The Origin and Evolution of Mammals - Moodle

interordinal divergences in the Late Cretaceous, but
most of the intraordinal splits at or after the end of
the Cretaceous. Only Xenarthra, Eulipotyphla,
Rodentia, and Primates have any intraordinal
divergences between their constituent groups
dated to the Late Cretaceous, these varying
between 77 and 70 Ma. Intraordinal divergences
within Cetartiodactyla and Chiroptera commenced
at 65 Ma, the time of the K–T boundary, and those
within Carnivora, Perissodactyla, and Lagomorpha
10–15 Ma after that event.
Another argument in favour of what amounts to
a decoupling of lineage splitting from the evolution
of modern diagnostic characters is that, even to this
day, three of the now recognised super-orders,
Afrotheria, Laurasiatheria, and Euarchontoglires
include small, insectivorous members, which presumably
have retained the general ancestral structure
of the respective stem-members. The very fact
that at one time tenrecs, shrews, and tree shrews,
were all included in a single order Insectivora
attests to the likelihood that the Cretaceous members
of the superorders would not expect to be
readily distinguished at a high taxonomic level.
Archibald (2003) has related the long fuse model
to the molecular dates (Fig. 7.26(b)). He agrees that
the placental stem group of fossil eutherians was a
Laurasian radiation, but furthermore that the origin
of the crown Placentalia also occurred in the north.
The existence of the Afrotheria and the Xenarthra is
explained by dispersals to those two continents
respectively, in the Late Cretaceous. The argument
is supported to the extent that relatives of the
Afrotheria may occur in Asia. If ‘condylarths’
such as Minchinella, or anthracobunids such as
Pilgrimella are indeed related to afrotherian orders,
then the origin of the Afrotheria may well be sought
in Asia. However, these phylogenetic associations
are far from agreed upon. Furthermore, there are no
serious suggestions as yet concerning the identity
of possible stem Xenarthans in the northern
continents.
The Palaeocene: explosive radiation
Irrespective of whether the majority of placental
orders arose in the Cretaceous, or not until the
early Cenozoic, there was certainly an explosive
LIVING AND FOSSIL PLACENTALS 279
appearance of new mammalian taxa right from the
start of the Palaeocene. Once the effects of the presumed
end-Cretaceous bolide impact had waned,
warm, greenhouse conditions pertained, with gentle
climatic gradients (Prothero 1997). Under these
circumstances, and in the absence of dinosaurs, a
mammalian fauna that was essentially modern in
ecological structure though not taxonomic constitution
was rapidly established. Alroy (1999b) plotted
the species numbers for successive time bins, and
after allowing for differing sample sizes, found an
increase from about 23 in the latest Cretaceous to
about 60 a mere 5 million years later. Equally significant,
while the majority of the new kinds of
Early Palaeocene mammals were still small-bodied
insectivores and omnivores, there were several new
groups with medium-sized (5–40 kg) and large
(�40 kg) members. The most abundant were the
‘condylarths’ of various kinds, which, in addition
to primitive ungulate herbivores, included the first
ecologically carnivorous placental mammals in the
form of actocyonids and mesonychids. Other new
placental orders were the specialised herbivorous
taeniodonts, tillodonts and pantodonts, all of which
were possibly derived from Cretaceous palaeoryctidans.
The arboreal plesiadapiforms, possibly stem
primates, and the earliest Carnivora were mainly
small, but included some cat-sized animals.
By far the best Early Palaeocene (65–60 Ma)
mammal record is from North America, followed
by that of China, but it is not clear exactly how the
new taxa related to one another biogeographically.
Some, such as the taeniodonts and the plesiadapiforms,
are assumed to have evolved in North
America because of their absence elsewhere other
than, later, Europe. Others were exclusively Asian,
such as mixodonts and anagalidans. However,
some Early Palaeocene taxa occurred in both continents,
but with inconsistent dates. The Chinese
pantodonts are slightly earlier and more primitive
than the American ones, but exactly the reverse is
true of the Carnivora.
The East of Eden hypothesis (Beard 1998) is that
the centre of origin and early radiation of placental
orders was the Asian landmass, and that on successive
occasions, throughout the Palaeocene and Early
Eocene, climatic conditions permitted the northwards
expansion of mammal faunas. At these times,