Reconstruction of Alnashetri cerropoliciensis. Image courtesy of Gabriel Díaz Yantén, Universidad Nacional de Río Negro.
Joel Kontinen
Not all dinosaurs were big, some were relatively small. They
weighed less than a small chicken.
The 95-million-year-old fossil of Alnashetri cerropoliciensis was
found at the La Buitrera site in northern Patagonia, Argentina, in 2014.
The first specimen of Alnashetri, found in 2012, was a
set of incomplete hindlimb bones, says Peter Makovicky at 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 were not even sure if it was a juvenile or fully grown,” he
says.
“With a whole skeleton, we suddenly had all the information
to understand how Alnashetri was similar or differed 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 revealed the dinosaur was an adult and at least 4
years old.
Alvarezsaurs were once thought to be early ancestors of
birds. However, it is now clear that, while Alnashetri might have had
some superficial resemblance to a bird, it and all the Alvarezsaurs were, in
fact, non-avian theropods. “The new discovery certainly underscores this,” says
Mackovicky.
Some evolutionists think that dinosaurs have been descended from
birds.
Previously, it was thought that all the tiny alvarezsaurs
had very short, stout forelimbs with a large thumb but shrunken side digits,
and tiny teeth. Palaeontologists thought these anatomical features evolved
alongside their shrinking body size because they only ate ants and
termites, says Makovicky. “But Alnashetri does not fit that
mould – it is among the smaller Alvarezsaurs, but neither its teeth nor its
forelimbs are reduced, because it represents a much earlier branch on the Alvarezsaur evolutionary tree.”
In fact, its forearms are more typical of other Theropods
rather than a specialist ant-eater, he says. “Alnashetri is tiny but is
otherwise built like a more typical Theropod – given its small size, it
probably ate its fair share of invertebrates, but probably had a wider range of
prey.”
That means palaeontologists still don’t fully understand why
these dinosaurs became so small. “We’re left with only a vaguer sense that Alvarezsaurs were successful at occupying the niches of very small predators,”
says Mackovicky.
Source:
James
Woodford 2026
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