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A small coelurosaur, a feather-tailed dinosaur that lived 99 million years ago,
approaches a resin-coated branch on the forest floor in an illustration.

Chung-tat Cheung
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 * Science
 * Explainer




WHY DID THE DINOSAURS GO EXTINCT?

Learn about the mass extinction event 66 million years ago and the evidence for
what ended the age of the dinosaurs.


ByVictoria Jaggard
Published July 31, 2019
• 6 min read
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Abundant fossil bones, teeth, trackways, and other hard evidence have revealed
that Earth was the domain of the dinosaurs for at least 230 million years. But
so far, not a single trace of dinosaur remains has been found in rocks younger
than about 66 million years. At that point, as the Cretaceous period yielded to
the Paleogene, it seems that all nonavian dinosaurs suddenly ceased to exist.

Along with them went fearsome marine reptiles such as the mosasaurs,
ichthyosaurs, and plesiosaurs, as well as all the flying reptiles known as
pterosaurs. Ancient forests seem to have flamed out across much of the planet.
And while some mammals, birds, small reptiles, fish, and amphibians survived,
diversity among the remaining life-forms dropped precipitously. In total, this
mass extinction event claimed three quarters of life on Earth.

3:32


Dinosaurs 101

Over a thousand dinosaur species once roamed the Earth. Learn which ones were
the largest and the smallest, what dinosaurs ate and how they behaved, as well
as surprising facts about their extinction.

Piecing together what happened has been a massive effort for paleontologists,
and theories for what killed the dinosaurs and the rest of the planet’s
Cretaceous inhabitants have ranged from the plausible to the downright zany. For
now, two leading ideas are battling it out within the scientific community: Were
dinosaurs victims of interplanetary violence, or more Earthly woes?




DEATH FROM ABOVE

One of the most well-known theories for the death of the dinosaurs is the
Alvarez hypothesis, named after the father-and-son duo Luis and Walter Alvarez.
In 1980, these two scientists proposed the notion that a meteor the size of a
mountain slammed into Earth 66 million years ago, filling the atmosphere with
gas, dust, and debris that drastically altered the climate.

Their key piece of evidence is an oddly high amount of the metal iridium in
what’s known as the Cretaceous-Paleogene, or K-Pg, layer—the geologic boundary
zone that seems to cap any known rock layers containing dinosaur fossils.
Iridium is relatively rare in Earth's crust but is more abundant in stony
meteorites, which led the Alvarezs to conclude that the mass extinction was
caused by an extraterrestrial object. The theory gained even more steam when
scientists were able to link the extinction event to a huge impact crater along
the coast of Mexico’s Yucatán Peninsula. At about 93 miles wide, the Chicxulub
crater seems to be the right size and age to account for the dino die-off.

In 2016, scientists drilled a rock core inside the underwater part of Chicxulub,
pulling up a sample stretching deep beneath the seabed. This rare peek inside
the guts of the crater showed that the impact would have been powerful enough to
send deadly amounts of vaporized rock and gases into the atmosphere, and that
the effects would have persisted for years. And in 2019, paleontologists digging
in North Dakota found a treasure trove of fossils extremely close to the K-Pg
boundary, essentially capturing the remains of an entire ecosystem that existed
shortly before the mass extinction. Tellingly, the fossil-bearing layers contain
loads of tiny glass bits called tektites—likely blobs of melted rock kicked up
by the impact that solidified in the atmosphere and then rained down over Earth.


VOLCANIC FURY

However, other scientists maintain that the evidence for a massive meteor impact
event is inconclusive, and that the more likely culprit may be Earth itself.



Ancient lava flows in India known as the Deccan Traps also seem to match nicely
in time with the end of the Cretaceous, with massive outpourings of lava spewing
forth between 60 and 65 million years ago. Today, the resulting volcanic rock
covers nearly 200,000 square miles in layers that are in places more than 6,000
feet thick. Such a vast eruptive event would have choked the skies with carbon
dioxide and other gases that would have dramatically changed Earth’s climate.

Proponents of this theory point to multiple clues that suggest volcanism is a
better fit. For one, some studies show that Earth’s temperature was changing
even before the proposed impact event. Other research has found evidence for
mass die-offs much earlier than 66 million years ago, with some signs that
dinosaurs in particular were already in a slow decline in the late Cretaceous.
What’s more, volcanic activity is frequent on this planet and is a plausible
culprit for other ancient extinctions, while giant meteor strikes are much more
rare. This all makes sense, supporters say, if ongoing volcanic eruptions were
the root cause of the world-wide K-Pg extinctions.


WHY NOT BOTH?

Increasingly, scientists trying to unravel this prehistoric mystery are seeing
room for a combination of these ideas. It’s possible the dinosaurs were the
unlucky recipients of a geologic one-two punch, with volcanism weakening
ecosystems enough to make them vulnerable to an incoming meteor.


RELATED: PHOTOS SHOW THE MYSTERY AND BEAUTY OF FOSSILS

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<p>This nearly whole, deep-black skull belongs to the most complete specimen of
<i>Tyrannosaurus rex</i> on display in Europe, an individual nicknamed Tristan
Otto. With 170 of its 300-odd bones preserved, this scientifically important but
privately owned skeleton is currently at the Museum für Naturkunde in Berlin,
Germany. Discovered in 2010 in Montana’s famed Hell Creek Formation of the late
Cretaceous, the 40-foot-long fossil took four years to excavate and prepare.</p>

This nearly whole, deep-black skull belongs to the most complete specimen of
Tyrannosaurus rex on display in Europe, an individual nicknamed Tristan Otto.
With 170 of its 300-odd bones preserved, this scientifically important but
privately owned skeleton is currently a...

Show moreShow more
Photography by Gerd Ludwig


But that notion depends a lot on more precise dating of the Deccan Traps and the
Chicxulub crater. In 2019, two independent studies looked at geochemical clues
from Deccan Traps lava and came to slightly different conclusions, with one
paper suggesting the volcanoes played a supporting role in the dinosaurs’ demise
by causing pre-impact declines, and the other saying the eruptions came after
the impact event and may have played only a small role in ushering along their
end.

This debate may rage for years, as scientists dig up new clues and develop new
techniques for understanding the past. But whether space invaders or loads of
lava are to blame, it’s clear that scientists studying the dinosaurs’ last gasp
are revealing vital lessons about the effects of dramatic climate change on
Earth’s inhabitants.

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