The Origin and Evolution of Mammals - Moodle

224 THE ORIGIN AND EVOLUTION OF MAMMALS
middle ear. Alternatively, the basal members of
related orders may have shared significant derived
characters, but these have subsequently been lost or
transformed out of recognition as the individual
orders evolved into their modern representatives.
Further purely morphological analysis of living
mammals is unlikely to lead to a recognition of
which of these two phylogenetic patterns is true, let
alone to resolve the interrelationships. Two centuries
of study is surely approaching saturation. In principle
the fossil record can improve the taxonomic
resolution by providing character combinations not
found amongst the living groups, and has indeed
contributed much to the debates on supraordinal
relationships, as discussed later. Unfortunately,
palaeontological evidence will always be hampered
by the bugbear of missing data: there are too many
cases of incongruence amongst dental and skeletal
characters for fossils alone to be regarded as reliable
trackers of phylogeny.
As with the marsupials discussed in Chapter 6,
analysis of molecular sequence data is proving the
likeliest source of information for solving the riddle
of placental interrelationships, and some remarkable
but by no means fanciful hypotheses have now
emerged, almost to the extent of providing a tree that
is fully resolved at the ordinal level. In one of the
first contributions, de Jong et al. (1993) analysed
the eye lens protein �A-crystallin and found that
the macroscelidean Elephantulus rufescens had three
amino acid replacements otherwise unique to the
paenungulates (elephants, sirenians, and hyraxes)
and the aardvark, suggesting a monophyletic group
for these very different orders, whose sole nonmolecular
common feature is that their history is
mainly African. Within a few years, this relationship
had become strongly supported by DNA
sequences, both mitochondrial and nuclear, and
even extended to include the Tenrecida (Springer
et al. 1997b; Stanhope et al. 1998). These are the tenrecs
of Madagascar and the related otter shrews of
mainland southern Africa, which together had been
regarded as a subgroup of lipotyphlan insectivores.
One final taxon of African mammals has been
added to what became named the Afrotheria.
Chrysochlorids are the golden moles of southern
Africa and, like the tenrecs, they too had been
firmly believed to be lipotyphlans. The study of
Stanhope et al. (1998) placed it firmly in the
afrotherian clade. The Afrotheria concept goes a
long way towards understanding the previously
poorly understood relationships of the Tubulidentata
and Macroscelidea. However, it contradicts all morphological
based classifications of Lipotyphla, by
removing the tenrecs and golden moles from their
position nested deeply within the group, and
related to the shrews and moles (Butler 1988).
Meanwhile, another radical rearrangement was
emerging. The relationships of the cetaceans had
been the subject of a long dispute, even to the extent
of whether they had evolved from a carnivorous,
or an herbivorous ancestor. On the basis of the
fossil evidence, the view prevailed that they had
evolved from a secondarily carnivorous group of
primitive, ‘condylarth’ ungulates known as mesonychids.
This consensus was actually disturbed long
ago, when, in a very early essay into molecular systematics,
Boyden and Gemeroy’s (1950) immunological
method suggested a relationship between
whales and artiodactyls. Little notice was taken at
the time. The recent era was marked by Graur and
Higgins (1994), who analysed the molecular evidence
then available, mainly protein sequences, and
also concluded that cetaceans were most closely
related to the Artiodactyla. Furthermore, their study
pointed to the egregious conclusion that whales
nested within the artiodactyls, as the sister-group of
the hippos. Since then, a very large quantity of DNA
sequence data bearing on the question has been
analysed, with results overwhelming confirming
the hippo-whale clade (Waddell et al. 1999; Gatesy
and O’Leary 2001). At first, morphological evidence
continued to fail to support even the overall group
Cetartiodactyla, as it had come to be named
(O’Leary 1999), although the similarities that do
exist between whales and hippos in particular, such
as the reduction of hair, development of subcutaneous
fat, and ability to suckle the young underwater,
were not lost on the proponents of the new
grouping. Far from being the convergences hitherto
supposed, they can be taken as evidence of a common
ancestor between the two that was already
adapted to a semiaquatic existence. Modern whales
are so extremely derived that the key characters that
would allow the recognition of their relationships,
notably limb structure and dental structure, are