The Origin and Evolution of Mammals - Moodle

the ankle bones to form an astragalus, increasing the
strength and precision of movements of the ankle.
Compared to the living amphibians, there are
many physiological, soft tissue, and behavioural
features characteristic of living amniotes, but of
course very little is known about how, when, and in
what sequence they evolved. Of all these, none is
more significant than the amniote egg. However,
understanding the evolution of this and the associated
reproductive mode of amniotes is frustratingly
difficult, not only because reproductive physiology is
generally indecipherable from fossils, but also
because the modern amniotes are very similar to one
another in this respect, and very different from their
closest living relatives, the Amphibia. (Stewart 1997).
At best, occasional individual amphibian species
demonstrate possible analogues of amniote features,
such as internal fertilisation, or the laying of terrestrial
eggs. A number of authors have stressed the primary
importance of increased egg size in tetrapods
that have direct terrestrial development compared to
amphibians with the typical aquatic larval phase.
Carroll (1970a) noted the relationship between body
size and egg size in plethodontids, a group of amphibians
that includes several that lay terrestrial eggs
from which miniature adults emerge directly, without
a larval phase. He inferred that the first amniotes
to lay eggs on land that still lacked extraembryonic
membranes must have been very small organisms
with a precaudal body length of about 40 mm,
because otherwise the eggs they laid would have
been too large for adequate gas exchange. Packard
and Seymour (1997) speculated that the initial evolutionary
innovation towards the amniotic egg was the
reduction of the jelly layers that surround a typical
anamniotic egg and their replacement by a yolk sac,
in order to increase the rate of gas diffusion from air
to embryo. At the same time, the evolution of a proteinaceous
egg membrane would have increased the
mechanical strength of the egg, as well as further
increasing its permeability. Given that these changes
permitted some increase in egg size, further increase
would have required more yolk provision, consequently
imposing meroblastic rather than holoblastic
development and requiring a vascular system for the
transport of food from the yolk sac to the embryonic
cells, an argument previously adduced by Elinson
(1989). On this argument therefore, the extended yolk
EVOLUTION OF MAMMAL-LIKE REPTILES 19
sac was the first of the extraembryonic membranes to
evolve. The allantoic sac was perhaps originally a
small extension of the embryonic gut that functioned
as storage of excretory products, and later the amniotic
folds evolved to create a space surrounding the
embryo so that the allantois could enlarge and
become a functional lung. At any event, the final outcome
of whatever sequence of evolutionary modifications
occurred was the full amniotic egg, capable of
developing on dry land and producing a miniature
adult no longer dependent on free-standing water.
The significance of this for the effective invasion of
land, and ultimately the origin of mammals can
hardly be exaggerated.
Other biological aspects of the origin of amniotes
are equally obscure, such as the origin of amniote
skin and scales, and their various protective and
supportive functions (Frolich 1997). Virtually nothing
is known of the evolution of the various waterconservation
adaptations.
Pelycosauria: the basal synapsid
radiation
The taxon Pelycosauria was introduced by E. D. Cope
in the nineteenth century for the North American
members of what subsequently became recognised
as the most primitive members of the Synapsida. By
the time of Romer and Price’s (1940) Review of the
Pelycosauria, which is one of the great classics of vertebrate
palaeontological literature, the term was
universally accepted. With the advent of cladistic
classification, the group Pelycosauria was recognised
as paraphyletic, but it continues to be widely
used for ‘basal’, or ‘non-therapsid’ synapsids, in
which informal, but extremely useful sense it has
been adopted here.
Reisz (1986) noted the synapomorphies of the
Synapsida, which are therefore the characters that
distinguish pelycosaurs from non-synapsid amniotes:
● temporal fenestra primitively bounded above by
the squamosal and postorbital bones
● occipital plate broad and tilted forwards
● reduced post-temporal fenestra
● postparietal bone single and medial
● septomaxilla with a broad base and massive
dorsal process.