The Origin and Evolution of Mammals - Moodle

226 THE ORIGIN AND EVOLUTION OF MAMMALS
three, has been long accepted on morphological
evidence: they share a unique kind of accessory
articulations between the vertebrae, from which the
taxon name comes, and the absence of enamel on
the teeth.
● Afrotheria. Proboscidea, Sirenia, and Hyracoidea,
together constituting the long accepted Paenungulata
of morphological based schemes, along with
Tenrecida (tenrecs and otter shrews), Tubulidentata
(aardvarks), Macroscelidea (elephant shrews), and
Chrysochlorida (golden moles).
● Euarchontoglires. Primates, Dermoptera (colugos),
and Scandentia (tree shrews) left over from the old
Archonta, combined with Rodentia and Lagomorpha
(rabbits and pikas), which constitute Glires.
● Laurasiatheria. Eulipotyphla (hedgehogs, moles,
and shrews), Chiroptera (bats), Pholidota
(pangolins), Carnivora, Perissodactyla (odd-toed
ungulates), and Cetartiodactyla (even-toed ungulates
and whales).
Murphy et al. (2001a) and Madsen et al. (2001)
differed over the interrelationship between these
four super-orders, and neither could produce a
well-supported hypothesis. However, they have
now combined their respective data sets, and
applied the powerful Bayesian methods in addition
to Maximum Likelihood methods to what has
become a 16.4 kilobase sequence (Murphy et al.
2001b). The result is a strongly supported inference
that the Euarchontoglires and Laurasiatheria are
sister groups of what they term Boreoeutheria.
Boreoeutheria is the sister-group of Xenarthra, and
Afrotheria is the basal group of living placentals.
The only other molecular analyses involving very
large data sets are those using the ever-increasing
range of taxa whose complete mitochondrial genome
has been sequenced (Arnason et al. 2002; Lin et al.
2002). These studies relatively weakly support
Xenarthra, Afrotheria, and most of Laurasiatheria,
but rodents and erinaceids tend to come out in
unexpected positions. The latter family, hedgehogs,
are found to be basal to all the placentals in
Arnason et al.’s tree, and in one of Lin et al.’s versions.
Corneli (2003) concluded that supraordinal
relationships as they appear in trees based on mitochondrial
genomes are very poorly supported
because the branches are too short. On the other
hand, he found that complete mitochondrial
sequences do resolve the relationships of families
within orders very well.
Murphy et al. (2001b) have generated the most
complete tree of the relationships of the subgroups
within the super-orders, most of which have good
statistical support both from this work and from
the further analysis by Springer et al. (2003), and
is the basis of the molecular-based cladogram in
Fig. 7.1(b).
There are important implications for the biogeographic
dimensions of the early diversification of placental
mammals inherent in this classification, which
are taken up later, after the fossil evidence for the
emergence of the recent orders has been reviewed.
Cretaceous fossils
Like the marsupials, the placental (eutherian) lineage
is one of the several groups of tribosphenidan
mammals that first appeared in the Early Cretaceous
fossil record. Primitive placental mammals can be
recognised by several dental characters. There are
only three molars in the upper and the lower jaws,
and the last of the premolars, particularly the upper
one, tends to be more complex than the others, and
is described as submolariform. The upper molar
teeth have a narrow stylar shelf so that the metacone
and paracone are well towards the buccal side
of the crown. These characters are associated with a
series of skull characters.
Eomaia (Fig. 5.17(a) and (b)) is the earliest
described placental (Ji et al. 2002). This astonishingly
well-preserved skeleton of a little 25 g mammal
even has impressions and carbonaceous traces
of its pelt. It is from the Barremian part of the Yixian
Formation of China, which gives it an age of about
125 Ma. The dental formula conforms to the presumed
primitive placental one of I5/4: C1/1:
PM5/5: M 3/3, and the last premolar shows little
molarisation. The postcranial skeleton indicates
that Eomaia was a climbing and perhaps permanently
arboreal animal. A few other Early Cretaceous
placental mammals have been found that are only
slightly younger than Eomaia, although none anywhere
near such complete specimens. The isolated
teeth of the Russian Murtoilestes date from around
the end of the Barremian some 120 Ma (Averianov