The Origin and Evolution of Mammals - Moodle

78 THE ORIGIN AND EVOLUTION OF MAMMALS
postcanine structure, and no postcranial characters
at all. Luo (1994) from his perspective makes use of
the inordinate number of 33 characters of the articular
and quadrate, and 20 of the petrosal bone, out
of his total of 82. It is certainly hard to believe that
these characters are all phylogenetically independent
of one another. Using a large number of postcanine
characters seems bound to bias the analysis
towards a relationship of diademodontoids and
tritylodontids, while postcranial characters generally
favour a relationship of tritylodontids to mammals
rather than to diademodontoids but may be
poor resolvers of the tritylodontid, tritheledontid,
and mammal trichotomy.
A possible source of resolution of the particular
question of the precise origin of Mammalia from
within the non-mammalian cynodonts lies in the
series of small forms, described above as the Rio
Grande do Sol tritheledontans from the early Upper
Triassic of South America. These differ from one
another in small details of the cranial structure and
dentition, and tentatively reveal detailed patterns of
the emergence of mammalian characters (Bonaparte
et al. 2003). With more knowledge of their structure,
morphological sequences showing in finer detail
the acquisition of fully mammalian characters may
be determined.
To complicate, or perhaps to elucidate matters
even further, Bonaparte and Crompton (1994)
described a juvenile specimen of possibly
Probainognathus, which has a number of more
derived, mammal-like character states, compared to
mature specimens. The prefrontal and postorbital
bones are small and the frontal bone borders the
Biarmosuchia Dinocephalia Anomodontia Gorgonopsia Therocephalia Cynodontia
Therapsida
Eutherapsida
Neotherapsida
orbit. The zygomatic arch is relatively more slender
and the braincase relatively larger. In the lower jaw,
the dentary extends backwards very close indeed to
the squamosal, and at the front end the symphysis is
horizontal and unfused. Even the postcanine teeth
resemble those of Morganucodon much more than do
adult Probainognathus teeth.
More will be said later about the possible role of
miniaturisation of the body in the process of the
origin of mammals. For the moment, this mosaic of
ancestral and derived mammalian characters seen
among the most progressive eucynodont groups,
tritylodontids, tritheledontids, therioherpetontids,
juvenile probainognathids, and presumably also
dromatherians and their like, reveals the possibility
that several lineages of Middle Triassic eucynodonts
were independently evolving reduced body
size with a concomitant convergence of several
basic characters associated with small body size,
but superimposed upon divergent characters associated
with differing diets etc. It is perhaps significant
that, as described in a later chapter, members
of the taxon Mammalia almost at their first appearance
consisted of at least four distinct groups. At a
far lower taxonomic level than was believed
50 years ago, maybe the ‘reptilian–mammalian’
boundary was crossed several times, if such
a cladistically incorrect statement may be forgiven.
Overview of the interrelationships and
evolution of the Therapsida
Rubidge and Sidor (2001) reviewed the interrelationships
of the principal therapsid groups and
Theriodontia
Eutheriodontia
Figure 3.26 Rubidge and
Sidor’s (2001) cladogram of the
principal therapsid groups.