Å®ÉúСÊÓƵ

A major shift in the oceans’ circulation wiped out a race of clams 67
to 68 million years ago, says an American palaeontologist. The shift may
also have made life more vulnerable to the catastrophe that soon after caused
the mass extinction in which the dinosaurs died out.

The clams, known as inoceramids, thrived for about 200 million years
in deep waters. But nearly all existing species became extinct in the late
Cretaceous, during the middle of the Maastrichtian Stage. This was 2 to
3 million years before the dinosaurs and most other living species vanished.

Ken MacLeod of the Smithsonian Institution’s National Museum of Natural
History in Washington DC has carried out a study of the remains of inoceramids
in Spain and France. When a clam died, its shell often broke up into thousands
or millions of fragments. Burrowing organisms such as worms and sea urchins
would then disturb the sediment and disperse the clams’ fragments. So the
more abundant these burrowing organisms were, the more spread out the clams’
fragments should be.

MacLeod discovered that the fragments became more spread out as the
clams became rarer, corresponding to the time when they began to die off.
This suggests that even as the inoceramids were becoming extinct, the burrowing
organisms were increasing (Geology, vol 22, p 139).

Other species are also known to have died out during the Maastrichtian.
‘The novelty of the correlation I’ve found is that the decline of the inoceramids
is correlated with a proliferation of something else,’ says MacLeod. ‘So
it’s not that what’s bad for one was bad for everyone; it’s that what’s
bad for one was actually good for others.’

The correlation, says MacLeod, holds the clue to what caused the extinctions.
During the Maastrichtian, the Earth cooled, the sea level dropped, and the
temperature difference between the poles and the equator increased. In response,
suggests MacLeod, ocean circulation patterns may have changed so that water
with a lot of dissolved oxygen flowed into the deep waters in which inoceramids
lived. This killed the clams, which preferred less oxygen, but nourished
the organisms that dispersed the clams’ shells.

The change in ocean circulation may have made the world’s ecosystems
more fragile. This might have accentuated the damage caused when the Earth
probably slammed into an asteroid or comet, triggering the extinctions at
the end of the Cretaceous. ‘Because of the change in ocean circulation,
it’s possible that the Earth was sensitised to a perturbation – things were
in flux and so species were more prone to extinction from a catastrophic
impact,’ says MacLeod.