Helen glanced down the arroyo, frowning. "Odd, though."

"What's odd?"

She pulled out the unknown fossil. "If this came from here, there's no way it's a shell. Not of a water dweller, anyway."

Joe nodded. "These are land formations; late Cretaceous, maybe even Maastrichtian."

"No 'maybe' involved, Joe. Look at where your hand is."

Joe looked at the rock wall he'd been leaning against. "What-"

He suddenly started laughing. "You can't be serious, Helen! It's like pulling three jackpots in a row at Vegas!"

"What is it?" Jackie asked, seeing the narrow, dark band both Joe and Helen were staring at. Then she whipped around, eyes wide.

"You mean…?"

"Yes." Helen was hardly able to believe it herself. "It looks like our fossil is sitting right smack on the K-T boundary."

"Where the comet-um, sorry." Jackie caught herself before finishing the sentence. She tended to forget that the Alvarez Hypothesis was still a touchy subject for a lot of paleontologists, even if she herself thought it was a darn neat idea.

"Yes, where the comet." Helen said the words with a half-snort, half-chuckle.

Fortunately for Jackie, Helen was less hostile to the Alvarez Hypothesis than most members of her profession. She didn't doubt at all that an impact had happened at the K-T Boundary, which marked the end of the Mesozoic Era. She simply questioned whether it had the worldwide cataclysmic effects that the hypothesis proposed. There were other impact craters about as big as the one in Yucatan, after all. The Manicouagan, to name just one. But they'd had no discernable ecological effects at all; not even regional ones, so far as anyone could determine.

Nor had anyone ever really explained, to Helen's satisfaction, exactly how the impact had killed off so many species. Nor the peculiar mechanism by which it had killed off some, but not others. In what mystifying manner, for instance, had it killed off all ammonites-but spared their close relatives, the squids and the octopi? These were the sort of nitty-gritty questions that paleontologists focused on, and that physicists tended to ignore.