The Origin and Evolution of Mammals - Moodle

The Late Cretaceous mammal record is extremely
sparse, with the sole exceptions of North America
and Mongolia, and the Garden of Eden hypothesis is
that the origins and divergences of the modern placental
orders occurred in a part of the world not at
present adequately sampled for Cretaceous mammal
fossils. The possibilities are the Gondwana continents
of Australia, South America, Antarctica, and
Africa, in all of which, there is extremely little in the
way of Cretaceous fossil mammals. On this view, the
early Cenozoic explosive appearance of the modern
orders was due to their immigration into the northern
continents of Laurasia.
The Garden of Eden hypothesis does have some
arguments in its favour, principally that the molecular
dates of the major divergences within placental
mammals coincide approximately with the
various phases of the break up of Gondwana.
Murphy et al. (2001b) suggested that while the basal
placentals (eutherian ancestors) occurred in the
northern continents, as indicated by the Early
Cretaceous fossil record, the Placentalia origininated
in Gondwana (Fig. 7.26(a)). The Afrotheria
diverged when Africa separated from the rest of
Gondwana about 105 Ma (Smith et al. 1994).
Around 5–10 Ma later, according to the molecular
dates, the Boreoeutheria differentiated, presumably
as a consequence of their ancestor entering North
America from South America. At this time, there
may have been several small islands that would
have offered a route northwards.
A second possibility is that the boreoeutherians
originated in India, which separated from Africa at
about the same time as did South America, becoming
another island continent. It is not certain when
India eventually collided with Asia (Hallam 1994).
Most have believed this to be Late Palaeocene
(Krause and Maas 1990; Prothero 1994), but
other evidence, including geomorphological and
palaeontological, suggests that at least faunal interchange
may have been possible a good deal earlier,
around the end of the Cretaceous (Jaeger et al.
1989). This hypothesis is consistent with the difficulty
of resolving the base of the supraordinal tree,
which is due to the very close dates of divergence
between Afrotheria, Xenarthra, and Boreoeutheria.
The only palaeontological support explicitly
claimed for the Garden of Eden hypothesis comes
LIVING AND FOSSIL PLACENTALS 277
from Rich et al. (1999; Woodburne et al. 2003), who
believe that the Early Cretaceous, 100 Ma Australian
mammal jaw Ausktribosphenos is an erinaceid-like
member of the Eulipotyphla, and therefore marks
the area of origin of placentals. Only more material
will confirm or refute this presently highly
disputed identification.
The Long Fuse hypothesis. The fourth possible explanation
for the incongruent dates has been proposed
most actively by Archibald and Deutschman (2001).
The Long Fuse hypothesis is that most of the
Cenozoic orders of placentals did diverge in the
Late Cretaceous; that they are in fact represented by
known fossils from that time; but that these have
been unrecognised because they lack the diagnostic
characters of the modern representatives. Rather,
they remained small, unspecialised mammals with
a basically insectivorous, or at most omnivorous
dentition. On this view, the post-Cretaceous explosion
of recognisable members of the modern orders
was actually their evolution from existing, primitive,
stem members. This, the hypothesis implies,
occurred in many orders during a small window of
time, and was possibly in response to the extinction
of the dinosaurs and therefore the release of the
habitats and niches from which mammals had hitherto
been competitively excluded. In this light, there
are several candidates for Cretaceous stemmembership
of modern orders. The Leptictida have
often been included with the Eulipotyphla
(Novacek 1986a), and the Zalambdalestidae interpreted
as stem Glires (Archibald et al. 2001). The
Zhelestidae possess incipiently ungulate-like molar
teeth and could conceivably include stem members
of any of the numerous Cenozoic ungulate groups
(Archibald 1999). The Palaeoryctida have from time
to time been allied to the Carnivora, and also some
of the extinct Cenozoic orders such as Creodonta,
Taeniodonta, Tillodonta, and Pantodonta. The evidence
for all of these putative relationships is certainly
not at all strong, often being based almost
completely on minor dental similarities, although of
course this in itself is a prediction of the very
hypothesis.
Springer et al.’s (2003) molecular study already
referred to (Fig. 7.25) is consistent with the Long
Fuse hypothesis, insofar as it places most of the