The Origin and Evolution of Mammals - Moodle

continuously growing, and form a grinding surface,
suitable for relatively soft vegetation. Appearing in
the Middle Eocene, glyptodonts radiated widely for
the remainder of the Cenozoic. The largest was the
Pleistocene genus Glyptotherium, which reached over
3 m in body length. They also dispersed into Central
and North America during the Plio-Pleistocene
(Gillette and Ray 1981).
The Phyllophaga are familiar today only as arboreal
forms, but most of the Cenozoic members of
the group were ground sloths. They date from
the Late Eocene, and by the Miocene had become
an abundant part of the South American mammalian
fauna. An undetermined form has also been
reported from the Eocene of Antarctica (Vicaíno
and Scillato Yané 1995). The mylodontids were
fairly large animals, bear-like in size and build and
with the forelimbs shorter than the hind limbs, indicating
their terrestrial rather than arboreal habit.
Early megatheriids were relatively smaller and possessed
equal-length fore and hindlimbs suggesting
a tree-living habit. However, later members of this
group, such as Megatherium, achieved enormous
body size, as large as a rhinoceros and, as has been
reconstructed many times, were no doubt capable
of an erect stance for feeding off high branches, with
the help of a massive balancing tail. The
Megalonychidae include genera that dispersed into
the Caribbean Islands in the Plio-Pleistocene, many
becoming dwarf forms.
The Vermilingua contains the South American
anteaters, which evolved a very long snout and
complete loss of the teeth. They were late to appear,
not occurring on that continent until the Middle
Miocene, and their fossil record remains generally
poor. One of the enduring puzzles of mammalian
palaeontology is the European Eurotamandua, which
consists of a beautifully preserved specimen discovered
in the Messel oil-shale (Storch and Richter
1992a). It has the elongated, edentulous snout, and
robust skeleton with powerful digging limbs of an
anteater of some description. It also possesses extra
articulations on two of its vertebrae, and a second
scapular spine, both of which characters suggest it
is a xenarthran (Storch 1981). Gaudin and Branham
(1998) performed a detailed cladistic analysis and
found that the most parsimonious position for
Eurotamandua is as the sister-group of Pilosa, which
is the xenarthran group composed of sloths plus
anteaters. On the other hand, Rose and Emry (1993)
believe that the characters shared by Eurotamandua
and xenarthran anteaters are convergent, and that it
has certain similarities instead to the pangolins,
and also (Rose 1999) to the palaeanodonts of North
America. The question of the relationships of
Eurotamandua is therefore not resolved. If it is a
xenarthran, then it is a unique representative of the
indigenous South American Eocene mammals that
mysteriously found its way into Europe by a very
obscure route, and so far as is known, unaccompanied
by any other taxon.
Although no other mammals, fossil or living can
be demonstrated to be xenarthrans, the possibility
must be entertained that the South American groups
of large placental herbivores, the Meridiungulata,
are members of the super-order. It would be no
more absurd than having to accept a relationship of
elephants with golden moles and tenrecs, within
Afrotheria, and would be consistent with their biogeography.
Probably only molecular evidence would
permit this hypothesis to be tested, but actually it is
not inconceivable that ancient DNA techniques
applied to Late Pleistocene specimens of toxodont
notoungulates might provide it.
Afrotheria
LIVING AND FOSSIL PLACENTALS 251
The wealth of molecular data supporting the
supraordinal group Afrotheria is consistent with
the palaeobiogeography of the constituent groups
(Fig. 7.15). The earliest known fossils of all the living
afrotherian orders occur in Africa, with one
exception, the Sirenia, which is readily explicable
by their marine habitat. There are also two extinct
orders that have long been associated with the paenungulates,
and which, if this is correct, are also
therefore afrotherians, an interpretation recently
confirmed by Asher et al. (2003). These are the large,
ungulate-like embrithopods, which are primarily
African but actually first appear in Europe, and the
semiaquatic Desmostylia whose exclusively northern
Pacific distribution is anomalous.
The fossil record of the relevant period of time necessary
for understanding the origin and early radiation
of mammals in Africa is very poor indeed
(Cooke 1978). Mammal-bearing Palaeocene deposits