The sloth family tree was once characterized by an immense variety of branches, body sizes, and lifestyles, spanning from small and limber tree climbers to lumbering bear-sized landlubbers.
The reason why the body size of sloths was once so diverse despite their current limited population of two small tree-dwelling birds has been a topic of debate for a long time. Experts now believe that this may be influenced by various factors, such as habitat preferences, diets, temperature changes, or pressure from large predators or humans.
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A new comprehensive analysis of sloth species, from living to extinct, suggests that a person's body size is most affected by their location in trees or on land, according to scientists.
The team analyzed the shapes of sloth fossils from 49 different lineages and used proteins and DNA in the bones to reconstruct the sloth family tree over time. They examined when lineages branched, shrank, grew, and relocated, and then compared these changes to global climatic changes.
According to the research, habitat preference played a crucial role in the gradual decline of sloth lineages over time, resulting in changes in their size over the course of evolution and diversification.
Around 35 million years ago, sloths were introduced during the Late Eocene Period, and they evolved solely on the isle of South America without the assistance of the vast mammalian predators that were prevailing in North America. The species eventually diversified into over 100 different genera, with some primarily inhabiting tropical forests and others occupying open grasslands.
As the climate changed and forested regions dwindled over time, sloth lineages could sometimes return to their original habitat or revert to their natural habitat, causing tree-crew to become smaller and more terrestrial forms to grow.
The prevalence of body size differences among other animal groups is not unique, as pointed out by Rachel Narducci, a paleontologist at the Florida Museum of Natural History in Gainesville.
The study suggests that the last common ancestor of all sloths, which has not been found in the fossil record, was probably a fairly large aquatic animal.
The new study by Juan Carrillo, a non-participating vertebrate paleontologist at the National Museum of Natural History in Paris, provides strong evidence for it, but cautions caution: There is no fossil evidence to support the appearance of the earliest sloths, and information about their earliest evolution is still fragmented.
The new study offers a favorable viewpoint on the evolution of these creatures, as stated by Greg McDonald, a former regional paleontologist from Fort Collins, Colo.
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Humans may have played a role in the extinction of most sloths, as suggested by recent studies that suggest human expansion was responsible for the decline of other large megafauna like mammoths.
During the Late Pleistocene Epoch, when humans began to move across the Americas, ground sloths became extinct around 15,000 years ago, and the larger genera were simply easier targets because they lived in the open grasslands, according to Narducci.
There are skepticism among researchers regarding whether humans were the primary reason for the rapid decline of sloths. Despite the abundance of evidence supporting the hunt for mammoths and other megafauna during the Late Pleistocene, there is limited information on human hunting.
Although the excavation in the early 2000s did not uncover the skeletal evidence of sloth bones from a quarry in the Pampas region of Argentina, scientists have still theologically guessed that the marks caused by cut marks from stone tools can be explained by butchering, which were suggested to be the cause in the ribs discovered in March 2019.
By Jessica Lucas