The Origin and Evolution of Mammals - Moodle

250 THE ORIGIN AND EVOLUTION OF MAMMALS
Hyaenodontids such as Sinopa (Fig. 7.14(c)) tended
to be more agile, with slender jaws, and a range
of body sizes comparable to the modern-day
carnivorans. In feeding specialisations too, they
radiated into a variety of highly active predators at
one end and presumed bone-crushing scavengers at
the other. The hyaenodontids also survived for far
longer than the oxyaenids, not finally disappearing
until the Miocene of Europe, Asia and Africa.
As already mentioned, the Carnivora actually
predated the creodonts, for the earliest members
were Pappictidops, a viverravid from the Early
Palaeocene Shanghuan Formation of China (Wang
et al. 1998; Lucas 2001) and the Early Palaeocene
Canadian genera Pristinictis and Ravenictis. At one
time, all the early, primitive carnivorans were
included in a group ‘Miacoidea’, which was assumed
to be basal to the whole order. However, recent
cladistic analysis indicates that different ‘miacoids’
are related to different lineages of the Carnivora
(Flynn 1998; Janis et al. 1998b). The family
Viverravidae (Fig. 7.14(f)), including the Early
Palaeocene members mentioned, are related to the
Feliformes on the basis of reduction of the number
of molars to two, and an elongated skull. The family
Miacidae (Fig. 7.14(e)), which do not appear
until the very end of the Palaeocene, are primitive
Caniformes as indicated, for example, by the relatively
shorter skull, and the loss of contact between
the calcaneum and fibula in the ankle. Surprisingly
therefore, the two respective main lineages of modern
Carnivora can be traced by fossils back to the
Early Palaeocene.
All the primitive, Palaeocene members were relatively
small carnivores, varying only from the size of
a weasel to that of a small cat, and it was not until
the rise of the modern families such as the canids,
felids, and ursids later in the Cenozoic that any
members of the order achieved the large body size
that certain mesonychids and creodonts had done
far earlier.
The origin and radiation of
the modern orders
The post-Palaeocene fossil record of placental
mammals and its associated literature is enormous,
and far beyond the scope of this work to deal with
in more than the barest outline. The times of first
occurrence, nature of the early members and their
possible relationships, and palaeogeographic distribution
are of the most immediate relevance for
understanding the Cenozoic mammalian evolution.
It is also, of course, irresistible to deal briefly with
the more unfamiliar, often quite bizarre kinds of
extinct mammals. Several modern orders appear
in the Palaeocene fossil record, undoubtedly the
Carnivora in the Early Palaeocene, and the Xenarthra,
Rodentia, Eulipotyphla, and possibly Perissodactyla
in the Late Palaeocene. However, even in these
cases, their main radiation did not commence until
the Eocene.
The molecular evidence for four super-orders of
modern placental mammals has been discussed
earlier in the chapter, and is taken as the basis for the
following account. As it happens, this radical new
phylogenetic hypothesis is surprisingly little in conflict
with conventional interpretations of the fossil
record, apart from two aspects. One is the extensive
taxonomic redistribution of the primitive ‘insectivorous’
orders; the other is the recognition that the
‘ungulate’ orders arose within two independent
radiations. Otherwise, the molecular story contributes
towards the formulation of new answers to
several long-standing questions that the fossils
alone have been unable to provide.
Xenarthra
Only the three living sub groups Cingulata (armadillos),
Pilosa (sloths), and Vermilingua (anteaters)
constitute the Xenarthra (Delsuc et al. 2001), now
that a relationship with either the old world
Pholidota, or the North America Palaeocene
Palaeanodonta has been discarded (Rose and Emry
1993). The earliest fossil record consists of Late
Palaeocene armadillo scutes of Prostegotherium in
the Itaboraí fissure deposits of Brazil. Armadillos
diverged throughout the Cenozoic of South
America, and a few genera reached North America
in the Late Pliocene faunal interchange. The
Glyptodonta were the most spectacular cingulate
xenarthrans, with their bony helmet, hugely exaggerated
carapace of interlocking scutes, and tail
massively protected by rings of bone. The cheek
teeth, all that remain of the dentition, are tri-lobed,