The Origin and Evolution of Mammals - Moodle

90 THE ORIGIN AND EVOLUTION OF MAMMALS
whose exact cladistic position is uncertain, such as
gorgonopsians.
Once an acceptable sequence of hypothetical
ancestral stages has been established, a decision
needs to be made about whether to adopt an atomistic
or an integrated approach to the biology of
these reconstructed hypothetical ancestral organisms.
Discussing the transition to mammals functional
system by functional system, which is the
atomistic approach, eases description and clarifies
explanation of particular morphological changes
from an existing state to a new one. The alternative
is the integrated approach of considering each stage
in its entirety, before moving on to the next. This
focuses on the interdependence of all the structures
and processes in the individual organisms, and
therefore clarifies the role of each functional system
in the overall biology of a particular organism.
Without this appreciation, it would be impossible
to explain the evolution of ‘mammalness’ as a
whole, even though this is the property that actually
evolved. An atomistic approach has been adopted
here, but the chapter concludes with a section in
which the evolution of the mammal as an integrated,
whole-organism is reviewed. This should
be seen as the culmination of the chapter and not
merely an appendage.
Feeding
Ancestral amniote grade
The ancestral amniote skull (Fig.4.2(a)) is reconstructed
on the basis of Westlothiana (Smithson et al.
1994). There was a homodont dentition of numerous
single-cusped teeth suitable for small prey such
as insects. No temporal fenestra had evolved, and a
relatively undifferentiated capiti mandibularis muscle
ran from its origin on the inner surfaces of the
temporal region of the skull behind the orbit, to an
insertion on the medial and dorsal surfaces of the
posterior part of the lower jaw. Pterygoideus musculature
from the dorsal surface and hind edge of
the palate was inserted on the medial face of the
lower jaw. The jaw articulation was a simple, transversely
oriented hinge joint, between the roller-like
quadrate condyle above and the trough of the articular
bone below. Although, with the abandonment
of suction feeding in water, kinetic movements of
the cheeks of the skull no longer occurred, the
pattern of skull bones still indicates the position of
the old hinge lines. The feeding mechanism by this
stage was a relatively weak and simple orthal closure
of the jaws, which would have disabled the prey.
Lauder and Gillis (1997) suggested that manipulation
of the food may have involved elaborate tongue and
hyoid musculature even by this early stage, though
there is little evidence about any details.
Basal pelycosaur grade
The major feature of the feeding system that had
evolved in the ancestral pelycosaur was the temporal
fenestra, but how exactly it developed remains
uncertain. The standard explanation for the function
of temporal fenestrae in amniotes is that of
Frazzetta (1969), who argued that the edges of the
bone surrounding a fenestra offer a stronger connective
tissue attachment for muscles than does the
flat, internal surface of the temporal bones of the
skull. The function of the fenestra is for the adductor
musculature to be larger and more powerful,
without tearing itself away from its anchorage on
the skull bones by its own force. Kemp (1980b, 1982)
proposed that the synapsid fenestra evolved by
expansion of the connective tissue that connected
the cheeks to the skull table, across the old kinetic
lines (Fig. 4.2(b)). As the connective tissue expanded,
it became the aponeurotic sheet covering the fenestra,
and more and more adductor mandibuli muscle
fibres would have attached to its under surface.
Thus the relationship between the temporal fenestra
and adductor musculature was established from
the start. The increased force produced by the jawclosing
musculature required strengthening of the
skull to resist it, and powerful braces between the
braincase, quadrate region, and skull roof evolved.
The supraoccipital expanded dorsally and the
paroccipital processes laterally, creating the characteristic
structure of the synapsid plate-like occiput.
This stage of cranial evolution had been reached
by Eothyris (Fig. 3.2(c) and4.2(b)), the pelycosaur
with the most primitive known arrangement of the
temporal region. By the more progressive, basic
eupelycosaur grade represented by varanopseids
(Fig. 3.4(d)), there had been additional remodelling
of the bones of the temporal region of the skull. The
supratemporal and postorbital bones were reduced