and the Marsupials - A Historical and Modern Perspective
| The oldest known Australian
marsupial fossils are from a 55 million year old, Early Eocene site at
Murgon in southern Queensland. The site has produced a range of marsupial
fossils, many with strong South American connections. The oldest
monotreme species yet known is Teinolophos
fossil jaw of Steropodon galmani, an Early Cretaceous, platypus-like
monotreme. It was the first Mesozoic mammal to be discovered in Australia.
The specimen has been fossilized in the form of opal, and was found at
Lightning Ridge, New South Wales. Courtesy: Australian Museum.
whose fossils are dated at 123 million years old (Early Cretaceous), from
Flat Rocks, Victoria. Another very old monotreme from Australia is
galmani, a somewhat platypus-shaped animal known from a small
section of a 110 million year old mandible. In 1991 and 1992, several
fossil teeth from a 61 million year old platypus were found at Punta Peligro
in southern Argentina (Pascual et al. 1992). Named Monotrematum
sudamericanum, the teeth
|are thought to actually belong to a member
of the genus Obdurodon, the remains of which have also been found
in Australia. Placentals are known from the Tertiary of Australia
as well, including some whales and a bat from the Middle Miocene (15 million
years ago), and rodents of the family Muridae from the Early Pliocene (4-5
million years ago).
In 1987, the aforementioned 55 million year old Murgon site also produced
an isolated tooth that is thought to be from a condylarth, which has been
porterorum. Condylarths are an ancient group of placental
mammals known from various other regions of the world, and the presence
of Tingamarra in Australia indicates that placentals were indeed
present there at the time when the continent broke away from Antarctica.
The world's oldest fossil bat was also found at the Murgon site.
Very old marsupial fossils
have been found on other continents, most notably, North America and Asia.
Perhaps some knowledge can be gleaned about the possible origin of the
Australian marsupials from the geographical and chronological distribution
of these non-Australian fossils. Until rather recently, the oldest
marsupial fossils were some isolated teeth found in 110 million year old
Cretaceous sediments from North America. In the year 2000
however, a new fossil (proto)marsupial, Sinodelphys
szalayi, was found in China's Liaoning Province. Dating back
to 125 million years ago, it is the oldest marsupial yet known, and may
indeed be the great-grandparent of all marsupials.
life reconstruction of the Late Cretaceous Sinodelphys szalayi,
the oldest marsupial yet discovered. Only one fossil specimen is
known, a slab and counterslab which is held in the collection of the Chinese
Academy of Geological Sciences. The species grew to only 15cm (5.9
in) long (including the tail), possibly weighed about 30g (1.05 oz), and
lived in much the same way as small modern opossums such as Monodelphis.
Courtesy: Carnegie Museum
of Natural History.
| Another fossil mammal
from Asia that has much increased our understanding of marsupial evolution
is 80 million year old Deltatheridium, from the Late Cretaceous.
At first, the species was known only from very fragmentary remains, and
there was considerable debate about whether it was a marsupial or a placental.
However, some well
preserved fossils of Deltatheridium were recently discovered
at Ukhaa Tolgod, Mongolia which have made it possible to firmly classify
it as a marsupial.
recently discovered mandibles of Deltatheridium, a Late Cretaceous
marsupial from the Gobi Desert of Mongolia. The animal had a length
of approximately 15cm (5.9in). Note the sharp cheek teeth with wide,
triangular crowns. Deltatheridium was insectivorous, and may
have also eaten small reptiles and other animals. Courtesy: University
In Europe, the oldest known marsupials are from the Eocene (about 50 million
years ago). They have been interpreted as having been immigrants
of North American stock before North America and Europe had become separated
by the forces of continental drift. At some time between the Late
Eocene and Early Oligocene epochs, one marsupial species apparently succeeded
in crossing over to North Africa, where its fossils were discovered at
a single locality in Egypt. Marsupials appear to have disappeared
from Europe during the Miocene (between 5-23 million years ago).