The Origin and Evolution of Mammals - Moodle

274 THE ORIGIN AND EVOLUTION OF MAMMALS
in the world. These include medium to large
bodied semiaquatic forms, some of which still
exist as capybaras. Sánchez-Villagra et al. (2003)
described a skeleton of the 700 kg, hippo-sized
Phoberomys from the Late Miocene of Venezuela, the
largest rodent of all. Others evolved relatively
long, cursorial limbs, as represented by the living
Patagonian hare, for example. The closest living
relatives of the hystricognaths are certain African
families, Ctenodactyla (gundis) and Anomaluridae
(spring hares), all of which constitute a group
Hystricomorpha. This suggests that the South
American radiation originated with dispersal from
West Africa across the Atlantic, some time during
the Oligocene.
Glires: Lagomorpha
The second branch of the Glires that includes living
members can be recognised by the presence of two
pairs of incisors, and the single rather than double
layer of enamel that covers them. The mastication
mechanism involves lateral rather than strictly
backwards and forwards jaw movements. On the
basis of these, and other cranial and postcranial
characters, the earliest undoubted lagomorphs do
not appear in the fossil record until the Middle
Eocene of North America. Like the rodents, however,
there are possible Palaeocene relatives from
Asia, this time in the form of the mimotonid
Mixodonta, such as Mimotona (Fig. 7.4(b)). In addition
to the lagomorph dental formula, there are
several features of the skull supporting the relationship
(Li and Ting 1993).
Overview of placental evolution
Given the relative completeness of the placental
fossil record, coupled with the biological and taxonomic
informativeness of many of the individual
specimens, theirs is possibly the single most important
part of the entire fossil record for illustrating
macroevolutionary patterns and inferring evolutionary
processes (Kemp 1999). From the pioneering
work of Simpson (1944, 1953) through that of
numerous subsequent workers such as Philip
Gingerich, Elizabeth Vrba, Christine Janis, John
Alroy, and so on, many of the concepts currently
important in macroevolutionary theory originated
in the study of extinct placental mammals. Adding
another dimension, the ever increasing volume of
molecular evidence bearing on phylogenetic relationships
has variously contradicted, corroborated,
or expanded the palaeontological perspective.
Combining the two disciplines, the evolutionary
story of the placentals, a story of great diversity and
disparity set against a background of shifting continents,
fluctuating climates, and changing biotas, is
becoming ever clearer.
The Cretaceous: origin and radiation
The earliest known stem placental Eomaia, and
stem marsupial Sinodelphys occurred 125 Ma in
China. As tabulated by Bromham et al. (1999),
molecular-based estimates of the time of divergence
of placentals from marsupials are extremely
variable, from as unbelievably high as 218 Ma to as
patently incorrectly low as 104 Ma, and subsequent
estimates continue to fall within this range. Cao
et al. (2000), using mitochondrial DNA, estimated
the date as 160 Ma. Springer et al. (2003), using a
large database of nuclear and mitochondrial,
sequences concluded only that the date lies
between 102 and 131 Ma, depending on such
variables as which constraints are used in the
model. Therefore the dating of Eomaia is consistent
with at least several of the molecular-based estimates,
especially this last quoted, which is the most extensive
and methodologically sophisticated to date.
It is a different matter for the origin and diversification
of the modern placental orders, however. As
far as the fossil record is concerned, it reveals the
history of the placentals throughout the Late
Cretaceous to have consisted of a modest radiation
of small, mostly insectivorous forms. The only specialised
kinds found either possessed broader,
lower-cusped molar teeth for a more omnivorous
diet, or sharper crested ones to deal with larger
invertebrate, and small vertebrate prey. However, it
is possible that what is revealed by the Cretaceous
fossil record is a very limited and distorted view of
what was really happening, because there is a very
considerable inconsistency between the fossil-based
and the molecular-based estimates of the dates of
divergences of the modern lineages. Not a single
fossil of a modern placental order can be shown