The Origin and Evolution of Mammals - Moodle

120 THE ORIGIN AND EVOLUTION OF MAMMALS
comparable-sized modern reptiles, and with no
indication of the beginning of the enlargement of
the forebrain that was later to dominate mammalian
brains. Quiroga (1980) described the cranial cast of
the Middle Triassic eucynodont Probainognathus
(Fig. 4.11(b)). According to his interpretation, there
actually was some expansion of the cerebrum and
the beginning of the development of the neocortex.
Even more extreme, Kemp (1979) reconstructed a
much larger brain in the basal cynodont Procynosuchus,
based on an acid-prepared skull (Fig. 4.11(c)
and (d)). His interpretation starts with a confident
reconstruction of the cerebellum, because the markings
on the internal walls of the hind part of the
braincase indicate that the cerebellum filled the cavity
completely. Given that in all vertebrates the cerebrum
is larger than the cerebellum, this gives a
minimum size for the cerebrum that is substantially
greater than Hopson’s and other’s tubular structure.
Therefore the forebrain, although still relatively narrow
as indicated by the impression of its dorsal surface,
but must have been considerably deeper than
other authors allow. At the maximum estimate possible,
the brain volume would have been at the
lower end of the size range for mammals.
There is, however, little consensus about the
evolution of the brain within the cynodonts, important
a topic as it is. Kielan-Jaworowska (1986)
inclined to the view that it was still very small compared
to mammals, although accepting Quiroga’s
(1980, 1984) interpretation that some enlargement
had occurred within the group. Rowe (1996) also
believed that little brain enlargement had occurred
in cynodonts, and indeed that the neocortex did not
develop significantly until much later, in the common
ancestor of modern mammal groups. On the
other hand, Allman (1999) accepted Kemp’s reconstruction
of a considerably larger, and especially
deeper brain for the group.
What is beyond dispute, however, is that the
earliest mammals themselves did have significantly
enlarged brains. As shown by a famous
endocast (Fig. 4.11(e)) of the Jurassic Triconodon
(Simpson 1927), and by the reconstruction of the
Morganucon brain by Kermack et al. (1981), and
those of Cretaceous multituberculates (Fig. 5.12)
and placentals (Fig. 4.11(f)) by Kielan-Jaworowska
(1986), brain size in Mesozoic mammals lay within
the lower part of the size range of the brains of living
mammals. This represents an overall increase of
some four or more times the volume of basal amniote
brains, and presumably involved the evolution of
the neocortex, the complex, six-layered surface of
the cerebral hemispheres that is one of the most
striking of all mammalian characters (Kielan-
Jaworowska 1986; Allman 1999).
Growth and development
The ancestral pattern of growth of amniotes is
described as indeterminate, because it is continuous
throughout life and there is no absolute adult size. It
is associated with polyphyodont tooth replacement,
in which there are several to many successive
replacements of each tooth. This process provides
the necessary increasing size of teeth and length of
tooth row as growth proceeds. In mammals, the
growth is determinate, with a rapid phase of juvenile
growth ending in adult size, after which no further
growth takes place. This is associated with
diphyodont tooth replacement, in which there is a
single juvenile, deciduous, milk dentition, followed
by a permanent adult dentition (e.g. Luckett 1993;
Kielan-Jaworowska et al. 2004). The mammalian
growth pattern is only possible with an extremely
high rate of parental provision of nutrition to the
young, in their case by lactation, although by
comparison with the similar growth pattern found
in birds, direct provision of foraged food can
achieve the same end.
Many direct studies of size ranges of specimens
have revealed an indeterminate growth pattern in all
non-mammalian synapsid groups (e.g. Abdala and
Giannini 2000, 2002). The pattern of tooth replacement
corresponds to this, for in all there is polyphyodonty,
although a specialised version is present in
the diademodontoid eucynodonts (Fig. 4.12(a)) in
which the rate of replacement was reduced, and the
anterior postcanines were not replaced as they were
shed. The total tooth number was maintained by the
addition of new teeth at the posterior end of the
tooth row (Hopson 1971). However, this is related to
the specialised form of the gomphodont teeth, which
are designed for true, accurate occlusion between
specific uppers and lowers, rather than reflecting the
evolution of lactation or mammalian growth pattern.