Snakes

Snakes differ from other living reptiles in terms of body shape and flexibility,and their evolutionary history has been a matter of ongoing debate.Whatever the precise relationships among snakes and other reptiles,it’s clear that snakes arose from lizard-like ancestors.The many divergent features of snakes,including their elongate (lengthened)and limbless body form and unusual lidless eyes,have resulted in two main competing explanations for snake origins.The dominant hypothesis is that snakes underwent a burrowing (underground digging)phase in their immediate ancestry or early history.The supporting evidence lies in the observation that several lineages of lizards (for example,slow worms and dibamids)have become elongate and have lost or substantially reduced their limbs,and all these burrow in sand or soil.Some of the peculiarities of snake eyes are the result of evolutionary loss of some ancestral features,and this is also known to have occurred to varying degrees in other burrowing vertebrates.

The main competing hypothesis is that snakes passed through a profoundly water-dwelling phase in their immediate ancestry. Although some scientists claim that snake eyes are similar in several features to those of other aquatic vertebrates,the main supporting evidence for the marine hypothesis comes from several snake fossils that retain hind limbs and that were fossilized in marine sediments deposited in the Cretaceous period (145 to 65 million years ago).Proponents of this marine hypothesis argue that elongation of the body and reduction of the limbs arose as an adaptation to swimming in water rather than burrowing on land. The marine hypothesis has been weakened by evidence that the Cretaceous limbed snakes are not close to the immediate ancestry of living snakes,but are instead probably embedded more deeply within the evolutionary tree of living snakes,so that their limbs might have partly re-evolved from small stubs such as are still found in,for example,python snakes.

Although the marine Cretaceous snakes are known from some spectacular and complete fossil specimens,the fossil record of snakes generally is irregular and largely fragmentary.Many snake fossils consist of no more than isolated vertebrae.Interpreting these fossils can be difficult because even vertebral variation within and among individuals and species of living snakes is not well- known in detail.It is particularly tricky to identify the earliest member of any major group of organisms based on fragmentary fossils,and this is true also for snakes.Geologically the oldest snake reported thus far is from Cretaceous rocks in North America dating to about 98.5 million years ago.An older fossil find from Spain (between 132 and 124 million years old)was claimed previously to be a snake,but its vertebral anatomy is not distinct enough from that of lizards to be sure.

Over the course of evolution,elongation of the snake’s body has necessitated the modification and rearrangement of its internal organs.The lungs in particular have undergone considerable modification.Some snakes have two lungs,but in these cases,the left is always smaller than the right.In some pythons,the left lung can be up to 85 percent of the length of the right lung,but in the majority of snakes the left lung has been either lost completely or become greatly reduced,as little as one percent of the right in some cases.In some aquatic snakes,such as the Asian/Australasian file snakes,the right lung is particularly large, extending backwards for nearly the entire length of the body.A possible additional respiratory (breathing)area in many snakes is provided by a special modification of the windpipe,known as the tracheal lung.

In most vertebrates,paired organs such as the kidneys and reproductive organs usually lie in the same position on either side of the body,but these are irregularly arranged in the elongate body of snakes.Among other organs affected by the radical change in body shape is the stomach,which has become greatly enlarged and in some snakes accounts for more than one-third of the total length of the body.There is also no urinary bladder;nitrogenous waste is eliminated not in a solution of liquid urea,as in humans and almost all other mammals,but in a semi-solid state as uric acid,as in lizards and birds.

 

 

1

Snakes differ from other living reptiles in terms of body shape and flexibility,and their evolutionary history has been a matter of ongoing debate.Whatever the precise relationships among snakes and other reptiles,it’s clear that snakes arose from lizard-like ancestors.The many divergent features of snakes,including their elongate (lengthened)and limbless body form and unusual lidless eyes,have resulted in two main competing explanations for snake origins.The dominant hypothesis is that snakes underwent a burrowing (underground digging)phase in their immediate ancestry or early history.The supporting evidence lies in the observation that several lineages of lizards (for example,slow worms and dibamids)have become elongate and have lost or substantially reduced their limbs,and all these burrow in sand or soil.Some of the peculiarities of snake eyes are the result of evolutionary loss of some ancestral features,and this is also known to have occurred to varying degrees in other burrowing vertebrates.

The word “divergent” in the passage is closest in meaning to

Aremarkable

Boriginal

Ccharacteristic

Dvarying

 

2

Snakes differ from other living reptiles in terms of body shape and flexibility,and their evolutionary history has been a matter of ongoing debate.Whatever the precise relationships among snakes and other reptiles,it’s clear that snakes arose from lizard-like ancestors.The many divergent features of snakes,including their elongate (lengthened)and limbless body form and unusual lidless eyes,have resulted in two main competing explanations for snake origins.The dominant hypothesis is that snakes underwent a burrowing (underground digging)phase in their immediate ancestry or early history.The supporting evidence lies in the observation that several lineages of lizards (for example,slow worms and dibamids)have become elongate and have lost or substantially reduced their limbs,and all these burrow in sand or soil.Some of the peculiarities of snake eyes are the result of evolutionary loss of some ancestral features,and this is also known to have occurred to varying degrees in other burrowing vertebrates.

According to paragraph 1,many scientists believe that snakes evolved from ancestors that burrowed underground because

Athe shape of the reduced limbs found in some snakes today suggests that they once were capable of digging in sand or soil

Bsome present-day lizards that burrow have also experienced the same loss of early features that has occurred in snakes

Csnakes have nearly the same degree of flexibility as some burrowing animals today

Dsnakes have thick eyelids that would protect their eyes from sand and soil when burrowing in the ground

 

3

The main competing hypothesis is that snakes passed through a profoundly water-dwelling phase in their immediate ancestry. Although some scientists claim that snake eyes are similar in several features to those of other aquatic vertebrates,the main supporting evidence for the marine hypothesis comes from several snake fossils that retain hind limbs and that were fossilized in marine sediments deposited in the Cretaceous period (145 to 65 million years ago).Proponents of this marine hypothesis argue that elongation of the body and reduction of the limbs arose as an adaptation to swimming in water rather than burrowing on land. The marine hypothesis has been weakened by evidence that the Cretaceous limbed snakes are not close to the immediate ancestry of living snakes,but are instead probably embedded more deeply within the evolutionary tree of living snakes,so that their limbs might have partly re-evolved from small stubs such as are still found in,for example,python snakes.

The word “retain”in the passage is closest in meaning to

Asuggest

Bkeep

Cwere created by

Dallow for

 

4

The main competing hypothesis is that snakes passed through a profoundly water-dwelling phase in their immediate ancestry. Although some scientists claim that snake eyes are similar in several features to those of other aquatic vertebrates,the main supporting evidence for the marine hypothesis comes from several snake fossils that retain hind limbs and that were fossilized in marine sediments deposited in the Cretaceous period (145 to 65 million years ago).Proponents of this marine hypothesis argue that elongation of the body and reduction of the limbs arose as an adaptation to swimming in water rather than burrowing on land. The marine hypothesis has been weakened by evidence that the Cretaceous limbed snakes are not close to the immediate ancestry of living snakes,but are instead probably embedded more deeply within the evolutionary tree of living snakes,so that their limbs might have partly re-evolved from small stubs such as are still found in,for example,python snakes.

In paragraph 2,the author refers to pythons to illustrate which of the following points about the origins of snakes?

AThe fossilized snakes of the Cretaceous period probably were close ancestors of today’s snakes.

BThe eyes of the ancestors of today’s snakes were similar to those found in other marine animals of the period.

CPresent-day snakes may not have evolved from ancestors that lived in watery environments.

DSome species of living snakes originated as marine animals and then evolved to become animals that burrowed on land.

 

5

Although the marine Cretaceous snakes are known from some spectacular and complete fossil specimens,the fossil record of snakes generally is irregular and largely fragmentary.Many snake fossils consist of no more than isolated vertebrae.Interpreting these fossils can be difficult because even vertebral variation within and among individuals and species of living snakes is not well- known in detail.It is particularly tricky to identify the earliest member of any major group of organisms based on fragmentary fossils,and this is true also for snakes.Geologically the oldest snake reported thus far is from Cretaceous rocks in North America dating to about 98.5 million years ago.An older fossil find from Spain (between 132 and 124 million years old)was claimed previously to be a snake,but its vertebral anatomy is not distinct enough from that of lizards to be sure.

The passage suggests that scientists have decided which of the following about an“older fossil find from Spain”?

AIt might be a lizard rather than a snake.

BIt might not be as old as originally thought.

CIt is the best-known example of the vertebral anatomy of early snakes.

DIt has too many vertebrae to be a lizard.

 

6

Over the course of evolution,elongation of the snake’s body has necessitated the modification and rearrangement of its internal organs.The lungs in particular have undergone considerable modification.Some snakes have two lungs,but in these cases,the left is always smaller than the right.In some pythons,the left lung can be up to 85 percent of the length of the right lung,but in the majority of snakes the left lung has been either lost completely or become greatly reduced,as little as one percent of the right in some cases.In some aquatic snakes,such as the Asian/Australasian file snakes,the right lung is particularly large, extending backwards for nearly the entire length of the body.A possible additional respiratory (breathing)area in many snakes is provided by a special modification of the windpipe,known as the tracheal lung.

According to paragraph 4,which TWO of the following modifications occur in the lungs of some snake species as a result of their elongate bodies?To receive credit,you must select TWO answer choices.

AThe left lung is either nonexistent or smaller than the right lung.

BSome python species have lost 85 percent of their right lung.

CSome aquatic snakes have a lung that is almost as long as their body.

DSome snakes no longer need a tracheal lung as an aid to respiration.

 

7

In most vertebrates,paired organs such as the kidneys and reproductive organs usually lie in the same position on either side of the body,but these are irregularly arranged in the elongate body of snakes.Among other organs affected by the radical change in body shape is the stomach,which has become greatly enlarged and in some snakes accounts for more than one-third of the total length of the body.There is also no urinary bladder;nitrogenous waste is eliminated not in a solution of liquid urea,as in humans and almost all other mammals,but in a semi-solid state as uric acid,as in lizards and birds.

According to paragraph 5,which of the following features of internal organs has NOT been affected by the elongate body of snakes?

AThe way nitrogenous waste is eliminated from the body

BThe position of the paired organs

CThe size of the stomach

DThe size of the kidneys

 

8

In most vertebrates,paired organs such as the kidneys and reproductive organs usually lie in the same position on either side of the body,but these are irregularly arranged in the elongate body of snakes.Among other organs affected by the radical change in body shape is the stomach,which has become greatly enlarged and in some snakes accounts for more than one-third of the total length of the body.There is also no urinary bladder;nitrogenous waste is eliminated not in a solution of liquid urea,as in humans and almost all other mammals,but in a semi-solid state as uric acid,as in lizards and birds.

What is the purpose of paragraph 5 in the passage?

ATo summarize the main idea of the passage

BTo give additional examples of the evolutionary effects introduced in paragraph 4

CTo emphasize that most vertebrates have not experienced the same evolutionary path as the one described in paragraph 4

DTo suggest reasons for the evolutionary changes discussed in paragraph 4

 

9

Look at the four squaresthat indicate where the following sentence could be added to the passage

The difference in lung sizes is not the only modification.

Where would the sentence best fit？Click on a square  sentence to the passage.

10

Snakes are reptiles that evolved from lizard-like ancestors to have a number of distinguishing features.

ATheir elongate bodies, lack of limbs, and lack of eyelids likely evolved from ancestors that burrowed in the ground, but some scientists claim that these features came from marine ancestors.

BThe discovery of snake fossils with reduced limbs in Cretaceous period marine sediments is evidence that the earliest snakes likely both burrowed and swam.

CAs it evolved over time, the snake’s body became elongated, which in turn caused changes in the structure and location of its internal organs.

DThe oldest known snakes lived about 98.5 million years ago, but since the fossil record of snakes is incomplete, the evolutionary tree of living snakes is difficult to piece together.

EDiscoveries of fossil specimens in Cretaceous rocks in North America and Spain have helped scientists understand the vertebral variation within and among species of living snakes.

FThough the placement of their paired organs differs, snakes and lizards both eliminate nitrogenous waste in the same manner, which is evidence of their close evolutionary relationship to each other.

 

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