Small, Stubby-Armed Dinosaurs Have Confounded Paleontologists. Are Answers Finally Within Reach?

Tyrannosaurs catch a lot of teasing for their tiny arms. The fiddly, two-clawed appendages just look a little silly on carnivores of such imposing stature. But they were hardly alone—another enigmatic dinosaur family had comparably small forelimbs. Called alvarezsaurs, many of these mysterious creatures had such unusual arms that paleontologists are still puzzling over how their ridiculously stubby, big-clawed limbs evolved.

A new description of an especially odd alvarezsaur encapsulates that mystery. Late last year, scientists at long last studied and described the partial skeleton of the small dinosaur, which paleontologists from the Russian Academy of Sciences had excavated from the Gobi Desert in 1979. Named Manipulonyx, the Cretaceous animal possessed some of the strangest hands ever recorded among the “terrible lizards.” On its hand is a large, single claw, flanked by two tiny fingers and a thumb-like spur.

“The hand of Manipulonyx is the weirdest bit of dinosaur anatomy I’ve ever seen,” says University of Edinburgh paleontologist Steve Brusatte, who was not involved in the study.

The bizarre nature of the hand and claws highlights an enduring puzzle about why such odd traits evolved. While alvarezsaurs have been interpreted as ant-eaters before, the authors of the new study propose that Manipulonyx was an egg thief that pilfered nests and clutched the stolen eggs close to its body as it ran away. If true, the dinosaurs’ behavior would represent the latest in a long line of major adjustments in paleontologists’ understanding of these vexing species. Investigating the egg-snatching hypothesis might finally help crack the long sought-after details regarding how these strange dinosaurs lived.

Such dinosaurs were wholly unknown to experts until 1991. That year, working from a partial skeleton found in the roughly 84-million-year-old rock of Argentina, paleontologist José Bonaparte described a new, turkey-size dinosaur. He called it Alvarezsaurus after the Argentine historian Gregorio Alvarez. Not much was known of the dinosaur. All Bonaparte had to work from were parts of the spine, hips, shoulder, feet and hand. The Cretaceous reptile was unique enough to merit a distinct name, but at the time, it seemed to be a relatively generic, vaguely raptor-like dinosaur.

No one knew that Alvarezsaurus was merely the first named member of an entire group of oddities. In fact, fossils of these bird-like dinosaurs had already been collected by experts but had been virtually forgotten about in museum collections. It took the discovery of another creature, this time from Mongolia’s Gobi Desert, for paleontologists to begin drawing the threads together. Named Mononykus, or “one claw,” in 1993, the small dinosaur had fused wrist bones and a single claw at the end of a short, stout arm.

Related dinosaurs soon followed, such as Patagonykus from Argentina in 1996 and Shuvuuia from Mongolia in 1998. In less than a decade, Alvarezsaurus went from a seemingly unremarkable little creature to the namesake for an assortment of dinosaurs that spurs scientific debate to this day.

To understand what alvarezsaurs really were, paleontologists first wanted to figure out their relationship to other dinosaurs. In the 1990s, when alvarezsaurs were initially coming to the attention of experts, paleontology was undergoing a rapid shift as researchers sought fossils that embodied the evolutionary transition from dinosaurs to modern birds. Alvarezsaurs resembled birds so much that determining exactly how they related to both birds and other dinosaurs was a priority—and a challenge.

Even before alvarezsaurs were officially named, experts had been struck by how bird-like they seemed. Alvarezsaur fossils excavated from the Gobi Desert in the 1920s were shelved with the label “bird-like dinosaur.” And when paleontologists named Mononykus, they first announced it as a flightless bird more closely related to modern avians than the iconic feathered dinosaur Archaeopteryx is.

The alvarezsaur Shuvuuia—from shuvuu meaning “bird” in Mongolian—was also placed closer to birds than other dinosaurs when it was unveiled. Its fused wrists, broad sternum for strong arm muscle attachments and the possible flexion of its skull bones were all traits that seemed bird-like. A Shuvuuia fossil described in 1999 was uncovered with feathers, putting alvarezsaurs among the first dinosaurs to be found with that trait.

But it wasn’t immediately clear how the strange, feathery animals evolved. The first dinosaurs from this group to be discovered had all lived near the end of the Cretaceous—it took years for experts to uncover older ancestors that began to fill out missing details, revealing that alvarezsaurs weren’t birds, after all.

In 2010, for example, paleontologists named the early alvarezsaur Haplocheirus, or “simple hand,” from Jurassic fossils found in China. The dinosaur lived tens of millions of years before the likes of Mononykus and Shuvuuia—an animal roughly twice the length and five times as heavy as some of its later relatives. Haplocheirus possessed longer arms with three-clawed hands and a mouth full of pointed teeth—marked differences from the Cretaceous species—but it also had subtle anatomical characteristics shared with later alvarezsaurs that solidified its place in the group.

From such a carnivorous ancestor, alvarezsaurs became smaller and more specialized over time. Some alvarezsaur bones evolved in the same way as birds; for example, chest bones modified for powerful muscles appeared in both groups. But while birds used them for flying, the little dinosaurs used them for digging—an example of convergent evolution, or a shared trait that appears independently in different parts of the tree of life. In this way, the alvarezsaurs evolved from species that resembled predatory raptors into an array of small, slender creatures with incredibly specialized arms.

Now, as dozens more alvarezsaurs have been found in the rocks of Asia, North America, South America and Europe, paleontologists are trying to answer questions about their behavior. Studies of Shuvuuia’s eyes and ears hint that it was a nocturnal animal with hearing acuity rivaling that of a modern barn owl. But what such dinosaurs might have been hunting in the night, experts still aren’t sure.

“The dietary ecology of alvarezsaurs has not been studied in as much detail as other dinosaur groups,” says paleontologist Michael Pittman of the Chinese University of Hong Kong, who was not involved in the new study. Early forms, such as Haplocheirus, seem to have been meat-eaters. The real mystery lies among the later forms, like Shuvuuia and the newly described Manipulonyx, which bore a heavily modified central finger with a massive claw.

In recent years, paleontologists have repeatedly speculated that alvarezsaurs were prehistoric ant-eaters. The short, powerful arms with large claws appeared to be a dinosaurian equivalent to the limbs of modern anteaters, pangolins and other mammals that dig into nests of social insects. Termites and ants made their nests in the Cretaceous just as they do today, sometimes even in dinosaur bones, so the food source was certainly there for any prehistoric predators of ants. The fact that early alvarezsaurs had pointed, gripping teeth and later species possessed only small teeth has been taken as another indication that the later dinosaurs were slurping up insects rather than pursuing meatier prey.

To date, however, paleontologists have not found direct evidence that alvarezsaurs were gobbling up ants or termites. The fossil evidence could still be out there. Perhaps a preserved termite nest with distinctive claw marks could confirm the connection—or, even better, experts could uncover insect carapaces packed into fossilized feces that could have been left by such a dinosaur. Gut contents would be the most conclusive evidence, preserving what an alvarezsaur was dining on prior to death. Paleontologists could even analyze the scratches on dinosaur teeth or look for geochemical clues about diet in the dinosaurs’ bones, Pittman says. But the researchers behind the new Manipulonyx fossils propose a different diet for the later alvarezsaurs.

Even though the arms of alvarezsaurs look like anatomical trowels, they were held close to the body of the dinosaur. To dig into a nest, a dinosaur like Manipulonyx would have had to crouch and stoop down to press its chest toward the earth—perhaps risking stings, bites or other retaliation from the insects. Instead, the new paper suggests, the arms of alvarezsaurs were best suited to gripping eggs and clasping them to the dinosaur’s side. Rather than raiding insect nests in the cool of the night, Manipulonyx might have snatched an egg away from a dinosaur nest, run off to a safe spot, punctured the shell with its claw and slurped up the goop inside.

Pittman calls the new hypothesis “thought provoking,” though he notes that egg manipulation might not have been about diet. Perhaps alvarezsaurs really did use their stout arms to move eggs—only the eggs might have been their own. Several theropod dinosaurs—such as parrot-like oviraptorosaurs and predatory troodonts—have been found sitting atop nests of eggs. Maybe alvarezsaurs evolved a unique way of tending their nests, related to their arms, that only the dinosaurs themselves could demonstrate.

“There are many ideas for why alvarezsaurids had such reduced arms and hands,” from eating insects to the egg-cracking hypothesis, Brusatte says. But the truth could be an explanation that no one has even thought of yet. “Maybe they were doing something with their arms that we can’t even envision, that was so unique, so bizarre, so singular.”

Alvarezsaurs have been found in association with fossilized eggs before. Egg-eating certainly is within the realm of possibility for these strange dinosaurs. But conclusive evidence has yet to be revealed. Given that earlier species like Haplocheirus were likely eating different foods than their later, more specialized relatives were, paleontologists aren’t just looking to uncover the diet of these dinosaurs—they hope to learn how the animals’ anatomy shifted along with their changing food preferences.

Experts didn’t know anything at all about alvarezsaurs 40 years ago. In that time, the group has been exhumed and argued over—and it’s still confounding researchers. The dinosaurs are big mysteries in small packages, sure to reveal new secrets about what the world was really like when feathery reptiles bobbed along the nighttime deserts in search of a snack.

With so little known about them, alvarezsaurs are open to interpretation. “Let your imaginations run wild,” Brusatte says, “at least until we find a fossil with gut contents or some more direct proof of what these wacky animals were eating.”