Argentine fossil rewrites evolutionary history of a baffling dinosaur clade

Nature

by Peter J. Makovicky

February 26, 2026

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A newly discovered fossil from Argentina is reshaping our understanding of the evolutionary history of alvarezsauroids, an enigmatic group of small-bodied dinosaurs known for their specialized anatomy and unique behaviors. The study describes Alnashetri cerropoliciensis, the most complete and smallest South American specimen of this clade to date. This discovery challenges previous theories about the evolution of these dinosaurs, particularly regarding their body size and geographic distribution. Alvarezsauroids are a diverse group of theropod dinosaurs that flourished during the Jurassic and Cretaceous periods. They are characterized by specialized forelimbs adapted for digging, tiny teeth, and heightened sensory capabilities, suggesting they were specialized hunters of small prey like insects. Previous research had suggested that alvarezsauroids underwent evolutionary miniaturization—shrinking in body size over time—as part of their adaptation to a diet focused on small prey. However, the new analysis of Alnashetri cerropoliciensis and other related fossils challenges this idea, finding no strong evidence for miniaturization but instead pointing to repeated evolution within a narrow body size range. The study also revises our understanding of alvarezsauroid biogeography. While earlier theories posited that South American species represented a paraphyletic grade compared to their Asian relatives (the Parvicursorinae), the new phylogenetic data reveal that these groups are more polyphyletic. This suggests a Pangaean origin for the group, with vicariance—geographic isolation due to tectonic movements—playing a key role in their evolutionary history rather than dispersal events. The discovery of Alnashetri cerropoliciensis is significant not only for its completeness but also for the insights it provides

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Originally published on Nature on 2/26/2026