The tiny predatory dinosaur weighed less than a chicken

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

95 million year old fossil Alnashetri cerropiciensis was found in 2014 at the La Buitrera locality in northern Patagonia, Argentina.

First specimen Alnashetrifound in 2012, was a set of incomplete hind limb bones, he says Petr Makovicky at the University of Minnesota, who was part of a study on the new fossil. With only fragmentary remains, no more could be said than that it was probably an Alvarezsaurus. “We weren’t even sure if it was a baby or a full-grown adult,” he says.

“With the whole skeleton, we suddenly had all the information to understand how.” Alnashetri was similar or different from other species and the 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 toes. Detailed analysis of the fossil bones revealed that the dinosaur was an adult and at least 4 years old.

It is estimated that it weighed only 700 grams when it was alive. “The specimen is really tiny, smaller than a chicken,” says Mackovický.

Alvarezsaurs were once thought to be the 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 underlines this,” says Mackovický.

Previously, all small alvarezsaurs were thought to have very short, stocky forelimbs with large thumbs, but contracted lateral toes 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 it doesn’t fit the mold – it’s a smaller alvarezsaur, but it doesn’t have reduced teeth or forelimbs because it represents a much earlier branch on the alvarezsaur evolutionary tree.”

In fact, its forearms are more typical of other theropods than of a specialized anteater, he says. “Alnashetri it’s tiny, but otherwise built like a more typical theropod—it probably ate quite a few invertebrates for its small size, but probably had a wider range of prey.”

That said, paleontologists still don’t fully understand why these dinosaurs got so small. “We were left with the vague feeling that alvarezsaurs were successful in occupying the niches of very small predators,” says Mackovický.

topics: