Alvarezsaurids, once misinterpreted by paleontologists as early flightless birds, are now recognized as non-avian dinosaurs with a penchant for eating ants. The prevailing theory suggested that these small-bodied theropods underwent evolutionary miniaturization to efficiently prey on ants and termites. However, a newly discovered fossil has prompted a reassessment of this hypothesis.
The fossil, belonging to a species named Alnashetri cerropoliciensis, reveals that not all alvarezsaurids were ant-eaters. This diminutive dinosaur, one of the smallest alvarezsaurids found, likely pursued insects and small mammals as a predator, according to Peter Makovicky, a paleontologist from the University of Minnesota.
The Oddball
Typically found in the Late Cretaceous rocks of Asia and South America, alvarezsaurids are characterized by short forelimbs with an enlarged thumb claw adapted for digging, small teeth, and sensory features similar to those seen in nocturnal birds. These adaptations seemed perfectly tailored for a lifestyle centered around termite mounds, supporting the theory that their small size was a result of this dietary specialization. But the evidence from Alnashetri cerropoliciensis suggests otherwise.
The fossil was discovered in the Candeleros Formation at Cerro Policía in Argentina's Río Negro Province and is estimated to be around 90 million years old. It represents the most complete and smallest alvarezsaurid skeleton found in South America, despite missing its skull roof, parts of its right arm, lower right leg, and much of its tail. Importantly, the fossil shows signs of medullary bone, suggesting it was a subadult nearing sexual maturity. Nonetheless, its adult weight was estimated at merely 700 grams.