Alvarezsaurids were mostly small-bodied theropods that paleontologists originally misinterpreted as early flightless birds, only to later recognize them as an ant-eating lineage of non-avian dinosaurs. For years, we suspected that Alvarezsaurids underwent a rare process of evolutionary miniaturization directly coupled to a diet of social insects like ants and termites. It was a tidy hypothesis: They got smaller to become more efficient at catching ants.
Now, a recently discovered fossil of one of the smallest alvarezsaurids ever found suggests that the evolution of miniature dinosaurs likely wasn’t as neat and linear as we thought. This new species, called Alnashetri cerropoliciensis, probably did not feed on ants at all. “It was a pursuit predator actively hunting insects and small mammals,” said Peter Makovicky, a paleontologist at the University of Minnesota.
The oddball
Alverezsaurids, found mostly in the Late Cretaceous rocks of Asia and South America, had short forelimbs tipped with a single oversized thumb claw built for digging. They also had minute teeth and sensory adaptations akin to those in modern nocturnal birds—everything necessary to work on termite mounds. “The explanation of their small body size has been tied to this specialization,” Makovicky explained.
The dinosaur he and his colleagues found, however, did not look like a specialized ant-eater.
The fossil of Alnashetri cerropoliciensis was unearthed from the Candeleros Formation at the Cerro Policía locality in Argentina’s Río Negro Province and is estimated to have lived roughly 90 million years ago. It currently stands as the most complete and smallest Alvarezsaurid skeleton found in South America.
While missing its skull roof, parts of its right arm, its lower right leg, and much of its tail, the skeleton preserves plenty of its crucial anatomy. Its bone tissue reveals that the alvarezsaurid was a subadult, likely approaching sexual maturity, as indicated by the presence of what appears to be medullary bone, a temporary tissue associated with egg-laying in modern birds. Despite being nearly fully grown, this dinosaur is estimated to have weighed a mere 700 grams.