'Exquisite' fossil of one of the smallest dinosaurs found in Argentina

In Argentina's Patagonia region, 95 million years ago, some huge dinosaurs roamed the landscape, including fearsome meat-eater Giganotosaurus, at about eight tons, and immense long-necked plant-eater Argentinosaurus, perhaps 70 tons. But this was no mere land of the giants, as a newly described fossil shows.

Researchers have found a well-preserved and nearly complete skeleton of one of the world's smallest-known dinosaurs, named Alnashetri cerropoliciensis. It was about the size of a crow and probably hunted small animals like lizards, snakes, mammals, and invertebrates.

The fossil, preserved ‌with the bones positioned as they would have been in ​life, offers insight into alvarezsaurs, an unusual family of dinosaurs within the group called theropods that spans all the meat-eating dinosaurs.

This specimen, given the nickname "Alna," was unearthed in sandstone at a site called La Buitrera in northern Patagonia's Rio Negro Province that has yielded many fossils of small- and medium-sized animals from the Cretaceous Period.

Alna was a small female that lived in a desert environment and died after reaching age four, so almost fully grown. After dying, ​Alna's body was quickly covered by a sand dune, accounting for its fine level of preservation.

Apart from birds, which evolved from small feathered dinosaurs, Alnashetri is the most diminutive dinosaur known from South America and rivals the smallest ones discovered globally.

"Alnashetri is truly tiny. Weighing in around 0.7 kg (1.5 pounds), it is smaller than a chicken," said University of Minnesota paleontologist Peter Makovicky, lead author of the research published on Wednesday in the journal. Nature. "It wouldn't even reach knee ‌height on an average adult person."

Alvarezsaurs were predominantly small, with stubby but powerful forelimbs, long and gracile hindlimbs, and lightly built skulls. The researchers suspect Alnashetri was feathered, based on fossils of other alvarezsaurs. Despite some birdlike traits, alvarezsaurs ‌were only distantly related to birds.

Alna dwelt in a locale called the Kokorkom, meaning "desert of the bones" in the local indigenous language of the Mapuche people.

"Although many of the inhabitants of the Kokorkom Desert were burrowers, Alnashetri was a lightweight animal that moved across the dunes on its slender legs. Its body resembled that of a rooster, but with a long tail," said paleontologist ‌and study co-author Sebastián Apesteguía of the Felix de Azara Foundation and National Scientific and Technical Research Council in Argentina, ‌or CONICET.

"Its arms were well-developed, though not long enough to allow it to fly, and its tail, although not ‌fully preserved, appears to have been as long (relative to body size) as that of any other typical carnivorous dinosaur," Apesteguía said, making Alnashetri ‌about 28 inches (70 cm) in length, most of it tail.

Alna's thin and fragile skeleton was so well preserved that the researchers were able to conduct a histological examination, studying microscopic bone structures.

"The level of histological detail is exquisite," Apesteguía said.

Its pointy teeth were numerous and strong, like those of a small Velociraptor. Later Alvarezsaurs from Argentina and other parts of the world possessed tiny teeth and reduced arms equipped with a large claw, presumably used for digging termite mounds as part of an insectivorous lifestyle.

Alna shows there were very small Alvarezsaurs without an insect-eating specialization, and that size reduction evolved multiple times in this lineage, Apesteguía said.

The first Alnashetri remains ever found were two incomplete legs discovered in 2004 at La Buitrera. The current specimen was discovered in 2014, before undergoing 12 years of preparation and study.

Patagonia is one of the world's hot spots for fossils of dinosaurs, large and small. La Buitrera has been a goldmine for fossils of small vertebrates such as the limbed early snake Najash, saber-toothed mammal Cronopio, and small herbivorous reptile Priosphenodon, as well as ​the small dinosaurs Jakapil and Buitreraptor.

"When we think of landscapes with dinosaurs, or through the lens of film fiction, we picture vast ​expanses with enormous beasts roaming in the distance. But these ​landscapes are almost always devoid of a crucial component of the ecosystem: medium and small animals," Apesteguía said.

"The era in which Alnashetri, one of the smallest dinosaurs, lived coincided with what ​we often call the 'age of the southern giants.' Alnashetri shows us that it wasn't a time of giants, but rather a time of immense biodiversity," Apesteguía said.