North American Marsupials
North America serves as the primary source of fossils revealing
early marsupial evolution. Marsupials first appear in the
Mid-Cretaceous. Kokopellia, from Utah, USA may be the oldest
marsupial at 100 Ma (Kemp, 2005, p. 196). Alphadon represents
the first undisputed marsupial. Alphadon from the Upper Cretaceous
of North America is known mostly from teeth and jaws. Alphadon is a member of the family Didelphidae and is often considered
a model archetype marsupial. Alphadon had the marsupial dental
formula of three premolars and four molars (placental mammals
have three molars). Like living marsupials these early forms
exhibited a tooth replacement related to their nursing habits.
Only the last premolar is replaced, anterior dentition is
not because of extended nursing (Benton, 2005, p 309).
In the late Cretaceous
marsupials underwent an adaptive radiation in North America.
These early marsupials were small
and adapted to various niches. Teeth indicate that various
marsupials were specialized as insectivores, carnivores,
and omnivores. Although more common than placentals, they
were not as diverse or numerous as the multituberculate mammals.
The K/T mass extinction event put a halt to the diversification
of marsupials in North America. In the Cenozoic North American
marsupials would be represented only by the didelphids
South American Marsupials
It is not clear if marsupials originated in North America
or Asia. In either case, it is clear they migrated to South
America and Australia. Today Opossums and their South American
relatives represent marsupials of the Western hemisphere;
however, in the past their diversity was much greater. South
American marsupials underwent an adaptive radiation in the
Early Paleocene, which resulted in three lineages. Multiple
niches were filled by this adaptive radiation.
Didelphimorphs (Order Didelphimorphia) radiated into small
to medium sized opossums specialized as insectivores and
omnivores. Didelphids went extinct in both Europe and North
America during the Miocene. South American forms migrated
back to Central and North America in the Pliocene/Pleistocene
during the Great American Biotic Interchange.
(Borhyaenoids) is an extinct order of specialized South
American carnivores. Many of these carnivores evolved body
and lifestyles like those of placental dogs, bears, and
cats (Benton, 2005, p. 315). These organisms serve as an
example of convergent evolution. Cladosictis from
the Late Oligocene to the Early Miocene of Patagonia was
an otter. Borhyaena also from the Late Oligocene
to Early Miocene of Patagonia was jaguar sized and rather
(Palmer, 2006, pp. 346-347). Thylacosmilus from
the Late Miocene to Early Pliocene of Argentina was like
saber-toothed cats of North America and Europe. The upper
canines, like their placental counterparts, were enormous.
Unlike its placental counterparts the canines grew continuously
and were protected by a vertical extension of the lower
jaw when the mouth was closed. Thylacosmilus was
one of the last borhyaenids to survive (Kemp, 2005, p.
is the order that includes living caenolestids or shrew-opossums.
Many extinct forms of this order had teeth adapted to
a rodent-like diet. Argyrolagus from the Late
Miocene to Late Pliocene of Patagonia had the body form
like a modern
day placental kangaroo rat (Palmer, 2006, p. 347).
Today, marsupials comprise the dominant native fauna of
Australia. Most people are familiar with wombats, possums,
koalas, and kangaroos. However, throughout the Cenozoic Australia
played host to a great diversity of marsupials. Marsupials
first appear in Australia in the Eocene. Australian marsupials
of the Early Cenozoic evolved without competition from placental
mammals and, unlike South America, their adaptive radiation
included the evolution of middle to large herbivores as well
as carnivores (Kemp, 2005, p. 208). However, as in South
America convergent evolution resulted in Australian marsupials
with body forms and lifestyles similar to placental mammals
on other continents. Marsupial equivalents of rodents, wolves,
tapirs, anteaters, and cats are easily recognized.
a few of the prehistoric megafauna of Australia. Thylacoleonidae
comprise a family of lion-like
marsupials. Thylacoleo from the Late Pliocene to the Late
Pleistocene of Australia had a leopard-like form. The incisors
were canine-like, while the cheek teeth formed large shearing
blades. The front paws were equipped with large claws on
semi-opposable thumbs. This marsupial family is unusual
in that the carnivorous forms evolved from a specialized
ancestor (Kemp, 2005, p. 213). Diprotodon of the Pleistocene
represents the largest known marsupial to ever have evolved.
The largest specimens were hippo-sized at 3 m long, 2.6
m high at the shoulder and weighing in at 2.78 metric tonnes.
Diprotodon had dentition specialized for browsing on relatively
soft vegetation (Kemp, 2005, p. 213). Palorchestes was
horse-sized herbivore resembling a tapir that ranged from
the Miocene to the Pleistocene. Large powerful claws may
have been used to dig for tubers and roots. The snout seems
to have been elongated into a short trunk. During the Pleistocene
kangaroos evolved into medium to large sized browsers and
grazers. Procoptodon was the largest kangaroo and stood
3 m tall. This genus of kangaroo had a short face with eyes
pointing forward. The hind foot had one functional toe
ended in a hoof-like structure. The forelimbs were adapted
for securing branches.
flourished and did not experience significant competition
from placental mammals until aborigines
arrived during the Pleistocene 40,000 years ago. Europeans
introduced more invasive placental mammals during the 1780’s.
Today, habitat destruction and competition from invasive
species endanger Australia’s marsupial populations.