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How North America got its plants and animals back

My latest Mind and Matter column in the Wall
Street Journal is about what happened to the cology of North
America after the asteroid impact of 66 million years ago:


Last week, just as a meteorite exploded over Russia, I used this
space for an email to Charles Darwin, wherever he is. I told him
about the now overwhelming evidence for an asteroid impact having
caused the extinction of dinosaurs. I thought he would be
interested because it is a striking exception to his
“uniformitarian” assumption that, in the past, evolution was shaped
by the same forces still operating on Earth today.

His ghost emailed back to say that he wanted to know what
happened after the impact. He always had a long-standing interest
in colonization—how animals and plants find and fill vacant lots in
nature. So here’s my reply:

To crdarwin@evolution.hvn

Dear Dr. D (sorry for my informality last week),

When the asteroid slammed into the Yucatán Peninsula 66,038,000
years ago, North America took the brunt of the impact, because the
asteroid came in from the southeast like a golf chip shot. Most of
what is now the American Midwest was then under shallow water
called the Bearpaw Sea, but the land on either side was devastated.
The shock wave destroyed almost every living thing for hundreds of
miles, then a “divot” of dust and mud buried the continent, quickly
followed by tsunamis that penetrated far to the north.

The Earth’s southern continents suffered far fewer extinctions,
though there was still a marked drop in the abundance and diversity
of animals and plants. Nonetheless, the effects of the collision
were felt globally. All around the world, the dinosaurs died
out—too big and specialized to cope, even if they survived the
blast. With them went the last of the flying pterosaurs, many kinds
of birds and mammals, lots of plants and a huge range of sea

The animals that suffered the fewest extinctions were ones like
alligators, turtles and salamanders. Perhaps protected by the
water, while feasting on the dead or on water plants that did not
suffer so much, they were also used to withstanding long fasts.

When the dust settled, as the paleontologist Tim Flannery
recounted in a fine book “The Eternal Frontier,” North America was
virtually devoid of flora and fauna, except in the far North and in
pockets to the west of the Sierra Nevada mountains. It must have
been a great brown, dusty and muddy waste. Pollen evidence shows
that ferns were the first plants to bounce back, just as they were
after the eruptions of Krakatau and Mount St. Helens. The pollen
and spore record shows a sharp “fern spike.” Soon ferns must have
dominated the landscape so much that they formed endless,
impenetrable thickets.

When trees did recolonize, they did so from the far north (the
North Pole was under land and ice-free at this time), meaning that
deciduous types of trees came to dominate even warm parts of the
North American continent. Mammals reappeared within 20,000 years,
probably from Asia via an Arctic land bridge. South America was off
doing its own thing, farther south than it is now, and could not
supply animals, apart from birds, to its northern neighbor.

All this we now know. The amazing thing about this story,
Charles, is that not a jot of it was known even when I was a boy,
let alone you. There is something almost miraculously exciting
about the way we can recover and relive the past adventures of the
planet in glorious detail thanks to modern science.

You once wrote, in a letter to your sister from the Falkland
Islands in 1834, of the pleasure of “finding a fine group of fossil
bones, which tell their story of former times with almost a living
tongue.” Today, almost 180 years later, the fossils, isotopes and
spores not only speak softly of slow and gradual changes but cry
out the story of a violent and truly terrible catastrophe that once
transformed life on Earth.

By Matt Ridley | Tagged:  rational-optimist  wall-street-journal