A nearly complete miniature dinosaur fossil from Argentina offers new insights into the evolution of small dinosaurs. The now described specimen of the species Alnashetri cerropoliciensis was an adult individual that weighed less than a kilogram, lived around 90 million years ago and belonged to the Alvarezsaurids – a group of small, bird-like dinosaurs, of which predominantly fragmentary remains were previously known. The well-preserved skeleton also helped to assign other relics to this group of dinosaurs and thus rewrite the history of the Alvarezsaurids.
The Alvarezsaurids were a group of bipedal dinosaurs that typically weighed less than five kilograms and were characterized by tiny teeth and short, stubby arms that ended in a single large thumb claw. These dinosaurs probably fed on ants and termites, whose mounds they dug up with the help of their claws. Fossils from this group were found in Asia and South America, although the South American finds in particular were usually only fragmentary and therefore difficult to interpret.
As small as a bantam chicken
Now a team led by Peter Makovicky from the University of Minnesota in Minneapolis has described for the first time an almost complete skeleton of a South American alarezsaurid. The approximately 90 million year old fossil, which paleontologists assign to the species Alnashetri cerropoliciensis, was discovered in 2014 in the La Buitrera site in northern Patagonia. The research team spent more than ten years analyzing the tiny bones and comparing them with other finds. According to the analysis, Alnashetri weighed less than a kilogram, just the size of a bantam chicken. However, microscopic examination of the bones confirmed that it was an adult animal, at least four years old.
“Alnashetri cerropoliciensis therefore represents the smallest South American taxon known to date,” explain Makovicky and his colleagues. Alnashetri’s teeth and arms were slightly longer in relation to his body size than those of his later relatives. Accordingly, the miniaturization of alvarezsaurids did not only occur in the course of evolution, as the team explains. Instead, tiny species existed early in their evolutionary history, even before what would later be considered typical features of the group had fully developed.
Distribution on the supercontinent Pangea
The newly described fossil also helps to classify other relics of this dinosaur group that were previously difficult to interpret. “The transition from fragmentary skeletons to a nearly complete and articulated animal is like discovering a paleontological Rosetta Stone,” says Makovicky. “We now have a reference point that allows us to accurately identify additional fragmentary finds and map evolutionary transitions in anatomy and body size.”
In museum collections, the researchers came across additional fossils from North America and Europe, which they were also able to identify as alvarezsaurids by comparing them with the new skeleton. This suggests that this group was widespread and probably developed when the continents were still connected as the supercontinent Pangea. Only the splitting of the land masses ensured that the Alvarezsaurids developed separately in different parts of the world. “This discovery largely reshapes our understanding of the evolution of this group of dinosaurs,” the researchers write.
Source: Peter Makovicky (University of Minnesota, Minneapolis, USA) et al., Nature, doi: 10.1038/s41586-026-10194-3