Transitional Forms Consider this Hypothesis: A C B TIME 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

Fossils

Permineralized Fossils These 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 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 event. Fossilization is a rare event. Fossilization of a whole organism is even rarer. Fossilization of a whole organism is even rarer. Finding fossils depends on finding exposed surfaces where fossils may have been deposited. Finding fossils depends on finding exposed surfaces where fossils may have been deposited.

Consider 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 fossils? Sue being excavated in South Dakota

How old are fossils? Have a look