The Sixth Extinction: An Unnatural History

were Hesperornithine birds, which were aquatic and for the most part
flightless. The same goes for lizards and snakes; around four-fifths of all
species vanished. Mammals’ ranks, too, were devastated; something like
two-thirds of the mammalian families living at the end of the Cretaceous
disappear at the boundary.
In the sea, plesiosaurs, which Cuvier had at first found implausible and
then “monstrous,” died out. So did mosasaurs, belemnites, and, of course,
ammonites. Bivalves, familiar to us today in the form of mussels and
oysters, suffered heavy casualties, as did brachiopods, which look like
clams but have a totally different anatomy, and bryozoans, which look like
corals but once again are totally unrelated. Several groups of marine
microorganisms came within a micron or two of annihilation. Among
planktonic foraminifera, something like ninety-five percent of all species
disappeared, including Abathomphalus mayaroensis, whose remains are
found in the last layer of Cretaceous limestone in Gubbio. (Planktonic
foraminifera live near the ocean surface; benthic species live on the ocean
floor.)
In general, the more that’s been learned about the K-T boundary, the
more wrongheaded Lyell’s reading of the fossil record appears. The
problem with the record is not that slow extinctions appear abrupt. It’s
that even abrupt extinctions are likely to look protracted.
Consider the accompanying diagram. Every species has what is known
as a “preservation potential”—the odds that an individual of that species
will become fossilized—and this varies depending on, among other things,
how common the animal is, where it lives, and what it’s made out of.
(Thick-shelled marine organisms have a much better chance of being
preserved than, say, birds with hollow bones.)
In this diagram, the large white circles represent species that are
rarely fossilized, the medium-sized circles those that are preserved more
frequently, and the small white dots species that are more abundant still.
Even if all of these species died out at exactly the same moment, it would
appear that the white-circle species had vanished much earlier, simply