The Sixth Extinction: An Unnatural History

“It was worth the trip just for that,” he exclaimed.
It’s unclear what aspect of the impact—the heat, the darkness, the
cold, the change in water chemistry—did in the ammonites. Nor is it
entirely clear why some of their cephalopod cousins survived. In contrast
to ammonites, nautiluses, for example, sailed through the extinction
event: pretty much all of the species known from the end of the
Cretaceous survived into the Tertiary.
One theory of the disparity starts with eggs. Ammonites produced
very tiny eggs, only a few hundredths of an inch across. The resulting
hatchlings, or ammonitellae, had no means of locomotion; they just
floated near the surface of the water, drifting along with the current.
Nautiluses, for their part, lay very large eggs, among the largest of all
invertebrates, nearly an inch in diameter. Hatchling nautiluses emerge,
after nearly a year’s gestation, as miniature adults and then immediately
start swimming around, searching for food in the depths. Perhaps in the
aftermath of the impact, conditions at the ocean surface were so toxic
that ammonitellae could not survive, while lower down in the water
column the situation was less dire, so juvenile nautiluses managed to
endure.
Whatever the explanation, the contrasting fate of the two groups