How do scientists mark the end of a geologic era and beginning of the next?

Click an item in the list or group of pictures at the bottom of the problem and, holding the button down, drag it into the correct position in the answer box. release your mouse button when the item is place. if you change your mind, drag the item to the trashcan. click the trashcan to clear all your answers. show the flow of energy in the following ecosystem by labeling each organism's link in the food chain. click once to select a label. click again to place the label in the correct location.