Last week I explained the larger divisions of the geologic time scale: the eon and the era. Now, it’s time for a smaller division: the period.
My sophomore year of college, I took a intro geology class and my professor was a bit eccentric. He taught us a mnemonic to remember the periods of the geologic time scale: Can Orville See Down My Pants Pocket? Tom Jones Can (see a) Pair (of) New Quarters. It might seem ridiculous, but it’s the one that I still use. The first letter of each word in the mnemonic refers to the first letter of each of the periods: Cambrian, Ordovician, Silurian, Devonian, Mississipian, Pennsylvanian, Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene, Quaternary. You’ll probably notice that most of the periods are named for where they were first studied.
The Cambrian was named after Cambria, the Latin word for Wales. It lasted from 541 million years ago until 485.4 million years ago. During the Cambrian, there was a rapid diversification of multicellular life known as the Cambrian Explosion. The Burgess Shale in Yoho National Park in Canada is an excellent example of the diverse marine fauna from that time.
The Ordovician was named for the Ordovices tribe of Northern Wales. It lasted from 485.4 million years ago until 443.4 million years ago. Jawed fish first appeared toward the end of the Ordovician and the Ordovician-Silurian boundary is marked by a mass extinction that killed off 60% of marine genera.
Like the Ordovician, the Silurian was named for the Silures tribe of southeastern Wales. It lasted from 443.4 million years ago until 419.2 million years ago. The first terrestrial vascular plants and first bony fish both originated in the Silurian. This period also inspired the named of the Silurians, a humanoid race in the television show Doctor Who.
Like the Cambrian, the Devonian was named for Devon, England. It lasted from 419.2 million years ago until 358.9 million years ago. The Devonian is often called ‘The Age of Fish’ because of the rapid diversification that occurred at that time. Much of what is now North America was under a shallow sea, but that didn’t stop the first seed plants from appearing.
Carboniferous breaks down into carbon and –iferous. Carbon comes from the Latin word ‘carbo‘ which means coal and iferous comes from the Latin word ‘ferous‘ which means bearing. It lasted from 358.9 million years ago until 298.9 million years ago. Terrestrial life became abundant and consisted mainly of amphibians and massive insects. Vast forests formed the coal that this period is known for. In North America, this period is commonly broken down into the ‘Mississippian’ and ‘Pennsylvanian’ subperiods.
The Mississippian was named for the Mississippi River valley where rocks of this age are exposed. It lasted from 358.9 million years until 323.2 million years ago. Ocean levels were high, which led to the prevalence of limestone sediments.
The Pennsylvanian was named for the state of Pennsylvania where rocks of this age are widespread. It lasted from 323.2 million years ago until 298.9 million years ago.
The Permian was named for the Perm region of Russia. It lasted from 298.9 million years ago until 252.2 million years ago. Instead of forests, arid deserts became widespread which led to the dominance of reptiles. Along with reptiles, other amniotes (creatures with an amniotic egg that doesn’t have to be laid in the water) began to diversify. Land consisted of the supercontinent Pangaea and the Permian ended with the largest mass extinction in Earth’s history that killed off 90% of marine species and 70% of terrestrial species.
The Triassic was named for the Greek word ‘triad‘ meaning three by German
geologist Friedrich August von Alberti because in Germany it was easily divided into three parts. It lasted from 252.2 million years ago until 201.3 million years and began ‘The Age of Reptiles’. Pangaea began to split into Laurasia in the north and Gondwana in the south. Both dinosaurs and mammals originated in the late Triassic. It ended with another, smaller mass extinction which helped the dinosaurs rise to dominance.
The Jurassic comes from the French word ‘Jurassique‘ which means from the Jura Mountains between France and Switzerland. It lasted from 201.3 million years ago until 145 million years ago. As Pangaea continued to split into Laurasia and Gondwana, the climate became more humid with rain forests and dinosaurs dominated the terrestrial fauna.
Cretaceous comes from the Latin word ‘creta‘ which means chalk. It lasted from 145 million years ago until 66 million years ago. The climate was warm and humid and as Laurasia and Gondwana continued to split apart inland seas formed. Flowering plants appeared and mammals and birds diversified. The Cretaceous ended with a mass extinction, likely triggered by a meteorite impact in the Yucatan peninsula, that wiped out the non-avian dinosaurs, pterosaurs and many marine creatures. Strangely, many of the dinosaurs from the movie Jurassic Park actually lived during the Cretaceous.
Paleogene breaks down into paleo and –gene. We already know that paleo means ancient and gene is a Greek word which means generation. It lasted from 66 million years ago until 23 million years ago and was the beginning of ‘The Age of Mammals’. The climate began to cool and become drier with greater seasonal differences, with periodic warm periods, largely because of the Antarctic Circumpolar Current. Mammals ran rampant and some took to the sea to become cetaceans (whales and dolphins).
Neogene breaks down into neo and –gene. Neo is a Greek word meaning new or recent and we already know that gene means generation. It lasted from 23 million years ago until 2.5 million years ago. The climate continued to cool, grassland spread across the interiors of continents and mammals began to take more modern forms. Hominids (human ancestors) spread throughout Africa. The continents assumed a more modern position. The Neogene is used by scientists as a proxy for climate prediction models because it was slightly warmer than today.
Quaternary comes from the Latin word for four and is from an archaic system that named geologic eras as ‘primary’, ‘secondary’, ‘tertiary’, ‘quaternary’, etc. It lasted from 2.5 millions years ago until the present day and began with an ice age in the Northern Hemisphere. The glaciers retreated about 10,000 years ago and carved the landscape that we know today, such as the Great Lakes. With the retreat of the glaciers came the extinction of many large mammal species such as mammoths, sabertooth cats and ground sloths. Humans spread across the globe and brought us the world we know today.
Next week we’ll talk about the smallest common division of the geologic time scale: the epoch.