Laboratory Notes for BIO 1003
© 30 August 1999, John H. Wahlert & Mary Jean Holland
INTRODUCTION TO EVOLUTION AND MORPHOLOGY:
Vertebrate arm bones are homologous
All arms have the same patternhumerus, radius and ulna, wrist, and handexcept for the fish at the lower right.
Two structures in different organisms are analogous if they serve the same function. They may also be homologous. Analogy, by itself, is not evidence of descent from a common ancestor.
Example: Birds, bats and bugs all fly. Their wing membranes are analogous and push against the air to give lift or to glide. But the structure of the wings is entirely different; they are not homologous. The wing membrane in birds is formed by feathers and supported by the arem. The analogous membrane in bats is skin that stretches between elongated fingers. Differences in basic structure indicate that flight was not inherited from a common ancestor.
The axial skeleton, the vertebral column, runs the length of the body from anterior to posterior. The pectoral and pelvic girdles are the parts of the skeleton to which the appendages (arms and legs in humans) are attached; together, the girdles and limbs are the appendicular skeleton. .
Arch of vertebral column - fighting gravity
The two sides of the arch lean toward the keystone in the middle. Its hard to squash an arch with downward pressure.
Land-living vertebrates with four legs (tetrapods) have an arched vertebral column—front and back lean together toward a middle vertebra. The arch resists the pull of gravity.
|An egg works two ways; the hen sits on the arch, but the hatching chick bursts up through it.|
Locomotion, moving from place to place, is important in all animals; can you discover from their skeletons how different animals move?
Connection of limbs to axial skeleton
The bones of the limbs are firmly but moveably attached to the pelvic and pectoral girdles by articulations, ligaments, and muscles.
|The pelvis is joined tightly to the vertebral column. Hind legs push the animal forward.|
|Forelimbs are not directly connected to the axial skeleton. The body is suspended in a sling of muscle between the dorsal edges of the scapulae (shoulder blades). The forelimbs keep the nose off the ground and keep pace with the speed of the hind limbs.
Many animals have freed up the forelimbs for manipulating the environment. Think of humans and kangaroos.
Compare several features, especially the position of the appendages in relation to the body. Is there a difference in support requirements for animals that live in water (aquatic) vs. animals that live on land (terrestrial)? Which appendages are attached to the axial skeleton in such a way to propel the axis of the body forward? What does a fish use for propulsion?
Observe that some terrestrial organisms have appendages positioned directly underneath the body (mammals). Others have the appendages positioned at the sides of the body (amphibians and many reptiles). Which one do you think is most efficient (would use the least muscle energy) in locomotion?
Compare the appendages of a fish, frog, alligator, mammals, and bird. Which limbs look most similar overall. What about the limbs are different? Do the limbs serve the same functions in all of these kinds of animals? Even though differences are obvious, are there any similarities? Compare two different terrestrial animals' hind limbs; can you match them bone for bone? Might the similarities support the hypothesis that the earliest vertebrates were fish and that terrestrial animals evolved from them?
Limbs and Life
The rat is a generalist and has equal lengths of upper and lower arm portions.
Fossorial (burrowing) life
The mole digs with its clawed, shovel-like forepaws. Arm bones have flaring processes for attachment of lots of muscle. Hind limbs are small and kick away the loosened soil.
Running on tiptoe
In a fastrunning animal, such as a deer or a gazelle, the legs are made longer by adding the hand and fingers and foot and toes to the length. Not only does a long limb sweep through a greater arc than a short limb, but each bone segment moves at the same time, so the arc is completed much more quickly than a single bone could achieve.
And then there are snakes
Snakes dont have legs; they undulate and push body segments against the ground. Did they ever have legs?
Boas and pythons have remnants of the pelvis and femurs. Early fossil snakes with short legs have been found.
But what about fish?
The limbs (fins) of fish are used for steering. The axial skeleton itself is the organ of propulsion. Fish dont need legs to hold them up against gravity. They are buoyant in water, which is much denser than air.
Look at the skulls. Group them by similarities, and note the differences. What criteria can you observe that suggest relationships?
Compare the teeth of various animals with the teeth in your mouth. Are the teeth of the alligator similar in size and shape throughout its dentition? Are they similar to your teeth? Your teeth are grouped as incisors in front, pointed canines, bicuspid premolars, and finally complex molars; can you make the same kind of groupings for the alligator's teeth? Do the upper and lower teeth in the alligator close together (occlude) in the same kind of relationship as your teeth? What is tooth replacement like in an alligator and in a mammal?
Look at any two mammal skulls available. Can the teeth of these animals be grouped into incisors, canines, premolars, and molars? Do the upper and lower occlude in a specific relationship as your teeth do, or are they like the alligator.
Review the information in your textbook about diets of animals: what is a carnivore, a herbivore, an omnivore? Which kind of diet to humans have? What are the teeth like in a carnivore, e.g., a cat, vs. a herbivore, e.g., a cow?
Teeth are clues to animal diets
When you compare the teeth of various animals in the lab, you see distinctly different designs.
Form and Function: Do you see evidence from tooth shape and size in mammals that the toughest jobs (those that require the greatest force) are done with the posterior teeth? Think about a nutcracker and the crushing force generated near the hinge. Do you use your own teeth the same way no matter what you are eating? Is there a difference in the way you bit into a peach and the way you bit into a whole carrot?
As in a nutcracker, the highest force is close to the pivot, the jaw joint. When you look at dentitions, including your own, the teeth that exert the most force are at the back of the jaw. In some mammals the incisors, farthest from the joint, are tiny and used mostly for grooming.
Carnivore vs. herbivore
Carnivore skulls are designed like scissors with the pivot in line with the teeth. Cutting edges are above and below the pivot.
Herbivore skulls are designed like these pliers with the pivot above the teeth. The teeth all meet at once and grind up plants.
Slicing teeth in the carnivore
Cutting edges, which align above and below the pivot, slice meat.
Upper and lower teeth occlude precisely to cut. The jaw joint has little room for wiggle since a malocclusion could crack these teeth, which are crucial for survival.
Your teeth have no edges that can be damaged, and you can slide your jaw from side to side.
Carnivores have a huge gape for biting prey. Can you open your mouth this far? Does a cow open its mouth so wide?
[This is my cat Tiger.]
Herbivores: Horse and cow
|Horses have upper nipping incisors. Eyes face obliquely to the sides. Teeth are tall and occlude for grinding.||Cows lack upper nipping incisors; lower incisors bite against a tough upper pad. Eyes face obliquely to the sides. Teeth are tall and occlude for grinding.|
Horse and cow teeth point to separate ancestry
|Horse premolars and molars have complex folds of enamel, the hardest tissue in the body, resist wear.|
|Cow premolars and molars have crescent shaped ridges of upper and lower teeth that shear across each other.|
Tooth size and occlusion
Cow as example: Upper teeth are stationary. Muscles raise and move the jaw thus bringing teeth into occlusion.
Crests of the lower teeth sweep through valleys of uppers, and crests of upper teeth press into valleys of the lower dentition. Red lines indicate direction.
|Hard enamel ridges break down the food. The moveable lower teeth are not as wide, cheek to tongue, as the uppers against which they chew.|
Primates – large brains, eyes forward
The jaw joint is high relative to the occlusal plane of the teeth. What does this suggest about the common ancestor of the primates?
Tooth occlusion: upper and lower teeth have cusps that fit into basins of opposing teeth. These have mortar and pestle crushing action and are similar to human teeth. The enamel is thicker and stronger than in human teeth. The jaw joint allows side to side movement.
|A ridge on the top of the skull and another at the back make a broad fossa or channel on each side. The ridges are surfaces for attachment of huge temporal muscles that will powerfully raise the jaw in chewing. Humans dont have any large ridges and the comparable temporal muscle is much smaller.|
Skulls of large herbivore and primate compared
Both are designed as herbivores; the cow has a long face to crop grass. Look at the eye sockets. Which mammals have the eye sockets facing forward? In most, the eyes face somewhat laterally. What happens to vision, when the fields of both eyes overlap? The cows eyes face obliquely to the side for nearly 360degree vision. The cow moves in a 2D world and needs to spot predators. The humans eyes face forward giving 3D vision. Human ancestors presumably lived in trees, a 3-D world. The cow has a brain that is smaller compared with head size.
Is it reasonable to think that when things look very similar they might be related? Under what circumstances might this not be the case?
If we look at a few individual specimens of a few kinds of vertebrates and then make general statements (inferences) about the groups from which they came (common ancestors), what errors may we be making? What is meant by a representative sample? How can one be obtained?
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