A rare paleontological discovery in Patagonia has revealed the remains of a dinosaur about 70 million years old, along with two fossilized eggs and an unusual imprint inside their shells. The find belongs to Bonapartenykus ultimus, a dinosaur from the Alvarezsauridae family, and is giving researchers new insights into the reproduction and life of these animals at the end of the Cretaceous period. The fossil includes a partial skeleton and two eggs found near articulated hind limb bones. According to the researchers, this is the first time that eggs of an alvarezsaurid have been found so close to the skeletal remains of an individual.

One possibility that paleontologists are considering is that the eggs may have still been in the female's oviducts when she died. However, scientists emphasize that this hypothesis cannot yet be considered proven. Bonapartenykus ultimus was about 2.6 meters long and is considered one of the largest members of the alvarezsaurid family. It is also considered the last known member of this group to live in Gondwana, the large land mass that during the Mesozoic Era included the territories of present-day South America, Africa, Antarctica, Australia and India.

Alvarezsaurids were relatively small, bipedal theropod dinosaurs with feathered bodies. They had bird-like heads, small jaws with teeth, and well-developed forelimbs. One of their most distinctive features was the presence of a large claw on the forelimbs. The new discovery is particularly important for understanding the evolution of this group in South America. The researchers estimate that basal alvarezsaurids continued to exist in this part of the world until the end of the Cretaceous, before the mass extinction that marked the end of the dinosaur era. But the most surprising part of the discovery is related to the eggs themselves. Analysis of the shells showed that their microstructure did not match the previously known categories of dinosaur eggs. For this reason, the researchers proposed a new family of eggs, called Arraigadoolithidae.

Further electron microscopy revealed fossilized structures within the shells' canals. The researchers interpreted these structures as evidence of a fungal infection in dinosaur eggs, a finding they say represents the first evidence of its kind reported in dinosaur eggs. The fossil was discovered by paleontologist Jaime Powell of the Argentine Museum of Natural Sciences, and the species was later described and named in honor of paleontologist José Bonaparte, who had discovered the first alvarezsaurid in Patagonia in 1991. The discovery, published in the scientific journal Cretaceous Research, provides a rare glimpse into the life and reproduction of a mysterious group of dinosaurs near the end of the Cretaceous period.