The Origin and Evolution of Mammals - Moodle

256 THE ORIGIN AND EVOLUTION OF MAMMALS
animal, known from the lower jaw and dentition,
plus a possible upper premolar (Gheerbrant et al.
2002). As far as can be judged, the dentition is similar
to that of Phosphatherium, and both resemble
a slightly younger, but much more completely preserved
Algerian proboscidean. This is Numidotherium
(Fig. 7.17(c)), which is represented by skull and
postcranial skeleton specimens (Mahboubi et al. 1984,
1986; Court 1995). It is small for a proboscidean, with
a height of about 1 m, but has the relatively large
head with pneumatised bones, nostrils high up on
the head, and graviportal limbs characteristic of the
order. One of the lower incisors and the canine were
lost, and the second upper incisors are the largest,
and therefore can be interpreted as incipient tusks.
The molar teeth are large and, like the Moroccan
genera, bilophodont.
The next oldest fossil proboscideans occur in the
Fayum and other Late Eocene localities of North
Africa, of which Moeritherium (Fig. 7.17(e)) is the best
known Like Numidotherium, it too has enlarged
second incisors and bilophodont molar teeth. It was
the size of a large pig and the long body, reduced tail
and short, stout limbs suggest that it was a semiaquatic,
hippo-like animal. It has actually been proposed
that it is not a proboscidean at all, but a
relative of the Sirenia, or even the extinct Desmostylia,
both of which are partially or fully aquatic mammals
(Coppens and Beden 1978). Barytherium is a second,
less well-known proboscidean from the Fayum. It
was far larger, as much as 4 tonnes in weight, and
standing 3 m at the shoulder.
Palaeomastodon is a third member of the Fayum
fauna. It possessed well-developed tusks, no trace
of canines, very large molars with three complex
lophs and thin enamel, and an extremely elongated
symphysis connecting the relatively short lower
jaws. With one exception, a hypothetical Oligocene
ancestral form resembling Palaeomastodon is
believed to have given rise to the subsequent proboscidean
radiation in Africa (Tassy 1996; Shoshani
et al. 1996). The exception are the deinotheres
(Fig. 7.17(d)), large animals that are first known in
the Late Oligocene of Ethiopia (Kappelman et al.
2003). They possess primitive molar teeth with only
two or three simple lophs, and thin enamel. The
tusks, uniquely, consist of the pair of lower incisors
extending ventrally, due to a downward reflection
of the symphyseal region of the lower jaws.
Deinotheres probably evolved directly from a primitive,
Moeritherium-like ancestor.
During the Late Eocene and through the Oligocene,
proboscideans were restricted to Afro-Arabia. In the
Middle Miocene, tectonic movement of the Arabian
peninsula created a connection across the Tethys Sea
between Africa and Eurasia, and at least three proboscidean
taxa crossed what has been termed the
‘Gomphothere Land Bridge’ (Agustí and Antón
2002). The elephantoid Gomphotherium (Fig. 7.17(f)),
the mammutid Zygolophodon, and the deinotheriid
Deinotherium (Fig. 7.17(d)) all appeared in various
parts of Eurasia in the mid Miocene. The former
two also reached North America at this time, from
which moment the elephants radiated to become a
significant part of the large herbivorous mammalian
fauna of that continent. Gomphotheres also briefly
existed in South America, after the formation of the
Panamanian land bridge in the Plio-Pleistocene.
Sirenia
The first fossil remains of the dugongs and manatees
come from the Early Eocene of Europe, and by
the Late Eocene this fully aquatic, herbivorous
group had a virtually worldwide distribution,
including the shores of Africa, Asia, North America,
and more doubtfully South America (Fischer and
Tassy 1993; Domning 2002b). Prorastomus from
Jamaica is the most primitive fossil skull of a sirenian
(Savage et al. 1994). It has, surprisingly, five
rather than the usual four premolars, which is presumably
a secondary reversion rather than retention
of the tooth formula of the Cretaceous ancestors of
placentals. The molars are bilophodont and therefore
comparable to those of early proboscideans. It
lacked the characteristic ventral deflection of the
rostrum found in all more progressive sirenians.
The postcranial skeleton of Prorastomus is unknown,
but that of a similar, slightly younger Jamaican form,
the Middle Eocene Pezosiren (Fig. 7.16(c)), is represented
by an almost complete set of postcranial
bones (Domning 2001). It was about 2 m in length
and had a relatively uncompressed neck, unlike
later sirenians. Several characters of the postcranial
skeleton indicate that Pezosiren was capable of terrestrial
locomotion, such as tall neural spines on the
anterior dorsal vertebrae for head support, a strong
connection between the sacral vertebrae and ilium,
and well built though relatively short limbs. These