Grass, shrubs and dryness: a year in the life of a giant sloth
A 27,000-year-old tooth reveals the secrets of a single animal. Dyani Lewis reports.
A massive fossilised tooth from central America has opened a window into life for a giant ground sloth that lived 27,000 years ago, according to a paper in the journal Science Advances.
Analysis of the tooth, prised from a clay ledge metres below the surface of a water-filled sink-hole in central Belize, reveals details about what the sloth ate, and the climate it lived in.
The tooth belonged to a Panamerican ground sloth – Eremotherium laurillardi – a six-metre-long that occupied a range that stretched from the southern states of the US to Brazil. The species is thought to have survived for almost two million years before its extinction around 11,000 years ago.
Jean Larmon from the University of Illinois at Urbana-Champaign, US, and colleagues measured the amounts of specific chemical elements that were incorporated into the tooth when the sloth was alive.
The isotopes carbon-13 and oxygen-18 are particularly useful. Carbon-13 values can tell scientists about the types of food the sloth consumed, and oxygen-18 levels reveal the aridity of the climate at the time.
Because amounts of these isotopes slowly change, or decay, over time, they can also reveal the age of any object containing them. But there’s a catch. The chemical signature differs depending on the composition of any given fossil.
In other animals, the hard outer coating of teeth – the enamel – is usually used in stable isotope analysis. But sloth teeth grow continuously – much like a rat’s incisors – and therefore lack it.
Larmon and colleagues instead took readings from different layers of the sloth tooth. The values varied. Using a technique that measures the amount of light emitted by a fossil, they ascertained that the most reliable values were from a hard layer called orthodentin.
By sampling along the length of the tooth’s orthodentin layer, the team built up a picture of the sloth’s life over a period of about a year.
The animal lived smack in the middle of the last glacial maximum – an ice age – and before humans arrived in the Americas.
At the time, Central America wasn’t covered in tropical forests as it is today. The region was much drier, covered in savannah and juniper scrub vegetation.
The sloth experienced two brief wet seasons, separated by a long dry season. Its diet changed with the seasons, suggesting it was an opportunistic feeder. This could have held it in good stead for adapting to the increasingly arid conditions of the period.
During the wet seasons, it most likely ate grasses and shrubs, but no leafy trees.
“It’s a really solid bit of work,” says palaeontologist Gilbert Price from the University of Queensland, Australia, who was not involved in the study.
“To understand the biology and the ecology of these animals when they were alive is absolutely critical.”
Armed with that information, he adds, scientists are better able to assess what factors – a changing climate, say, or the arrival of humans – eventually led to the species’ extinction.
The reasons for the demise of large animals, collectively known as megafauna, is an enduring mystery. And while this study describes details of just one individual’s life, it contributes to a greater understanding of its prehistoric environment.