The size and shape of the vertebrate skull can reveal a great deal about an animal's lifestyle and evolutionary relationships. Consider your own skull. If you put your finger in your ear and move your jaw up and down, you can feel the space near the hinge of your jaw. Nestled in this space are the tiny bones that make your hearing possible: the malleus, incus, and stapes. All mammals have these three ear bones, but reptiles such as this T. rex don't.
Where did ear bones come from?
Analyze the morphological data shown here and write a hypothesis to explain the origin of mammalian ear bones. (The cynodont shown is one of many extinct synapsid amniotes that lived early in the lineage that gave rise to mammals.)

As a genetic counselor, you advise couples about the possibility of genetic disease in their offspring. Today you met with an engaged couple, both of whom are phenotypically normal. The man, however, has a brother who died of Duchenne-type muscular dystrophy, an X-linked recessive condition. His fiancée, whose family has no history of the disease, is worried that the couple's sons or daughters might be afflicted. The sister of this man is planning to marry his fiancée's brother. How would you advise this second couple?

As a genetic counselor, you advise couples about the possibility of genetic disease in their offspring. Today you met with an engaged couple, both of whom are phenotypically normal. The man, however, has a brother who died of Duchenne-type muscular dystrophy, an X-linked recessive condition. His fiancée, whose family has no history of the disease, is worried that the couple's sons or daughters might be afflicted. How would you advise this couple?

The size and shape of the vertebrate skull can reveal a great deal about an animal's lifestyle and evolutionary relationships. Consider your own skull. If you put your finger in your ear and move your jaw up and down, you can feel the space near the hinge of your jaw. Nestled in this space are the tiny bones that make your hearing possible: the malleus, incus, and stapes. All mammals have these three ear bones, but reptiles such as this T. rex don't. Where did ear bones come from? How is the opossum related to you?

Select True or False for each statement.

T/F An opossum is an animal, but I am a human.

T/F An opossum is a mammal, but I am a human.

T/F An opossum is a marsupial, but I am a placental mammal.

T/F The opossum and I are both tetrapods.

The size and shape of the vertebrate skull can reveal a great deal about an animal's lifestyle and evolutionary relationships. Consider your own skull. If you put your finger in your ear and move your jaw up and down, you can feel the space near the hinge of your jaw. Nestled in this space are the tiny bones that make your hearing possible: the malleus, incus, and stapes. All mammals have these three ear bones, but reptiles such as this T. rex don't. Where did ear bones come from? The illustration of the opossum skull shows that the ear bones are completely separated from the jawbone (as they are in all mammals).

Pose a hypothesis to explain why this separation could be an adaptation that contributed to the radiation of mammals into diverse niches, including a nocturnal lifestyle.

Match each description on the left with the corresponding term on the right Closest relatives of chordates

a. annelids

b. nematodes

c. sponges

d. arthropods

e. flatworms

f. cnidarians

g. molluscs

h. echinoderms

i. chordates

The size and shape of the vertebrate skull can reveal a great deal about an animal's lifestyle and evolutionary relationships. Consider your own skull. If you put your finger in your ear and move your jaw up and down, you can feel the space near the hinge of your jaw. Nestled in this space are the tiny bones that make your hearing possible: the malleus, incus, and stapes. All mammals have these three ear bones, but reptiles such as this T. rex don't. Where did ear bones come from? Gene expression patterns can be used to test hypotheses based on morphology. For example, the regulatory gene Bapx1 is expressed in the hinge of the developing lower jaw in fishes and reptiles.

Where would you predict Bapx1 expression to occur in mammals?