The fossil record is chock-full of ground-dwelling sloths ranging
from medium-sized to mammoth (literally). But these days, there are only
a few small sloths, and they all live in trees. Researchers
reconstructing sloth evolution found that extinct sloths developed large
body sizes at an amazing rate. Existing sloths are the black sheep of
the sloth family: not only do they not reflect the overall evolutionary
trends of the group, but they also obscure the strong (but long gone)
signal in the fossil record of ever increasing body size.
Sloths were incredibly diverse in the past, with more than 50 known
species distributed among eight families. Animals in at least half of
those families weighed over 1,000 kilograms. Megatherium americanum grew to the size of an elephant, and the claws of Eremotherium eomigrans
were about a third of a meter long. They showed up around 50 million
years ago, but multiple megafaunal extinction events—from 2.6 million to
11,700 years ago—cut their diversity down by about 90 percent.
All six sloth species today belong to just two genera, Bradypus and Choloepus, each in their own family: four three-toed sloths in Bradypodidae, and two two-toed sloths in Megalonychidae.
The two lineages may seem similar, but they diverged from one another
about 30 million years ago. So oddly enough, they must have
independently evolved their small size—no more than six kilograms—and
arboreality, or tree-living.
Using a data set of all 57 species of known living and fossil sloths, a trio of researchers led by John Finarelli of University College Dublin
examined changes in body mass through sloth evolution. The team found a
clear trend for the evolution of larger and larger body sizes through
time—and they evolved at an extremely fast rate. The work was published in BMC Evolutionary Biology this week.
Megatherium’s family saw an average mass increase of 129
kilograms per million years—one of the fastest rates of body size
evolution known for mammals. Even the family Megalonychidae (which includes today’s two-toed sloths) had an average body mass increase of 2.6 kilograms per million years.
The findings suggest that environmental conditions at the time, such
as climate or species competition, must have really favored larger body
sizes. Whatever the cause of their eventual decline, only
small, arboreal sloths survived these events, completely reversing the
millions-year-old trend towards larger and larger sloths, the BMC blog reports.
“If we ignore the fossil record and limit our studies to living
sloths, as previous studies have done, there's a good chance that we'll
miss out on the real story and maybe underestimate the extraordinarily
complex evolution that produced the species that inhabit our world,"
says study co-author Anjali Goswami of Univeristy College London in a news release.
And sloths may not be the only group where modern-day species are
unrepresentative of overall evolutionary trends. The diversity of
hyenas, elephants, and rhinos, for example, are only a fraction of what
it was in the past.
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