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4,540 Ma
Formation
of Earth4,000 Ma
First life
3,200 Ma
Earliest start of
photosynthesis530 Ma
Cambrian
explosion4
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*58 DISCOVERMAGAZINE.COM
Ma = Millions of years ago
*The Archean
and Proterozoic
eons/eonothems
both fall under the
informal label of
Precambrian Time.EVERYTHING
WORTH
KNOWING
CHRONOSTRA-WHAT? The long word
may be new to you, but you’ll find
the concept familiar. Case in point:
Jurassic Park. You can thank chro-
nostratigraphy for the name, even
though any dino nerd will tell you it
should have been called Cretaceous Park.
That’s because most of the animals in
the park, including T. r e x, lived around
the end of the Cretaceous, tens of mil-
lions of years after the Jurassic. How
do paleontologists know that? Thank
chronostratigraphy for that one, too.
During 18th-century mining explora-
tions and early 19th-century fossil digs,
expeditioners noticed similarities in
rocks over large geographic areas. And so
stratigraphy — the study of layers (strata)
of rock in relation to each other
— was born. Chronostratigraphy
is a modern offshoot of this discipline,
organizing these dateable rock layers into
chronological units. The standardized
system gives geologists, paleontologists,
and researchers from many other fields a
framework of how our planet, and life on
it, has changed over time.
Let’s get deep about time.
BY GEMMA TARLACH
Chronostratigraphy
Units Big and Small
Eonothems/Eons: The entirety of Earth’s existence
is formally divided into just three of these
largest of units. Two of them — the Archean and
Proterozoic — are informally lumped together as
Precambrian Time. A third span, the Hadean, is
also included in the Precambrian; it represents
our planet’s infancy, starting some 4.6 billion
years ago. But due to the dearth of dateable
geologic material from that far back in time,
scientists disagree on whether the Hadean
should be recognized as a formal eon. However
you divvy up Precambrian Time, it ended a mere
541 million years ago. The eon that followed, the
Phanerozoic, is still going strong today.
Erathems/Eras: These units broadly reflect
evolving complexity among living things. The
Phanerozoic, for example, is divided into three
erathems, or eras: Paleozoic (“old life,” from
rapidly diversifying multicellular organisms to
the first land vertebrates), Mesozoic (“middle
life,” including dinosaurs, early mammals and
the first flowering plants) and Cenozoic (“recent
life,” basically everything that survived or evolved
after the mass extinction that ended the Mesozoic66 million years ago).
Systems/Periods: Generally 30 million to
80 million years long, though the current
Quaternary began just 2.58 million years ago.
Series/Epochs: About 13 million to 35 million
years long.
Stages/Ages: The smallest of the units, they last
2 million to 10 million years. Paleontologists and
other researchers using chronostratigraphy, or
its twin, geochronology, typically report results
based on stage (or age) rather than a larger unit.
2
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