The Origin and Evolution of Mammals - Moodle

(a)
(b)
(i)
(ii)
(iii)
(iv)
(v)
a g
canine
2
4
3
3
1
3
Diademodon
2
2
3
The extreme version of this condition is seen in the
tritylodontids, which do not replace any of their
postcanines at all, but only discard them from the
front and add to them at the back (Kühne 1956).
Other cynodonts, including the basal form
Thrinaxodon and the highly derived tritheledontid
Pachygenelus (Crompton and Luo 1993), retained the
primitive pattern of polyphyodonty, consisting of
waves of replacement from front to back affecting
teeth alternately along the jaw.
It is not until the basal mammal Morganucodon
that the combination of determinate growth and
diphyodonty is known to have evolved, as was
demonstrated by Parrington (1971), who found
specimens amongst the hundreds of fragments that
were either juvenile growth stages or, the great
majority, identical-sized adults. The incisors, canines,
s
Postcanines
1 2 3 4
2 3 4
2 3 4 5
3 4 5
3 4 5
Sinoconodon
Figure 4.12 Tooth replacement (a) Diademodon (Kemp 1982) and
(b) Sinoconodon. a-anterior teeth; g-gomphodont teeth. s-sectorial teeth
(Crompton and Luo 1993).
EVOLUTION OF MAMMALIAN BIOLOGY 121
and anterior postcanines are replaced once, and
posterior postcanines are added sequentially at the
back, not replaced, and therefore can properly be
referred to as molar teeth. Given its correlation with
growth pattern, it is assumed by this stage that lactation
had evolved. However, the story is complicated
by the situation in Sinoconodon, which is basal
to Morganucodon. It still had indeterminate growth,
for specimens are found that range in skull length
from 2.2 to 6.2 cm, corresponding to an estimated
body mass range of 13–517 g (Kielan-Jaworowska
2004). The tooth replacement pattern is also more
primitive in Sinoconodon, as indeed it had to be in
order to allow for the very considerable growth in
size of what must have been sub-adults not
dependent on lactation for their growth. The incisors
and canines still show alternate, multiple replacements.
The postcanine teeth are not replaced, and
there is loss of anterior and addition of new posterior
postcanines maintaining the relatively short
postcanine tooth row of only three or four teeth
(Fig. 4.12(b)). This condition in Sinoconodon is therefore
intermediate between the primitive tritheledontid
and the fully mammalian conditions. It may
be speculated that the state of evolution of lactation
was also intermediate, with maternal provision of
milk limited to an early neonate stage only, after
which the juvenile was weaned and relied on its
own foraging, or perhaps on a more limited conventional
food supply provided by the mother.
Temperature physiology
Nothing is more fundamental to the life of mammals
than their endothermic temperature physiology,
if only because it entails a 10-fold increase in
daily food requirements. Such a huge cost must be
balanced by an equally large benefit for endothermy
to have evolved and been maintained. Yet surprisingly
there is no consensus about exactly how, why,
or when endothermy evolved in the course of the
evolution of the mammals. The fact that the birds
share a virtually identical mode of endothermic
temperature physiology with the mammals adds
little elucidation: the same contentious issues apply
to them. The problem arises because of the complex
nature of endothermy. It has two distinct primary
functions in modern mammals, and it also involves