Weighing Less Than 1 kg: The Smallest Dinosaur in the World Forced a Rewrite of Evolution 0 6

Researchers have identified a 90-million-year-old fossil that helped unravel the ancient mystery of a strange group of prehistoric animals. The discovery was made by Peter Makovicky in collaboration with Argentine paleontologist Sebastián Apesteguía from the University of Minnesota, as reported by Focus citing Science Daily.

During the study, scientists were able to describe nearly a complete skeleton of Alnashetri cerropoliciensis — this dinosaur belonged to a peculiar group of bird-like theropods known as alvarezsaurs. It is noteworthy that these dinosaurs are known for their tiny teeth and unusually short forelimbs ending in a single enlarged claw on the thumb.

For decades, scientists have been trying to decipher the mystery of the evolution of this group, as most well-preserved fossils have been found in Asia. At the same time, fossil remains from South America have often been incomplete, leaving significant gaps in the evolutionary history.

However, in 2014, in northern Patagonia, Argentina, scientists discovered a complete skeleton of Alnashetri. The species was initially named a few years earlier based on fragmentary remains, but the new skeleton provided a much clearer picture of the animal's unusual body structure.

It took a decade to carefully study the specimen: scientists meticulously cleaned and assembled the fragile bones to prevent damage to the small and delicate skeleton. According to the lead author of the study, Professor Peter Makovicky from the Department of Earth and Environmental Sciences at the University of Minnesota, the complete skeleton provided them and their colleagues with a starting point that allowed for the accurate identification of smaller finds and mapping of evolutionary transitions in anatomy and body size.

The results showed that Alnashetri differed from its later relatives in several ways. Members of this species had longer arms and larger teeth — evidence that some alvarezsaurs had already evolved to very small body sizes before developing specialized features that later species used for an ant diet.

Furthermore, the scientists conducted a microscopic examination of the bones and found that the animal was an adult and at least four years old. Notably, these dinosaurs are considered some of the smallest non-avian dinosaurs — moreover, they remained tiny throughout their lives. Even the largest representatives of the group reached the size of an average human, which is very small compared to most dinosaurs. The weight of the dinosaurs was also only about 900 grams, making it one of the tiniest dinosaurs discovered in South America.

The authors of the study also found that these animals appeared significantly earlier than previously thought, and the widespread distribution of the species likely occurred when the continents were still connected in the supercontinent Pangaea.