The Origin and Evolution of Mammals - Moodle

18 THE ORIGIN AND EVOLUTION OF MAMMALS
Tulerpeton (Lebedev and Coates 1995), which, if correctly
interpreted, indicates that the living Amniota
must have separated from their closest living relatives,
the Amphibia, at least 360 Ma. If Tulerpeton is
rejected, the next candidate for earliest stem-amniote
is the 333 Ma Lower Carboniferous Westlothiana
(Fig. 3.2(a) and (d)), which is known from complete
skeletons (Smithson et al. 1994). It was a small,
short-limbed, superficially very reptile-like animal
with a number of amniote characters such as the
structure of the vertebral column and certain details
of the skull bone pattern. There is a considerable
diversity of later Carboniferous tetrapods that are
widely, if not universally considered stem-group
amniotes (Clack and Carroll 2000; Heatwole and
Carroll 2000). The seymouriids, once believed to be
the ancestral reptile group, include quite large
highly terrestrially adapted forms. Probably the
majority view at present is that the apparently herbivorous
diadectids along with the large, crocodilelike
limnoscelids together constitute the closest
relatives of amniotes (Laurin and Reisz 1995, 1997;
Berman et al. 1997; Gauthier et al. 1988). Other analyses
place the diadectomorphs as the sister group of
the Synapsida alone, and therefore regard them as
actual members of the Amniota (Panchen and Smith
1988; Berman 2000).
Whatever the exact relationships might be, it is
clear that Amniota was the one lineage within the
complex radiation of early tetrapods that achieved a
high level of terrestrial adaptation. Assuming that
the protorothyridids and synapsids are indeed the
two sister groups constituting a monophyletic
Amniota, the structure of the hypothetical common
ancestor of the two can be inferred from a comparison
of the most primitive, Carboniferous members
such as Hylonomus and Archaeothyris, respectively. It
was a relatively small, superficially lizard-like animal,
with a presacral body length of maybe 10 cm.
The classic view of the evolution of the skull in
amniotes was that the synapsid skull such as that of
Archaeothyris evolved from an anapsid skull such as
that of a protorothyridid by little more than opening
of a temporal fenestra low down behind the orbit
(e.g. Carroll 1970b). However, Kemp (1980b, 1982)
noted that the protorothyridid skull also possessed
uniquely derived characters, absent from the synapsids.
Notably, the postorbital, supratemporal, and
tabular bones were all reduced in size. The implication
is that the common ancestral amniote skull must
have combined the absence of a temporal fenestra
with large postorbital, supratemporal, and tabular
bones. This configuration is found in certain early
tetrapods such as the limnoscelids and probably
Westlothiana (Fig. 3.2(a)), both considered stemgroup
amniotes. Significantly, in both these latter
cases the posterior part of the skull appears to have
retained at least some degree of flexibility between
the skull table and the postorbital cheek region. This
in turn is probably part of a type of cranial kinetism
found in the most primitive of tetrapods, which is
associated with suction feeding in water. Thus the
hypothetical ancestral amniote was by inference as
much an aquatic feeder as a fully evolved terrestrial
one. The strengthening of the hind part of the skull
that is necessary for an actively biting animal feeding
on land must have evolved subsequently, differently,
and therefore independently in the two respective
amniote groups, explaining the difference in cranial
structure between them. In addition to the repatterning
of the temporal bones, other features of the
evolution of the amniote skull are related to strengthening
of the skull. The back of the skull, the occiput,
was braced firmly against the cheek to resist the
increasing forces generated by the increased jaw closing
musculature, whilst elaboration of the tongue
and hyoid apparatus in the floor of the mouth may
well have occurred to assist in the capture and oral
manipulation of food (Lauder and Gillis 1997). To
judge by the homodont dentition of sharp teeth
rounded in cross-section, insects and other invertebrates
constituted the principal diet.
There were new features of the postcranial skeleton
of the ancestral amniote, although as Sumida
(1997) shows, evolution of the characteristic amniote
postcranial skeleton was quite gradual, with characters
accumulating in several of the stem-amniote
groups, and leading eventually to a highly terrestrially
adapted form (Fig. 3.2(e)). Early on there was
the evolution of the typical amniote vertebral structure
in which the spool-shaped pleurocentral
element and neural arch are firmly connected,
and small ventral intercentra lie between adjacent
pleurocentra. The limbs became relatively larger.
The most characteristic postcranial feature acquired
at the fully amniote level is the fusion of three of