Early Alvarezsaurid Discovery Rewrites Dinosaur Evolution

A newly analyzed fossil of Alnashetri cerropoliciensis, an early member of the Alvarezsauroidea group, is challenging long-held beliefs about the evolution of these unique theropod dinosaurs. Researchers have found that this Late Cretaceous dinosaur doesn’t fit the expected pattern of miniaturization and specialized features seen in later Alvarezsaurids, forcing a re-evaluation of the clade’s evolutionary history.

Challenging the Established Model

Traditionally, Alvarezsaurids were thought to have steadily evolved towards smaller body sizes and highly specialized forelimbs adapted for digging and insect-eating. Still, Alnashetri, dating back to the Cenomanian stage (approximately 97 million years ago) of the Late Cretaceous, presents a different picture. The discovery indicates that the miniaturization observed in later Alvarezsaurids wasn’t a consistent, linear process across the entire lineage.

A Roadrunner-Like Dinosaur

Unlike the stubby-armed, insectivorous forms that came later, Alnashetri possessed relatively long limbs, suggesting it was a swift runner. Paleontologist Dr. Makovicky likened its build to that of a roadrunner, indicating a more generalized lifestyle than the highly specialized adaptations of later Alvarezsaurids. Its forelimbs were 61 percent of the length of its hindlimb, significantly longer than the less than half-length forelimbs seen in later species.

Teeth and Jaws: Further Deviations

Further complicating the evolutionary narrative are Alnashetri’s jaws and teeth. While it retained the three-fingered hands with a robust first digit characteristic of the group, its teeth were not the extremely modest, crushing teeth found in later Alvarezsaurids like Enthusiasm or Jaculinykus. Instead, Alnashetri had non-serrated teeth set in sockets, but of a more typical size for a theropod.

Fluctuating Body Mass and Independent Miniaturization

The findings suggest that extreme miniaturization in Alvarezsaurids didn’t necessarily co-evolve with the development of smaller arms for digging or specialized teeth for insectivory. Instead, the evolutionary model now incorporates fluctuating body mass throughout the group’s history. Alnashetri achieved its 700-gram frame independently of the other, highly specialized alvarezsaurid species.

Discovery Details

Alnashetri cerropoliciensis is known from the holotype MPCA-477, consisting of articulated hind limbs discovered in 2005 at the “Hoyada de los esfenodontes” sublocality of La Buitrera, Río Negro Province, Argentina. The fossils were recovered from the Candeleros Formation. A more complete skeleton was discovered in 2014 and described in 2026. The site has also yielded fossils of other Cretaceous creatures, including the rhynchocephalian Priosphenodon avelesi, the snake Najash rionegrina, and the dromaeosaurid Buitreraptor gonzalezorum.

Source: Wikipedia – Alnashetri