Fossil teeth of giant ground sloths reveal their unique role in prehistoric ecosystems

Imagine a sloth. You probably picture a medium-sized, arboreal creature hanging from a branch. Today’s sloths—prominently featured on children’s backpacks, stationery, and lunch boxes—are slow-moving creatures, living undeniably in the rainforests of Central and South America.

But their massive Pleistocene ancestors that inhabited the Americas up to 35 million years ago were nothing like the sleepy tree-huggers we know today. The giant ground sloth—weighs several thousand pounds and stands taller than a single-story building. They play important and diverse roles in shaping ecosystems across the Americas, roles that disappeared with their loss at the end of the Pleistocene.

In our new study, published in the journal Biology Lettersour goal is to reconstruct the diets of two species of giant ground sloths that now live side by side in southern California. We analyzed samples recovered from La Brea tar pits colloquially known as the Shasta ground sloth (Nothrothrips chastanis) and Harlan’s ground sloth (Pyramelodon harlanii).

Our work sheds light on the lives of these fascinating creatures and the consequences of their extinction in Southern California 13,700 years ago.

Dental Challenges

Studying the diets of extinct animals often feels like putting together a jigsaw puzzle with only one piece of the puzzle. Stable isotope analyzes have revolutionized how paleontologists reconstruct the diets of many ancient organisms. By measuring the relative proportions of light and heavy carbon isotopes in tooth enamel, we can tell what kind of food an animal ate—for example, grass versus trees or shrubs.

But the teeth of giant groundhogs lack enamel, the most unnecessary and hard outer layer on most animal teeth — including our own. Instead, sloth teeth are made primarily of dentin, a more porous and organic-rich tissue that easily changes its chemical composition with fossilization.

Stable isotope analyzes are less reliable in sloths because the chemical composition of dentin can be altered postmortem, thereby discarding isotopic signatures.

Another technique researchers use to gain information about an animal’s diet relies on analyzing microscopic wear patterns on its teeth. Analysis of the microstructure of teeth can determine whether an animal ate mostly hard foods such as leaves and grasses or hard foods such as seeds and fruit pits. This technique is also difficult when it comes to raw fossil teeth because signs of wear may be preserved differently in soft dentin than in hard enamel.

Before studying fossil sloths, we examined dental microwear patterns in modern xenarthrans, a group of animals that includes sloths, armadillos, and anteaters. This study revealed that dentin microware can reveal dietary differences between leaf-eating sloths and insect-eating armadillos, giving us confidence that these tools can reveal dietary information from ground sloth fossils.

Different nutritional niches were revealed

Previous research suggested that giant groundhogs were either herbivorous grazers or leaf-eating browsers, based on the size and shape of their teeth. However, more direct measures of diet – such as stable isotopes or dental microarrays – are often lacking.

Our new analyzes reveal contrasting dental signatures between two common ground sloth species. Harlan’s ground sloth, the larger of the two, had a dominant deep pit texture on microarray specimens. Such wear is indicative of chewing hard, mechanically challenging foods such as nuts, seeds, cookies, and fruit pits. Our new evidence is consistent with skeletal adaptations that suggest powerful digging abilities, consistent with food both above and below ground.

In contrast, the Shasta ground sloth exhibited a tooth microstructure similar to that of leaf-eating and woody plant-eating herbivores. This pattern supports previous studies of its fossil dung, which demonstrated a diet rich in desert plants such as yucca, agave and saltbush.

We then compared the microarray structures of sloths to those of ungulates such as camels, horses, and bison that lived in the same region of southern California. We confirmed that neither sloth species’ feeding behavior completely overlapped with that of other herbivores. Giant ground sloths did not perform the same ecological functions as other herbivores that shared their landscape. Instead, the two ground sloths shared their niches and played complementary ecological roles.

Extinction caused ecological damage

Harlan’s ground sloth was a megafauna ecosystem engineer. It excavates soil and gnaws underground, thereby affecting soil structure and nutrient cycling, even dispersing seeds and fungal spores over wide areas.

Anecdotal evidence suggests that some anachronistic fruits—like the strange, controversial and softball-sized Osage orange—were dispersed by ancient megafauna such as giant ground sloths. When the Pleistocene megafauna went extinct, the loss contributed to the regional restriction of these plants, as no one was around to spread their seeds.

The broader consequence is clear: megafaunal extinctions wiped out key ecosystem engineers, triggering cascading environmental changes that continue to affect habitat resilience today. Our results resonate with growing evidence that conserving today’s living large herbivores and understanding the diversity of their ecological niches is critical to maintaining functional ecosystems.

Studying the teeth of lost giant groundhogs has shed light not only on their diet but also on the lasting environmental legacy of their extinction. Today’s sloths, though fascinating, only hint at the profound environmental influence of their prehistoric relatives. People who have shaped landscapes in ways we are only beginning to appreciate.

This article is reprinted from The Conversation under a Creative Commons license. Read the original article.