Discovery of Alnashetri cerropoliciensis: Insights into Alvarezsaurs

Researchers have unveiled a nearly complete skeleton of *Alnashetri cerropoliciensis*, one of the smallest known non-avian dinosaurs, discovered in the La Buitrera fossil site in northern Patagonia, Argentina. Weighing approximately 0.7 kg (1.5 pounds) and measuring about 70 cm (28 inches) in length, this dinosaur is comparable in size to a crow. The fossil, which dates back 95 million years, provides critical insights into the evolutionary history of the alvarezsaurs, a unique family of theropod dinosaurs known for their small stature and specialized anatomical features.

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The discovery of *Alnashetri* challenges previous assumptions regarding the evolution of alvarezsaurs. Traditionally, these dinosaurs were believed to have evolved their diminutive size as an adaptation for an insectivorous diet, primarily targeting ants and termites. However, the new findings indicate that *Alnashetri* possessed longer arms and larger teeth than its later relatives, suggesting that miniaturization occurred before the development of specialized feeding adaptations. This indicates a more complex evolutionary trajectory for the alvarezsaurs than previously understood.

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The *Alnashetri* skeleton was first identified from fragmentary remains in 2012, but the nearly complete specimen found in 2014 allowed researchers to conduct a thorough analysis. The exceptional preservation of the fossil, attributed to rapid burial under sand, enabled detailed histological examinations of its bone structure. This level of preservation is rare and provides a clearer picture of the anatomy and lifestyle of these small dinosaurs.

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*Alnashetri* inhabited a desert environment, known as the Kokorkom, meaning "desert of the bones" in the indigenous Mapuche language. It likely preyed on small animals such as lizards, snakes, and invertebrates. The anatomical features of *Alnashetri*, including its slender limbs and well-developed forelimbs, suggest it was agile and adapted to its environment, moving across sandy terrains.

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The findings from *Alnashetri* highlight the biodiversity of the Cretaceous period, a time often characterized by large dinosaurs. According to Peter Makovicky, a lead researcher on the study, "Alnashetri shows us that it wasn't a time of giants, but rather a time of immense biodiversity." This discovery emphasizes the importance of small and medium-sized animals in prehistoric ecosystems, which are frequently overlooked in favor of larger species.

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Peter Makovicky noted the significance of the fossil, stating, “Going from fragmentary skeletons that are hard to interpret to having a near complete and articulated animal is like finding a paleontological Rosetta Stone.” Sebastián Apesteguía, another co-author, remarked on the exquisite preservation of the specimen, which has allowed for unprecedented insights into the evolutionary history of alvarezsaurs.

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Researchers are continuing their work at the La Buitrera site, which has proven to be a rich source of fossils, including other small vertebrates. The team anticipates further discoveries that could shed light on the evolutionary patterns of alvarezsaurs and their ecological roles during the Cretaceous period.

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• “Alnashetri is truly tiny. Weighing in around 0.7 kg (1.5 pounds), it is smaller than a chicken,” — Peter Makovicky, Paleontologist, University of Minnesota

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• “The level of histological detail is exquisite,” — Sebastián Apesteguía, Paleontologist, Felix de Azara Foundation

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• “' Alnashetri shows us that it wasn't a time of giants, but rather a time of immense biodiversity," he concluded.” — Sebastián Apesteguía, Paleontologist, CONICET

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