Evolution__3rd_Edition

672 PART 5 / Macroevolution
Molecular evidence suggests an
earlier origin
The fossil–molecular conflict has
inspired several lines of research
Tertiary: this is when we find the earliest fossil Carnivora, ungulates (Perissodactyla
and Cetartiodactyla), ant-eaters, elephants, and primates. The dinosaurs had gone
extinct in the mass extinction that preceded the mammal radiation. The fossil evidence
fits the pattern of Figure 23.11a exactly, implying independent, rather than
competitive, replacement.
A Tertiary origin for the modern mammal orders is also supported by the fossil
record for more ancestral mammals. Eutherian mammal fossils are known from the
Cretaceous. Until recently (see below), the oldest clearly eutherian mammal fossils
were from about 80 million years ago. These fossils do not belong in any of the modern
mammal orders. They are classified as relatives of the moderns orders, connected by
deep branches in the mammal tree. It would have taken some time for the modern orders
of mammals to have evolved from ancestral eutherians. If the eutherians originated
around 80 million years ago, it makes sense that the modern groups evolved around
55 million years ago. That leaves 20 million years for the evolutionary change from
ancestral to modern eutherian forms. The modern orders could hardly have existed
before 80 million years ago, if that was when the earliest ancestral eutherians lived.
However, when the molecular differences among the modern mammal orders are
measured, and the rate of the molecular clock calibrated, the inferred time for the common
ancestor of these groups is much older than the early Tertiary. The molecular date
for the common ancestor is about 90–100 million years ago. The molecular evidence
implies that the modern mammal orders a Carnivora, Primata, Proboscidea, and so on
a already existed in the mid-Cretaceous. Indeed they already existed before the earliest
known fossil of any eutherian mammal.
If the molecular date is correct, the mammal groups that now occupy the niches of
dinosaurs coexisted with dinosaurs for the last 30 million years of the Cretaceous. This
does not prove that mammals competed with dinosaurs, or that any such competition
contributed to the demise of the dinosaurs. The Cretaceous mammals may have been
small in size and ecologically different from their modern descendants, so they would
not have competed with dinosaurs. Or they may have been numerically too rare to
affect the dinosaurs. All that is uncertain. For now, the main point is that the molecular
evidence has spoiled the clear Figure 23.11a-like pattern for dinosaurs and mammals,
and weakened the case for an independent replacement.
The conflict between molecular and fossil dates for the modern mammal orders
is another example of a common kind of conflict in modern evolutionary biology
(see Section 15.13, p. 460, for hominins, and Section 18.4, p. 535, for the Cambrian
explosion). The modern groups of birds a gulls, ducks, and passerines, for instance a
are a further example. The fossil evidence suggests that they radiated after the
Cretaceous extinction, in the early Tertiary, but the molecular clock suggests they
originated far earlier. As with the hominin and Cambrian explosion examples, the
conflict could be resolved in three ways. The molecular evidence may be wrong, the
fossil evidence may be wrong, or the two may be reconciled.
For the mammal orders, the molecular evidence has been analyzed and reanalyzed,
but has not been seriously challenged. Fossil research has been more revealing. One
approach has been to estimate, statistically, how likely it is that the main mammal
orders existed in the Cretaceous but left no fossils. The estimates are made using independent
evidence about the completeness of the fossil record. For one lineage, we can
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