Worksheet

From Jaw to Ear: What a Clinic Visit Revealed About Embryos

Grade 8 practice comparing jaw and ear bone development in seven vertebrate embryos and nostril position in dolphin and pig fetuses to infer relationships.

Grade 8ScienceInteractive · Printable

Aligned toMS-LS4-3

Play online Play as a game

PrintLetter-size worksheet Answer keyPrintable answers

Assign to classAssign to your classes, or get a join code

What’s included

Single worksheet
Auto-graded online
1 NGSS standard
Printable answer key
11 questions
Classroom-ready

Details

Format: Interactive · Printable
Grade: Grade 8
Subject: Science
Question types: Fill in the blank, Multiple choice, Circle the answer, Sorting, Short answer

About this worksheet

What students practice

Students compare what the first-arch jaw rods become in fish, reptile, bird and mammal embryos.
Students read fetal measurements to describe how a dolphin's nostrils move as it develops.
Students explain how embryos show relationships that adult anatomy hides.

How to use

Look at the ear bones on a diagram or model first, then read the passage together and let students work through both tables on their own.

Curriculum

NGSS standard covered

MS-LS4-3

Compare patterns in embryo development across species to find relationships not seen in adult anatomy.

Preview

OwlSheets.com

Name Date

From Jaw to Ear: What a Clinic Visit Revealed About Embryos

Compare how jaws, ear bones and nostrils develop to find hidden relationships.

Part 1: From jaw rods to ear bones

Read the passage and Table 1, then answer questions 1 to 5.

Three Tiny Bones

1Eighth grader Ikaika Brennan had an earache that would not go away, so his doctor sent him to an ear, nose and throat clinic. While she checked his ear, Dr. Ottoline Mwangi-Ross showed him a model of the middle ear. Sound makes the eardrum vibrate, and three tiny bones called the hammer, the anvil and the stirrup pass the vibrations inward. Then she surprised him: "Two of those bones started out as part of your jaw." She lent Ikaika a set of notes from the medical school where she teaches, so he could see what she meant.

2The notes first compared adult animals. In an adult mammal, each half of the lower jaw is a single bone, and the jaw hinges directly against the skull. Adult reptiles and birds have several bones in each half of the lower jaw. Their jaw hinges between two small bones at the back of the jaw, one in the lower jaw and one in the skull, and their middle ear has only one bone, shaped like a rod.

3Next the notes described embryos. Early embryos of fish, reptiles, birds and mammals all grow ridges along the sides of the neck called pharyngeal arches. In the first arch, rods of cartilage form to support the growing jaws: one for the upper jaw and a long one for the lower jaw. In fish, reptile and bird embryos, bones form at the back ends of these rods, where they meet, and become the jaw hinge. In mammal embryos, the back ends of the same rods form the hammer and the anvil. For a while they sit at the back of the jaw, and then they separate from it and move into the middle ear. Later, most of the rest of the lower-jaw rod disappears.

4Opossums show one step of this in a newborn. A Virginia opossum is born after only about 13 days of development and keeps growing in its mother's pouch. At birth, its mammal jaw hinge has not formed yet, so the baby opens and closes its mouth at a hinge between the bones that will become its hammer and anvil. A few weeks later the new hinge forms, and those two bones move into the ear.

5The notes ended with dolphins. Adult dolphins breathe through a single blowhole on top of the head, and their skin is smooth and hairless. In a dolphin fetus, two nostrils first form near the tip of the snout, as in most mammals, and a few whisker hairs grow on the snout; the whiskers usually fall out soon after birth. A museum at the medical school keeps preserved dolphin and pig fetuses of different sizes. Students there measured how far back each nostril sat, as a percent of the head's length from the snout tip. Their results are in Table 2.

Table 1: The First-Arch Jaw Rods in Seven Embryos
Embryo	First-arch jaw rods	Back ends of rods become	Adult middle-ear bones
Zebrafish	Forms	Jaw hinge bones	0 (no middle ear)
Alligator	Forms	Jaw hinge bones	1
Chicken	Forms	Jaw hinge bones	1
Opossum	Forms	Hammer and anvil	3
Mouse	Forms	Hammer and anvil	3
Human	Forms	Hammer and anvil	3
Dolphin	Forms	Hammer and anvil	3

Use Table 1. Count the embryos in each group.

In of the seven embryos, the back ends of the jaw rods become jaw hinge bones. In of them, they become ear bones.

In mammal embryos, the hammer and anvil form from the same parts of the jaw rods that become jaw hinge bones in alligator and chicken embryos. Which inference does this support best?

A. Mammal and reptile embryos are the same until birth, and their bones change after it.
B. Mammals share ancestors with reptiles, and their ear bones began as jaw bones.
C. Mammals hear better than reptiles, so each mammal grows extra bones in its ear.
D. Mammal ear bones form first in the ear and later move into the jaw.

Use paragraphs 2 and 3. Why do embryos show this relationship more clearly than adult skulls do?

A. Embryos of different animals are the same species until birth.
B. Adult reptiles have the same three ear bones as adult mammals.
C. In adults, the matching bones have different places and jobs.
D. Adult mammals lose their ear bones, so embryos are easier to study.

Use paragraph 4. A newborn opossum opens and closes its mouth at a hinge between the bones that will become its hammer and anvil. What does this most likely show?

A. The young opossum uses the bones as a jaw hinge before they become ear bones.
B. The newborn keeps this hinge for life instead of forming a new one.
C. The opossum's ear bones form first in the ear and later move into the jaw.
D. The baby opossum hears through its mouth until the ear bones grow in.

Use paragraphs 1 to 3. Sort each feature by when it can be seen in a human.

Three small bones in the middle earA hammer and anvil at the back of the jaw rodsA lower jaw half made of a single boneA long cartilage rod supporting the lower jaw

Seen in an adult human	Seen only in a human embryo

Part 2: Where a dolphin's nostrils start

Use Table 2 and paragraph 5 to answer questions 6 to 11.

Table 2: How Far Back the Nostrils Sit (% of head length from the snout tip)
Fetus length	Dolphin nostrils (%)	Pig nostrils (%)
2 cm	4	4
5 cm	15	3
10 cm	35	2
20 cm	55	2
Newborn	70	2

Use Table 2. Compare the dolphin's nostrils at 2 cm with a newborn dolphin's.

From the 2 cm fetus to the newborn, the dolphin's nostrils moved back by percentage points of the head's length.

Which statement best describes the pattern in Table 2?

A. Both started near the snout tip, then the dolphin's nostrils moved back.
B. The pig's nostrils moved back as fast as the dolphin's as both fetuses grew.
C. The dolphin's nostrils started far back and moved toward the snout tip.
D. The dolphin's nostrils moved back the most between 2 cm and 5 cm.

An adult dolphin's blowhole looks nothing like a pig's snout. What does the 2 cm row of Table 2 suggest?

A. Pigs evolved from dolphins that moved out of the sea and onto land.
B. Each dolphin is a pig fetus until it grows to about 2 cm long.
C. Dolphin nostrils form on top of the head first, then slide forward to the snout.
D. Dolphins share ancestors with land mammals that had nostrils at the snout tip.

Circle the two observations that are evidence that dolphins are related to hairy land mammals, even though adult dolphins look very different.

A dolphin fetus grows a few whisker hairs on its snout.An adult dolphin breathes through one blowhole.A pig fetus's nostrils stay near its snout tip.A 2 cm dolphin fetus has its nostrils near the snout tip.An adult dolphin's skin is smooth and hairless.

10.

A beluga is a whale that also breathes through a blowhole on top of its head. Based on Table 2 and paragraph 5, what would you predict about a beluga fetus?

A. Its nostrils form on top of the head and stay there as it grows.
B. Its nostrils form near the snout tip, then move back as it grows.
C. Its nostrils form at the snout tip and stay there until birth.
D. It forms gills first, which later turn into a blowhole.

11.

Ikaika's friend says, "Dolphins have fins and smooth skin and live in the sea, so they must be more closely related to fish like the zebrafish than to land mammals like pigs and mice." Write a claim that answers the friend. Give at least two pieces of evidence from the embryo notes, Table 1 or Table 2, and explain why embryos can show relationships that adults hide.