In December 2025, Russian scientists published an analysis of a 67-million-year-old dinosaur fossil that was found in the Gobi Desert in 1979. The fossil specimen belonged to a small, two-legged dinosaur named Manipulonyx reshetovi, a part of the Alvarezsauridae family.

Alvarezsaurids looked like your standard therapod dinosaur. With many species being around the size of a dog, they scurried around the Cretaceous Period on two long legs and boasted stubby forearms. 

“The configuration of every alvarezsaurid forelimb is a little bit different,” said Tracy Thomson, a research assistant in the UC Davis Department of Earth and Planetary Sciences. “Some of them have a single digit. Some have two digits. Even though they’re all really reduced, they are all different." 

Paleontologists have long debated the functional purpose of alvarezsaurid forelimbs. In their study, the Russian researchers examining Manipulonyx reshetovi suggested that the species specialized in egg eating, using its stubby digits and long claws to grasp and puncture eggs. “These animals were nocturnal hunters of dinosaur eggs,” the researchers wrote.   

That explanation seemed far-fetched to Thomson, who did his Ph.D. dissertation on the functional morphology of vertebrate claws. 

“Thinking of all the things that modern animals use their claws for, never once did I ever come across an animal that used its claws to open an egg,” Thomson said. “So that got my juices flowing.” 

Rather than being used to crack open eggs, Thomson thinks Manipulonyx reshetovi could have used their stubby forelimbs for social signaling. The hypothesis, which Thomson is laying out in a paper he’s currently writing, is based on observations of modern-day animals and their behaviors.

“If you’re going to propose that as a hypothesis, it needs to be grounded in probabilities of what modern ecosystems and animals do,” he said. 

Why strictly egg eating is rare among modern vertebrates 

The first problem Thomson highlighted is that strictly feeding on eggs, also known as obligate ovivory, is relatively rare in modern vertebrates. The list is exclusive to snakes with about 19 species specialized for egg eating. 

“You don’t need limbs to eat eggs and snakes are the proof of that,” Thomson said.  

Other vertebrates, including mongooses, raccoons and seriema birds, are opportunistic ovivores.  

“Seriemas will pick up the egg in its beak and throw it on the ground really hard, and they’ll get at the eggs that way,” Thomson said. “The mongoose, what they’ll do, is they’ll grab an egg and hike it like a football through their hind limbs at a rock or a wall and break the egg.”

“But again, no specialized claws,” he added. “It’s behavioral what they’re doing, but there’s no special structure involved.”  

This lack of evidence for modern-day vertebrates using claws to crack open eggs led Thomson to wonder, what do modern animals use their forelimbs and claws for that’s specialized?

Modern animal clues: pond turtles and raptors 

Combing through the scientific literature, Thomson created a meticulous database that documents how myriad vertebrate animals use their forelimbs and claws. 

And he may have found a model for alvarezsaurid’s stubby forelimbs and claws in modern pond turtles and birds of prey. 

Male pond turtles use their forelimbs and claws to engage in a courtship behavior called titillation.

“The males will actually touch and stroke and vibrate their forelimbs around the head of the female,” Thomson said. “The males have these really long claws that they use to touch the female in specific ways.”

Thomson has identified around 26 species of pond turtles that engage in this courtship behavior, and each ritual is species-specific.  

He’s also identified 66 species of raptors that use their claws in a social behavior called cartwheeling.

“What these raptors do is they will approach each other, they'll fly upside down, grab each other's talons, and then they'll just cartwheel down like a pinwheel, freefalling down and before they hit the ground, they'll release each other and fly off,” Thomson explained.   

The behavior serves multiple social functions, being used for courtship, family-bonding and even adversarial confrontations. 

“Throughout the animal kingdom, social signaling is all over the place,” Thomson said. “And it can be multimodal. There’s visual, there’s audio, there’s touch, there’s smell, all of these things are coming together in very specific, complex ways so that animals can identify and communicate with each other. They can greet each other, they can bond, they can fight, they can mate, all of these complex social interactions.” 

Using modern animals to understand extinct species

For Thomson, the differences in sizes and shapes of alvarezsaurid forelimbs and claws hints at a social function.  

“I would propose that the default explanation should be social interaction, whatever that is, because there’s so much of it throughout the animal kingdom, and it’s so diverse and so complex,” he said. 

Rather than infer how an extinct creature used a part of its body based solely on fossils, Thomson advocates for using modern animals as analogs for understanding the functional morphology of extinct creatures. 

“We as human beings are sometimes so desperate to understand what's going on that we will even make up or accept explanations that may not be justified or even reasonable, that may be “just so” stories that that aren't really testable in a scientific way,” Thomson said. “They’re untestable narrative explanations.” 

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