The Tertiary and Quaternary, the original periods of the Cenozoic, were divided into epochs by Charles Lyell in 1833, and the names he gave are still retained, although they are now assigned to the Paleogene, Neogene, and Quaternary Periods.
He used the same root word, “cene,” meaning recent, that is the prefix of the era, Cenozoic, but Lyell used it as a suffix when he named the epochs. He had analyzed assemblages of fossils from the different Cenozoic formations, and using the percentages of species that still were alive at present, he assigned names to the time periods that were marked by increasingly modern or recent fossil assemblages. He named the Eocene, which means “dawn-recent,” for rocks with fewer than 5% of modern species present, Miocene, “less recent,” for those that had 20-40% of modern species, and Pliocene, “more recent,” for those that were 40% to 90% modern. Since Lyell’s time, we’ve added Paleocene, “ancient-recent,” with essentially no modern species, Oligocene, “little or few-recent,” Pleistocene, “most recent,” and Holocene, “entirely recent.”
The Paleocene, which began with the end of the Cretaceous, and the Eocene and Oligocene make up the Paleogene Period. The Miocene and Pliocene are the Neogene Period, and the Pleistocene and Holocene are the epochs of the Quaternary Period. The boundary between the Paleogene and Neogene is put at 23 million years ago, and the Quaternary started 2.5 million years ago and basically coincides with the glacial age.
So, that makes the Paleogene about 43 million years long, pretty typical for a geological period, and the Neogene is 21 million years long, which is on the short side. The Quaternary, at 2.5 million years, is an exceedingly short time span for a period, but it’s OK because we know so much about it, and because it does represent a time when things changed a lot, the modern ice ages. There is some argument as to whether the Quaternary should be seen as a full-fledged period or should be an epoch of the Neogene. All semantics and technicalities. For now I’m going with what I perceive to be the internationally recognized scheme that has the Quaternary as a period, not that it really matters.
I realize that that’s a lot of jargon and detail. I’ll try over the course of the month when I use names like Oligocene or whatever to keep it clear where we are as we progress through the Cenozoic.
To the Pliocene Skull – by Bret Harte
(A GEOLOGICAL ADDRESS)
Speak, O man, less recent! Fragmentary fossil!
Primal pioneer of pliocene formation,
Hid in lowest drifts below the earliest stratum
Of volcanic tufa!
Older than the beasts, the oldest Palaeotherium;
Older than the trees, the oldest Cryptogami;
Older than the hills, those infantile eruptions
Of earth's epidermis!
Eo--Mio--Plio--whatsoe'er the 'cene' was
That those vacant sockets filled with awe and wonder,--
Whether shores Devonian or Silurian beaches,--
Tell us thy strange story!
Or has the professor slightly antedated
By some thousand years thy advent on this planet,
Giving thee an air that's somewhat better fitted
For cold-blooded creatures?
Wert thou true spectator of that mighty forest
When above thy head the stately Sigillaria
Reared its columned trunks in that remote and distant
Tell us of that scene,-- the dim and watery woodland,
Songless, silent, hushed, with never bird or insect,
Veiled with spreading fronds and screened with tall club mosses,
When beside thee walked the solemn Plesiosaurus,
And around thee crept the festive Ichthyosaurus,
While from time to time above thee flew and circled
Tell us of thy food,--those half-marine refections,
Crinoids on the shell and Brachipods au naturel,--
Cuttlefish to which the pieuvre of Victor Hugo
Seems a periwinkle.
Speak, thou awful vestige of the earth's creation,
Solitary fragment of remains organic!
Tell the wondrous secret of thy past existence,--
Speak! thou oldest primate!
Even as I gazed, a thrill of the maxilla,
And a lateral movement of the condyloid process,
With post-pliocene sounds of healthy mastication,
Ground the teeth together.
And from that imperfect dental exhibition,
Stained with express juices of the weed nicotian,
Came these hollow accents, blent with softer murmurs
"Which my name is Bowers, and my crust was busted
Falling down a shaft in Calaveras County;
But I'd take it kindly if you'd send the pieces
Home to old Missouri!"
—Richard I. Gibson
Bret Harte photo in public domain
Time scale by Richard Gibson