Thank you to Wondrium for Supporting PBS 70 million years ago, a chicken-sized dinosaur roamed what’s now Mongolia.

Today, we call it Mononykus, and it was a little over a meter long and light-bodied thanks to its hollow bones.

It was probably covered with shaggy feathers and each of its stubby arms ended in just one functional claw.

Now, Mononykus was a theropod, a group of dinosaurs famous for being fierce predators -- picture T. rex or a Velociraptor.

But those short arms with their single claws might point to a pretty different lifestyle for this animal.

We often think of dinosaurs as either preying on other dinos or mammals, or as plant-eaters -- but in ecosystems today, those aren’t the only two options.

So why would we expect dinosaurs to have only been carnivores or herbivores, with the occasional omnivore thrown in the mix?

Well, it looks like Mononykus and its relatives might’ve carved out their own ecological niche - one that only really became possible with the rise of social insects, like ants and termites, in the Cretaceous Period.

Because, some paleontologists think these dinosaurs were specialized insect-eaters.

And the origin and spread of social insects would not only push these dinos toward a new diet, but it would also come with a surprising consequence.

Instead of getting bigger, like many other theropods in the Cretaceous, these dinos shrunk.

Mononykus belonged to a group of dinosaurs called the alvarezsaurs, which are thought to have originated in Asia and would ultimately include 21 species over time.

They first appeared in what’s now China during the Late Jurassic Period, about 160mya And they were already small compared to most other theropod dinosaurs.

Most of the alvarezsaurs we’ve found weighed less than 50 kilograms.

But the earliest members of this group looked pretty different from Mononykus.

They had longer, chunkier forearms and teeth that were specialized for eating meat.

One of these early alvarezsaurs was Haplocheirus.

It could reach a little over 2 meters long and weigh 41 kg on average - about the weight of a large dog, like a male German Shepherd.

With its long legs and long forearms with three clawed fingers on each hand, Haplocheirus could chase down and grab prey.

But its skull features point to a relatively weak bite, so Haplocheirus might've already been feeding on smaller prey than many of its theropod cousins.

And in the same area lived Aorun.

It also had three claws on each hand and more finely serrated teeth.

We’ve even found a third alvarezsaur from this same formation, dating to around the same time as the other two.

While it’s clear that alvarezsaurs were already diverse during the Late Jurassic, they were all probably carnivores like most other theropods -- they just seemed to be after smaller prey.

And in the Early Cretaceous Period, their evolution would take an interesting turn.

Beginning about 130 million years ago, we find two more alvarezsaurs in China.

They were probably smaller than Haplocheirus, but not tiny, and they still had three claws on their hands.

And one of these dinos had an enlarged thumb with an extra-stout claw, dwarfing the other two, shorter fingers.

Paleontologists aren’t sure what they ate, as their forearms were in between those of their carnivorous ancestors and the later specialized insectivores.

But they did have two enlarged muscle attachment areas - one on their humerus and one on their ulna - that are similar to what we see in digging mammals today.

These might’ve helped them make powerful arm movements.

Paleontologists have ruled out burrowing, though, because their forearms were too short and their hind limbs were too long for a burrower.

Instead, they were probably used for digging into harder surfaces - maybe taking the first steps toward foraging for insects.

Those two dinosaurs fill the gap in the fossil record between their Jurassic ancestors and the Late Cretaceous alvarezsaurs, and their transitional features hint at what was to come.

By the Late Cretaceous, alvarezsaurs had dispersed to other continents.

And that’s when we see another major change in their anatomy: these dinosaurs were shrinking in body size and losing their smaller fingers altogether.

Around 86 million years ago, Alvarezsaurus lived in what’s now Argentina alongside other members of its family that had radiated in South America.

And the whole group is named after this species, because it was the first one discovered.

Alvarezsaurus had tiny arms and just one functional clawed finger.

While there is some debate about it, paleontologists think that Alvarezsaurus was an insectivore that used its finger claw to rip into logs in search of termites.

And overall, alvarezsaur body size dropped dramatically from the Jurassic to the Late Cretaceous.

The ancestral Jurassic alvarezsaurs in China are estimated to have averaged 23 kg in body mass.

By 85 million years ago, these dinosaurs averaged less than 5 kg, and most were less than 2kg.

And those second and third fingers had all but disappeared on many species, including Alvarezsaurus in Argentina and Mononykus in Mongolia, leaving just one functional large-clawed finger.

Not only that, but in Mononykus, carnivorous teeth had been replaced by tiny, simple teeth like those of some ant-eating mammals today, including aardvarks.

So, by the Late Cretaceous, the alvarezsaurs had miniaturized, while evolving stubby front arms with a single, clawed finger and simple teeth.

But why?

Their miniaturization happened really quickly by evolutionary standards -- a three-fold decrease in size over 25 million years.

And it’s the opposite of what happened in many other groups of dinosaurs in the Cretaceous.

They all got bigger by orders of magnitude, not smaller.

For example, the tyrannosaurids outgrew their competition and became apex predators.

The only other group of dinosaurs known to have shrunk in size are those from the lineage that evolved into birds.

So, what pushed the alvarezsaurs towards developing smaller body sizes?

Well, a study published in 2021 proposed that it was the transition to eating insects that shrunk them down and drove the adaptations in their hands, claws, and teeth.

When flowering plants arose and started to spread during the Cretaceous, it spawned an explosion of insects, including social ones, like termites and ants.

Paleontologists call this period of increased diversification of life on land the Cretaceous Terrestrial Revolution.

Termites evolved from cockroaches in the Early Cretaceous, but based on fossil evidence, their social lifestyle started to flourish later as part of this Revolution.

And ants also got a boost during the rise of flowering plants.

Ants first show up as fossils in the Early Cretaceous, but they radiated and became more abundant between 100 and 60 million years ago.

And these thriving social insect colonies that appeared in the Mid to Late Cretaceous might have made a new lifestyle possible for some dinosaurs.

The shrinking of the alvarezsaurs lines up neatly with the diversification of ants, termites, and bees.

Not only that, but as they got smaller, they split into even more species than ever before.

And their short, clawed forelimbs became adapted to dig into wood to find ants or termites underneath.

For example, one species from Mongolia about 80 million years ago is pegged as a termite specialist that dug into nests, based on its short arms and broad, flat hands, tipped with a single claw.

And another species that lived in Canada 68.5 million years ago looked like it was adapted to feed on wood-nesting termites, using its strong but short arms to dig them out of logs.

Trace fossils of termite borings were also found in the same formation.

It isn’t just their hands and arms that give us evidence of this changing diet, either.

Other features provide support for these dinosaurs as insect-eaters, including their long, narrow jaws.

Many ant-eating mammals today, including pangolins and armadillos, have long, narrow snouts that they can stick into insect nests.

And the alvarezsaurs seem to have had weak jaw bones, probably because chewing becomes less important when you’re eating ants.

Also, like today’s social insect specialists, alvarezsaurs had no teeth in the front of their mouth.

This arrangement allows the tongue to flick out and return covered with ants or termites.

So, while the first alvarezsaurs were carnivores, the rise of social insects during the Cretaceous Terrestrial Revolution seems to have pushed them into a new niche -- one that was new for them and new for dinosaurs.

They evolved into smaller-bodied insectivores with delicate jaws, short limbs, and single claws.

Their hands became similar to those of anteaters and other insect-eating mammals today, which have an enlarged finger with a pick-like claw to break open social insect nests.

And alvarezsaurs may have reduced competition with other theropod dinosaurs that had gotten bigger and become apex predators by becoming insect-eaters.

Which is pretty incredible, when you think about it.

Insects going from solitary life to living in organized, social colonies is, by itself, a major transition in the history of life on our planet.

And for these tiny creatures to have influenced the evolution of a group of dinosaurs is definitely worthy of wonder.

And while the alvarezsaurs wouldn’t survive the end-Cretaceous extinction event, their new lifestyle would live on -- in the many insect-eating birds and mammals that are part of our world, today.