The slow-moving sloths were descended from giant beasts that could snatch food from tigers’ mouths?

The sloth family has a say in matters like "our ancestors were also wealthy."

Unlike today's sloths that like to hang motionless on branches, their prehistoric relatives were completely different. This giant beast with footsteps like thunder is today's protagonist - the American giant ground sloth (Megatherium americanum). The American giant ground sloth lived in the American continent between the early Pliocene and the late Pleistocene. Its size was the largest among the land vertebrates of the same period, and only large elephants and giant rhinos could compare. The American giant ground sloth is also a representative species of the giant fauna of the American continent in the Pleistocene.

Drawing of a giant ground sloth skeleton from Extinct monsters: a popular account of some of the larger forms of ancient animal life | Paul K / Flickr

Although the giant ground sloth is very different from existing sloths in terms of body shape and habits, the evolutionary lineage shows that this land giant and the slow sloth are indeed a family. In biological classification, both belong to the large evolutionary branch of Mammalia, Heteroarthromorpha, Phalaropedia, and Slothia. However, the giant ground sloth claims to belong to the Megatheriidae family, and the American giant ground sloth we usually refer to in the context refers to the type species under the genus Megatheriidae.

The Behemoth Appears

As the first prehistoric giant to be discovered and understood by humans, the fossils of the giant ground sloth were first discovered in Argentina in 1788. The bone fragments were then collected and sent to the National Museum of Natural Sciences in Madrid, Spain for display, and have been preserved to this day. However, people at the time knew nothing about these huge remains until the appearance of a famous paleontologist, Georges Cuvier, the father of paleontology and comparative anatomy.

Portrait of Cuvier | Wikimedia Commons

After applying the most advanced anatomical theories at the time, Cuvier gradually pieced together the information carried by these fragmented fossils, and preliminarily sorted out the appearance characteristics and kinship evolutionary relationships of this prehistoric giant. He then published two papers in 1796 and 1804 to explain this major discovery and officially named this species "American Ground Sloth" (M. americanum).

Through in-depth research and comparison, Cuvier found that there was a very subtle evolutionary relationship between the giant ground sloth and the existing sloths, but it was obvious that this giant beast could not hang on branches like sloths, so he boldly believed that "the giant ground sloth is a large terrestrial animal, and its claws are like modern anteaters, mainly used to dig caves and find food." Although this theory was later denied by research, in the 18th and 19th centuries when natural science was just beginning, his hypotheses were enough to stir up a thousand waves, and the "giant beast trend" set off by the American giant ground sloth continued until the discovery of dinosaur fossils decades later.

Illustration from Cuvier's Recherches sur les ossemens fossiles | picryl

Food Myths

As more and more giant ground sloth fossils are discovered in various parts of the Americas, our image of this giant has become clearer and clearer, but the feeding habits of giant ground sloths are still a controversial topic. As a "giant" among the entire sloth suborder and even all terrestrial mammals, an adult American giant ground sloth can be up to 6 meters long from head to tail and weigh 4 tons, which is enough to be compared with a modern adult African elephant! Such a huge size undoubtedly requires sufficient food intake to support it.

So what do they eat? By studying the skeletal structure of the giant ground sloth, we can find that their skeleton is very strong, with a wide and thick pelvis and a strong tail. It is speculated that when eating, they can use their thick hind legs and tail to support their bodies like a tripod, and then stand up to eat fresh branches and leaves on high branches. With the specialized hooked claws on their forelimbs and their clever tongue, the giant ground sloth can easily eat leaves that other herbivorous animals cannot touch, thereby obtaining more and better food resources.

Reconstruction of a giant ground sloth standing up to eat leaves | Robert Bruce Horsfall / Wikimedia Commons

Not only that, the footprint fossils left by the American giant ground sloth also revealed that their hind limbs also had long claws, which means that their feet could not actually step completely flat on the ground to walk, but could only move through the outer edge of the feet. Therefore, this giant beast probably also had good bipedal movement function.

However, in most cases, the American ground sloth still feeds on low-lying vegetation (such as agave leaves and grass plants). Their well-developed cheek muscles and teeth are very suitable for chewing and cutting the leaves and rhizomes of these plants. However, after swallowing a large amount of food, the American ground sloth also needs a long time to digest these difficult-to-swallow plant fibers.

Giant ground sloth fossil, Kensington Museum of Natural History | B / Flickr

Northern Expedition Warriors

As a native species originating from South America, the American giant ground sloth can be said to have caught up with a good opportunity, and their population has continued to prosper and grow. At that time, the South American continent was relatively more arid and semi-arid than the temperate North America. The dense woodlands and grasslands throughout became the home of the giant ground sloth family, and their territory eventually expanded to the North American continent. The American giant ground sloth population successfully moved north through the Isthmus of Panama, and became one of the few representative species in the "Great Migration of South and North America" ​​that entered North America from South America.

Although North America, where many animals compete for supremacy, is by no means a peaceful paradise, adult American ground sloths have almost no natural enemies in nature due to their unrivaled size advantage. Therefore, paleontologists believe that whether American ground sloths are social animals or loners, they should be typical diurnal animals, mainly active during the day to forage for food, and return to caves and other hidden places to rest after nightfall, leading a peaceful life.

Reconstruction of a giant ground sloth | Wikimedia Commons

Although the image of giant ground sloths as docile herbivorous giants has been deeply rooted in people's minds, there are still hypotheses that question this.

Some research theories believe that the triceps attachment point (i.e., antecubital process) of the American ground sloth is shorter, which means that their forelimbs are more inclined to speed rather than strength when moving, which is closer to many carnivorous animals. It is speculated that the giant ground sloth may wave the sharp claws of its forelimbs to grab the prey captured by other predators (such as saber-toothed tigers) and take it for itself. If this theory is confirmed, then the American ground sloth is likely to be an opportunist, and occasionally it will rely on its huge size and bluffing claws to scare off other predators and get a free meal.

However, so far, scientists have not found any bone residues in the feces fossils of giant sloths. Therefore, this hypothesis is not recognized by the mainstream academic community due to the lack of reliable evidence.

The once-prosperous American ground sloths were not spared during the extinction event at the end of the Pleistocene. As climate change brought about dramatic changes in the environment and shrinking habitats, the population of giant ground sloths gradually declined. According to fossil records, the last population of American giant ground sloths may have survived until 10,000 years ago, when the dawn of human civilization was already shining on the horizon.

These two species thus took completely different directions at the intersection of time and space. One fell in the torrent of life evolution, while the other rose up and eventually completely changed the face of the planet.