The Cenozoic Era

Geology 103

• Following the Mesozoic extinctions, – marine invertebrates diversified giving rise to the

present-day familiar marine fauna • Overall, we can think of the Cenozoic Era – as a time during which Earth's flora and fauna

became increasingly familiar• Cenozoic rocks are more easily accessible – at or near the surface, so we know more about

Earth and life history for this time than for any previous eras.

Increasingly Familiar

• Cenozoic rocks are especially widespread in western North America – although they are also found along the Gulf and Atlantic coasts

• As a result, we have a particularly good fossil record for many organisms– Several of national parks and monuments in the West feature

displays of fossil mammals, including:• Agate Fossil Beds National Monument in Nebraska,• Badlands National Park in South Dakota, • John Day Fossil Beds National Monument in Oregon• Hagerman Fossil Beds National Monument in Idaho

Good Fossil Records

• Mammals evolved during the Late Triassic, – and some Mesozoic mammals retained

characteristics of their ancestors, the cynodonts. • By Cenozoic time, mammals had clearly

differentiated from their ancestors.

Evolution of Mammals

• Angiosperms (flowering plants) continued to dominate land plant communities and now constitute more than 90% of all land plants

• Birds evolved during the Jurassic, perhaps earlier– the families now common appeared during the

Paleogene and Neogene, reached their maximum diversity during the Pleistocene Epoch, and have declined slightly since then

Other Biological Events

What periods are in the Cenozoic?The basic problem is that there aren’t many “big” mass extinctions in this era!

Zuni transgression ends; Tejas begins

Antarctica separates from Australia, heads to the South Pole (Eocene)

Eocene thermal maximum• Around 55 my ago,

temperatures worldwide increased to levels similar to the early Mesozoic

• Cause is uncertain, but may have involved increased atmospheric methane due to warming of ocean floor releasing methane from methane clathrates

• Caused many reptilian and mammalian orders to go extinct

Climate story of the Cenozoic is cooling

Grasses: a new type of photosynthesisC4 plants evolved in the Oligocene as a response to lowering CO2 levels, drought conditions and other environmental stresses. Arose in many families of plants simultaneously; a good example of convergent evolution

Whales: another study of convergent evolution

• Eocene hippo-like terrestrial mammals to Oligocene marine mammals

• Evolution of whales stumped Darwin

• Molecular biology and discovery of transitional forms helped explain history

Cope’s rule

• Edwin Drinker Cope (1870) proposed this rule to describe the changes in size of organisms as they evolved

• As time progresses, organisms of the same clade increase in size

• Horses over the Cenozoic are a good example

Himalayan orogeny occurs in Pliocene and continues today

• India detaches from Gondwana and heads north

• Collides with Asia about 10 my at a (tectonically) rapid rate

• Raises Himalayas, Tibetan Plateau, Tien Shan

• Causes northern subtropical jet stream to split around the uplift, and repositions the high and low pressure areas over the Pacific

Cenozoic climate trend is drier

5.96 my – Strait of Gibraltar closes5.96 – 5.33 my – Messinian salinity crisis5.33 my – Zanclean flood event

Closing of Panama isthmus

• Subduction-related volcanism closes isthmus about 3 my

• Allowed ice sheets in Northern hemisphere to form

• More salty water in Atlantic• Water sinks because it cools

as it heads north• Releases heat as it sinks, so

polar oceans never get warmer water

Ice ages start up about 2 my ago

• Why then?• Several factors need to

be in place: polar continent, north/south-oriented oceans, split in the sub-tropical jet stream

• At that point, subtle factors can determine whether the glaciers build up or not

Milankovitch cycles

• Milutin Milankovitch (University of Belgrade, Serbia) proposes that “orbital parameters” that vary in a cyclic fashion over tens of thousands of years will determine that amount of solar insolation that reaches the Earth’s surface

Evidence of Pleistocene glaciations