Source: Ars Technica

Diet Plan

Small size seems to have come before a change in diet for a tiny dinosaur lineage.

Alvarezsaurids were mostly small‑bodied theropods that paleontologists originally misinterpreted as early flightless birds, only to later recognize them as an ant‑eating lineage of non‑avian dinosaurs. For years, we suspected that alvarezsaurids underwent a rare process of evolutionary miniaturization directly coupled to a diet of social insects like ants and termites. It was a tidy hypothesis: they got smaller to become more efficient at catching ants.

Now, a recently discovered fossil of one of the smallest alvarezsaurids ever found suggests that the evolution of miniature dinosaurs likely wasn’t as neat and linear as we thought. This new species, called Alnashetri cerropoliciensis, probably did not feed on ants at all.

“It was a pursuit predator actively hunting insects and small mammals,” said Peter Makovicky, a paleontologist at the University of Minnesota.

The oddball

Alvarezsaurids, found mostly in the Late Cretaceous rocks of Asia and South America, had short forelimbs tipped with a single oversized thumb claw built for digging. They also possessed minute teeth and sensory adaptations akin to those of modern nocturnal birds—everything necessary to work on termite mounds.

“The explanation of their small body size has been tied to this specialization,” Makovicky explained.

The dinosaur he and his colleagues discovered, however, did not look like a specialized ant‑eater.

The fossil of Alnashetri cerropoliciensis was unearthed from the Candeleros Formation at the Cerro Policía locality in Argentina’s Río Negro Province and is estimated to have lived roughly 90 million years ago. It currently stands as the most complete and smallest alvarezsaurid skeleton found in South America.

Although the specimen is missing its skull roof, parts of its right arm, its lower right leg, and much of its tail, the skeleton preserves plenty of crucial anatomy. Its bone tissue reveals that the alvarezsaurid was a subadult, likely approaching sexual maturity, as indicated by the presence of what appears to be medullary bone—a temporary tissue associated with egg‑laying in modern birds. Despite being nearly fully grown, this dinosaur is estimated to have weighed a mere 700 g.

The real surprise, though, came when researchers realized that Alnashetri wasn’t a highly specialized, late‑stage alvarezsauroid. Instead, despite living in the Late Cretaceous, it occupies an early‑branching position among basal members of the clade.

This combination of tiny size and early‑branching status fundamentally breaks our previous model of how these animals evolved. If the miniaturization of alvarezsauroids was strictly tied to their lifestyle as stubby‑armed insect‑eaters, an early‑diverging species like Alnashetri should display transitional features on a steady, clade‑wide march toward that extreme endpoint. But it did not look that way.

“It’s a very long‑limbed animal, so it was probably fairly fast. My best analogy would be something like a roadrunner from the American West,” Makovicky said.

Arms and Teeth

Late Alvarezsaurids had tiny, robust forelimbs that were less than half the length of their femurs. Alnashetri, however, sported comparatively long forelimbs that were 61 % of the length of its entire hindlimb. While it possessed three‑fingered hands with a robust first digit—a hallmark of its group—it still retained slender second and third digits, unlike its later cousins.

Other features that challenge the established evolutionary model of miniature dinosaurs are Alnashetri’s jaws and teeth. Its dentition includes non‑serrated teeth set into sockets, but, importantly, these teeth are not extremely small, as they were in late Alvarezsaurids such as Shuvuuia or Jaculinykus.

“This decoupled the evolution of small body size from anatomical specializations,” — Makovicky

The team concluded that extreme miniaturization in Alvarezsaurids did not necessarily co‑evolve with either the evolution of smaller arms (more suitable for digging) or small teeth built for crushing ants and/or termites. Instead of a clade‑wide trend in which the entire lineage steadily shrank over time, a new evolutionary model that includes Alnashetri suggests that Alvarezsaurid body mass fluctuated repeatedly. Alnashetri achieved its ≈ 700 g frame independently from other highly specialized alvarezsaurid species.

But Alnashetri didn’t just upend the understanding of how Alvarezsaurids evolved their tiny bodies; it also redrew the map of where they lived.

Museum Tour

Before Makovicky’s study, it was a mystery why alvarezsaurids were found almost exclusively in Late Cretaceous rocks of Asia and South America. The prevailing hypothesis suggested that the group dispersed back and forth between these two landmasses relatively late in their history. However, placing Alnashetri—a remarkably basal member—into the evolutionary tree created a massive ghost lineage. The phylogenetic analysis linked geographically close South‑American species to much older, geologically distant Asian taxa such as Bannykus and Xiyunykus, implying that the group must have diverged back in the Jurassic.

Re‑examining Museum Collections

To explain this chronological and geographic gap, Makovicky and his colleagues combed historical museum collections for early alvarezsaurids that might have been misidentified. Their effort paid off:

• North America: A small, fragmentary theropod from the Upper Jurassic Morrison Formation was re‑identified as an early alvarezsaurid.
• Europe: A Lower Cretaceous specimen from the Isle of Wight received the same reassignment.

Both taxa display distinctive features—most notably specialized ball‑and‑socket joints in the neck vertebrae—that are unique within the alvarezsaurid clade. These re‑identifications completely reshaped the biogeographical narrative.

A New Biogeographic Model

If alvarezsaurids were already roaming North America and Europe in the Jurassic and Early Cretaceous, they were not merely executing a late‑stage migration between Asia and South America. Makovicky’s team proposes a widespread Pangaean distribution, with early alvarezsaurids present across the globe before the supercontinent fully fractured.

Consequences of this model:

1. Late‑Cretaceous distributions (the pattern we see in the fossil record) result from populations becoming isolated as continents drifted apart.
2. Regional extinctions eliminated alvarezsaurids from areas such as North America and Europe.
3. Surviving pockets persisted in Asia and South America, explaining their later abundance there.

Remaining Questions

Makovicky’s work opened many new avenues of inquiry. If at least some alvarezsaurids did not evolve their miniature bodies as an adaptation for eating ants, what drove their diminutive size?

Messy Evolution

“We sort of falsified this nice narrative where Alvarezsaurid body‑size change was driven by ecology, but unfortunately, we don’t have anything good to replace it,” Makovicky acknowledged.

The classic story of alvarezsaurids—a lineage steadily shrinking in lockstep as it committed to a life of termite‑hunting, finally migrating across the Late Cretaceous globe—was neat and logical, but it’s apparently gone now.

“That’s science. Sometimes you can falsify a hypothesis without necessarily finding a better one to support,” Makovicky added.

His team is already busy looking for evidence documenting a new, more complex and messier version of alvarezsaurid evolutionary history. “We have a couple of angles we’re pursuing,” he said.

First angle: CT scanning Alnashetri

The team is taking a closer look at Alnashetri’s anatomy using CT scans. The goal is to treat Alnashetri as a starting point for understanding the stepwise evolution of its ant‑eating, specialized cousins. Most of this meticulous scanning is currently happening in Argentina.

Second angle: A larger alvarezsaur

“By pure luck, we found another alvarezsaur in the same general area,” Makovicky said. This specimen is larger than Alnashetri and has slightly shorter forelimbs.

“It’s still being prepared, but I think it will give us the next chapter in the story of how alvarezsaurids evolved,” he explained. “It’s probably a few years out in the making.”

Makovicky’s work on Alnashetri is published in Nature:
https://doi.org/10.1038/s41586-026-10194-3

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About the author

Jacek Krywko is a freelance science and technology writer who covers space exploration, artificial intelligence research, computer science, and all sorts of engineering wizardry.

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