Page 33 - Reptiles & Dinosaurs (Britannica Illustrated Science Library)

28 DINOSAURS REPTILES AND DINOSAURS 29

Living Life to the Limit Other Proposed Theories 2 FROM HERE…
During the Cretaceous Period, intense
Not all scientists agree with the idea that a volcanic activity on Earth caused frequent,
large meteorite caused the mass extinction copious eruptions of lava and ash that
xtinctions of living beings on Earth have occurred in a series of drastic of the dinosaurs. Rather, they suggest that the exterminated the dinosaurs. Over 386 square
episodes throughout history, from the Cambrian Period to the Cretaceous. Chicxulub crater was formed 300,000 years miles (1,000 sq km) of volcanic rock
E The most famous chapter is associated with the total disappearance of the before the end of the Cretaceous Period. These deposited on the Deccan Plateau in India lend
scientists claim that terrestrial events, such as
credence to this scientific hypothesis of the
dinosaurs about 65.5 million years ago. This mass extinction of these large volcanic eruptions, were more likely to have Cretaceous extinction.
reptiles is so important that it was used by scientists to indicate the end of the 6 miles (10km) caused the Cretaceous extinction. According to …OR FROM THERE
intermediate positions, the eruptions may have
Cretaceous Period and the beginning of the Tertiary, a designation known as the DIAMETER OF THE ASTEROID been caused by a large meteorite impact. 3 As the Solar System crosses the galactic
plane of the Milky Way, every 67 million
K-T boundary (“K” is the abbreviation for Cretaceous). Natural phenomena of that caused the Chicxulub years it changes the paths of meteoroids and
terrestrial or extraterrestrial origin are possible causes of the crater in Mexico comets in the Oort cloud. These bodies could
enter the inner Solar System as meteors and
disappearance of these gigantic animals of the Mesozoic Era. possibly strike the Earth as meteorites.

Fatal Meteorites
In its long geological history, the Earth has witnessed following periods: Cambrian-Ordovician, Ordovician-Silurian,
1 several mass extinctions. Some scientists argue that Devonian-Carboniferous, and Permian-Cretaceous. However,
the cause could be the same in all cases, and they scientists have yet to determine a convincing factor that could
point to extraterrestrial phenomena as the most likely cause. be the cause in all cases. The Devonian extinction exterminated
However, this hypothesis has been widely criticized. From the 50 percent of all species, much like the one occurring at the K-
Paleozoic Era 570 million years ago to the Cretaceous Period, it T boundary. However, the largest extinction of all occurred in
has been determined that there were five or six mass the Permian Period, in which 95 percent of all species were
extinctions on Earth, which mark the boundaries between the eliminated.

CLUES IN CHICXULUB
In the town of Chicxulub, on the Yucatán
Peninsula in Mexico, a depression was found
that measured 112 miles (180 km) across. This
enormous imprint was evidence of the violent
impact of an extremely large meteorite that
crashed into the Earth.
MIXED ROCKS 50
Samples taken from the
Chicxulub crater show a mixture OF ALL SPECIES
of terrestrial minerals (dark 50% became extinct at
POST-EXTINCTION LAYER areas) and meteorite the K-T boundary.
Sediments of microfossils minerals (light areas). million
from eras later than that of
the dinosaurs
LOCATION OF THE CRATER ATOMIC BOMBS
like the one dropped on
CUBA
Outer Edge of Hiroshima equal the force
FIREBALL LAYER the Crater Yucatán
Dust and ash from the Channel of the impact of one
meteorite's impact meteorite measuring 6
Cancún miles (10 km) in diameter.
Campeche Mérida
Bay
EJECTION LAYER Cozumel
Materials from the
crater that settled MEXICO
during several months
112 miles (180 km)
0 100 200 miles
PRE-EXTINCTION GUATEMALA
(0 160 320 km)
LAYER
Sediments with DIAMETER OF THE CHICXULUB CRATER on the Yucatán Peninsula
microfossils from
the era of the
dinosaurs