The Origin and Evolution of Mammals - Moodle

published a cladogram based on recent literature
that reflects the current majority view (Fig. 3.26).
It may reflect the most parsimonious arrangement
of the characters considered, have relatively little
homoplasy, and would perhaps measure acceptably
high on tests such as consistency index and bootstrapping,
but what it does not in itself reveal is how
few, minor, and sometimes ill-defined are the characters
supporting some of the nodes:
Therapsida: very strongly supported by around
40 characters of the dentition, skull, and postcranial
skeleton. Not since Olson (1962) proposed an
independent origin of anomodonts from caseid
pelycosaurs has anyone seriously challenged the
monophyly of the therapsids.
Eutherapsida: in contrast, very few diagnostic
characters supporting it. Rubidge and Sidor (2001)
quote just three: lateral bowing of the zygomatic
arch indicating an increase in the size of the temporal
fenestra; loss of the olecranon process of the
ulna; only three phalanges in the fifth pedal digit.
Neotherapsida: also very little support, with four diagnostic
characters mentioned, all uninspiringly triviallooking:
the vaguely described ventral expansion of
the squamosal obscuring most of the quadrate from
hind view; epipterygoid broadly contacting the underside
of the parietal; epiphyses on the atlas vertebra;
enlarged obturator foramen in the pelvic girdle.
Theriodontia: given its long acceptance, at least
since Watson and Romer (1956), surprisingly little
evidence for the relationship of these three carnivorous
groups, with just four characters: freestanding
coronoid process of the dentary; quadrate
and quadratojugal bones reduced in height and
lying in a recess in the squamosal; dorsal process of
premaxilla reduced; skull reduced in height.
Eutheriodontia (Therosauria of Kemp 1982): slightly
better diagnosed group than the previous three.
Hopson (1991) noted five unique characters: very
narrow intertemporal region; reduction of the postorbital
bone; posteriorly elongated zygomatic arch;
dentary thickened postero-ventrally below angular
bone; broadened epipterygoid. Several other characters
found in this group are shared with at least some
anomodonts, such as a reduced postfrontal bone and
loss of palatal teeth, though the cladistic analysis
implies that they are convergent.
EVOLUTION OF MAMMAL-LIKE REPTILES 79
The great majority of the characters found in therapsids
are uninformative, being either plesiomorphic
for Therapsida, or autapomorphic for the individual
taxa. Although this may be the best cladogram, and
therefore the basis of the best classification available,
it does not inspire great confidence in its truth.
The situation is reminiscent of the relationships of
the placental mammal orders, where supraordinal
groupings based on similarly weak morphological
evidence have been shown to be highly inconsistent
with the ever-better supported groupings based on
molecular data. Unfortunately, no such alternative
source of evidence is accessible for non-mammalian
therapsids.
There is, however, another possible approach to
the problem, which is to consider a functional scenario.
A cladogram of relationships is evaluated by
noting the implied evolutionary changes between
nodes, and then assessing the likelihood of those
transformations in terms of their structural and
functional plausibility. As a methodology this is
fraught with difficulties, relating less to the principle
than to the practical extent to which degrees of
plausibility can be objectively evaluated. However,
the case of the interrelationships of therapsids is
possibly one where this approach is powerful.
Much of the case for the systematic arrangement of
therapsids subgroups concerns the organisation of
the temporal fenestra. Increase in its size is the most
prominent character of the Eutherapsida, but it is
highly improbable that this really is a homologous
state in all the eutherapsid groups (Kemp 1988).
The structure of the temporal fenestra, associated
lower jaw, and inferred adductor musculature of
Biarmosuchus is sufficiently similar to that of a
sphenacodontid pelycosaur to render it safe to
assume that it represents the ancestral therapsid condition.
Compared to it, the modification of the temporal
fenestra found in a primitive dinocephalian
such as Titanophoneus consists of a dorsal expansion
in such a way that adductor musculature could
invade a broad dorso-lateral exposure of the postorbital
and squamosal bones. Turning to the
Anomodontia (Fig. 3.11), the genera Anomocephalus,
Patronomodon, Ulemica, and Eodicynodon constitute a
sequence illustrating the morphological transition in
structure of the temporal fenestra from the broad
temporal roof and short, rod-like zygomatic arch of