18. During the nineteenth century, biologists knew that South America and Africa contained similar fossils. These fossils led them to believe that a temporary land
bridge between South America and Africa allowed ancient species to move between
both continents. In the twentieth century, evidence showed that South America and
Africa were once part of the same larger continent and that there were no land bridges
between them. How should biologists respond to the new geological finding?

relative dating involves things like higher layers are most recent than rocks lower in the sequence. this can be extended to the known sequence of fossils. rocks containing rabbit fossils are later than those containing dinosaur fossils.  

absolute dating methods mainly use radioactive elements that have known rates of decay and produce recognizable products. for example potassium 40 decays to produce argon. once a rock has solidified the argon is trapped. by measuring the argon and potassium0 you can get an accurate number for the age.  

some other methods are good for archaeological ages. dendrochronology involves recognizing the sequence of tree rings. the orientation of magnetized particles in baked clay can be used to date a fire.

hope this .. .

it is called the mitotic spindle