PRIMITIVE MAMMALS IN AN EGG OR A POUCH? (2)

The topic of my presentation is primitive mammals. According to Solomon,
Villee and Davis (1985) mammals are thought to have evolved from a group
of reptiles called therapsids. These were dog-like carnivores with legs
adapted for running and differentiated teeth (considered a mammalian
trait). 'Non-reptilian' characteristics of the members included in this
group were pelvic and thoracic skeletal feutures which permitted the
legs to be positioned directly underneath the body
(http//tidepool.st.usm.edu/Crswr/therapsid.html). This development set
the stage for the evolution of the primitive mammals. In determining the
lower branches of the evolutionary tree of mammals, the following
features are studied:
1) The synapsid opening in the skull (a large hole behind the eye
socket for muscles that extend to the jaw, also found in early
relatives of mammals)
2) the three middle ear bones (used to classify all mammals)
3) the placenta
These traits correspond to eating, hearing, and reproduction functions,
and each represents an evolutionary branch.
(www.amnh.org/exhibitions/permanent/fossils/primitive.html)

Solomon, Villee and Davis (1985) states that by the end of the
Cretaceous period there were three main groups of mammals. These are
currently divided into three subclasses:
Prototheria. egg-laying mammals, also called monotremes. These are
represented by the extant duck-billed platypus
Metatheria.pouched mammals, also known as marsupials. These are
represented by opossums, kangaroos etc.
Eutheria. placental mammals.
The fossil record describes the Cretaceous (146-65.5 MYA) as a period
when the first fossils of many insect groups, modern mammal and bird
groups, and the first flowering plants were found
(www.ucmp.berkeley.edu/mesozoic/cretaceous/cretaceous.html). This
presentation will deal with origins and evolutionary developments of
primitive mammals. It will also explore the conditions/developments that
enabled the survival of the Prototheria and Metatheria (primitive
mammals).

Dane McDonald
Department of Biodiversity and Conservation Biology
University of the Western Cape
Private Bag X17
Bellville 7535

Dane McDonald
Department of Biodiversity and Conservation Biology
University of the Western Cape
Private Bag X17
Bellville 7535