The Origin and Evolution of Mammals - Moodle

Living placentals and their
interrelationships
The vast majority of living and fossil mammals are
placentals. Today there are about 4,400 species, which
are traditionally organised into 18 Orders (Table 2.4),
with an extra one if the Pinnipedia are separated
from the Carnivora, and a twentieth if the recently
extinct Malagasy order Bibymalagasia is recognised
as such. There have been many attempts to discover
supraordinal groupings from amongst these Orders
based on morphological characters, though few proposals
have been universally accepted. It is only with
the advent of increasingly large sets of molecular
sequence data in the last few years that a reasonably
robust resolution looks imminent, although these
contemporary analyses are remarkably and controversially
at odds with the traditional ones.
Novacek et al. (1988) summarised the then current
situation regarding supraordinal classification of placentals,
a time at which morphology was still dominant
but molecular data was at the threshold of
significance (Fig. 7.1(a)). They accepted a basal group
Edentata that combined the Xenarthra of the New
World with the Pholidota of the Old, based on a few
cranial characters, loss of the anterior teeth, and
reduction of the enamel of the remaining ones. This
left the rest of the living placentals as a monophyletic
group Epitheria, sharing such apparently minor characters
as the shape of the stapes bone in the ear. They
found very little resolution within the Epitheria,
and concluded that there was a polychotomy of
no less than nine lineages arranged as a ‘star’ phylogeny.
No remnant of the previously recognised taxon
Ferungulata, created by Simpson (1945) for the
Carnivora plus the ungulate orders Artiodactyla,
Perissodactyla, Proboscidea, Hyracoidea, Sirenia,
and Tubulidentata remained. On the other hand,
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CHAPTER 7
Living and fossil placentals
three supra ordinal taxa of earlier authors did survive.
One was Gregory’s (1910) Archonta, consisting of
generally conservative forms and by now composed
of the Primates, Dermoptera, Scandentia, and
Chiroptera, but excluding the Lipotyphla. The second
was Glires, originating with Linnaeus (1758) and
widely accepted ever since, for the Rodentia and
Lagomorpha; Novacek et al. (1988) tentatively placed
the Macroscelidea as the sister-group of the Glires.
The third supraordinal taxon recognised was, like
Glires, well-established if not universally accepted.
The Proboscidea and Sirenia constitute a group
Tethytheria and the addition of its supposed sistergroup
Hyracoidea creates the Paenungulata.
Even for the supraordinal taxa that were proposed,
morphological characters supporting them
are few in number, fine in level of detail, and frequently
challenged. Rose and Emry (1993), for
example, rejected the Edentata, and Pettigrew et al.
(1989) dismembered the Archonta by separating the
Microchiroptera (echo-locating bats) and tree shrews
on the one hand, from the Macrochiroptera (fruit
bats), Primates, and Dermoptera on the other. Few
authors have accepted uncritically the relationship
of macroscelideans to rodents plus lagomorphs, and
Fischer and Tassy (1993) continued to argue for a
relationship of hyracoids with perissodactyls,
rather than with the elephants and sirenians as
Paenungulata. In evolutionary terms, this general
paucity and triviality of supraordinal diagnostic
characters points to one of two conclusions. Possibly
the living placental orders all diverged at a low
taxonomic level, from within a radiation of primitive,
insectivorous forms that differed from one
another in little more than such things as the course
of various minor foramina, nerves and blood vessels
in the skull, or the fine details of the structure of the