Historical Geology Understanding Our Planet\'s Past

The introduction of hard skeletal parts has been called the greatest dis-

continuity in Earth history. It signaled a major evolutionary change by

accelerating the developmental pace of new organisms. Nearly all major

groups of modern animals appeared in the fossil record. For the first time,

animals sported shells, skeletons, legs, and sensing antennae. Furthermore, a

stable environment enabled marine life to flourish and disperse to all parts

of the world.

The period follows on the heels of the great Precambrian ice age,the

worst the world has ever experienced, when ice sheets covered half the world.

It was also a time when oxygen concentrations rose to significant levels. After

the ice retreated and the seas began to warm, life took off in all directions.The

Cambrian saw the highest percentage of experimental organisms than any

other interval of geologic history, with perhaps three times more phyla in exis-

tence than today.

At the beginning of the Cambrian, an ocean turnover might have

brought unusual amounts of nutrient-rich bottom water to the surface. Due

to an increase in seawater calcium levels, early soft-bodied creatures developed

skeletons as receptacles for the disposal of excess amounts of this toxic min-

eral from their tissues. As concentrations of calcium in the ocean further

increased, animal skeletons became more diverse and elaborate.

Levels of atmospheric oxygen appeared to rise in concert with the skele-

tal revolution. The higher oxygen levels increased metabolic energy, enabling

Figure 45Early
Cambrian marine fauna.

Historical Geology