The Origin and Evolution of Mammals - Moodle

have postcranial adaptations for arboreal leaping,
such as an elongated calcaneum and opposable first
digits. Several authors have argued that omomyoids
are basal members of the Haplorhini, on the
basis of the relatively large orbits, loss of the first
premolars, and details of tooth structure and the
ear region of the skull (Ross et al. 1998; Gunnell and
Rose 2002). Some go so far as to relate them to the
tarsiers in particular; certainly the short snout,
large, forwardly directed eyes, and highly agile
skeleton give a very strong impression of a tarsier,
as reviewed by Martin (1993).
The modern groups of Strepsirhini have a poor fossil
record, even for primates. There is no certain
lemuriform at all prior to the Late Pleistocene of
Madagascar, with the dubious and controversial
exception of the Oligocene Bugtilemur from Pakistan
(Marrivaux et al. 2001) The other strepsirhine branch,
the lorisiformes, have been recorded in the late
Middle Eocene of the Fayum region of Egypt, but
only as isolated teeth: Karanisia is a possible loris, and
Saharagalago a possible galago (Seiffert et al. 2003).
For the living haplorhines, the earliest member of
the modern tarsiiform family Tarsiidae has been
identified from teeth attributed to the modern
genus Tarsius, found in Middle Eocene deposits of
China (Beard et al. 1994).
The diversity of the adapiforms, omomyoids,
lorisiforms, and tarsiiforms show that the Eocene
was a time of considerable radiation of primitive
primates, producing a significant fauna of small,
arboreal mammals. This observation might have
been of only marginal interest had the radiation not
also included basal members of the anthropoids, the
monkeys, apes, and humans. What at the time was
in effect another ‘prosimian’ grade of Eocene primates
appears in the same Middle Eocene deposits of
China that yielded Tarsius (Beard et al. 1996; Dagosto
et al. 1996). Eosimias is known only from dentitions
and some probably associated limb bones, and
would indeed warrant little attention except for its
probable position as the most basal member of the
Anthropoidea (Ross et al. 1998; Ducrocq 2001). It
was a small animal, the lower jaw length being
about 2.6 cm. Characters that support its anthropoid
affinity are mostly dental as no cranial and very little
postcranial material has yet been found. The incisors
are reduced in height, spatulate, and vertical
LIVING AND FOSSIL PLACENTALS 271
rather than slightly procumbent, and the talonid of
the last lower molar is much reduced. Isolated teeth
of equally ancient basal anthropoids occur in the
Glib region of Algeria (Godinot and Mahboubi
1992), indicating that from the start anthropoids
occurred in Africa as well as Asia.
The exact relationship of the Anthropoidea to the
other Eocene primates remains a matter of considerable
debate, and virtually every possibility has
been proposed by one author or another (Martin
1993). Dagosto (2002) notes that hypotheses linking
them to, respectively, adapiforms, omomyoids,
tarsiids, omomyoids plus tarsiids, or to no known
group have all been proposed by one author or
another within the last decade or so.
From the Late Eocene onwards, basal anthropoids
radiated in Africa and Asia (Beard 2002a).
Several genera are found in the Late Eocene and
early Oligocene of the Fayum deposits, notably
Apidium, which is represented by partial skulls
(Simons 1995). Siamopithecus from Thailand
(Ducrocq 1999) is an Asian representative although
so far known only from the dentition. The interrelationships
amongst these several lineages of
Eocene anthropoids are very unclear at present
(Simons 1992; Ross et al. 1998; Ducrocq 2001), and it
is not even established whether the respective taxa
from the two continents are sister-groups, or part of
a contiguous fauna.
This uncertainty about exact relationships is also
true of the two modern groups of anthropoids. The
oldest of the New World monkeys, the Platyrrini, is
Branisella, represented by Late Oligocene teeth and
fragmentary jaws from Bolivia. The oldest actual
skull so far described is that of Chilicebus, which is
Early Miocene in age and a member of the existing
family Cebidae (Flynn et al. 1995). No African or
Asian platyrrini have been found, although
Simons (1997; Takei et al. 2000) noted some dental
similarities between Branisella and the Fayum
Proteopithecus. They proposed a relationship
between the two which, if true, offers taxonomic
support for the view that the origin of the New
World monkeys was a dispersal event directly from
Africa during the Oligocene. However, not all
accept the proposed relationship, so the question is
not closed, and an American or Asian origin for
Platyrrini remains a possibility (Dagosto 2002).