90-million-year-old fossil of ‘missing link’ bird-like dinosaur discovered

They described the discovery of a near complete skeleton of Alnashetri cerropoliciensis as the “missing link” for a mysterious group of prehistoric animals.

The identification of the fossil was made by an international team led by University of Minnesota Twin Cities researcher Professor Peter Makovicky and his Argentinean colleague Dr Sebastian Apesteguía.

Makovicky described the discovery as a "palaeontological Rosetta Stone”.

He explained that 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.

But, 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 known for its Cretaceous fossils.

The species was originally named a few years earlier based on fragmentary remains.

But the 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.

Study lead author Makovicky 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.”

He says the discovery of the 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.

Makovicky says that proves that some alvarezsaurs evolved to be tiny long before they developed the specialised features thought to be adaptations for an "ant-eating" diet.

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

The animals are not just among the tiniest non-avian dinosaurs but they never get any bigger, according to the study published in the journal Nature.

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 the animals originated much earlier than expected when the continents were still connected as the "supercontinent" Pangaea.

The researchers said 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 important species, including primitive snakes and tiny sabre-toothed mammals.

Dr. Apesteguía, a researcher at Universidad Maimónides in Buenos Aires, Argentina, 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."

He says the team's work is far from over as they continue to discover and study fossils from the same area where they found 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,”