Fossils of hummingbird-sized ants challenge what we know about insect sizes: ScienceAlert

More than 47 million years ago, giant carnivorous ants swarmed the prehistoric forest floors of North America in search of prey.

‘Huge’ is no exaggeration either. Some ancient colonies living in what is now the state of Wyoming were ruled by hummingbird-sized queens.

They are not even the largest ants that have ever walked the surface of the Earth. The largest known queen ant to have ever lived was a relative found in fossil form in Germany. She had the body mass of a wren, was more than 5 centimeters (2 inches) long, and had wings that span 16 centimeters. Her army of workers is believed to have hunted everything in their wake, including perhaps lizards, mammals and birds.

Like modern ants, these ancient insects were more than likely ectothermic, meaning they struggle to survive without an appreciable amount of heat in their environment. How far the temperature can drop before they don’t thrive depends largely on their body size.

While animals that can change their own temperatures resist colder climates by maximizing their mass and minimizing their skin, animals that must absorb heat from their surroundings do better with more surface area and less volume. Today, larger queen ants are found closer to the tropics, for example.

The fossil of an extinct giant ant from Wyoming versus a hummingbird for size. (Bruce Archibald)

So how did ancient giant ants cross the frigid Bering land bridge that once connected Russia to Alaska to get from Europe to Wyoming?

In 2011, researchers suggested that this temperate land bridge once included a climate-controlled ‘gate’. During brief periods of global warming, that gate may have opened to allow cold-blooded organisms, such as ants, to pass comfortably from one continent to another.

A recently discovered fossil of an ancient ant queen now complicates this hypothesis.

Giant ant Germany
A fossil of a giant ant queen found in Germany, the largest ant ever found. (Wikimedia Commons/CC BY 4.0)

It also belongs to the same genus as the giant ants found in Wyoming and in Germany are called Titanomyrma.

But this one was found in British Columbia, Canada – the first fossil of its kind to appear in such a cold climate.

Scientists can’t be sure of its size due to its squashed nature, but there’s a chance it was as big as its Wyoming counterpart.

“If it was a smaller species, was it adapted to this region of cooler climate by reduction in size and giant species were excluded, as we predicted back in 2011?” wonders paleontologist Bruce Archibald from Simon Fraser University (SFU).

“Or were they huge and our idea about the climatic tolerance of giant ants and how they crossed the Arctic was wrong?”

Allenby Fossil
The giant fossil queen ant Titanomyrma discovered in the Allenby Formation. (Bruce Archibald)

The Canadian Titanomyrma is not in good condition, meaning it cannot be assigned to a specific species, but it is close in age to other fossils of its kind found in Europe and Wyoming.

Depending on which way it was compressed, the organism could have originally been either 3 or 5 centimeters long.

The shorter estimate would make it 65 percent smaller than its Wyoming counterpart, supporting the idea that giant ants require warm climates and could only have been released through the Bering Land Bridge’s climate gateway during a period of global warming.

The larger measurement suggests that these ancient ants had more cold tolerance than we thought and could have crossed the land bridge at any time.

The only way to distinguish between these scenarios is to find more fossils.

“Remake our ideas Titanomyrma’s ecology, and so of this ancient spread of life, needs revision?” asks Archibald.

“For now, it’s a mystery.”

The study was published in The Canadian Entomologist.

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