Dinosaurs and Parental Care

From fossil evidence alone the question of whether or not dinosaurs cared for their young is very difficult to answer. Because?behaviors are not preserved in the fossil record, we can only make inferences from indirect evidence. Parental care can be?divided into two types of behavior: prehatching (building nests and incubating eggs—for example, sitting on top of them so?as to warm the eggs and encourage hatching) and post hatching (feeding the young and guarding the nests). Most of our?evidence comes from alleged dinosaur rookeries (places where nests are built). Several have been excavated in eastern?Montana, where a large concentration of dinosaur nests was found at a place now called Egg Mountain. Most of these probably?belonged to the hadrosaur Maiasaura. Preserved in these nests are the bones of baby dinosaurs. The finds at Egg Mountain?

and other sites around the world document that dinosaurs laid their eggs in nests.

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The nests at Egg Mountain are reported to be equally spaced, separated by a space corresponding to the length of an adult?Maiasaura. From this arrangement scientists have inferred that the nests were separated in this way to allow incubation in a?tightly packed nesting colony. Although this interpretation is open to challenge, the discovery of Oviraptor adults on top of?Oviraptor egg clutches (as determined by embryos in some eggs), is relatively powerful evidence that at least these dinosaurs incubated their nests.

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Evidence for parental care following hatching is much more controversial. Behavioral speculation based on indirect fossil?evidence is dangerous because the data is not always as unambiguous as might appear. At Egg Mountain, many nests contain?baby dinosaur bones. Not all the dinosaurs in the nest are the same size. Many of the small bones found in the nests are?associated with jaws and teeth, teeth that show signs of wear. It seems reasonable to assume that the wear was caused by the?chewing of the coarse plants that were the hatchlings’ diet. Because the young were still in the nest, this food may have been?brought to the rookery by foraging adults. This line of reasoning suggests that these animals had an advanced system of?parental care. A closer look at the evidence clouds this interpretation. Analysis of dinosaur embryos indicates that worn?surfaces are present on the teeth of juveniles even before hatching. Just as a human baby moves inside the mother before birth,?modern-day archosaurs also grind their teeth before birth, wearing the surface in some spots. Thus, the fossil evidence for an?advanced parental care system in extinct dinosaurs is suggestive but inconclusive, and it is hard even to imagine the sort of paleontologic discovery that could settle this debate for good.

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The strongest evidence that extinct dinosaurs had some form of advanced parental care system is based on an understanding?of the phylogenetic relationships among dinosaurs and their closest living relatives. Living dinosaurs (birds), even primitive?ones such as ostriches and kiwis, exhibit parental care, so some form of parental care can be inferred to have existed in the?last common ancestor of all birds. Although unappreciated, crocodiles are reptiles that are also caring parents. They build?nests, guard the nests, and in some cases dig their young out of the nest when they hear the chirping young ones hatching.?The young even communicate with each other while still in the egg by high-frequency squeaks (as birds do). Some evidence?suggests that this squeaking is a cue for the synchronization of the hatching. Since birds and crocodiles share a common? ancestor, the simplest explanation for the characteristics they share (such as nest building and some form of parental care) is?that they evolved only once—that these attributes were present in their common ancestor and passed on to its descendants.?

Because extinct dinosaurs also descended from that ancestor, the simplest and most general theory is that extinct dinosaurs?also shared these characteristics, even though they cannot be directly observed, and we cannot be sure how elaborate their?

parental care was.

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【Paragraph 1】

From fossil evidence alone the question of whether or not dinosaurs cared for their young is very difficult to answer. Because behaviors are not preserved in the fossil record, we can only make inferences from indirect evidence. Parental care can be divided into two types of behavior: prehatching (building nests and incubating eggs—for example, sitting on top of them so as to warm the eggs and encourage hatching) and post hatching (feeding the young and guarding the nests). Most of our evidence comes from?alleged?dinosaur rookeries (places where nests are built). Several have been excavated in

eastern Montana, where a large concentration of dinosaur nests was found at a place now called Egg Mountain. Most of these probably belonged to the hadrosaur Maiasaura. Preserved in these nests are the bones of baby dinosaurs. The finds at Egg Mountain and other sites around the world document that dinosaurs laid their eggs in nests.

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1. The word “alleged” in the passage is closet in meaning to

A. scattered

B. supposed

C. isolated

D. exposed

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2. Paragraph 1 answers which of the following questions about parental care in dinosaurs?

A. Which type of parental care was more important for the survival of dinosaur young, prehatching care or post hatching care?

B. Why were dinosaur remains in eastern Montana preserved rather than destroyed over time?

C. Did Maiasaura hadrosaurs provide types of parental care not provided by other dinosaurs?

D. What evidence supports the view that Maiasaura females laid their eggs in nests?

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【Paragraph 2】

The nests at Egg Mountain are reported to be equally spaced, separated by a space corresponding to the length of an adult Maiasaura. From this arrangement scientists have inferred that the nests were separated in this way to allow incubation in a tightly packed nesting colony. Although this interpretation is open to challenge, the discovery of Oviraptor adults on top of Oviraptor egg clutches (as determined by embryos in some eggs), is relatively powerful evidence that at least these dinosaurs incubated their nests.

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3. According to paragraphs 1 and 2, the fossil record most clearly shows that dinosaurs engaged in which of the following behaviors?

A. Laying eggs in nests

B. Hiding eggs

C. Feeding young

D. Storing food

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【Paragraph 3】

Evidence for parental care following hatching is much more controversial. Behavioral speculation based on indirect fossil evidence is dangerous because the data is not always as unambiguous as might appear. At Egg Mountain, many nests contain baby dinosaur bones. Not all the dinosaurs in the nest are the same size. Many of the small bones found in the nests are associated with jaws and teeth, teeth that show signs of wear. It seems reasonable to assume that the wear was caused by the chewing of the coarse plants that were the hatchlings’ diet. Because the young were still in the nest, this food may have

been brought to the rookery by foraging adults. This line of reasoning suggests that these animals had an advanced system of parental care. A closer look at the evidence clouds this interpretation. Analysis of dinosaur embryos indicates that worn surfaces are present on the teeth of juveniles even before hatching. Just as a human baby moves inside the mother before birth, modern-day archosaurs also grind their teeth before birth, wearing the surface in some spots. Thus, the fossil evidence for an advanced parental care system in extinct dinosaurs is suggestive but inconclusive, and it is hard even to imagine the sort of paleontologic discovery that could settle this debate for good.

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4. According to paragraph 3, the patterns of wear found on the teeth of young dinosaurs may indicate which of the following?

A. Baby dinosaurs were eating food brought to them by their parents.

B. Early development of jaw and teeth varied according to a dinosaur’s size.

C. Dinosaurs went foraging for food at an early age.

D. Baby dinosaurs did not begin to eat solid food until after they left the nest.

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5. In paragraph 3, why does the author mention that baby archosaurs ground their teeth inside the egg?

A. To support the claim that baby dinosaurs in the egg shared certain behaviors with human babies before birth

B. To contrast the behavior of baby archosaurs with that of other types of dinosaurs

C. To cast doubt on the claim that adult dinosaurs fed their hatchlings in the nest

D. To explain why the teeth of baby archosaurs were more worn than those of other juveniles

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【Paragraph 4】

The strongest evidence that extinct dinosaurs had some form of advanced parental care system is based on an understanding of the phylogenetic relationships among dinosaurs and their closest living relatives. Living dinosaurs (birds), even primitive ones such as ostriches and kiwis, exhibit parental care, so some form of parental care can be inferred to have existed in the last common ancestor of all birds. Although unappreciated, crocodiles are reptiles that are also caring parents. They build nests, guard the nests, and in some cases dig their young out of the nest when they hear the chirping young ones

hatching. The young even communicate with each other while still in the egg by high-frequency squeaks (as birds do). Some evidence suggests that this squeaking is a cue for the synchronization of the hatching.?Since birds and crocodiles share a common ancestor, the simplest explanation for the characteristics they share (such as nest building and some form of parental care) is that they evolved only once—that these attributes were present in their common ancestor and passed on to its descendants.?Because extinct dinosaurs also descended from that ancestor, the simplest and most general theory is that extinct dinosaurs also shared these characteristics, even though they cannot be directly observed, and we cannot be sure how elaborate their parental care was.

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6. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage?

Incorrect choices change the meaning in important ways or leave out essential information.

A. The simplest explanation for the similarities between birds and crocodiles is that they evolved at the same time.

B. A common ancestor is probably the source of the shared traits of crocodiles and birds.

C. The originally similar traits of birds and crocodiles increased after evolving through a shared ancestor.

D. Only one shared pattern of behavior—that of nest building—was present in the common ancestor of birds and crocodiles.

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7. Paragraph 4 answers all of the following questions about crocodiles EXCEPT

A. What is the evidence that crocodiles are caring parents?

B. Why do crocodile parents communicate with the young inside their eggs?

C. What is a possible reason for the high-frequency sounds that crocodiles make inside their eggs?

D. How do crocodiles participate in the hatching process of their young?

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8. In paragraph 4, the author discusses birds and crocodiles in order to

A. contrast patterns of parenting behavior in both living and extinct animals

B. provide evidence that sophisticated parental care behaviors evolved only relatively recently

C. demonstrate that parental care behaviors have continued to evolve since the time of the dinosaurs

D. support the theory that extinct dinosaurs probably inherited some kind of parental care system

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【Paragraph 1】

From fossil evidence alone the question of whether or not dinosaurs cared for their young is very difficult to answer. ■Because behaviors are not preserved in the fossil record, we can only make inferences from indirect evidence. ■Parental care can be divided into two types of behavior: prehatching (building nests and incubating eggs—for example,

sitting on top of them so as to warm the eggs and encourage hatching) and post hatching (feeding the young and guarding the nests). ■Most of our evidence comes from alleged dinosaur rookeries (places where nests are built). ■Several have been excavated in eastern Montana, where a large concentration of dinosaur nests was found at a place now called Egg Mountain. Most of these probably belonged to the hadrosaur Maiasaura. Preserved in these nests are the bones of baby dinosaurs. The finds at Egg Mountain and other sites around the world document that dinosaurs laid their eggs in nests.

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9. Look at the four squares【■】that indicate where the following sentence can be added to the passage.

Evidence of the former is easier to find than that of the latter.

Where would the sentence best fit?

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10.【Directions】An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some answer choices do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage.

This question is worth 2 points.

Scientists must use indirect evidence to determine whether extinct dinosaurs cared for their young.

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Answer Choices

A. Because baby dinosaur bones and eggs were very delicate, there are relatively few preserved as fossils, so little is known about dinosaur young.

B. Fossils from sites like Egg Mountain indicate that dinosaurs built nests, and perhaps that they incubated their eggs and fed their hatchlings.

C. Fossil evidence such as the spacing of nests may indicate advanced parental care but can have different interpretations.

D. Tightly packed Oviraplor rookeries indicate that dinosaurs may have tended to nest in large colonies in order to better protect both eggs and hatchlings.

E. Discovery of hadrosaur bones of different sizes in the same nest may indicate that, in some species, older siblings took care of younger ones.

F. The strongest evidence comes from extinct dinosaurs’ nearest living relatives, birds and crocodiles, who do engage in many forms of parental care.

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