Complete Patagonian Skeleton of Alnashetri cerropoliciensis Rewrites the Evolutionary History of Enigmatic Alvarezsauroid Dinosaurs

A 90-million-year-old Alnashetri skeleton reveals alvarezsauroids shrank before they became ant-eaters. Discover how Pangea shaped dinosaur evolution.

By: AXL Media

Published: Feb 28, 2026, 8:48 AM EST

Source: The information in this article was sourced from Sci.News

Decoding the Enigmatic Alvarezsauroidea

The Alvarezsauroidea represent one of the most baffling lineages of theropod dinosaurs, characterized primarily by their bird-like features and small body sizes. Historically known from Jurassic and Cretaceous deposits in Asia and South America, these dinosaurs are distinguished by specialized forelimbs, minute teeth, and heightened sensory capacities. Paleontologists have long interpreted these features as adaptations for myrmecophagy, or a diet consisting primarily of ants and termites. However, the exact sequence of their evolutionary transformation has remained a subject of intense debate. The discovery of a remarkably complete skeleton of Alnashetri cerropoliciensis in Patagonia is now providing a definitive timeline for these physical changes.

The Smallest Giant of the Late Cretaceous

Living approximately 90 million years ago during the Cenomanian age, Alnashetri cerropoliciensis was a diminutive inhabitant of what is now the Río Negro province in Argentina. While first described in 2012 from fragmentary remains, the new skeleton found in the La Buitrera fossil area allowed for a comprehensive microscopic analysis. Researchers from the University of Minnesota Twin Cities confirmed the specimen was an adult, at least four years of age, yet it weighed less than 0.9 kg. This puts the species among the smallest dinosaurs ever recorded in South America, reinforcing the hypothesis of evolutionary miniaturization within the clade.

Challenging the Adaptive Sequence

One of the most significant revelations from the Patagonian fossil is the disconnect between size and dietary specialization. Unlike their later relatives, which possessed stubby arms and a single large thumb claw for digging into insect nests, Alnashetri cerropoliciensis featured relatively long arms and larger teeth. This indicates that alvarezsauroids evolved their tiny stature well before the development of the digging tools associated with their ant-eating niche. The findings, published in the journal Nature, suggest that miniaturization was an independent evolutionary step that preceded the more radical morphological changes seen in Late Cretaceous species.