Cerda (Instituto de Investigación, National University of Rio Negro, Provincial Museum). Federico A. Gianechini of the San Luis Interdisciplinary Biology Institute.
Funding for this research was provided by the National Council for Scientific and Engineering Research (CONICET), the Field Museum, National Geographic, the University of Minnesota, the National Science Foundation, and the Fulbright U.S. Scholars Program.
