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Ch. 32 - Deuterostome Animals
Freeman - Biological Science 8th Edition
Freeman8th EditionBiological ScienceISBN: 9780138276263Not the one you use?Change textbook
All textbooksFreeman 8th EditionCh. 32 - Deuterostome AnimalsProblem 11a
Chapter 32, Problem 11a

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.)

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1
Begin by understanding the evolutionary context: Mammals evolved from synapsid ancestors, which include the extinct group known as cynodonts. These ancestors provide clues about the transition from reptilian jaw structures to mammalian ear structures.
Examine the morphological data: Look at the jaw and ear structures of cynodonts and compare them to both modern reptiles and mammals. Note any similarities and differences, particularly in the bones near the jaw hinge.
Identify key bones involved: In reptiles, the jaw is composed of several bones, including the articular and quadrate bones. In mammals, these bones have evolved into the malleus and incus of the middle ear.
Formulate a hypothesis: Based on the morphological data, hypothesize that the mammalian ear bones (malleus and incus) originated from the jaw bones (articular and quadrate) of synapsid ancestors. This transition likely provided an evolutionary advantage in hearing sensitivity.
Consider the role of natural selection: Propose that natural selection favored individuals with more efficient hearing, leading to the gradual modification of jaw bones into ear bones over time, enhancing auditory capabilities in early mammals.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Evolutionary Adaptation

Evolutionary adaptation refers to the process by which organisms change over time to better suit their environment. In the context of mammalian ear bones, this concept involves understanding how certain structures in ancestral species, like the jawbones in reptiles, evolved into the specialized ear bones in mammals, enhancing their hearing capabilities.
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Morphological Homology

Morphological homology is the study of similarities in the structure of different organisms due to shared ancestry. The presence of similar bone structures in the jaws of reptiles and the ear of mammals suggests a common evolutionary origin, indicating that these bones have been repurposed over time to serve different functions in different species.
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Synapsid Evolution

Synapsid evolution traces the lineage of amniotes that eventually led to mammals. Synapsids, like the cynodonts mentioned, exhibit transitional features that provide insight into the evolutionary changes leading to modern mammals. Understanding this lineage helps explain how certain jawbones in early synapsids evolved into the ear bones found in mammals today.
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Related Practice
Textbook Question

Consider the evolution of protostomes and deuterostomes (this chapter):

Compare adaptations in protostomes and deuterostomes for living on land.

Recall that changes in the expression patterns of tool-kit genes in arthropods such as insects enabled a dramatic diversification of their segmented appendages and bodies.

Design an experiment to test whether this concept applies to vertebrates.

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Textbook Question

Genetic diversity in living human populations is highest in Africa and decreases as a function of distance traveled by the human migration that left Africa many millennia ago. Draw a graph to show this trend. (Hint: What is the independent variable? What is the dependent variable? What kind of graph is appropriate for this kind of data? Add a label where you would expect to find a data point for the Yanomamö tribe of the Amazon rain forest in South America. Why is it important to use indigenous people for this study?

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Textbook Question

Humans possess which of the following traits? Select True or False for each trait. T/F triploblasty T/F parthenogenesis T/F viviparity T/F metamorphosis

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Textbook Question

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.

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Textbook Question

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.

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Textbook Question

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?

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