The small predatory dinosaur weighed less than a chicken

A nearly complete skeleton of a dinosaur that weighed less than a small chicken has provided new insights into the evolution of alvarezsaurs, which are among the smallest dinosaurs that ever lived.

The 95 million year old fossil of Alnashetri cerropoliciensis was found at the La Buitrera site in Northern Patagonia, Argentina, in 2014.

The first copy of Alnashetrifound in 2012, was a set of incomplete hind leg bones, says Peter Makovicky of the University of Minnesota, who was part of the study on the new fossil. With only fragmentary remains, it was impossible to say more than that it was probably an alvarezsaur. “We weren’t even sure if it was a juvenile or fully grown,” he says.

“With a complete skeleton, we suddenly had all the information to understand how Alnashetri was similar or different from other species, and a key to understanding how the unusual anatomy of alvarezsaurs evolved, says Makovicky.

The new fossil has very long, slender hind limbs and surprisingly long forelimbs that retain three well-developed fingers. Detailed analysis of the fossil bones showed that the dinosaur was an adult and at least 4 years old.

It is estimated to have weighed only 700 grams when alive. “The specimen is really small, smaller than a chicken,” says Mackovicky.

Alvarezsaurs were once thought to be early ancestors of birds. However, it is now clear that while Alnashetri may have had some superficial resemblance to a bird, it and all alvarezsaurs were actually non-avian theropods. “The new discovery certainly underscores this,” says Mackovicky.

Previously, all the tiny alvarezosaurs were thought to have very short, powerful forelimbs with a large thumb, but shrunken lateral digits and tiny teeth. Paleontologists thought these anatomical features evolved along with their shrinking body size because they only ate ants and termites, Makovicky says. “But Alnashetri doesn’t fit that mold—it’s among the smaller alvarezsaurs, but neither the teeth nor the front legs are reduced, because it represents a much earlier branch of the alvarezsaur evolutionary tree.”

In fact, the forearms are more typical of other theropods rather than a specialist anteater, he says. “Alnashetri is small, but is otherwise built like a more typical theropod – given its small size, it probably ate its share of invertebrates, but probably had a wider range of prey.”

That means paleontologists still don’t fully understand why these dinosaurs got so small. “We are left with only a vague sense that alvarezsaurs were successful in occupying the niches of very small carnivores,” says Mackovicky.

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