The Origin and Evolution of Mammals - Moodle

coexist with other characters that are aquatic adaptations,
including a dorsal position of the nostrils and
a long row of very dense ribs.
Desmostylia
The desmostylians are a group of semiaquatic
mammals as large as hippos. They were limited taxonomically
to about half a dozen genera, temporally
to the Late Oligocene and Miocene, and geographically
to the east and west margins of the northern
Pacific Ocean (Domning et al. 1986; Domning 2002a).
Complete skeletons of Paleoparadoxia (Fig. 7.16(b))
demonstrate well-developed fore and hind limbs,
with broad, presumably paddle-like hands and feet,
suggesting an amphibious, seal-like existence. The
incisors and canine teeth are well developed, and
there is a long diastema. The cheek teeth resemble
those of the early proboscideans, particularly
Moeritherium. Clementz et al. (2003) analysed stable
isotope contents of the bones of Desmostylia, and
concluded that they could forage on sea grass, and
also on land during terrestrial excursions.
That desmostylians are paenungulates is not seriously
disputed; a relationship with the tethythere
grouping of proboscideans and sirenians is usually
preferred (Fischer and Tassy 1993; Asher et al. 2003).
Embrithopoda
The embrithopods are best known as the celebrated
Late Eocene Arsinoitherium (Fig. 7.16(d)) from the
Fayum. It was a huge, rhinoceros-sized animal with
the skull adorned with a pair of massive horns.
Neither tusks nor enlarged canines are present, and
the cheek teeth are bilophodont and hypsodont in
form (Court 1992). Teeth of earlier and more primitive,
hornless forms (Fig. 7.16(e)) occur in Romania
and central Turkey (Maas et al. 1998).
As with the desmostylians, there is more or less
complete agreement that the Embrithopoda are part
of the paenungulate radiation, but not about precisely
where they fit. Tassy and Shoshani (1988) regarded
them as the most basal paenungulate group. Court
(1990), Fischer and Tassy (1993) and Asher et al. (2003)
have them closer to the Proboscidea than to the
Sirenia, within the Tethytheria.
Macroscelidea
Fossil elephant shrews occur only in Africa, the
earliest record being teeth of Chambius from
LIVING AND FOSSIL PLACENTALS 257
the Middle Eocene of Tunisia, followed by the Late
Eocene Herodotius from the Fayum (Simons et al.
1991; Butler 1995). They have four-cusped, bunodont
molars lacking lophs, similar to what is seen
in later macroscelideans.
Comparison of the dental characters of these
primitive forms has been made with hyopsodontid
‘condylarths’ by a number of recent authors
(Simons 1991 et al; Butler 1995). Most recently,
Tabuce et al. (2001) have also supported the ‘condylarth’
origin of the Macroscelidea, and proposed
that the dentition of the primitive members is close
to the ancestral state for paenungulates. However,
since the non-afrotherian Perissodactyla appear in
their cladogram as relatives, convergent evolution
of dental characters must have occurred at least at
some level, which considerably blurs the picture.
Tubulidentata
Represented today by Orycteropus afer, the single
species of aardvark, the fossil record of this group is
scarcely any richer. The four or five known genera
are all placed in the same family by McKenna and
Bell (1997) and they date from the Oligocene of
Europe and Early Miocene of Africa.
There are subfossil bones of a very peculiar mammal
in Madagascar, named Plesiorycteropus,
preserved along with giant lemurs, elephant birds,
etc. in Quaternary deposits. Representatives of most
of the bones are present, except for the facial region,
jaws, and dentition. It was a heavily built, digging
animal, probably with a long narrow snout and
lacking teeth. Most commentators have regarded
Plesiorycteropus as a tubulidentate. However, McPhee
(1994) failed to find any characters uniquely shared
with Orycteropus, and his cladistic analysis did not
group them exclusively together. He concluded
that Plesiorycteropus is sufficiently distinct from any
other placentals as to warrant its own order,
Bibymalagasia. In the most recent analysis, Asher
et al. (2003) found that Plesiorycteropus is at least a
member of Afrotheria.
Afrosoricida
Seiffert and Simons (2000) have described lower
jaw fragments of a Late Eocene form from the
Fayum fauna, which they claim may be related
to chrysochloridans and/or tenrecs, or possibly to the
West Indian eulipotyphlan family Solenodontidae.