“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 the species known from the end of Cretaceous survived into to 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 ammonately, had no means of locomotion. They just floated on 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 a years gestation as miniature adults and then immediately start swimming around searching for food in the depths. Perhaps in the aftermath of the impact conditions on the surface of the ocean were so toxic that ammonately could not survive, while lower down in the lower column the situation was less dire. So juvenile Nautilus managed to endure. Whatever the explanation, the contrasting fate of the two groups face a key point. Everything and everyone alive today is descendent from an organism that somehow survived the impact. But it does not follow from this that they or we, are any better adapted. In times of extreme stress the whole concept of fitness at least in a Darwinian sense loses its meaning. How could a creature be adapted either well- or ill- for conditions that it had never before encountered in its entire evolutionary history?”
The contemporary rate of extinction of species is estimated at 100 to 1,000 times higher than the background extinction rate, the historically typical rate of extinction (in terms of the natural evolution of the planet);. also, the current rate of extinction is 10 to 100 times higher than in any of the previous mass extinctions in the history of Earth. One scientist estimates the current extinction rate may be 10,000 times the background extinction rate, although most scientists predict a much lower extinction rate than this outlying estimate
Mass extinctions are characterized by the loss of at least 75% of species within a geologically short period of time.[The Holocene extinction is also known as the “sixth extinction” (beginning ~200K years ago), as it is possibly the sixth mass extinction event, after the Ordovician–Silurian extinction events (445 and 415 million years ago wiped out as much as 85 percent of all animal species on Earth), the Late Devonian extinction (The term primarily refers to a major extinction, the Kellwasser event (also known as the Frasnian-Famennian extinction), which occurred around 372 million years ago, at the boundary between the Frasnian stage and the Famennian stage, the last stage in the Devonian Period. Overall, 19% of all families and 50% of all genera became extinct), the Permian–Triassic extinction event (251.9 million years ago. It is the Earth’s most severe known extinction event, with the extinction of 57% of biological families, 83% of genera, 81% of marine species and 70% of terrestrial vertebrate species. It was the largest known mass extinction of insects. The scientific consensus is that the main cause of extinction was the large amount of carbon dioxide emitted by the volcanic eruptions that created the Siberian Traps, which elevated global temperatures, and in the oceans led to widespread anoxia and acidification) the Triassic–Jurassic extinction event, and the Cretaceous–Paleogene extinction event (The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary (K–T) extinction) was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the exception of some ectothermic species such as sea turtles and crocodilians, no tetrapods weighing more than 25 kilograms (55 pounds) survived. It marked the end of the Cretaceous Period, and with it the Mesozoic era, while heralding the beginning of the Cenozoic era, which continues to this day.