The Origin and Evolution of Mammals - Moodle

navicular facet, 19 or more thoracic vertebrae,
and the third molar tooth larger than the second in
both upper and lower tooth rows. It is on the basis
of this limited morphological evidence, that the two
extinct orders, Embrithopoda and Desmostylia are
regarded as members too, although hardly surprisingly
there is no agreement about their exact respective
relationships, either to one another or to other
paenungulates.
From time to time certain Asian fossils have been
tentatively attributed to the Paenungulata, to the
extent of implying an Asian rather than an African
origin for the group as a whole (Beard 1998; Fischer
and Tassy 1993). Minchenella is represented by lower
jaws from the Late Palaeocene of China. Its molar
teeth are ‘condylarth’-like but there are minor features
somewhat reminiscent of those of the primitive
proboscidean Moeritherium. Anthracobunids
(Fig. 7.15(b)), which may be related to Minchenella,
are an Early to Middle Eocene family centred on
Pakistan (Wells and Gingerich 1982). Depending
on fine details of molar morphology (Tassy and
Shoshani 1988), anthracobunids have been interpreted
by various authors, respectively, as basal members
of Proboscidea, or Desmostylia, or Tethytheria
(Proboscidea plus Sirenia).
The interrelationships of the four other modern
orders of placentals that have been shown to be
members of Afrotheria are not yet well resolved, and
elucidation is little helped by the very poor fossil
record of all four. Two of them, Chrysochlorida, the
golden moles, and Tenrecida, the tenrecs and otter
shrews, are probably sister-groups and included in a
super-ordinal taxon Afrosoricida (Scally et al. 2002;
Asher et al. 2003). The other two are Macroscelidea,
the elephant shrews, and the solitary species of
Tubulidentata, the aardvark.
Hyracoidea
The hyracoids are represented today by only about
a dozen species in three closely related genera, all
of quite small body size. Surprisingly therefore,
they were at one time the most abundant medium
and large-sized herbivores in Africa, playing this
role long before the perissodactyls and artiodactyls
arrived on the continent (Rasmussen 1989).
Hyracoids also dispersed into much of Asia. The
earliest record is from the Late Eocene and Oligocene
LIVING AND FOSSIL PLACENTALS 253
of Fayum, where they ranged in size from Thyrohyrax,
which was about the size of a modern hyrax,
through Megalohyrax (Fig. 7.16(a)) with a 40 cm
long skull (Thewissen and Simons 2001), to the
giant Titanohyrax which was as large as a small
rhinoceros, and bigger than any contemporary proboscideans.
The morphology of the cheek teeth was
also enormously variable, from simple bunodont to
fully lophodont and selenodont molars. As the
major, principally forest dwelling group of medium
to large-sized browsers, there were species equivalent
to the pigs antelopes, and small rhinos of today
(Rasmussen and Simons 2000).
By the Miocene, hyracoids had declined in diversity,
an event presumably connected with the arrival
in Africa of several artiodactyl and perissodactyl
families. By this time, a second radiation had
occurred, the pliohyracines, which spread from
Africa throughout Europe and Asia. In these northern
areas they underwent a modest increase in diversity,
evolving high-crowned, hypsodont cheek teeth
for feeding on more abrasive plant material. Most of
the hyracoids had become extinct by the Pliocene,
although one highly specialised lineage, procaviids,
survived as the living members of the order.
Proboscidea
As mentioned, anthracobunids (Fig. 7.15(b)) are
represented by dentitions of mid-Eocene age from
India and Pakistan, which have one or two proboscidean
characters (Wells and Gingerich 1983),
and accordingly they have been accepted by several
authors as the most basal members of the group
(e.g. Fischer and Tassy 1993; Shoshani et al 1996).
However, earlier-dated, undisputed proboscideans
have since been discovered in North Africa. These
are from the earliest Eocene in age and are part of
the Ouled Abdoun Basin fauna of Morocco.
Gheerbrant et al. (1996; 1998) described isolated
upper teeth of Phosphatherium (Fig. 7.17(a)). They
were from a relatively small animal, with a body
weight estimated from tooth size of 10–15 kg. The
molar teeth have the true lophodont condition of
crests running transversely and uninterrupted by
conules, that characterises proboscideans, and in
this form there are two such lophs, bilophodonty,
which is the primitive proboscidean condition.
Daouitherium (Fig. 7.17(b)) is a very much larger