DIVERSIFICATION OF LIFE 545
major subdivisions and had diversifi ed a little
in the Late Cretaceous (see p. 466), but none
of them were larger than a cat. Within the
10 myr of the Paleocene and early Eocene, 20
major clades evolved, and these include the
ancestors of all modern orders, ranging from
bats to horses, and rodents to whales. During
this initial period, overall ordinal diversity
was much greater than it is now: it seems that
during the early parts of a radiation the found-
ing clades may radiate rapidly, and many
body forms and ecological types arise. Half of
the dominant placental groups of the Paleo-
cene became extinct soon after, during a phase
of ecospace fi lling and competition, until a
more stable community pattern became estab-
lished 10 myr after the KT mass extinction.
The radiation of the placental mammals is
frequently described as an adaptive radiation,
and the adaptation that drove this could have
been their warm-bloodedness, their differenti-
ated teeth, their intelligence or their parental
care. Any one of these, or indeed all of them,
could be a reasonable explanation for the
radiation and later success of placental
mammals. But it is important to remember
that these are hypotheses that must be tested.
First, the marsupials were also around in the
Late Cretaceous, and they share all of these
characters with the placentals, so why did the
marsupials not radiate as much as the placen-
tals? Second, Mesozoic mammals had pos-
sessed most of these features since the Late
Triassic, so why didn’t they radiate then? The
supposedly superior mammals originated at
the same time as the dinosaurs, but somehow
the dinosaurs prevailed fi rst and kept the
mammals at bay for fully 160 myr (see p.
454). It is important not to get carried away
with assumptions and rhetoric, and it is espe-
cially important not to confuse pattern and
process.
Biotic replacements
Biotic replacements are an obvious feature of
the history of life. These are times when one
group of plants or animals replaces another.
The replacement of brachiopods by bivalves
is a famous example. This had always been
seen as a progressive process: the common
view is that brachiopods are less adaptable
than bivalves, and they clearly succumbed to
long-term competition, perhaps lasting for
tens or hundreds of millions of years. Com-
petition is generally defi ned as any interaction
between individuals of the same species or
different species, in which one individual
gains an advantage and the other suffers. A
simple example is competition among baby
birds in the nest: the big bruiser wins the
attention of its parents and gets more food,
and the timid smaller sibling loses out. Com-
petition is a process, a hypothesis, and it must
not be assumed in any example until it can be
demonstrated.
Gould and Calloway (1980) looked closely
at the brachiopods versus bivalves example,
and they decided quite rightly not to assume
simply that the replacement was driven by
competition. Their studies suggested that the
take-over was more complex. Brachiopods
and bivalves had maintained fairly constant
diversities through the Paleozoic, with bra-
chiopods being more diverse (Fig. 20.8). The
Permo-Triassic (PT) mass extinction 251 Ma
(see pp. 170–4) drove their diversities right
down. The bivalves recovered, and began to
radiate rapidly during the Triassic and Juras-
sic, whereas the brachiopods have remained
at the same low post-extinction diversity level
ever since.
Other major biotic replacements have
shown similar outcomes on close study. For
example, the replacements of various major
plant groups through time (see Chapter 18)
used to be regarded as competitive, but there
is limited evidence for that. The replacement
of various tetrapod groups by the dinosaurs
in the Late Triassic used to be seen as a process
in which dinosaurs outcompeted their slower-
moving and less rapacious predecessors.
Restudy suggests that there was an extinction
event, perhaps mediated by climate and fl oral
change, and the initial success of the dino-
saurs was more good luck than a demonstra-
tion of their overall superiority (see p. 454).
The story of the “Great American inter-
change” (see pp. 43–4), which followed the
closure of the isthmus of Panama, used to be
that the North American mammals thundered
into South America and slaughtered their
inferior southern cousins. Close study of the
data suggests the interchange was balanced,
and that most invaders found new things to
do and did not drive anything to extinction.
Perhaps the majority of biotic replacements
were passive, mediated by extinction events