The small and medium-sized sloth lemurs of Madagascar were incredibly convergent with modern tree sloths, and the biggest member of the group likewise seems to have been the closest they came to evolving a ground sloth equivalent.
Archaeoindris was the largest known lemur – and one of the largest primates – similar in size to a modern gorilla at about 1.5m tall (4′11″). It would have been a slow-moving animal which fed mostly on leaves, and while it was still capable of climbing around in larger trees it was probably much too bulky for upside-down suspension like its smaller relatives, and would likely have had to regularly traverse the ground to reach new feeding sites.
It seems to have been a fairly rare member of the ecosystem, living only in the Central Highlands, and the last known remains date to just over 2000 years ago – around the same time that humans first reached that area of the island. Sadly a combination of factors such as the giant lemurs’ slow reproductive rate, habitat loss, and hunting pressure was too much for their population to recover from all at once, and they probably went extinct very soon after that date.
The island of Madagascar has been isolated from other landmasses for almost 90 million years, and as a result there are many lineages present there found nowhere else on Earth.
Lemurs are one of the island’s most famous residents, having arrived from Africa via a rafting event sometime early in the Cenozoic and evolving to fill the ecological niches occupied elsewhere by monkeys and apes. But while there are around 100 lemur species alive today, there used to be more before the arrival of humans – subfossil remains from the last 25,000 years hint at an ecology with even greater diversity, and types of lemurs much larger than any still living today.
The sloth lemurs, as their name suggests, resembled modern sloths in a remarkable case of convergent evolution. With long limbs and long hooked fingers and toes they were adapted for swinging through trees and hanging from branches, feeding on a wide range of plant material such as leaves, fruit, and seeds.
Palaeopropithecus was one of the larger members of this group, and the most specialized for sloth-like upside-down suspension. Three different species have been identified, with the biggest (Palaeopropithecus maximus) possibly measuring around 1m long (3′3″).
It probably spent almost its entire life in the trees, and would have been slow and awkward on the ground. Malagasy folklore about a creature known as the tretretretre or tratratratra, which couldn’t navigate on smooth flat surfaces, may even represent a cultural memory of Palaeopropithecus from before its extinction – which may have happened as recently as within the last 500-1000 years.