Travel Reference
In-Depth Information
The fi rst chunks to split of were Africa and India, calving of towards the north-
east. Showing an early independent streak, India promptly began a surprisingly
swift journey up the entire length of the nascent Indian ocean towards Laurasia.
The gap between Africa and the rest of Gondwana widened, accom-
panied by seafl oor spreading in an arm of the ocean that would eventually
become the Atlantic. The remainder of Gondwana, a complex region made
up of nascent South America, Antarctica, and Australia, was still a single
landmass, but it too began to break up. At the same time this whole chain
of nascent continents moved south, so that the center of it, which would
become Antarctica, entered the polar region. Antarctica remained warm,
because the whole planet was warm, but its nights got longer and longer.
Big-eyed nocturnal dinosaurs evolved in Antarctica.
South America fi nally broke away completely from Antarctica and Aus-
tralia about 70 million years ago, near the beginning of the Age of Mammals.
It entered a long period of isolation as a gigantic island. Australia and Ant-
arctica then reached an irrevocable parting of the ways roughly 35 million
years ago.
It was at about the time that Gondwana began to break up that three great
lineages of mammals emerged from mammal-like ancestors called ther-
and have survived down to the present only in Australia and New Guinea.
This entire order of mammals now survives only as duck-billed platypuses
and two species of echidnas—snul y, Hoover-like spiny anteaters.
The second lineage, the Marsupials, raise their young in pouches. At the
start of its life the little marsupial embryo is nourished briefl y by a yolk-like
placenta inside its mother's womb, but soon crawls out of the womb and
attaches itself to a nipple in the pouch. Marsupial reproduction is adapted to
desert life—under unfavorable conditions some kangaroos can evert their
pouches and spontaneously abort the tiny joeys. This leaves the mothers
free to start a new family when food is more plentiful.
The third group, the placental mammals including ourselves, have
evolved a placenta, a mass of vascularized tissue that permits transfer of
nutrients between mother and baby. The baby's development takes place
entirely inside the mother's protected womb.
