The Origin and Evolution of Mammals - Moodle

ecognised four subgroups, based mainly on differences
in jaw structure, although there are very few
characters defining these taxa so that they are not at
all well supported. A more recent and more detailed
cladistic analysis is that of Angielczyk (2001), which
includes 20 dicynodont taxa and 40 characters. It
disagrees with many of King’s (1988) relationships,
but again most of the clades are very weakly supported.
Many of the nodes of his cladogram collapse
with only a single additional step, and with as little
as three additional steps virtually all the internal
structure of the cladogram becomes unresolved.
Thus convergent evolution and character reversal
was extensive and it should be stressed again that the
40–50 genera accepted nowadays, are all relatively
conservative. Were they a modern herbivorous
mammal group, dicynodonts might well have been
incorporated into a single family with a disparity
perhaps equivalent to that of the Bovidae. Given this
lack of taxonomic resolution of Dicynodontia, the
scheme of King (1988) based on evident differences in
the feeding structures is followed, although more
from convenience than conviction.
Endothiodontoidea. Endothiodon has a highly atypical
dentition. It is a large form found in the
Tapinocephalus Assemblage Zone of South Africa
and also in beds of equivalent age in Zambia, South
America, and India (Ray 2000). The skull reaches
over 50 cm in length with a distinctively high, very
narrow sagittal crest. The lower jaw is distinguished
by the very deep anterior part attached to the relatively
slender postdentary part. The upturned front
tip of the fused dentaries worked against the broad,
deeply concave plate formed from the fused premaxillae.
Canine tusks are absent, but the postcanine
dentition is remarkably and presumably secondarily
well developed. There are about 10 long, slender
upper teeth, fairly widely spaced and forming
a single row that does not oppose the lower teeth,
but must have bitten against a presumed horny pad
lying in a longitudinal groove lateral to the lower
teeth. The lower dentition consists of smaller, tightly
packed teeth forming two or three irregular rows.
Only the more lateral ones were functional, biting
against a horny pad presumably present in life on
the heavily rugose palatine bone, internal to the
upper tooth row. The more medial of the lower teeth
EVOLUTION OF MAMMAL-LIKE REPTILES 47
are young teeth, just erupting and preparing to
move laterally to replace existing functional teeth as
they are discarded. An area of vascularised bone
lying lateral to the upper tooth row has been interpreted
by Cluver (1975) as the line of attachment of
a muscular, food-retaining cheek. Several authors
have speculated on the diet of Endothiodon. King
(1990) suggested that the teeth had a slicing action,
perhaps by direct unilateral occlusion of uppers and
lowers. Latimer et al. (1995) proposed that they
browsed on riverine vegetation using cheek pouches
to collect the food. Cox (1998) rather ingeniously
speculated that conifer cones were stripped by the
action of the tip of the mandibles acting against the
premaxillae, followed by maceration of the seeds
using the postcanine dentition.
Kingorioidea. Kingoria (Cox 1959) lacks all signs of
postcanine teeth which, if its cladistic relationships
are correctly inferred, were lost independently of
the loss in other lineages. Some specimens of
Kingoria retain but others have lost the pair of tusks.
The horny, biting surfaces inferred from the structure
of the bones of the upper and lower jaws differed
considerably from those of other dicynodonts.
The secondary palate formed from the premaxillae
is smooth and flat, and the horn appears to have
been restricted to the lateral margins of the adjacent
maxillae. Instead of being sharp, these lateral edges
are rounded, as are the corresponding anterior parts
of the dentaries, implying a crushing rather than a
shearing action. However, there are some sharp
margins, a short one immediately in front of the
caniniform process of the upper jaw, and another at
the upturned anterior tip of the joined dentaries.
Hotton (1986) attempted to explain this curious
combination of features with the suggestion that
Kingoria fed on a food source that was of small
size, soft, and abundant. King (1988) thought that
perhaps its food consisted of invertebrates, grubbed
up from mud and swallowed more or less without
any chewing.
Kingoria also has some distinct specialisations in
its postcranial skeleton, reminiscent of the incipiently
mammalian condition seen in cynodonts
(Cox 1959; King 1985). The shoulder girdle has
developed a particularly strongly everted spine
extending from the acromion process up the front