A tiny dinosaur species, Alnashetri cerropoliciensis, may be the missing link to understanding a group of bird-like dinosaurs called Alvarezsaurs, after the first complete fossil was discovered in Argentina, according to paleontologists.
University of Minnesota Twin Cities paleontologist Peter Makovicky co-led the research with Sebastian Apesteguía of Argentina’s Universidad Maimónides, with the pair presenting their 90-million-year-old fossil discovery in a recent paper published in Nature.
The dinosaur group Alvarezsaurus is defined by its small teeth and short arms, which terminate in a single claw, features that researchers suspect were adaptations to an ant-eating diet.
La Buitrera Fossil Site
Mystery has shrouded the Alvarezsaurs for decades. While well-preserved specimens have been identified in Asia, their presence in the South American fossil record has been limited to incomplete occurrences that are difficult to interpret. In 2014, paleontologists uncovered the first complete Alnashetri fossil. They made the find in a region of Patagonia, Argentina, called La Buitrera, known for its trove of Cretaceous-era fossils.
“After more than 20 years of work, the La Buitrera fossil area has given us a unique insight into small dinosaurs and other vertebrates like no other site in South America,” said Apesteguía, a researcher at Universidad Maimónides in Buenos Aires, Argentina.
An Incomplete Fossil Record
Prior to this discovery, clues in fragmentary fossils alerted paleontologists to the species’ existence, even giving it its name, but it wasn’t until the 2014 find that researchers had their first full view of Alnashetri. It took a decade of delicate work to safely assemble the dinosaur’s unusual anatomy.
“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 on the paper and a professor in the University of Minnesota Department of Earth and Environmental Sciences. “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.”
This “missing link” provides paleontologists with valuable information on how the
Alvarezsaurus spread over the globe, growing more diminutive as they evolved. Alnashetri’s small stature, but relatively long arms and large teeth, show that size was established within the Alvarezsauria lineage well before some of its most distinguishing features evolved.
Digging Deeper on Alvarezsaurus
Based on microscopic analysis, researchers determined that the specimen represents an adult of the species, aged 4 years or older. While the largest Alvarezsaurus was small for a non-avian dinosaur, at only about the size of a human, Alnashetri is one of the smallest dinosaurs ever discovered in South America, weighing a mere 2 pounds, based on paleontologists’ calculations.
After analyzing the Alnashetri fossil, the team examined other Alvarezsaur remains in North American and European museum collections. Those comparisons demonstrated that these dinosaurs date back to the time of the supercontinent Pangea, much earlier than expected. The discovery centers on how the group spread across land in the distant past, rather than on later ocean voyages.
“We have already found the next chapter of the Alvarezsaurid story there, and it is in the lab being prepared right now,” added Makovicky.
Research continues, both in the lab on Alnashteri, and on the broader effort toward adding to the expanding Cretaceous-era fossil record through ongoing digs at La Buitrera.
The paper, “Argentine Fossil Rewrites Evolutionary History of a Baffling Dinosaur Clade,” appeared in Nature on February 25, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.