The Origin and Evolution of Mammals - Moodle

242 THE ORIGIN AND EVOLUTION OF MAMMALS
are molarised and the cheek teeth have a welldeveloped
and characteristic bilophodont form.
Until quite recently (Prothero et al. 1988),
dinoceratans were believed to be ungulates related
to the paenungulate group of elephants, hyraxes,
and sirenians, a view based on a number of
ambiguous postcranial characters. Lucas (1993) has,
however, argued the case for a sister-group relationship
with the Pyrotheria, which is an exclusively
South American, Eocene order. He notes a
number of detailed similarities in the molar teeth.
Dinoceratans, particularly the more advanced
uintatheres, must have been browsing forms with a
rhinoceros-like mode of life. The loss of the upper
incisors suggests that a mobile proboscis was present.
Why such apparently uniquely adapted forms
should have gone extinct as early as they did is mysterious;
they had no obvious direct competitors.
Arctostylopida
Arctostylopidans are small, rabbit-sized, and exclusively
Late Palaeocene mammals. All are Asian apart
from the single North American genus, Arctostylops
(Fig. 7.9(e)). Little more than dentitions are known
for the group, which are characterised by the even
size of all the teeth, including reduced canines and
molarised premolars. Lophs are developed on the
molars, which develop as longitudinal shearing
edges. They were, perhaps, comparable in feeding
habits to the modern day hyraxes. Several authors
had placed arctostylopidans as a basal group of the
South American ungulate order Notoungulata, but
Cifelli et al. (1989; Cifelli and Schaff 1998) have
argued that the dental similarities are superficial
and probably convergent. Thus their affinities are at
present unclear.
Meridiungulata: South American Ungulates
One of the more remarkable features of the whole
placental fossil record is the revelation of a great radiation
of advanced ungulates in South America, from
the Early Palaeocene right through until the Plio-
Pleistocene, 60 million years later. Five orders are
recognised, mostly restricted to that continent
although their presence in the Eocene of Antarctica
(Reguero et al. 2002), and the dispersal of a few into
North America towards the end of their existence are
recorded. All have the fundamental ungulate
characteristics of large, grinding, lophodont premolar
and molar teeth, and fully hoofed feet.
McKenna (1975) formalised the commonly held view
that all five orders constitute a monophyletic group,
by creating a super-order Meridiungulata, and there
has been fairly general acceptance of the concept,
despite the lack of clearly defined characters supporting
it. However, the issue has become complicated
by Muizon and Cifelli’s (2000) cladistic analysis
of dental characters that supports a relationship
between the Early Palaeocene Tiupampan ‘condylarths’
such as Pucanodus, the North American
mioclaenid ‘condylarths’ (Fig.7.10(a)), the South
American didolodontids (Fig. 7.10(b)), which
are advanced ‘condylarths’, and the Meridiungulate
order Litopterna (Fig. 7.10(c) and (d)). The implication
to be drawn is that a mioclaenid entered South
America from the North around the start of the
Palaeocene, and radiated into the somewhat
more progressive didolodontids, and the fully ungulate
litopterns. None of the other four Meridiungulate
orders can be shown to belong to this same group,
although neither is there strong evidence that they do
not. They all have teeth too specialised to reveal their
relationships clearly, and therefore any of them could
have originated from a different ‘condylarth’ grade
ancestor. Indeed, on the basis of certain details of
tooth structure, Schoch and Lucas (1985; Lucas 1993)
proposed that the two of the Meridiungulate
orders, Pyrotheria (Fig. 7.10(g)), and Xenungulata
(Fig. 7.10(h)), are related to the Dinocerata of the
northern continents (Fig. 7.9(d)), and referred to this
grouping as the Uintatheriomorpha. They then
claimed that these uintatheriomorphs are related to
the anagalidans (Fig. 7.4(a)), a group of small omnivores/herbivores
from the Asian Palaeocene. If true,
the relationship would imply that there was a
second, independent immigration into South
America from Laurasia of a progenitor of ungulate
taxa, this one not even a ‘condylarth’.
For the time being, the Meridiungulate hypothesis
that all five orders of South American ungulates
form a monophyletic group that could have been
derived from a single dispersal of a mioclaenid-like
ancestral ‘condylarth’ cannot be satisfactorily refuted
and therefore remains the simplest explanation for