Å®ÉúСÊÓƵ

NEW evidence from the pattern of trilobite extinctions confirms the
idea that mass extinctions are qualitatively different from the normal ‘background’
extinctions which occur all the time. The idea, developed over the past
couple of years by David Jablonski, a palaeontologist at the University
of Chicago, has always needed testing. Now,a Canadian palaeontologist, Stephen
Westrop, has comeup with strong evidence thatdifferent rules do indeedapply
during mass extinctions(Paleobiology, vol 15, p 46).

Westrop, at Brock University, Ontario, has studied trilobites from North
America and a particular mass extinction in the Cambrian period. These armoured
arthropods teemed in the Cambrian seas 500 million years ago. According
to Westrop, patterns of disappearance of trilobite families imply that,
during such mass extinctions, creatures that are able to adapt more easily
to changing conditions fare no better than their less adaptive counterparts.
Although species may be particularly successful during times of normal background
extinction because, for instance, the individuals are better at foraging
or at defending themselves against predators, this counts for nothing when
‘The Big One’ arrives.

Westrop has found that this shows up in the fossil record of trilobites
in a distinctive way. ‘Some species,’ says Westrop, ‘persist through long
periods of time outside mass extinctions, while others come and go quickly.
This longevity must depend on species’ adaptations, the longer-lived ones
being somehow able to escape the threat of background extinction more effectively.’

If these same adaptations had the same kind of protective effect during
mass extinctions, the normally long-lived species should be more common
after the Big Ones. ‘That’s not what you see,’ says Westrop. ‘There is no
effect of species’ longevity on the survival of families through mass extinctions.’

Jablonski has studied the fate of various mollusc families at the Cretaceous/Tertiary
extinction 65 million years ago. He sees the new rule this way: ‘Families
are more likely to survive mass extinctions if they have a broad geographic
distribution, no matter how that is achieved.’ Broad geographic distribution
of a family can result, for instance, from its species each having widespread
populations, or from having widely distributed species, each of which has
only a narrow geographic range.

One particularly striking difference in the fate of families between
regimes of normal and background extinction relates to the size of the family,
or how many species it includes. ‘High species richness is good insurance
against extinction in normal times,’ says Jablonski. ‘But in mass extinctions,
family size makes no difference. Geographic range is all.’

Westrop draws the same kind of conclusions from his trilobite data.
‘It’s impressive that the patterns are so similar,’ comments Jablonski,
‘given that I’m dealing with clams from 65 million years ago and he’s looking
at trilobites from 500 million years ago . . . It tells you that ecosystems
work the same wherever they are and whenever they are.’

This novel view of a qualitative difference between background and mass
extinctions has important implications for the history of life on Earth.
If survival through extinctions of all kinds were indeed governed by the
quality of species’ adaptations, then it would be legitimate to aknowledge
that the ones that make it through are superior (in a Darwinian sense).
But, if, as the work of Jablonski and Westrop suggests, this rule applies
only during background extinctions and is suspended at mass extinctions,
then survivors of these events must be counted as more lucky than superior.

The same Cretaceous/Tertiary extinction that wrought havoc among some
of Jablonski’s molluscs while leaving others relatively unscathed, also,
of course, changed life on Earth in another important way: it extinguished
the dinosaur, and mammals – particularly placental mammals – took centre
stage.

Recently, there has been much speculation that the dinosaurs were already
in decline and were simply delivered a coup de grace by the extinction event.
But biologists have always assumed that the placental mammals came to predominate
over their marsupial cousins because of some kind of physiological superiority
they possessed. Now it seems that this may not have been necessary at all.

According to William Clemens of the University of California, Berkeley,
marsupials were geographically restricted to regions near the equator when
this big extinction event hit, whereas placental mammals enjoyed a much
broader range. This fact alone, according to Jablonski’s ideas, could explain
why the marsupials were nearly eclipsed, while the placental mammals fared
better and subsequently boomed. Part of that boom was the adaptive radiation
of the primates – including Homo sapiens.