Shells, Wings and Tails: What Refuge Embryos Reveal

Compare ribs, fingers and tails in five embryos from refuge animals.

Part 1: Two shells and a wing

Read the refuge notes and the embryo table. Then answer questions 1 to 5.

Survey Season at Cypress Bend

1Cypress Bend Wildlife Refuge protects marshes and woods along the Gulf of Mexico coast. Every spring and summer, refuge staff run animal surveys. They count alligator nests along the canals, mark river cooter turtles that crawl up the banks to lay eggs, count great egrets in their nesting colony, and record green anole lizards and nine-banded armadillos along the trails. This year, eighth grader Cyrus Adeyemi joined the surveys as a summer volunteer with the refuge biologist, Dr. Wynona Bellamy.

2On rainy days, Dr. Bellamy showed Cyrus photos and models of embryos of the same five kinds of animals from a university collection. No refuge nests were opened for the study. For the first part of development, the five embryos look much alike: each has a curled body, four small limb buds and a tail. Differences show up as the bones begin to form.

3The turtle surprised Cyrus most. An adult turtle's upper shell looks like nothing on any other reptile. In the embryo, though, the shell grows from parts that every reptile has. The turtle's ribs do not curve down around the body as they do in the alligator and the lizard. Instead, they grow straight out sideways under the skin of the back, then widen and join with the backbone to form the shell. The armadillo also has a hard shell as an adult, but its embryo's ribs curve down like a lizard's. Its shell forms later, from small bony plates that grow in the skin.

4The egret embryo has its own surprises. An adult egret's wing ends in a few small finger bones that are partly joined into one stiff piece, and its short tail ends in a knob of joined bones that holds the tail feathers. In the embryo, the wing starts with three separate fingers, and the tail is long, with many separate bones. As the embryo grows, the finger bones partly join and the tail shortens, and its last bones join together.

5Dr. Bellamy reminded Cyrus that biologists compare early stages because animals that share a pattern of development often share a common ancestor, even when the adults look very different.

Cyrus's Embryo Table
Embryo	Ribs	Front-limb fingers	Tail	Shell forms from
American alligator	Curve down around the body	Separate, stay separate	Long, many separate bones	No shell
Green anole	Curve down around the body	Separate, stay separate	Long, many separate bones	No shell
River cooter	Grow straight out under the back	Separate, stay separate	Short, separate bones	Ribs and backbone
Great egret	Curve down around the body	3 separate, later partly joined	Long, then short with end bones joined	No shell
Nine-banded armadillo	Curve down around the body	Separate, stay separate	Long, many separate bones	Bony plates in the skin

Use the table.

The ribs curve down around the body in of the five embryos. A shell forms in of them, but from parts of the body (write same or different).

Use paragraphs 2 and 3. What does the turtle embryo suggest about how turtles are related to other reptiles?

A. Each turtle grows its shell because it needs protection after it hatches from the egg.
B. Turtles share ancestors with other reptiles, and their shell is built from reshaped ribs.
C. Turtles are closest to armadillos, since the adults of both animals have a hard shell.
D. Turtles are not related to other reptiles, since no other reptile has a shell like theirs.

Adult turtles and adult armadillos both have hard shells. Which conclusion does the embryo evidence support best?

A. The armadillo shell came from turtles, since turtles appear in the table before armadillos.
B. The armadillo shell is reshaped ribs too, since its embryo's ribs curve down like a lizard's.
C. The shells likely evolved separately, since one forms from ribs and one from skin plates.
D. The two shells are the same structure, since both protect the animal's back and sides.

Use paragraphs 2 and 4 and the table. Sort each egret feature by where it can be seen.

Three separate fingers in the wingA knob of joined bones at the end of the tailA long tail of many separate bonesSmall finger bones partly joined into one stiff pieceFour small limb buds on a curled body

In the embryo, not the adult	Still seen in the adult

Use paragraph 4 and the table. What do the egret embryo's fingers and tail suggest?

A. Each egret loses its fingers because it uses its wings for flying instead of grabbing.
B. Birds likely came from ancestors with separate fingers and long tails of separate bones.
C. Egrets grow long tails before hatching because they need them to balance inside the egg.
D. Egrets are closest to armadillos, since the embryos of both have tails of separate bones.

Part 2: Predictions and claims

Use the whole passage and the table to answer questions 6 to 10.

Circle the two observations that are evidence that turtles are related to other reptiles even though adult turtles look very different.

Early turtle embryos have a curled body, four limb buds and a tail, like alligator and lizard embryos.Adult river cooters crawl up the canal banks to lay eggs.The turtle's shell is built from ribs, a set of bones the alligator and the lizard also form.The adult turtle's upper shell looks like nothing on any other reptile.The armadillo's shell forms from bony plates in its skin.

A softshell turtle has a flat, leathery shell. Based on the pattern in the table, what would you predict about its embryo?

A. It forms its shell from plates in the skin, since its shell is soft and leathery instead of hard.
B. Its ribs curve down around the body, and its shell forms from plates in the skin, like the armadillo's.
C. Its ribs curve down at first and then turn into a shell after the turtle hatches.
D. Its ribs grow straight out under the skin of the back, as the river cooter's ribs do.

Wood ducks also nest at the refuge. What would you most expect to find in a wood duck embryo?

A. Three separate wing fingers that later partly join, and a tail that shortens
B. Bony plates growing in the skin that later join into bands over its back
C. Ribs that grow out under the skin of the back to form a shell over the body
D. A short tail of joined bones from the start, since adult ducks have short tails

Cyrus writes, "For a while, an egret embryo is a lizard." Which revision is the most careful?

A. "Egret and lizard embryos share early features, a sign of shared ancestry."
B. "An egret embryo looks like an adult egret, since it already has feathers and a beak."
C. "An egret embryo turns into a lizard for a few days so that it can grow its long tail."
D. "An egret embryo is a lizard until it hatches, and then it changes into a bird."

10.

A refuge visitor says, "Turtles and armadillos must be close relatives, because they are the only animals here with hard shells." Write a claim that answers the visitor. Give at least two pieces of evidence from the table or passage, and explain why embryos can show relationships that adults hide.