Amazingly complete skeleton of an Alvarezauroid dinosaur species Arunachetri seropolisiensis Two Alvarezauroid specimens from Patagonia, Argentina, and two Alvarezaurid specimens from the Northern Hemisphere reveal how a once-mysterious lineage of theropod dinosaurs evolved and spread before continental dispersion, calling into question long-held assumptions about its origins.
Arunachetri seropolisiensis They lived in what is now Argentina during the Cenomanian period of the Late Cretaceous, about 90 million years ago.
The species was first described from fragmentary remains in 2012. Alvarez Sauroidea.
These small dinosaurs are famous for their small teeth and stubby arms ending in one large thumb claw.
“The Alvarezauroidea is an enigmatic clade of primarily small theropod dinosaurs known primarily from the Jurassic to Cretaceous periods of Asia and South America,” said paleontologist Peter Makowiecki of the University of Minnesota, Twin Cities, and colleagues.
“Late Cretaceous Alvarezauroids are interpreted to be carnivorous (mainly eating ants), with specialized forelimbs adapted for burrowing, microscopic supernumerary teeth, and enhanced sensory abilities.”
“They are hypothesized to exhibit evolutionary miniaturization coupled with dietary specialization.”
new, almost complete skeleton Arunachetri seropolisiensis It was discovered in the La Buitrera fossil region in Rio Negro, northern Patagonia.
Microscopic analysis of the specimen confirmed that the animal was indeed an adult, at least four years old.
Weighing less than 0.9 kg (2 lb), it is one of the smallest known South American dinosaurs.
Unlike later relatives, Arunachetri seropolisiensis It had long arms and large teeth.
Paleontologists say this proves that some alvaresaurids evolved into smaller animals long before they developed special features that may have adapted them to an ant-eating diet.
By identifying previously discovered alvaresaurid fossils in museum collections in North America and Europe, the researchers also proved that these dinosaurs originated much earlier than expected, existing at a time when the continents were still connected as the supercontinent Pangea.
Their distribution was caused by the breakup of Earth’s landmass, and they are unlikely to cross oceans.
“Our biogeographical analysis infers the Pangean ancestral distribution of Alvarezsauroroidea, and the early history of this clade was dominated by surrogates,” the scientists said.
their paper Published in today’s magazine nature.
_____
PJ Makowiecki others. Argentina fossil rewrites the evolutionary history of a mysterious dinosaur clade. naturepublished online on February 25, 2026. doi: 10.1038/s41586-026-10194-3