T HE EvoluTion of BiologiCAl DivERsiTy 495
This discussion illustrates a fundamentally important aspect of every scien-
tific discipline, including evolutionary biology: scientists discuss the ways in
which their data could possibly be misinterpreted and lead to false conclusions,
and they devise ways of avoiding that.Diversity through the Phanerozoic
The most complete fossil record has been left by skeletonized marine animals
(those with hard parts such as shells or skeletons). Jack Sepkoski accomplished the
heroic task of compiling data from the paleontological literature on the stratigraphic
ranges of more than 4000 skeletonized marine families and 30,000 genera through-
out the 541 My since the beginning of the Cambrian period [63, 64]. Using this
database, he plotted the diversity of families throughout the Phanerozoic, creating
one of the most famous graphs in the literature of paleobiology (FIGURE 19.4). T he
graph shows a rapid increase in the Cambrian and Ordovician, a plateau through-
out the rest of the Paleozoic, and a steady, almost fourfold increase throughout the
Mesozoic and Cenozoic. This pattern is interrupted by decreases in diversity caused
by mass extinction events (see Chapter 17). Similar studies of the terrestrial fossil
record show that the end-Permian extinction was followed, after a delay of more
than 15 My, by a great diversification of dinosaurs, crocodilians, and synapsid proto-
mammals [8]. In the Cretaceous, flowering plants proliferated and largely replaced
gymnosperms, and insects exploded in diversity. Life on land became more diverse
than in the sea [74].
Since Sepkoski first summarized the history of marine diversity, other paleo-
biologists have applied various corrections for sampling errors and the biases that
we have noted (FIGURE 19.5) [2, 21]. They have found a decline in diversity in the
Devonian instead of a Paleozoic plateau, an increase in the Permian before the
end-Permian extinction, and a less steep, but still pronounced, increase through
the Mesozoic and Cenozoic to the present time [10].
Much of the increase in the number of taxa reflects the evolution of morpho-
logically and ecologically new forms of life. This aspect of diversity has increased
from the early Paleozoic to the present; among marine animals, for example, theFutuyma Kirkpatrick Evolution, 4e
Sinauer Associates
Troutt Visual Services
Evolution4e_19.04.ai Date 12-12-2016Q: Should x-axis be changed to (My) for consistency with other gures?500 400 300 200 100 0Time (Mya)Number of families900800
7006003004005001000200Includes families
rarely present in
the fossil recordReliable
fossil recordC O S D C P Tr J K Pg NgPaleozoic Mesozoic CenozoicFIGURE 19.4 Taxonomic diversity of
skeletonized marine animals dur-
ing the Phanerozoic. The number of
families entered for each geological
stage includes all those whose known
temporal extent includes that stage.
The blue curve includes families that
are rarely preserved; the green curve
represents only families that have a
more reliable fossil record. There are
approximately 1900 marine animal
families alive today, including those
rarely or never preserved as fossils.
(After [63].)19_EVOL4E_CH19.indd 495 3/22/17 1:42 PM