How do they choose which dinosaurs to kill in "A Sound of Thunder"? How do they mark them?

The only dinosaurs that are killed are about to die anyway.

The time travel safari company Time Safari, Inc. allows its customers a unique experience.  They get to go back in time and kill a dinosaur.  Of course, there are other packages available to different time periods and for different animals, but the dinosaur is what Eckels comes for.

The company has a special dispensation from the government to shoot dinosaurs, but it is also...

The only dinosaurs that are killed are about to die anyway.

The time travel safari company Time Safari, Inc. allows its customers a unique experience.  They get to go back in time and kill a dinosaur.  Of course, there are other packages available to different time periods and for different animals, but the dinosaur is what Eckels comes for.

The company has a special dispensation from the government to shoot dinosaurs, but it is also under strict guidelines.  These guidelines are in place to protect the future.  If you kill the wrong animal at the wrong time, you could change the past and that changes the present.

Lesperance’s job is to track a dinosaur through its life cycle and determine which dinosaurs are about to die.  Then they take their hunter to that specific time and place to kill it.  It was about to die, so killing it won’t hurt anything.

“I track them through their entire existence, noting which of them lives longest. Very few. …  Life’s short. When I find one that’s going to die when a tree falls on him, or one that drowns in a tar pit, I note the exact hour, minute, and second.”

There is a special path laid that ensures that the hunters do not affect anything in the past.  When Eckels accidentally steps off the path onto the butterfly, he causes the Butterfly Effect.  Killing that one butterfly completely changes the future.

It’s his shoes! Look at them! He ran off the Path. That ruins us! We’ll forfeit! Thousands of dollars of insurance! We guarantee no one leaves the Path. He left it. Oh, the fool!

Travis is worried about the company losing its license.  If the government finds out that the butterfly is dead, it might revoke it.  However, when they return to the present, they realize it is far worse than that.  The writing on the sign is slightly off from modern English, and a different man is elected president.  Stepping on one butterfly changed everything.