The Origin and Evolution of Mammals - Moodle

176 THE ORIGIN AND EVOLUTION OF MAMMALS
reason to doubt that they shared a common mammalian
ancestor (Hopson and Crompton 1969;
Kermack and Kielan-Jaworowska 1971).
There were two characters quoted in support of
a relationship between Morganucodon and monotremes.
The first was an alleged similarity between
the molar teeth. In fact, echidnas completely
lack teeth and Ornithorhynchus only possesses them
as a juvenile, shedding them and replacing them
with horny ridges, shortly after emerging from the
burrow. These juvenile teeth are three in number
and are large, thinly enamelled and have a crown
pattern consisting of two or three high, transverse
ridges or lophs connecting labial with lingual cusps.
They were interpreted as based on a linear arrangement
of three cusps, and hence derivable from
the ‘non-therian’ molar teeth of a Morganucodon-like
ancestor, but specialised by extreme transverse
widening. The second supposedly shared character
was the structure of the sidewall of the braincase.
In living therians this is formed largely from the
alisphenoid bone, in monotremes from an anterior
lamina of the petrosal bone. In Morganucodon both
these bones are present but, as in monotremes, the
anterior lamina constitutes the sidewall itself. Thus,
relying on these two similarities, an early dichotomy
into non-therian and therian mammals was envisaged,
with monotremes and therians the respective
living members of the two.
In due course, however, it was shown that the
differences between the two groups in the structure
of the sidewall of the braincase were quite trivial. In
both cases the bone that forms it is an intramembranous
ossification that later in development
simply fuses with either the periotic bone behind as
in monotremes, or the alisphenoid bone below as
in therians. Furthermore, the alleged similarities in
tooth structure came to be recognised as far too
vague to carry much weight. Compared to this very
weak evidence for a monotreme–Morganucodon relationship,
there are several characters shared by
monotremes and therians that can be demonstrated
to be more derived than in Morganucodon (Kemp
1983). These include the loss of contact of postdentary
bones to the dentary to form ear ossicles, the
fusion of the elements of the first vertebra to form a
complete atlas ring and of the cervical ribs to the
vertebrae, and the enlargement of the cerebellum.
All cladistic analyses now place monotremes closer
to living therians than to morganucodontids.
Up to this point, the fossil record of monotremes
had not proved helpful. Apart from Pleistocene
remains, the only fossil known was Obdurodon
(Fig. 5.21(a)), a Miocene ornithorhynchid that had
retained an adult dentition. Even the beautiful,
complete skull found in 1985. (Archer et al. 1992;
Musser and Archer 1998) added nothing useful to
the debate on monotreme relationships, for the teeth
(Fig. 5.21(b)) are very similar to the juvenile teeth of
Ornithorhynchus, and equally difficult to compare
with those of other taxa. A palaeontological breakthrough
came with the description of a fragment
of lower jaw bearing three molar teeth that had
been found in the opal mine of Lightning Ridge,
New South Wales (Archer et al. 1985). Steropodon
(Fig. 5.21(c)) dates from the Albian stage of the
Early Cretaceous. Its molars have a pattern of cusps
that can be identified as equivalent to the three
trigonid and three talonid cusps of a tribosphenic
lower tooth, although modified in a monotremelike
fashion by development of strong transverse
ridges (Kielan-Jaworowska et al. 1987; Rich et al.
2001b). Whether the dentition of Steropodon was a
modified tribosphenic tooth as such, or evolved
from a pre-tribosphenic stage was debatable (Kielan-
Jaworowska et al. 1987). At any event, not only was
Steropodon the first Australian Mesozoic mammal to
be discovered, but also it is one of the best examples
of that rare phenomenon of a new fossil intermediate
resolving an existing dispute about relationships.
Monotremes are highly derived tribosphenidans.
A few more monotremes have been discovered.
Teinolophos (Fig. 5.21(d)) is also from the Albian of
Australia, and the sole specimen to date consists of
a single antero-posteriorly compressed molar tooth
in a minute partial lower jaw. Rich et al. (2001b)
interpreted it as closely related to Steropodon. The
first monotreme to be discovered outside Australia
is Monotrematum (Fig. 5.21(e)). It is of Palaeocene
age and occurs in Patagonia. So far only isolated
molar teeth have been found, but these resemble
quite closely those of Obdurodon, indicating that it is
an ornithorhynchid (Pascual et al. 1992, 2002).
To this point in the story of the origin of tribosphenidans,
the timing of the appearance in the
fossil record of the different degrees of expression