The Origin and Evolution of Mammals - Moodle

form of the tooth, must reflect subtle differences in
niche amongst this new, spectacular richness of
entomological resources, comparable to what is
seen in the insectivorous mammals of today.
Thus the Late Cretaceous became dominated by
two mammalian groups, the multituberculates and
tribosphenidans. However, the vast preponderance
of information about Mesozoic mammal evolution
comes from the Laurasian continents of North
America, Europe, and Asia, and to a small degree
North Africa. It would be misleading to assume
that the events outlined above also involved the
southern continents. Indeed, evidence is accruing
that it would be just plain wrong. Prior to the Late
Cretaceous, close to nothing is known at present,
apart from the pitifully few australosphenidans.
Late Cretaceous mammals occur in the Los Alamitos
Formation of Argentina, and are composed entirely
of non-tribosphenidan mammals. There are indigenous
groups of eutriconodontans, symmetrodontans,
dryolestidans eupantotheres, and the strange,
hypsodont teeth of the uniquely Gondwanan group
Gondwanatheria. Presumably this mammalian
fauna evolved in complete isolation from the familiar
northern fauna, and no southern tribosphenidan
is known for certain prior to the Cenozoic.
The general biology of the Mesozoic
mammals
The cladistic relationships of the non-mammalian
cynodonts indicates quite clearly that the origin of
mammals during the Upper Triassic involved
amongst other things fairly extreme miniaturisation.
Sinoconodon was the largest of the early forms, with
a skull length up to 6 cm, which is roughly the size
of the skull of the European hedgehog. The postcranial
skeleton is unknown, but, continuing the
comparison with the hedgehog, it would be
expected to have been about 25 cm in presacral
length, and to have weighed about 800g. It may
also be of significance that Sinoconodon retained the
primitive condition of continuous growth, rather
than the typically mammalian pattern where growth
is restricted to a relatively brief juvenile phase. The
great majority of the rest of the Mesozoic mammals
were considerably smaller, being of the general
body size of soricid shrews and murid mice and
THE MESOZOIC MAMMALS 183
rats, with body weights in the range of 200 g down
to the 3 g of a minute modern shrew such as
Microsorex hoyi (Eisenberg 1981). The docodontan
Henkelotherium is typical. It had a skull length of
4 cm, and a presacral body length of 11 cm, which
is roughly equivalent to a body weight of about 20 g.
The smallest Mesozoic mammal so far described is
the sub-adult specimen of Hadrocodium, which has
a skull length of only 1.5 cm and an estimated body
weight of 2 g (Luo et al. 2001b). There were also a
few relative ‘giants’. The largest are the Early
Cretaceous eutriconodontans such as Gobiconodon
ostromi. This had a 10 cm skull and a 35 cm presacral
body length, giving it a body size roughly
comparable to Didelphis virginiana, a mammal
whose body weight is around 1.5 kg. A few of the
advanced tribosphenidan mammals of the Late
Cretaceous also achieved somewhat increased
body size. The largest is probably Zalambdalestes
lechei, but even this was reconstructed by Kielan-
Jaworowska (1978) with a skull length of no more
than 4.6 cm, and a presacral body length of 14.9 cm.
The hypothetical ancestral mammal, as well as
being small, is universally agreed to have been
insectivorous, endothermic, and nocturnal. The
inference that its primary diet consisted of small,
terrestrial invertebrates is based on the sharpcusped,
shearing nature of the postcanine teeth,
which were unsuitable for dealing with any significant
volume of plant material. The hypothesis that
it was endothermic has been considered at length
earlier, and it is difficult to imagine that the whole
integrated suite of mammalian morphological characters
was not associated with insulation and a
high metabolic rate. Preservation of the impression
of the pelt of the Early Cretaceous placental Eomaia,
and of hair of a Late Palaeocene multituberculate,
powerfully confirms this belief. Nocturnality is
more speculative, but is implied by the form of the
brain and sense organs. The enlargement of the
cerebrum to become the primary control centre in
mammals, at the expense of the optic lobes, suggests
that the importance of the sense of olfaction
found in modern nocturnal insectivores was
already present. The increased sensitivity of the
sense of hearing, as witnessed by the evolution of
the ear ossicle system may also point to nocturnality,
although it has to be said that amongst other