Evolution in the Fossil Record

Goals

• An evolutionary biologist’s geology toolkit• Understand the nature of the rock record• Understand the nature of the fossil record• See what kinds of data fossils can give us

Earth, from outer space

• Some facts about Earth:– Age: ~4.6 billion years– Weight: 6x1024 kg (heavy!)– Hot on the inside, cooler on the outside– Spins on its axis and makes orbits around the…– Sun– Is home to a lot of water

Earth, from outer space

• How do we know that there is life on Earth?• Life has caused planet-scale phenomena• Life and the planet: a give and take

Earth, from inside

Earth, from the ground:Plate tectonics

• Proposed in 1915 but no mechanism suggested• Accepted in 1960 when mechanism proposed• Data supported this mechanism

Earth, from the ground:Plate tectonics

Earth, from the ground:Plate tectonics

Earth, from the ground

This map illustrates the break-up of the supercontinent, Rodinia, which formed 1100 million years ago. The Late Precambrian was an "Ice House"

World, much like the present-day

Earth, from the ground

Animals with hard-shells appeared in great numbers for the first time during the Cambrian. The continents were flooded by shallow seas. The

supercontinent of Gondwana had just formed

Earth, from the ground

Vast deserts covered western Pangea during the Permian as reptiles spread across the face of the supercontinent. 99% of all life perished during the extinction event that

marked the end of the Paleozoic Era

Earth, from the ground

During the Cretaceous the South Atlantic Ocean opened. India separated from Madagascar and raced northward on a collision course with Eurasia.

Notice that North America was connected to Europe, and that Australia was still joined to Antarctica.

Earth, from the ground

We are entering a new phase of continental collision that will ultimately result in the formation of a new Pangea supercontinent in the future

Earth, from the ground

New subduction zones along the eastern coasts of North America and South America will begin to consume the ocean floor separating North America

from Africa

Earth, from the ground

The next Pangea, "Pangea Ultima" will form as a result of the subduction of the ocean floor of the North and South Atlantic beneath eastern North

America and South America. This supercontinent will have a small ocean basin trapped at its center

Rock

Igneous rock

here be no fossils

Rock

Sedimentary rock

here be fossils

Rock

Metamorphic rock

here be damaged fossils

Rock

The Geological time scaleCorrelation of rock: relative ages

• Strata: horizontal bands = a point in time• Older are lower, vertically• Younger are higher, vertically

The Geological time scaleCorrelation of rock: relative ages

• Strata: horizontal bands = a point in time• Older are lower, vertically• Younger are up higher, vertically• Strata often differ in the kinds of fossil

organisms they host

CorrelationFossil organisms can be used to correlate rocks

Many geological time points are defined by the organisms found within

There are, for example, 67 global ammonite zones for the 55 myr long Jurassic

When does the Cretaceous end? When the ammonites disappear

Absolute age of rocks

• Once we have the relative order of stages, how do we assign absolute dates?

Ash beds are used for absolute dates. The rocks they occur in are dated. Fossils are assigned to rocks, not dates.

½ life = 0.7 billion years

Gaps in the rock record

• Why gaps?

• Sea level fluctuates– Imagine a point just offshore right now that is receiving sediment.

Organisms can become trapped in this sediment to form fossils– Now imagine sea level drops. That point will no longer receive sediment – Now imagine sea level rises gain, this time higher than ever. That point

will experience erosion, and all fossils that may have been laid down before will disappear from the rock record

– In any particular rock column, there may be only pulses of sedimentation

Fossils

Issues– How complete is the fossil record? – If we glean patterns from fossil data, can we trust them?

FossilsThe fossil record is incomplete

1. Delicate, they can decay rapidly

2. Sediments formed sporadically

3. Fossil bearing sediments must become rock

• Rock formation

• Rock persist

• Rock found by paleontologists

FossilsThe fossil record is incomplete

FossilsTwo basic questions:

1. Provides evidence for evolutionary change?

2. Evolution is gradual versus the idea that changes are discontinuous

Fossils:missing links

• Are fossils ancestors?

• Probably not.

Evolutionary Change within Species

Changes in the mean number of ribs in eight lineages of trilobites

Evolutionary Change within Species

Changes in the mean values of fin rays in fossil sticklebacks

Evolutionary Change within Species

Changes in the mean values of dorsal spines in fossil sticklebacks

Origins of Higher Taxa

Origin of Amphibia

Eusthenopteron, a member of the group of lobe-finned fishes from which tetrapods arose

Origin of Amphibia

Origin of Birds

Skeletal features of (A) Archaeopteryx, (B) a modern bird, and (C) a theropod dinosaur

Origin of Birds

A phylogeny of some groups of theropod dinosaurs

Origin of Birds

Estimated body weights of fossil hominins

Hominin Fossil Record

Hominin Fossil Record

The approximate temporal extent of named hominin taxa in the fossil record

Hominin Fossil Record

Frontal and lateral reconstructions of the skulls of a chimpanzee and some fossil hominins

Hominin Fossil Record