"At least five mass extinctions,
most presumably caused by asteroids
that struck the earth,
have transformed global ecology
in the half-billion years
since the emergence of multicelled life,
lopping entire branches
from the evolutionary tree
and allowing others to flourish.
.

The greatest “great dying,”
251 million years ago,
erased 95 percent of species
in the oceans
(and most vertebrates on land).

followed by
an explosion of complexity
in marine life,

Moreover, it happened quite suddenly,

The shift to complicated, interrelated ecosystems
was more like
a flip of a switch
than a slow trend.

marine life before the biggest global die-off,
the Permo-Triassic extinction,
was evenly split
into two types of communities:
simple ones,
in which most species were anchored
in place
and got by
without interacting with neighbors

and
complex ones,
with many interrelationships.

now
complex communities filled with grazers,
scavengers,
predators,
burrowers
and other mobile creatures
have been
three times as common
as simple ones,

The shift essentially took the oceans
from a norm in which anchored
(or sessile) creatures,
including brachiopods and sea lilies,
filtered food carried
in currents
to one dominated by
roaming (or motile) fauna
like snails, urchins and crabs.

it is not clear
why this particular extinction spasm
had this permanent effect
on the character of communities,
while others did not.

motility was an enduring characteristic of the more variegated biological webs.

“The increased diversity of mobile species
would have contributed
to more complex ecological communities,”

“With sessile guys,
everybody is just living next to one another
and that’s it.
With mobility and higher metabolism,
you bump into each other
more often,
both literally and figuratively,
and you end up
with a greater number of potential interactions.”