Life of the Oceans - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 159

truly included a land-based extension. This was in its infancy, though,
while its oceanic parent was already very old: not that it still didn't
have a few tricks up its sleeve—to take back, for instance, part of what
it had given.
Life on land originated, thus, from marine antecedents. In time
some land animals would return to the sea. They would become the
most magnificent organisms that have ever lived.
Magnificent Whales
It was some 16 million years after the calamity of the end-Cretaceous
extinction event, and after the demise on land of the dinosaurs, and in
the sea of marine reptiles such as the ichthyosaurs and plesiosaurs,
and of the beautiful coiled ammonites. The oceans had changed in
other ways too. Planktonic micro-organisms with calcium carbonate
skeletons had evolved, their tiny skeletons now raining down end-
lessly on to the sea floor to make up the pale oozes that, over a cen-
tury ago, turned up in bucket after bucket of HMS Challenger 's
laborious dredging programme (see Chapter 3).
Along the luxurious tropical coastline of an ancient India lived
Pakicetus , a four-legged mammal. Pakicetus looked rather like a long-
legged large rat: one big enough to frighten the living daylights out of
your average domestic cat. It stood knee-high to a human, and was a
little over a metre long. Pakicetus may have lurked in the shallows, hid-
den in water up to its snout and able to pounce, wolfishly, on both
marine and land-based prey. The ancestors of Pakicetus were hoofed
animals that kept firmly to the land. Its relatives though, were any-
thing but. The telltale structure of the ears of Pakicetus are the clue to
its family connections. The overall shape of the skull in this animal
indicates that it heard sound like other land-based mammals. But
Pakicetus possessed a very robust tympanic bulla—the bit of bone in
its skull used to conduct sound to the ear—and in this case adapted