Mad about Science: Chalicotheriidae

By Brenden Bobby
Reader Columnist

Multicellular life has had about 1.6 billion years to experiment on Earth. In that time, it has created all manner of weird lifeforms that were, however briefly, perfectly suited to the environments in which they existed. To our eyes, these creatures seem alien — how on Earth was Hallucigenia, from the Cambrian epoch, perfectly suited for any environment?

Some of the most unusual animals to ever dominate the Earth came from the family, Chalicotheriidae. These creatures were strange — almost like a cross between a horse, a gorilla and a sloth. They were odd-toed ungulates that didn’t possess hooves, but instead bore long, sharp claws similar to a sloth; the body structure of gorillas; and the heads and appetite of a horse.

Though extinct as of about 1 million years ago — recent enough to have directly interacted with human progenitors including Homo erectus and Homo habilis — they are most closely related to present-day horses and rhinos.

Chalicotheres inhabited virtually every continent except for Antarctica, becoming particularly prolific throughout much of Asia and North America.

They began appearing in Asia around 48 million years ago, giving them a solid 47 million-year run. We don’t know why they started to go extinct, but it happened gradually over the course of about 10 million years as species began to disappear from North America and Europe until a handful of species remained in modern-day China.

Despite their close relation to grass-grazing ungulates, it’s speculated that members of Chalicotheriidae were sustained mostly by trees. Their claws were large and powerful, and they were capable of standing upright to pull down branches. We know they were exclusively herbivorous, so their massive claws were adapted for a mix of self- defense and food acquisition. 

The largest of these impressive beasts measured more than 8.8 feet at the shoulder and weighed somewhere close to 2,000 pounds. It was likely that you wouldn’t want to tangle with one of these things — it wouldn’t eat you, but the claws could do some damage.

While sloths are pretty cute, their claws are formidable, being less like our fingernails or even a horse’s hoof, but akin to a protrusion of bone. They’re very strong and can be very sharp. A friend of mine from high school worked with sloths in Costa Rica, and she always said that getting stitches from handling the sloths was like initiation at the facility.

Chalicotheriidae claws weren’t nearly as long, but they had several hundreds of pounds of solid muscle attached to them. Imagine a gorilla with hunting knives duct-taped to its fingers, and there you have Chalicotherium.

These creatures were huge and bulky, but they likely weren’t very fast. It’s hypothesized that they moved at the speed of a cow. They walked utilizing quadrupedal locomotion in the manner of the great apes, by folding their claws back into their palms and using the knuckles of their forelimbs to stride. Despite the fact that our ancestors may have interacted with these animals, we have absolutely no idea what they sounded like. Given their size, they likely made low, resonant sounds similar to hippopotamuses or cows — if they made any vocalizations at all.

Despite their long evolutionary run, Chalicotheres were outcompeted as Earth’s climate shifted over the past several million years. Temperatures fell and climates became drier, which meant there were fewer forests and more grasslands.

Their entire evolutionary niche was built around pulling down limbs from trees, which were disappearing. Meanwhile, modern equines and bovids were rapidly evolving to exploit new savannah, pushing their strange cousins into the receding forests.

While some species may have existed in or near Bonner County throughout their long evolutionary history, not a single member saw Lake Pend Oreille. The lake formed between 13,000 and 15,000 years ago, nearly 1 million years after the last of their kind went extinct in China.

It is interesting to note that many of the features that made Chalicotheres go extinct were features that are still highly valued in animal species today. Many of the great apes still share their rough body shape and ability to walk on all fours using their knuckles. The biggest difference is that primates are much more adaptive when it comes to their diet. 

Primates can eat a huge variety of foods, while Chalicotheres could likely only sustain themselves on specific types of leaves.

Their head shape is still utilized by bovids and equines, but those have adapted to eat plentiful grasses to sustain their large body mass. The ultimate downfall of the Chalicotheres was their inability to adapt and consume other kinds of foods to sustain themselves. They were too large, with too many specialized evolutions to survive in a shifting world.

This is ultimately the lesson of evolution: While hyperspecialized tools are striking, identifiable and great at exploiting an environmental niche that can lead to tremendous gains and global proliferation, if that specialization is incapable of changing as the planet’s climates shift, it will all end in extinction. There’s a reason that sharks have been on the Earth longer than trees — they aren’t picky about what they eat, and they’re really good at eating.

Stay curious, 7B.