taxonomy relies on the selection of certain character states, the
presence or absence of which groups species together. As species with
a common ancestry evolve and diverge through time, anatomical features
that their common ancestor shared will either be conserved or lost.
Genetic mutations occur over time within a species' genome and alter
or modify that species' phenotype. Therefore, if two species share
many highly specialised anatomical features (termed homology),
that are unlikely to have arisen by chance, then it is likely that
they once shared a common ancestor which also possessed those traits,
thereby meaning those species are relatively closely related.
major anatomical domains utilised by systematists to define orders
and families of placental mammals are the teeth, auditory region and
proximal tarsus (ankle). Of the three major anatomical domains utilised
in mammalian morphology, arguably the most revealing is the teeth.
Indeed, Colbert (1991) stated that if all the eutherian mammals were
extinct, and represented only by fossil teeth, "their basic classification
would be essentially the same as the classification now drawn up on
the knowledge of the complete anatomy of the animals". Others are
less complimentary of the benefits of classification using dentition.
Novacek (1992) for example, accuses "the abiding fascination with
tooth structure" of being responsible for many confusing and highly
incongruous higher mammalian relationships. Dental traits, it would
appear, seem prone to rapid and substantial parallel changes.
major problem of comparative morphology as means of determining phylogenetic
relationships is homoplasy. Homoplasy (or analogy) is the expression
of a very similar trait in two or more species that has arisen by
some reason other than inheritance from a common ancestor. Such reasons
include convergent and parallel evolution.
the 1950's phylogenetics using comparative morphology was no longer
the groundbreaking science it had been, and research was concentrating
only on the smaller branches of the tree. Phylogeneticists it seemed
had exhausted the available morphological data, and the techniques being
used were not stringent enough in their distinction between homology
and analogy. In reference to the decline of morphological phylogenetics,
Paterson declared "the clues are poor, the trail is cold". Morphological
studies may not therefore yield any further dramatic insights into the
large-scale interordinal relationships of placental mammals, but comprehensive
research is still carried out to determine intraordinal relationships.
The future of using comparative morphology for phylogenetics may lie
in utilising both morphological and molecular data. In this way, the
phylogenticist receives a more rounded perspective of possible systematic
relationships, and hopefully further helps to reduce the problems of