Lab 8 Vertebrate homologies:Layout 1.qxd

LABORATORY
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Back in the 1890’s, Darwin and others proposed that natural selection allowed for the survival of
some individuals over that of others. Variations in anatomical and physiological structures may have
lead to this differential survival of organisms. Since these beneficial adaptations were inherited we
can use the observed variations in the skeletons of vertebrates as evidence for evolution. We see
important similarities as though they were variations on a basic theme. This kind of similarity due
to common ancestry are called homology. In some vertebrate structures we do not observe this
kind of similarity but rather commonality of function of structures which is called analogy.
Skeletons of vertebrates have changed relatively slowly over time. This slow change helps reveal
relationships among species since skeletons, including teeth, are often the only parts of the animal
that are preserved through fossilization.
In this lab we will:
• Learn how to recognize different tooth types in animals and understand dental
formulas.
• Learn how to extract information about the diet of animals based on the tooth
type and structure of the teeth
• Learn the major parts of the mammalian skeleton
• Use your knowledge of evolutionary relationship to find similarities and
differences among the structures of present day animals
ADAPTATION OF TEETH
Vertebrate Homologies: Evidence
for evolution?
Begin by examining the human skull and comparing its teeth with that diagram shown.
Teeth are adapted for particular eating habits. Notice the four type of teeth in the diagram. Most
mammals are heterodontic, having teeth of different shapes and sizes while other animals are
homodontic, having teeth that are the same shape and size (example: dolphins).
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Incisors: Well developed chisel-shaped teeth with a
cutting edge. These teeth are primarily used for obtaining
food.
Canines: Conical shaped teeth that are adapted for
stabbing, holding, or tearing.
Premolars: Teeth that contains several points (cusps)
for tearing and slicing
Molars: Largest of the teeth that may contain points in
carnivores. In herbivores they are usually flat and
adapted for grinding cellulose in plant cell walls. Both
the premolars and molars are used primarily for chewing.
The dental formula is a shorhand notation expressing
the number of each kind of tooth in the jaws. Teeth in
the upper jaw are noted above the line; those in the lower
jaw are noted below the line. Using the human as an
example, the dental formula would be written as such:
incisors(4)-canine(2)-premolars(4)-molars(6) in upper jaw = 16
incisors(4)-canine(2)-premolars(4)-molars(6) in lower jaw = 16
Total number of teeth = 32
The basic design of teeth and jaws in mammals is similar. However, there are adaptations based
on the primary food source of the mammals.
Omnivores, such as pigs and bears and humans, may eat anything, including vegetation and meat.
The molars and premolars are adapted for both grinding and crushing.
Carnivores, such as cats, dogs, and wolves are meat eaters. They have teeth that are welldifferentiated
and adapted for securing and eating flesh. The canines are long and sharply-pointed
for seizing and tearing prey. The premolars and molars are flattened laterally and have sharp edges
for cutting. The skull is also adapted for the attachment of strong muscles required for meat eating.
Insectivores, such as moles, some species of bats, shrews, and hedgehogs do not have well
differentiated teeth. All of the teeth are short and have sharp points (cusps).
Herbivores, such as cows and deer are plant eaters. The incisors are flat and sharp for cutting off
vegetation or, in the case of browsers, the upper incisors may be absent. The canine teeth may also
be absent in herbivores, leaving a space between the incisors and the cheek teeth (premolars and
molars). The cheek teeth are broad and flat to provide a large surface area for grinding and mashing
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plants. There are many varieties of herbivores. They include grazers such as sheep, cows, and
horses that feed on grasses, gnawers such as rats, beavers, and rabbits that feed on hard plant
materials like seeds and bark, and browsers such as deer, elephants, and giraffes, which eat low
shrubs and trees.
COMPARISON OF ANIMAL SKULLS
Examine the skulls of the animals available in the laboratory. Choose four different skulls, recognize
the adaptations of each skull, infer the diet, and determine the dental formula. Record your findings
in the following table. For each of the animal skulls your instructor may direct you to use your
textbooks or additional resources in the laboratory to find the scientific and common name of
each animal you choose.
****Circle the animal you investigate
Table 9-1: Dental formula data table
Number of teeth type in the upper &
lower jaw
Species**** Incisors Canines Premolars
(bicuspids)
Homo sapiens
(humans)
Rabbit
Beaver
Cow
Horse
Deer
Dog
Cat
Raccoon
Mole
Small Bat
Shrew
4
4
2
2
4
4
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Molars
Diet Dental Formula
(tricuspids)
6
6
Ominvore
4-2-4-6 = 16
4-2-4-6 = 16

ADAPTATION FOR LOCOMOTION
One of the distinquishing characteristics of animals is their ability to move independently within
their environment. Locomotion, movement from one place to another, is an essential function
in animal life. Vertebrates have adapted to the various barriers to locomotion in their
environment so that they can perform their daily activities. There are three basic types of
locomotion that are observed in animals.
In appendicular movement, legged locomotion, the body of the animal achieves motion by
the movement of its appendicular skeleton (see diagram of the human skeleton). The animal
uses muscles to move the appendages forward or backward in order to achieve locomotion.
Important aspects of legged locomotion are stance (the way the body is supported by the legs),
the number of legs, and the shape of the foot.
There are also many forms of gaits which are ways of moving the legs in order to move such as
walking, running, or hopping. Digitigrades are animals that stand and walk on their digits
(toes/fingers). These animals include cats, dogs, and wolves. Because of the small footprint of
the limbs they are for the most part faster and quieter than other types of animals.
Unguligrades are animals that walk on the “tips” of their toes. These tips correspond to the
nails of the animal. Animals in with this type of motion include horses, cows, goats, and other
hooved animals. The arrangment of the bones of the limbs of unguligrades enables them to
extend their stride length and so move faster. Plantigrades walk with their metatarsals and
metacarpals flat on the ground. Humans, rats, bears, and raccons are examples of plantigrades.
This type of locomotion is less specialized than the previous two types of locomotion.
However, some plantigrades are capable of bipedal locomotion, walking on two limbs (usually
the legs) in an upright position.
In axial movement such as slithering the animal uses its axial skeletal to move. Snakes, fish,
and several species of limbless lizards are examples of slithering animals.
In rolling movement some animals are also capable of locomotion by rolling their entire body.
Species of caterpillar, salamanders, and mamamals use rolling as a defence mechanism to quickly
move away from being attacked.
HOMOLOGOUS VS. ANALOGOUS STRUCTURES BETWEEN
SPECIES
In this experiment we will examine the various skeletons in the classroom and compare the size,
type, and arrangment of the bones. Using these comparisons choose one of the full skeletons in
the classroom that you feel is the most different from our human model. Keep in mind that
humans are considered general creatures since we have not evolved to fulfill a specific nich.
Your analysis should focus on looking at the number of differences compared with the number
of similarities between the type of bones, arrangement of bones, and relative size of bones
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etween the human and the animals you choose. A diagram of the human skeleton is
provided at the end of the laboratory exercise for identification of the various types of
bones.
After you have analyzed your first skeleton repeat the same analysis with a different skeleton that
you feel is most similar to your human model. Record your analysis on the data sheet on the
following page.
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**Choose one animal from each category**
Grazer Insectivore Carinivore Omnivore
Atrributes Human ________ _________ _________ __________
Type of
Locomotion
Plantigrade
Number of Digits 5
Fused Radius and
Ulna
Scapula (flat or
elongated)
Fused Tibia and
Fibula
Sternum
Clavicles
Patella present
Pelvic Girdle
Fused vertebrae
Fused Carpals and
Metatarsals
TABLE 9.2 SKELETON COMPARATIVE DATA TABLE
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