Geological Period   Millions of   *Percent Marine
in which            Years ago      Extinctions for:
Extinctions Occur
                              Families      Genera
Late Ordovician      435         27           57
Late Devonian        365         19           50
Late Permian         245         57           83
Late Triassic        220         23           48
Late Cretaceous       65         17           50
Late Eocene           35          2           16
*Rough estimate of percent of all families and genera of marine animals with hard parts (so we have fossil evidence of their existence) rendered extinct; numbers are given to nearest 5 million years (Column 2). Extinctions of land families and genera are thought to be roughly comparable. Data courtesy J. John Sepkoski, Jr., Univesity of Chicago. As described in Chapter 16, there is active debate about some of these numbers; the Pemian and Cretaceous mass extinctions are the most reliably determined. Other mass extinctions used to derive the disputed periodicities described in Chapter 16 are not tabulated here.

The extinction of what biologists call a "family" of plants or animals represents a major loss. Think of the variety of dogs in the world, from Chihuahuas to Great Danes. They are all one species, because they are all interfertile. Biologists distinguish a broader category called a genus (plural, genera) that here embraces not only dogs but wolves and jackals as well. This genus, Canis, is in turn part of a much larger category, a family, that includes foxes; together they all are the Canidae. A family is a major group of beings. The human family, the family of the hominids, includes most of those primates who walked around on two feet and tried to puzzle things out over the last few million years, even though we might not recognize many of them as human were we to encounter them on the street. The loss of a family is the hacking off a limb of the tree of life.

In the table above, six of the most spectacular known interruptions in the history of the Earth are listed, along with an indication of how massive these extinctions were. It is with a sense of shock that we look at the last columns of this table. It shows, for each of the six catastrophes, the percent of all families and genera of known marine organisms that were lost. At the Ordovician and Devonian boundaries, fully half of the genera became extinct, less for the Eocene event, almost as much for the Triassic. In the Permian catastrophe more than half of all the families of life on Earth were lost, more than three-quarters of the genera, and more than 90 percent of all species. One of the most recent of these planetary disasters was at the end of the Cretaceous Period, some 65 million years ago, when almost one out of five families, half the genera and three-quarters of the species disappeared.

...

Scientists look closely at the Cretaceous extinctions because they concern us particularly. A critical turning point in the evolutionary path to humans can be glimpsed amoung all those reptile bones. The Cretaceous catastrophe wiped out every family, every genus, and every species of dinosaur, and they were as varied and successful as the mammals are today. It is as if all the mammals were to roll over, their appendages stiffly in the air--every shrew and whale of us, and every person. All the flying and swimming reptiles died as well, and more than a hundred families of beings that live in the oceans. It was a catastrophe enormously beyond anything humans have ever known, at least so far.

The first mammals appeared at about the same time as the first dinosaurs. The dinosaurs were the lords of the Earth, the largest creatures, the most powerful, the ones who would capture your attention on any Cretaceous landscape. The mammals, our ancestors, were then tiny, furtive, scampering creatures, cautious, mouse-like, spending much of their time keeping out of the way of the thundering reptiles. A few dozen little mammals would have made a meager lunch for a middling-sized carnivorous dinosaur. For over 100 million years our ancestors were at an apparent evolutionary cul de sac, living at the margins and in the shadows of a world dominated by dinosaurs. If you surveyed that late Cretaceous landscape--in which the trees and flowers looked pretty much as they do today, but where the dominant animals were all reptiles--you would not have bet much on the chances of our ancestors.

Comet p 273, 275.