Imagine holding a dinosaur smaller than a turkey, its bones so light they almost float out of your hands. For decades, creatures like this teased paleontologists because they were rare, fragmentary, and frustratingly elusive.
Now, buried in the red rocks of Patagonia, researchers have unearthed a nearly complete skeleton of one such animal. In a new study published in Nature, researchers say the fossil is prompting them to rethink how an entire branch of dinosaurs evolved.
That’s the story of Alnashetri cerropoliciensis, a 90-million-year-old dinosaur that may unlock the mysteries of Alvarezsaurid, a group of bird-like dinosaurs so strange they almost look like cartoon caricatures, with tiny teeth and stubby arms ending in a single large thumb claw.
“Going from fragmentary skeletons that are hard to interpret, to having a near complete and articulated animal is like finding a paleontological Rosetta Stone,” said Peter Makovicky, lead author in a press release. “We now have a reference point that allows us to accurately identify more scrappy finds and map out evolutionary transitions in anatomy and body size.”
A Patagonia Fossil That Rewrites Alvarezsaur Origins
For years, the best Alvarezsaurid fossils were from Asia, while South American specimens were too fragmentary to give a full picture. That left researchers guessing 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 uncovered much of a skeleton, remarkably intact.
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, researchers in Argentina and the U.S. have spent thousands of hours carefully removing rock from the fragile bones.
Read More: Unusually Small Dinosaur Fossil Helps Fill a 70-Million-Year Gap in Ornithopod Evolution
This Tiny Dinosaur Was Small Before It Specialized
Microscopic analysis shows that this individual was fully grown, at least four years old, yet weighed less than two pounds. That alone makes Alnashetri one of the smallest dinosaurs known from South America.
Bones of Alnashetri.
(Image Credit: Peter Makovicky, University of Minnesota)
Most later Alvarezsaurids had compact bodies and stubby forelimbs, features many researchers assumed evolved early as adaptations for digging into insect nests. But Alnashetri had relatively long arms and larger teeth. Instead of a creature specialized from the start, this dinosaur was more generalized. It hadn’t yet evolved the extreme body plan of its later cousins.
In other words, tiny size didn’t come from gradual shrinking over time. It came first, and the digging adaptations came later.
With this nearly complete skeleton in hand, the research team turned to museum collections around the world. Some fossils that had been hard to classify suddenly made sense, now recognizable as close relatives of Alnashetri.
That realization pushed the origins of the entire family back in time, to when Earth’s continents were still joined together in the supercontinent Pangaea. Instead of rafting across oceans or migrating long distances, these dinosaurs simply spread across connected landmasses that slowly drifted apart over millions of years.
This kind of broad distribution, shaped by continental breakup rather than unlikely epic journeys, helps explain why Alvarezsaurids appeared on multiple continents.
More Alvarezsaur Fossils Are Already in the Lab
La Buitrera has already given up snakes, mammals, and now this tiny dinosaur. And according to the team, more fossils from the same valley are waiting in labs, patiently being prepared.
Makovicky said this is just the beginning. “We have already found the next chapter of the alvarezsaurid story there, and it is in the lab being prepared right now.”
In the tale of evolution, even the smallest characters can change everything we thought we knew.
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Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article references information from a recent study published in Nature: Argentine fossil rewrites evolutionary history of a baffling dinosaur clade
