Answer Key to the Short Answer Section
Exam 1
Spring 1998

1. Marsupials are extremely undeveloped at birth:

The forelimbs, however, are well developed for grasping. This aids the neonate in moving from the birth canal to the pouch or nipple.

One hypothesis for why marsupial young are expelled from the uterus in such an undeveloped state is immunological incompatibility between the fetus and the mother. The immune system is designed to distinguish self from non-self. Since half of the fetus's genome is derived from the father, the fetus's genetic make-up is different from the mother's and the mother's immune system will classify the fetus as non-self. The shell membrane protects the fetus from the mother's immune system for the first 2/3 of pregnancy, but after it is shed parturition soon follows.

2. You could have drawn a diagram for this or summarized it verbally.
Similarities: 2 ovaries, 2 fallopian tubes, double (biphid) uterus.
Differences: Monotremes have a cloaca (convergence of the urinary, reproductive and digestive tract into one common opening and lack a vagina. Marsupials have two (lateral) vaginas and form a medial vagina just prior to giving birth. The infants are born through the medial vagina.

3. Heterodont dentition is the condition of having teeth of different shapes for different functions. Mammals have four basic types:

Incisors Squarish, sharp Nipping
Canines Elongated, connical, pointy Stabbing and grasping prey
Premolars Shape varies from sharp (for shearing flesh) to broad, cusped surface (for smashing and grinding)
Molars Similar to premolars in shape tearing, smashing grinding

4. The family Peramelidae (bandicoots) has a chorioallantoic placenta. The rest of the marsupials have a choriovitelline placenta. The choriovitelline placenta is less vascularized than the chorioallanoic placenta. The portion in contact with the uterine lining is convoluted (to increase the surface area in contact with the uterus) but it is not deeply imbedded in the uterine lining. A small amount of material (uterine milk) diffuses from the uterine tissue to the fetus, but most of the nourishment for the fetus comes from the yolk sac. The chorioallantoic placenta is more highly vascularized. The fetal and maternal tissues are more highly interconnected due to the many finger-like projections (villi) that grow into the uterine lining. Gases and nutrients diffuse from the maternal capillary beds, across the placental membranes and into the fetal capillaries, and vice versa.

5. This answer could also have been diagrammed (in part). One type of mammary gland consists of glandular tissue (that produces milk), milk ducts (in which the milk collects) and a nipple (an external protrusion that the infant sucks on to obtain milk). The other major type consists of glandular tissue, a cistern (large compartment for collecting and storing milk) and a teat (the external projection that is suckled by the infant). Monotremes have no nipples or teats, but instead secrete milk on to the fur or skin, where it is lapped up by the infants.


heterodont dentition This allows increased efficiency of food capture and processing, thereby increasing the total amount consumed and the efficiency of nutrient extraction from that food. this is necessary to support the high metabolic rate necessary to sustain homeothermy.
muscular diagphram The contraction of this muscle causes air to rush into the lungs. This increases the efficiency of gas exchange in the lungs.
sweat glands secrete a watery substance (sweat) onto the surface of the skin. The evaporation of sweat helps to cool the body when environmental temperatures are high.
enucleate red blood cells The loss of the nucleus leaves more room for hemoglobin molecules, thus increasing the capacity of red blood cells to transport oxygen to the body tissues.
sebaceous glands Secrete an oily substance that coats and lubricates hairs. This keeps the hairs from becoming brittle and increases their ability to insulate the body against heat loss. In semi-aquatic animals, sebaceous gland secretions also waterproof the hair, which keeps water from contacting the skin. This would be a major loss of heat.


  1. The opposable, clawless hallux and long digits enable the didelphid marsupial to grasp branches. It is arboreal.
  2. The macropod food shows syndactyly (loss and fusion of digits), with the 4th digit serving as the main base of support. This allows a strong pulpulsive force for saltatory locomotion. The reduced 3rd and 5th toes may serve to stabilize the foot.
  3. The dasyurid shown is terrestrial and shows an elongated foot with fairly short digits. There is no evidence of syndactyly. This is an unspecialized foot for generalized terrestrial locomotion (this particular example is a hopping animal, but the foot shape is also typical of plantigrade terrestrial locomotion). FYI, I gave credit to everyone who made a reasonable attempt to explain the significance of foot shape in the dasyurid.

8. Characteristics of therapsids that we think of as typically mammalian could include:

Characteristics that distinguished early mammals from therapsids could include:

The development of homeothermy (with fur for insulation) may have allowed early mammals to forage at night and escape predation pressure from the dinosaurs. Their relatively small body size probably increased their ability to hide effectively as well.

9. Note: This question is asking about how to differentiate groups of mammals from each other, NOT how to differentiate mammals from other types of organisms.

Locomotor adaptations are strongly affected by ecology. Animals that have similar mode of locomotion may show highly similar morphologies due to convergence (ie, when two species carry seemingly identical traits because they have experienced similar evolutionary pressures --sharing the same lifestyle, etc.-- rather than by inheritance from a common ancestor). Thus a system that differentiates groups of mammals based on mode of locomotion is not likely to show phylogenetic relationships accurately because shared characteristics are likely to be convergent rather than homologous (inherited from a common ancestor). Homologous traits are useful for the construction of phylogenetic trees, because they point to common ancestry. Use of convergent traits would lead to an innacurate rendering of evolutionary relationships.

Some types of characters that may be more useful: genetics, physiology, morphology, paleontology, geography, etc. The problem of convergence still exists, but with careful selection of the traits used we hope to minimize it.