Evolution, 4th Edition

434 CHAPTER 17

TABLE 17.1 The geological time scale

The Cenozoic era embraces seven epochs, Paleocene through Holocene. The older literature refers to the first five epochs (66–2.58

Mya) as the Tertiary period, and to the Pleistocene and Holocene (or Recent) (2.58 Mya–present) as the Quaternary period. Geologists

now recognize, instead, the Paleogene (Paleocene through oligocene, 66–23 Mya), Neogene (Miocene through Pliocene, 23–2.58

Mya), and Quaternary periods.

Era

Period

(abbreviation)

Epoch

Millions of

years from

start to present

Major events

Quaternary (Q) Holocene 0.012 Continents in modern positions; repeated glaciations and changes of

sea level; shifts of geographic distributions; extinctions of large mammals

and birds; evolution of Homo sapiens, spread out of Africa; rise

of agriculture and civilizations

Pleistocene 2.58

Neogene (Ng) Pliocene 5.33 Continents nearing modern positions; increasingly cool, dry climate;

Miocene 23.03 grasslands spread; modern families of mammals and birds; first apes

Paleogene (Pg) oligocene 33.9 Radiation of mammals, birds, snakes, angiosperms, pollinating insects,

Eocene 56.0 bony fishes

Paleocene 66.0

Cretaceous (K) 145 Most continents separated; continued radiation of dinosaurs; increasing

diversity of angiosperms, mammals, birds; mass extinction at end of

period, including last ammonoids and nonavian dinosaurs

Jurassic (J) 201 Continents separating; diverse dinosaurs and other reptiles; first birds;

diverse mammals; gymnosperms dominant; evolution of angiosperms;

ammonoid radiation; Mesozoic marine revolution

Triassic (Tr) 252 Continents begin to separate; marine diversity increases; gymnosperms

become dominant; diversification of reptiles, including first dinosaurs;

transitional mammal-like forms; modern corals, teleost fishes

Permian (P) 299 Continents aggregated into Pangaea; glaciations; low sea level; increasingly

“advanced” fishes; diverse orders of insects; amphibians decline; reptiles,

including early mammal-like forms, diversify; major mass extinctions,

especially of marine life, at end of period

Carboniferous (C) 359 Gondwana and small northern continents form; extensive forests of early

vascular plants, especially lycopsids, sphenopsids, ferns; early orders of

winged insects; diverse amphibians; first reptiles

Devonian (D) 419 Diversification of bony fishes; trilobites diverse; origin of ammonoids,

tetrapods, insects, ferns, seed plants; mass extinction late in period

Silurian (S) 443 Diversification of agnathans; origin of jawed fishes (acanthodians, placo-

derms, osteichthyes); earliest terrestrial vascular plants, arthropods

ordovician (o) 485 Diversification of echinoderms, other invertebrate phyla, agnathan verte-

brates; mass extinction at end of period

Cambrian (–C) 541 Marine animals diversify; first appearance of most animal phyla and many

classes within relatively short interval; earliest agnathan vertebrates;

diverse algae

Ediacaran 635 Animal fossils (Ediacaran fauna); inferred lineages of sponges, cnidarians,

bilaterians

Cryogenian 720 inferred (from DNA) animal lineages

(others) 2500 Earliest eukaryotes (ca. 1900–1700 Mya)

4000

origin of life in remote past (first fossil evidence at ca. 3500 Mya); diver-

sification of prokaryotes (bacteria and archaea); photosynthesis gener-

ates oxygen, replacing oxygen-poor atmosphere; evolution of aerobic

respiration

Source: Geological names and dates are from the international Commission on Stratigraphy, http://www.stratigraphy.org

CENOZOIC

MESOZOIC

PALEOZOIC

PROTEROZOIC

ARCHEAN

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