A 90-million-year-old fossil unearthed in Argentina is helping scientists rewrite the evolutionary history of a group of peculiar bird-like dinosaurs – and settle a long-running debate about how they spread across the ancient world.
The fossil, described in Nature, belongs to Alnashetri cerropoliciensis, a member of the alvarezsaurs: small dinosaurs notorious for their tiny teeth and stubby arms ending in a single large thumb claw.
Most well-preserved alvarezsaur fossils have been found in Asia, making their presence in South America difficult to explain, given the oceans separating the two landmasses.
The near-complete skeleton – recovered from the La Buitrera fossil area in northern Patagonia, a site also known for primitive snakes and tiny sabre-toothed mammals – has proven unusually revealing.
“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 lead author Peter Makovicky, a professor at the University of Minnesota.
Unlike its later relatives, Alnashetri had longer arms and larger teeth. That means alvarezsaurs shrank in body size before developing the smaller limbs and teeth more typical of the group, and which are thought to be better adapted for eating ants and termites.
“Our study suggests that alvarezsaurs as a group are small for dinosaurs with species ranging from crow-sized to man-sized,” Makovicky told BBC Science Focus. “Body size seems to fluctuate within this narrow size window, with no set pattern or trend.”
The find also resolved the intercontinental puzzle. Detailed anatomical study of Alnashetri led the team to reexamine collections around the world. "We found other alvarezsaurs hiding in plain sight," Makovicky said.
“These species from the Jurassic of North America and the Early Cretaceous of Europe helped prove that alvarezsaurs were inhabiting most of the continents before the major rifts between the northern and southern hemispheres occurred.”
Around 200 million years ago, Earth's landmasses were fused into a single supercontinent called Pangaea, which gradually broke apart over tens of millions of years – carrying animals with it as the continents drifted to their current positions.
The team says a further specimen from the same site is already being prepared, though Makovicky is giving little away about what it shows. “The new specimen reinforces some of our findings about body size and specialisation,” he said. “Not willing to go much further than that at this time.”
Read more:
