The Origin and Evolution of Mammals - Moodle

80 THE ORIGIN AND EVOLUTION OF MAMMALS
the first stage, to the fully dicynodont condition
of the last. Now the initial stage in the sequence,
represented by Anomocephalus, is quite similar to
Biarmosuchus except for an antero-posterior expansion
and there is no extension of the attachment area
of the temporalis muscle dorsally onto the external
surface of the intertemporal region. Furthermore, at
no subsequent stage does the temporal fenestra
resemble the basic dinocephalian condition, or that
of any of the other supposed eutherapsids. The morphology
corroborates the hypothesis that the
Anomodontia were not related to any other derived
subgroup of Therapsida, but modified their adductor
jaw musculature independently via a different
route from the ancestral condition.
A unique arrangement of the temporal fenestra
also evolved in the gorgonopsians, this time by
extensive posterior and lateral expansions of the
temporal fenestra, but virtually no medial expansion,
leaving a very broad, flat intertemporal roof.
It was associated with modifications of the lower
jaw, jaw articulation, and adductor musculature.
These included an invasion by musculature of the
lateral surface of the reflected lamina of the angular
of the lower jaw, in a manner not matched at all in
the Therocephalia. Functionally, it is difficult to see
how either the gorgonopsian or the therocephalian
arrangement could have been ancestral to the other,
and therefore enlargement of the temporal fenestra
and associated remodelling of the adductor musculature
must have occurred independently in the two,
by different routes from an ancestral, Biarmosuchuslike
condition. It further follows that since the discrete
coronoid process of the dentary is a functionally
integral part of the modified arrangement of the
adductor musculature, it too must have been
evolved separately in these two respective taxa. The
same statement is likely to be true of the reduced
quadrate bone, and in any case the detailed structure,
mode of attachment, and function of the
quadrate is quite different in gorgonopsians compared
to therocephalians. The unavoidable conclusion
from this approach is that the association of
gorgonopsians and therocephalians in Theriodontia
is based on convergent characters. As yet there is no
morphological sequence of increasingly derived
gorgonopsian skulls, or therocephalian skulls comparable
to the anomodont sequence, but surely
when they are discovered, they will demonstrate an
independent transformation of the two lineages all
the way from a Biarmosuchus-like ancestor.
Only the Eutheriodontia, consisting of the
Therocephalia plus Cynodontia stands up to functional
scrutiny of the temporal fenestra and associated
structures. The mode of enlargement of the
fenestra is similar in the two, consisting mainly of a
medial expansion, though followed later by a lateral
expansion in the cynodonts alone. However,
discovery of a morphological sequence of primitive
to derived therocephalians might well produce a
surprise on this front and demonstrate convergence
of the feeding structures in the two.
In the meantime the picture that emerges is one of
the rapid diversification of several therapsid lineages
from a hypothetical Biarmosuchian-grade ancestor,
each developing a more sophisticated feeding
mechanism, but doing in a variety of different ways.
Although Rubidge and Sidor’s (2001) cladogram
may still be the best, given the taxonomic characters
to hand, it should not be forgotten how very weakly
supported are several of its groupings. In this light,
the reference classification of Table 2.1 is fairly
non-committal.
The palaeoecology and evolution of
synapsida
The rise of the pelycosaurs as the first group of
tetrapods to dominate the terrestrial environment;
their mid-Permian complete replacement by therapsids;
the subsequent dominance by the latter of
the terrestrial tetrapod biota during the Late
Permian; the huge, sudden drop in diversity at the
end of the Permian; and the eventual replacement
of therapsids by other taxa in the later part of the
Triassic. These high points of the synapsid story all
illustrate one way or another the profound, intimate
relationship between the evolution of the
group and its environment.
Palaeogeography and palaeoecology of the
pelycosauria
The pelycosaurs appear in the fossil record in
Pennsylvanian (Upper Carboniferous) deposits
of North America during Westphalian times