Evidences of past life
Ediacara—soft-bodied forms, worms, jellyfish, anemones
Trace fossils—burrows, slime trails
Small shelly fauna—tiny pieces of calcareous material
Burgess Shale fauna—trilobites, cephalochordates, etc.
Are all related? There are some thoughts that the ediacara fossils represent one episode of experimentation, but that they left no descendents. Perhaps their members were done in by the dramatic change in the Earth’s environment brought about by the “Snowball” Earth phenomenon. Others feel that the organisms that arose in the Cambrian arose from the ediacaran creatures. At this point it is difficult to prove one idea or the other.
What is known is that all major bauplans are formed by the Cambrian (perhaps except for the bryozoa which are found from Ordovician age on).
Why no new bauplans?
Developmental restrictions
May be conditions are not right
Limitation on available niches
New Topic--segmentation
Pseudosegmentation—tapeworm
Segmentation or metamerism—defined as a serial repetition of body parts
Theories
Budding but retention of bud with bud and source of bud having integrated life processes.
Advantages of segmentation
A segmental organism if injured can stand the loss of a body part (loss of back end of body by earthworms)
More efficient locomotion. Because of compartmentalized coelomic pouches, each segment can move independent of the other.
Looking ahead
Monday some more stuff on segmentation. Ecdysis