The Origin and Evolution of Mammals - Moodle

182 THE ORIGIN AND EVOLUTION OF MAMMALS
immediately manifest. Within a brief window of
time, around the end of the Triassic, at least five
very distinct groups of mammals occur in the fossil
record, namely Sinoconodon, morganucodontans,
Kuehneotherium, haramiyidans, and the possible
symmetrodontan Woutersia. Furthermore, the phylogenetic
position of the docodontans, basal to
Kuehneotherium, indicates that a sixth distinct form
may have been present, even though no certain representatives
of it have been found prior to the
Middle Jurassic. The age of these early mammals
ranges through the Norian and into the Rhaetian, a
time period spanning approximately 220–205 Ma.
Whether this initial fossil diversity was an artefact
due to a significant period during which no fossils
of the early stages are preserved, or whether there
really was a rapid radiation at the time is unknown.
However, the range of small, ‘near-mammalian’
tritheledontans that occur in the Upper Santa Maria
Formation of Rio Grande do Sol, southern Brazil,
suggest a possible explanation. They are believed to
be Carnian in age, and so slightly older than the mammals.
A tritheledontan-grade mammalian ancestor
may therefore have dispersed from Gondwana into
some part of Laurasia, in which new geographical
area it was free to rapidly radiate and fill a set of
newly available niches.
The next major event in mammalian history was
the Jurassic radiation into a bewildering variety of
forms, all small but differing in tooth structure to an
extent that indicates a wide range of adaptations to
different food resources suitable for small-bodied,
endothermic animals. The primitive form of the
molar tooth is a single row of three main cusps
arranged linearly from front to back of the tooth,
surrounded by a cingulum of cuspules at the base of
the crown. They were designed for a simple shearing
action between crests and presumably functioned
mainly in dealing with insect cuticle. From this basic
form, several different modes of tooth action
evolved, often convergently in different groups.
Triangulation of the main cusps developed on a
number of occasions. Kuehneotherium was the earliest,
but the condition also occurred at least to some
extent in the morganucodontan Megazostrodon,
the eutriconodontan Gobiconodon, and the symmetrodontans.
The greater interlocking between the
crests in triangulated teeth may perhaps have given
them a better facility for dealing with relatively
larger insect prey that required more extensive mastication
before swallowing.
More radical modification of the primitive molar
tooth form involved the evolution of a crushing
action in addition to the primitive shearing action,
was achieved by the development of a lingual
extension of the upper tooth, coupled with a labial
extension of the lower. One short-lived version is
found in the docodontans, which possessed molar
teeth virtually as complex in design as the tribosphenic
tooth. Presumably it was an adaptation
for an omnivorous diet, able to deal with highenergy
vegetable matter such as seeds, nuts, tubers
and fruits, as well as invertebrates. A much more
radical design of crushing teeth was manifested
during the Jurassic by the appearance of the multituberculates,
with their highly specialised, rodentlike
dentition, surely capable of dealing adequately
with these plant products. The very high diversity
of the group indicates that there was specialisation
into many specific food and habitat niches within
the group, and several modes of locomotion have
also been documented in different forms, including
parasagittal, arboreal, ricochetal, and burrowing.
The ecological comparison of the group with modern-day
rodents remains as compelling as ever.
A third category of dietary specialisation occurred
in several of the eutriconodontans, which evolved
what was for Mesozoic mammals the relatively
large body size of a cat, strongly shearing teeth, and
therefore must have been small carnivores.
Initially the Cretaceous Period saw a continuation
of the diversity of the non-tribosphenidan groups of
mammals, with the multituberculates increasingly
dominant. However, the most important evolutionary
innovation, towards the end of the Early
Cretaceous, was the tribosphenic molar tooth. These
are designed to provide two phases of mastication,
a generalised puncture crushing when the food is
reduced to a pulpy mass, and a precise shearing,
in which resilient parts are finely shredded. The
origin of the tribosphenic tooth coincided with the
beginning of the spectacular Late Cretaceous explosive
radiation of flowering plants, and the associated
diversification of the insects feeding on them.
The several lineages of tribosphenidans, with their
many generic and specific variations in the exact