The nearly complete skeleton of a small dinosaur, weighing less than a domestic chicken, has provided significant insights into the evolution of Alvarezaurus, one of the smallest dinosaur species ever documented.

This 95 million-year-old fossil, identified as Arunashetri seropolisiensis, was unearthed in 2014 at the La Buitrera site in northern Patagonia, Argentina.

The first specimen of Arunashetri consisted of incomplete hind limb bones found in 2012. Peter Makowiecki from the University of Minnesota contributed to the research on this new fossil. At the time, only fragmentary remains were available, leaving the classification as a probable alvaresaurid. “We didn’t even know if it was a juvenile or an adult,” says Makowiecki.

“With the entire skeleton now complete, we suddenly have all the information required to understand how Arunashetri functioned. Its anatomical structure is both similar and distinct from other species, providing key insights into the evolution of Alvarezsaurus’s unusual anatomy,” notes Makowiecki.

The new fossil features very elongated and slender hind limbs, along with unexpectedly long forelimbs equipped with three well-developed fingers. Detailed analysis revealed that this dinosaur was an adult, estimated to be at least four years old.

It is believed that it weighed only 700 grams in its lifetime. “These specimens are incredibly small, even smaller than a chicken,” explains Makowiecki.

Previously, Alvarezsaurus was thought to be an early ancestor of birds. Recent findings clarify that while Arunashetri may have superficially resembled a bird, it, along with all Alvarezsaurus, are indeed non-avian theropods. “This new discovery confirms our understanding,” states Makowiecki.

It was once believed that all small alvaresaurids had short, robust forelimbs with prominent thumbs but reduced lateral digits and small teeth, interpreted as evolutionary adaptations for a diet of ants and termites. However, as Makowiecki points out, “Arunashetri does not fit that mold. Although it is a smaller member of the Alvarezauridae family, it signifies a relatively early branch in the evolutionary tree, hence its teeth and forelimbs remain quite substantial.”

In fact, he adds, its forelimbs are more characteristic of other theropods rather than anteater specialists. “Arunashetri, while smaller, structurally resembles a typical theropod. Considering its size, it likely consumed a significant variety of invertebrates, along with a broader prey spectrum,” he further explains.

This indicates that paleontologists still lack a full understanding of why these dinosaurs evolved to such small sizes. “We are left with a vague understanding that Alvarezsaurids successfully occupied a very small predator niche,” concludes Makowiecki.