90-Million-Year-Old Fossil Reveals a Fully Grown Dinosaur That Weighed Less Than 2 Pounds

Imagine holding a dinosaur smaller than a turkey, its bones so light they almost float out of your hands. For decades, creatures like this have teased paleontologists because they were rare, fragmentary and frustratingly elusive.

Today, buried in the red rocks of Patagonia, researchers have unearthed the almost complete skeleton of one of these animals. In a new study published in NatureResearchers say the fossil prompts them to rethink how an entire branch of dinosaurs evolved.

This is the story of Alnashetri cerropoliciensisa 90-million-year-old dinosaur that could unlock the mysteries of Alvarezsaurid, a group of bird-like dinosaurs so strange they almost look like cartoon caricatures, with small teeth and short arms ending in a single large thumb claw.

“Going from difficult-to-interpret fragmentary skeletons to a nearly complete, articulated animal is like finding a paleontological Rosetta Stone,” lead author Peter Makovicky said in a press release. “We now have a reference point that allows us to accurately identify more disjointed findings and map evolutionary transitions in body anatomy and size.”

A Patagonian fossil that rewrites the origins of the Alvarezsaur

For years, the best Alvarezsaurid fossils came from Asia, while South American specimens were too fragmentary to paint a complete picture. This left researchers guessing about how this group evolved, where they came from and how their anatomy developed.

Then, in 2014, a research team working at the La Buitrera fossil site in northern Patagonia discovered a large portion of a remarkably intact skeleton.

The site is one of South America’s most important windows into the Cretaceous period, known for preserving delicate animals that rarely fossilize. Over the past decade, Argentine and American researchers have spent thousands of hours carefully removing rocks from fragile bones.

Learn more: Unusually small dinosaur fossil helps bridge 70 million-year gap in ornithopod evolution

This little dinosaur was small before it specialized

Microscopic analysis shows that this individual was adult, at least four years old, but weighed less than two pounds. This alone makes Alnashetri one of the smallest dinosaurs known from South America.

Most later Alvarezsaurids had compact bodies and truncated forelimbs, features that many researchers assumed evolved early as adaptations for digging insect nests. But Alnashetri had relatively long arms and larger teeth. Instead of a specialized creature from the start, this dinosaur was more generalized. It had not yet developed the extreme body plan of its later cousins.

In other words, short stature is not the result of gradual shrinkage over time. That came first, and the digging adaptations came later.

With this almost complete skeleton in hand, the research team turned to museum collections around the world. Some fossils that had been difficult to classify suddenly took on meaning, now recognizable as close relatives of Alnashetri.

This realization pushed the origins of the entire family back in time, to when Earth’s continents were still united in the supercontinent Pangea. Instead of rafting across oceans or migrating long distances, these dinosaurs simply spread across connected landmasses that slowly separated over millions of years.

This type of wide distribution, shaped by continental fragmentation rather than improbable epic voyages, helps explain why Alvarezsaurids appeared on multiple continents.

Other Alvarezsaur fossils are already in the laboratory

La Buitrera has already abandoned snakes, mammals and now this little dinosaur. And according to the team, other fossils from the same valley are waiting in the laboratories, patiently prepared.

Makovicky said this is just the beginning. “We have already discovered the next chapter in the history of Alvarez saurids there, and it is currently in the laboratory being prepared.”

In the tale of evolution, even the smallest characters can change everything we thought we knew.

Learn more: 84-million-year-old horned dinosaur fossils rewrite Europe’s prehistoric record

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