The Origin and Evolution of Mammals - Moodle

280 THE ORIGIN AND EVOLUTION OF MAMMALS
taxa crossed the temporarily dry Bering Straits to
enter North America from its northwestern corner.
While the evidence is too sparse and ambiguous to
support the East of Eden hypothesis for the first half
of the Palaeocene, there is strong evidence that a
succession of at least three, closely spaced waves of
immigration by this route subsequently took place,
leading to the spread of several new placental
groups by earliest Eocene times. Between about 57
and 54 Ma, a period spanning the Palaeocene–
Eocene boundary, several taxa known from earlier
in the Palaeocene of Asia first occur in the North
American fossil record (Beard 2002b; Bowen et al.
2002). These included the dinoceratans, arctostylopidans,
tillodonts, and hyaenodontid creodonts
amongst the archaic groups. It also includes members
of the modern orders Perissodactyla, Artiodactyla,
Rodentia, and Primates. The palaeoclimatic evidence
is also compatible with the East of Eden
hypothesis. A brief, intense period of global warming
occurred at this time. It is indicated by a shift in
13 C/ 12 C ratio that is possibly associated with the
release by volcanic activity of methane and CO 2 ,
causing a greenhouse warming effect (Pearson and
Palmer 2000). According to Kent et al. (2003), the
event may have been triggered by a cometary
impact. The average world temperature reached its
maximum Cenozoic level, and the associated climate
would be expected to have allowed the expansion
of mammalian faunas into higher northern
latitudes, and to open up a dispersal route from the
Asian to the North American continents via the temporarily
dry Bering Straits. The East of Eden
hypothesis also predicts that there would have been
at least some dispersal in the opposite direction,
from North America to Asia. There are, in fact, no
clear examples of this except possibly the Carnivora,
and nor is obvious why it was in Asia rather than
elsewhere that new groups were originating in the
first place. Beard (1998) suggested that both phenomena
might be related simply to the much larger
area of the Asian landmass compared to the North
American.
The Palaeocene mammalian fauna of Europe is
sufficiently similar to that of contemporary North
America that they are presumed to have been interconnected
through a northern Atlantic route. With
the onset of the warm conditions of the Palaeocene,
dispersal through what is now northern Canada and
Greenland is perfectly plausible, and there is neither
systematic nor palaeobiogeographical evidence that
any Asian taxa arrived directly from the east.
Of the Palaeocene mammals in the southern continents,
vastly less is known. Early and Middle
Palaeocene mammals are virtually unknown in
Africa, and therefore there is no palaeontological confirmation
of the taxonomic inference discussed earlier,
that the Afrotheria were diversifying at that time,
in isolation of the rest of the world. A little more is
revealed about the early history of placentals in
South America. Pantodonts and ‘condylarths’ were
present in the Early Palaeocene, and presumably had
arrived from the north along with the marsupials, at
or soon after the end of the Cretaceous. Xenarthran
fossils are absent prior to the Late Palaeocene, when
they occur alongside the first of the South American
ungulates. The latter, Meridiungulata, may have
originated monophyletically from an immigrant
North American ‘condylarth’. However, it is also
possible that some of them evolved from an immigrant
dinoceratan that also entered during the
Palaeocene.
The Indian subcontinent is another Gondwanan
landmass that may, for all that is known, have had
an important role in the story. During most of the
Cretaceous it was an isolated plate, drifting northwards
from Africa, and eventually colliding with
the Asian continent. The earliest time at which
mammals might have been able to disperse from
India to the main Asian landmass is unclear. It may
have been around the Palaeocene–Eocene boundary,
or possibly significantly earlier, even close to
the Cretaceous–Palaeocene boundary. Any placental
groups that are first recorded in the Asian
Palaeocene could conceivably have originated in
isolation in India, and, as mentioned above, this
could have included either or both the Laurasiatheria
and Euarchontoglires. Unfortunately, there are as
yet no suitable Palaeocene fossils from the Indian
subcontinent to test this idea.
The Eocene: flourishing
The very warm conditions arising at the end of
the Palaeocene continued into the Eocene, and are
associated with the time of maximum placental