A new study of fossils from a bird-like dinosaur, called Alnashetri, provides new insight into how its lineage evolved, shrank and spread across the ancient world. Credit: Gabriel Díaz Yantén, Universidad Nacional de Río Negro.

A new study says a 90-million-year-old fossil Alnashetri originated when the continents were still connected as the supercontinent Pangaea.

A team co-led by University of Minnesota Twin Cities researcher Peter Makovicky and Argentinean colleague Sebastian Apesteguía from the Universidad Maimónides in Buenos Aires, Argentina has identified a 90-million-year-old fossil that provides the ‘missing link’ for a mysterious group of prehistoric animals. 

The study details the discovery of a complete skeleton of Alnashetri cerropoliciensis.

Alnashetri belongs to a group of bird-like dinosaurs, known as alvarezsaurs, that are famous for their tiny teeth and stubby arms ending in a single large thumb claw.

For decades, they have remained a mystery because most of the well-preserved fossils were found in Asia, while records from South America were fragmented and difficult to interpret.

In 2014, the almost complete fossil of Alnashetri was discovered in the northern part of Patagonia, Argentina, at a site that is world-renowned for its exquisite Cretaceous fossils.

The species was originally named a few years prior based on fragmentary remains, but this newer, more complete specimen allowed the team to finally map the group’s strange anatomy.

The team spent the last decade carefully preparing and piecing together the fossils to avoid damaging the small bones.

Lead author Peter Makovicky, a professor in the University of Minnesota Department of Earth and Environmental Sciences, said: “Going from fragmentary skeletons that are hard to interpret, to having a near complete and articulated animal is like finding a paleontological Rosetta Stone.

“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.”

Alnashetri was rapidly covered by an advancing sand dune that preserved it almost intact for 90 million years. Credit: Peter Makovicky, University of Minnesota.

Microscopic analysis

The discovery of this nearly complete skeleton opens up a new understanding of how its lineage evolved, shrank and spread across the ancient world.

Unlike its later relatives, Alnashetri had long arms and larger teeth. This proves that some alvarezsaurs evolved to be tiny, long before they developed these specialised features thought to be adaptations for an ‘ant-eating’ diet.

Microscopic analysis of the bones confirmed the animal was indeed an adult of at least four years old.

These animals are not just among the tiniest non-avian dinosaurs, but they never get any bigger – the largest species are the size of an average human, very small for dinosaurs, and Alnashetri itself weighed less than 2lbs making it one of the smallest dinosaurs known from South America.

By identifying previously found alvarezsaurs fossils in museum collections from North America and Europe, the team proved these animals originated much earlier than expected when the continents were still connected as the supercontinent Pangaea.

Their distribution was caused by the breakup of the Earth’s landmasses, not unlikely treks across oceans.

The well-preserved fossil was recovered from the La Buitrera fossil area, a site that has yielded other scientifically critical animals, including primitive snakes and tiny sabre-toothed mammals.

Apesteguía said: “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.”

Their work is far from over, as the scientists continue to discover and study fossils from the same area where they discovered Alnashetri.

Makovicky added: “We have already found the next chapter of the alvarezsaurid story there, and it is in the lab being prepared right now.”

The study is published in Nature.