A nearly complete skeleton of a small predatory dinosaur discovered in Argentina has revealed new details about the evolution of alvarezsaurs. The 95-million-year-old Alnashetri cerropoliciensis weighed just 700 grams and challenges previous ideas about their anatomy and diet. Researchers suggest it had a broader range of prey than previously thought.
In 2014, palaeontologists unearthed an almost-complete skeleton at the La Buitrera site in northern Patagonia, Argentina. This fossil belongs to Alnashetri cerropoliciensis, a member of the alvarezsaur group, which includes some of the smallest dinosaurs known. The specimen dates back 95 million years.
The first evidence of Alnashetri came in 2012, consisting of incomplete hindlimb bones. As Peter Makovicky at the University of Minnesota notes, those fragments made it hard to determine if the dinosaur was juvenile or adult, or to compare it fully with other species. The new find changes that. "With a whole skeleton, we suddenly had all the information to understand how Alnashetri was similar or differed from other species, and a key to understanding how the unusual anatomy of alvarezsaurs evolved," says Makovicky.
Analysis shows the dinosaur was an adult, at least four years old, with long, slender hind limbs and forelimbs that are longer than expected, ending in three well-developed fingers. It weighed only 700 grams, making it smaller than a chicken. "The specimen is truly tiny, smaller than a chicken," Makovicky adds.
Alvarezsaurs were once considered bird ancestors but are now classified as non-avian theropods. Earlier views held that their small size, short stout forelimbs with a large thumb and reduced side digits, and tiny teeth evolved for an ant- and termite-only diet. However, Alnashetri, from an earlier evolutionary branch, has unreduced teeth and forelimbs more like typical theropods. Makovicky explains, "Alnashetri is tiny but is otherwise built like a more typical theropod – given its small size, it probably ate its fair share of invertebrates, but probably had a wider range of prey."
This discovery leaves questions about why alvarezsaurs shrank so much. As Makovicky puts it, "We're left with only a vaguer sense that alvarezsaurs were successful at occupying the niches of very small predators."
The findings appear in Nature (DOI: 10.1038/s41586-026-10194-3).