The Origin and Evolution of Mammals - Moodle

pubis is excluded from the acetabulum. Krebs
(1991) believed that Henkelotherium was adapted
for an arboreal existence, as indicated by the long,
flexible tail, the relative length of the penultimate
phalanges, and the recurved claws.
The dryolestidans were abundant and diverse
during the Upper Jurassic and Early Cretaceous of
Europe and North America. More unexpectedly,
they underwent a considerable radiation into several
indigenous families during the South American
Late Cretaceous (Fig. 5.15(d)), where they form
the dominant part of the mammalian fauna of the
Campanian-aged Los Alamitos Formation of
Patagonia (Bonaparte 1990, 1994).
One of the earliest ‘eupantotheres’, and also
the first to be discovered is Amphitherium, from the
Middle Jurassic Stonesfield slate of Oxfordshire
(Fig. 5.16(d)). As yet it is only known from lower
jaws and teeth. What the lower molar teeth show is
a relatively large protoconid and smaller metaconid
and paraconid cusps, forming a roughly equilateral
triangle. The talonid is more developed than in
dryolestidans, to the extent of bearing a distinct
cusp and extending backwards to overlap the next
molar behind.
Vincelestes (Fig. 5.16(a)) is known from several
skulls and postcranial remains from the Early
Cretaceous of Argentina (Hopson and Rougier 1993).
It was a relatively large form, with a short, robustly
built skull about 7 cm long. The side wall of the braincase
is composed of more or less equal sized anterior
lamina of the petrosal and alisphenoid, which more
closely resembles the condition in the basal mammal
Morganucodon than the tribosphenidans. No
description of the postcranial skeleton has yet been
published (Rougier 1993). The dentition is unusual,
with reduction of the dental formula to I4/1: C1/1:
PM2/2: M3/3. The upper canine is huge, the first and
last postcanines small. The lower molars have low
cusps and a small talonid with only a single cusp. The
upper molars are a curious shape, being very wide
laterally, and narrow medially. The phylogenetic position
of Vincelestes is not certain because of the specialisation
of its dentition, and the lack of cranial material
in most other ‘eupantotheres’ for comparison.
The dentition of the Upper Jurassic Peramus
(Fig. 5.16(b) and (d)) is very well known, and is
important as the ‘eupantothere’ stage whose teeth
are most similar to those of the Tribosphenida
(Clemens and Mills 1971). There are eight postcanine
teeth, but whether four or five of these
should be regarded as premolars is debated, with
a growing consensus that the primitive number
of premolars is actually five (Novacek 1986b;
Martin 2002). The lower molars have a talonid that
bears two cusps, the hypoconid and hypoconulid,
connected by an oblique crest. In view of developments
to come, the talonid is described as incipiently
basined. The upper molars are triangular and
although no protocone has evolved, there is a narrow
cingulum on the lingual side of the crown that
is the incipient homologue of the protocone. The
functional effect of these changes was to enhance
the shearing action between the hinder part of the
upper tooth and the talonid of the lower tooth, as
analysed by Crompton (1971).
Tribosphenida
THE MESOZOIC MAMMALS 167
Aegialodontidae
If the discovery of such an oddity as Shuotherium
illustrates the unexpectedness of Mesozoic mammal
research, then the discovery of a single, worn,
and incomplete molar tooth referred to as Aegialodon
illustrates the rewards of patience (Kermack et al.
1965). It emerged from a very large volume of the
Early Cretaceous Wealden deposits of Cliff End in
Sussex, and its importance lies in the small, but distinctly
basined talonid on the back of the tooth. All
the advanced holotherian mammals, including the
modern marsupials and placentals as well as several
extinct Mesozoic taxa possess a new large cusp
on the upper molar teeth called the protocone, which
occluded with an enlarged talonid on the corresponding
lower molar. The talonid is described as
basined because crests on all sides enclose it. This
new arrangement adds a crushing action to the
largely shearing action of more primitive therian
teeth such as those of Amphitherium and Peramus, in
which the smaller talonid is still open on the lingual
side. Although no upper molar of Aegialodon is yet
known, a protocone must have been present, for in
no other way could the pattern of wear facets on the
talonid be explained (Crompton 1971). A couple of
decades later, Dashzeveg and Kielan-Jaworowska
(1984) described a very similar form from the Early