 |
Jefferson's Ground Sloth
This ground sloth (Megalonyx jeffersonii) is
one of the most unusual of North American Ice Age mammals. Its bones are so peculiar that
whenever I encounter an "unrecognizable" large mammal bone in the Old Crow Basin
of the northern Yukon, Megalonyx is the first animal I consider. This
long-haired, ox-sized mammal was one of several, including the American mastodon (Mammut
americanum), the giant short-faced bear (Arctodus simus) and
flat-headed peccary (Platygonus compressus), to enter the Yukon and Alaska
from southern North America. The main mammalian invasion of Canada came from Eurasia via
the "Bering Isthmus".
This species is of historical as well as paleontological
significance, for Thomas Jefferson's lecture on Megalonyx ("great
claw") to the American Philosophical Society in 1797 marked the beginning of
vertebrate paleontology in North America. Appropriately, Megalonyx jeffersonii
was named for the third president of the United States.
Jefferson's ground sloth, the largest known species of Megalonyx,
when fully grown was about the size of an ox (2.5 to 3.0 m long). The more southerly
specimens are smaller than those from the north: such latitudinal clines are seen in many
living mammals. The skull is short, broad and deep with a blunt muzzle (Figure 1).
Evidently chewing muscles were well-developed, and the deep, heavy mandible lacks the
spout-like front characteristic of megathere ground sloths (Megatheriidae).
Normally there are five teeth on each side of the upper jaw
and four on each side of the lower jaw. The teeth are peg-like (Figure 2) and a space
(diastema) separates the blunt, broad, ovate caniniforms (fang-like teeth) from the wider
cheek teeth. As in living tree sloths of South and Central America (e.g., Bradypus
tridactylus), milk teeth are absent. Compared to other ground sloths, Jefferson's
has broad, wing-like processes on the heel bone (calcaneum), and a short third upper foot
bone (metatarsal), and v-shaped fifth metatarsal and an outward-bowed fibula. Megalonyx
and other ground sloths had well-developed collar bones (clavicles) and sternal ribs.
 Megalonyx jeffersonii, like humans, had plantigrade
hind feet -- the weight being borne on the sole rather than the outside of the foot as in
other ground sloths. Further, the three central claws of the hind foot were well-developed
and touched the ground, presumably allowing these animals to move more easily and with
greater stability than other ground sloths. Could that feature, perhaps with greater cold
tolerance (unusually thick hair?) have enabled Megalonyx jeffersonii to
occupy a greater latitudinal range than other North American ground sloths? The strong,
broad tail -- a stabilizer when reaching up -- and curving front claws may have assisted
the species to hook down high, leafy forage.
Of the two ground sloth families (Megalonychidae
and Megatheriidae), the former first appeared in the early Oligocene (about 35 million
years ago) of Patagonia, and the former in the late Oligocene (some 30 million years ago)
beds of South America. The former group includes megatheres such as the heavily-built Megatherium
and Eremotherium, reaching a length of 6 m; and the nothrotheres such as the
slighter Hapalops-Nothrotheriops line, reaching a length of about 1.2 m. The
earliest known North American megalonychid, Pliometanastes protistus that
lived in Florida about 8 million years ago, shows no close relationship to Megalonyx.
Megalonyx, a widespread North American genus, lived from the late Hemphillian
(about 5 million years ago) to the close of the last (Wisconsinan) glaciation.
Megalonychids increased in size as time progressed. The
first species were small and may have been partly arboreal (tree dwelling), whereas the
Pliocene (about 5 to 2 million years ago) species were approximately half the size of the
late Pleistocene Megalonyx jeffersonii. Some West Indian species were cat to
black bear sized -- their dwarf condition reflecting their restricted island environment.
 Several species of Megalonyx have been named; in fact it has
been stated that "nearly every good specimen has been described as a different
species". In some cases, even isolated teeth -- a highly variable element in sloths
-- were diagnosed as the types (individual specimens considered characteristic of the
group) of some species. A broader perspective on the group, accounting for age, sex,
individual and geographic differences, indicates that only three species are valid (M.
leptostomus, M. wheatleyi, and M. jeffersonii) in the late Pliocene
and Pleistocene of North America. Probably the Blancan-Nebraskan (about 3 to 1.2 million
years ago) narrow-mouthed ground sloth Megalonyx leptostomus was ancestral to
Wheatley's ground sloth (Megalonyx wheatleyi) of Irvingtonian-Yarmouthian age
(about 1.2 to 0.6 million years ago), which in turn gave rise to the larger, later Megalonyx
jeffersonii.
Jefferson's ground sloth first appears during the second
last (Illinoian) glaciation (probably more than 150,000 years ago), and has been reported
from about 80 localities including the eastern two-thirds of the United States (as well as
the coastal shelf off New Jersey) and parts of western North American (central Mexico to
California to Washington and inland Idaho). Megalonyx jeffersonii is the only
ground sloth that roamed as far north as the Yukon Territory and Alaska. In Canada, it is
known from Saskatchewan, Alberta (Medicine Hat -- perhaps of last [Sangamonian]
interglacial age -- about 130,000 years ago, and Edmonton), British Columbia (Quesnel
Forks); Yukon (Old Crow River) and the Northwest Territories (Lower Carp Lake). Considered
together, the Alaskan (Fairbanks area), Yukon and Northwest Territories records suggest
that the species occupied a rather broad east-west range in northwestern North American
during a warm phase of the late Pleistocene. Since the Old Crow specimens are smaller than
most Wisconsinan (about 90,000 to 10,000 years ago) fossils, perhaps the species reached
the region during the last interglacial.
In more southerly regions of North America, Jefferson's
ground sloth lived in woodlands. Its broad, blunt caniniforms suggest a leaf-stripping
adaptation, so Megalonyx jeffersonii probably browsed on leaves, twigs and
perhaps Megalonyx jeffersonii became extinct about 9,400 years ago.
C.R. Harington July, 1993 |
