Transitional FormsTransitional FormsConsider this Hypothesis:Consider this Hypothesis:

A

C

BTIM

E

What would the organism at position “C” look like?

How about the one at position “A”? “B”? “D”?

Organism A represents a transitional form from C to dogs.

Organism B represents a transitional form from A to dogs.

What is organism “D”?

What is organism “C”?

Are A, B, C & D alive today?

How could we find evidence for the existence of A, B, C or D?

65 MYBP

D

FossilsFossils

Permineralized FossilsPermineralized FossilsThese are what you often think

of when you think of fossils

These fossils form when the organism is buried in sediment and dissolved minerals in cells and in the sediment precipitate to form a mineralized structure.

Fossil FormationFossil Formation

Animal dies and is protected from scavanging

Animal becomes covered by sediment and permineralization occurs

Geologic pressure converts sediments to rock

Shifts in geologic formations or erosion may alter the shape of the rock formation, ultimately exposing the fossil

Lots of different

processes may affect whether

a fossil is formed Fossilization is a rare Fossilization is a rare

event.event. Fossilization of a Fossilization of a

whole organism is whole organism is even rarer.even rarer.

Finding fossils Finding fossils depends on finding depends on finding exposed surfaces exposed surfaces where fossils may where fossils may have been deposited.have been deposited.

Consider this treeConsider this tree

A

B

C

D

Based on what you know about how fossils form, how likely do you think it is that you could find fossils for every ancestor of the organism called lizards at each of the time positions shown?

Where would I look for Where would I look for fossils?fossils?

Sue being excavated

in South Dakota

How old are fossils?How old are fossils?

Have a look

http://cstl-csm.semo.edu/overmann/bi151/protected/geologic_time.htm