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IELTS READING LEVEL 7.5 – Sea Monsters

Sea Monsters

by Stephen Fraser


A new wave of fossils reveals the oceans’ prehistoric giants.

Way back when Tyrannosaurus rex shook the ground, another giant reptile lurked in the prehistoric oceans. A 50-foot predator, Mosasaurus was a real sea monster.

Mosasaurus and T. rex never battled or even met. But the marine giant is now stealing some of the spotlight that T. rex and its fellow dinosaurs have enjoyed for so many years. A new wave of findings has drawn some amazing portraits of the aquatic denizens of the Age of Reptiles.

“Over the last 10 to 20 years, we have begun to look more closely at fossils found in marine sediments,” says Mike Everhart, a paleontologist at the Sternberg Museum of Natural History in Hays, Kan. “In doing so, we’ve discovered that some of these creatures were very large, very scary predators-something you wouldn’t want to share your ocean with!”


From Land To Sea

Only a few reptiles-turtles, sea snakes, and saltwater crocodiles-inhabit today’s oceans, which are dominated by mammals and fish. But the seas of the Mesozoic Era (251 million to 65 million years ago) swarmed with reptiles, some of them as big as whales. Marine reptiles were actually the first big prehistoric reptiles discovered by fossil hunters.

The earliest marine reptiles evolved from land reptiles roughly 240 million years ago (mya). Earth’s climate was getting warmer then, and so were the oceans, which favored the evolution and spread of the ectothermic (cold-blooded) reptiles.

Unlike most of today’s reptiles, the prehistoric marine reptiles wereviviparous-the females produced live offspring instead of eggs. “The reason is simple,” says Mike Caldwell, a paleontologist at the University of Alberta in Canada. “If you give live birth you can live anywhere in oceanic environments and are not bound to come ashore to lay eggs.” One fossil of a prehistoric marine reptile, now on view in a German museum, shows the animal giving birth.


Mosasaur. Simon Danaher

No longer tied to the land, the marine reptiles could fully adapt to living in the ocean and compete with sharks and other big fish. “The interesting fact is that just about every animal in the ocean is a predator-from the smallest minnow to the biggest mosasaur-while almost all land animals are herbivores [plant eaters],” says Everhart.

Three Groups

Paleontologists have sorted the prehistoric marine reptiles into three main groups


Ichthyosaurs. Thomas Miller


Ichthyosaurs. The first group was the ichthyosaurs. The earliest ones had long, supple bodies and probably rippled through the water like eels. Later ichthyosaurs evolved fins and tails and “looked like our present-day dolphins,” says Caldwell. Ichthyosaurs were built for speed.

The largest known marine reptile was a whalelike ichthyosaur, Shonisaurus. It was as long as two school buses.


Pliosaur. Chris Butler/Photo Researchers, Inc


Plesiosaurs. Next to evolve, about 200 mya, were the plesiosaurs. Plesiosaurs moved like turtles:

They flapped their paddle-like limbs to propel themselves through the water.

Plesiosaurs had small heads, broad bodies, and short tails. Over time, many of them evolved fantastically long necks. One of them, the 14-meter (46-foot) Elasmosaurus, had a neck that was half the length of its entire body and contained 72 vertebrae (bony segments). Today’s mammals-even giraffes-have just seven neck vertebrae.

The long-necked plesiosaurs were slow swimmers. They probably cruised just below the ocean surface, swinging their long necks to angle their heads beneath unsuspecting fish and snap them up.

Another group of plesiosaurs, the pliosaurs, evolved in a whole different direction. Their necks remained short, but their bodies grew bulkier with heads like those of crocodiles. “These guys were the big, hulking monsters of the group, with huge teeth and a bonecrushing bite,” says Everhart. They preyed on fish, ichthyosaurs, and other plesiosaurs.


Mosasaurs. The ichthyosaurs and pliosaurs disappeared about 90 mya. Replacing them at the top of the food chain were the mosasaurs, huge lizards related to today’s Komodo dragons. Mosasaurs had long heads, short necks, and long, sinuous tails, which they used to propel themselves like snakes.

“More than likely, mosasaurs were very aggressive animals, capable of pursuing and killing all kinds of prey,” says Everhart.

If mosasaurs were still alive, “ocean travel would be safe in larger vessels,” he adds. “But you wouldn’t want to go fishing, sailing, surfboarding, windsurfing, or just plain swimming anywhere mosasaurs lived.”


Endless Questions

Along with the dinosaurs, the giant marine reptiles became extinct 65 mya. But their fossilized remains are abundant around the world.

“Mosasaurs were first discovered in Europe, but the most and some of the best have been found here in Kansas, which used to lie under a prehistoric sea,” says Everhart. “The first major fossil I ever collected turned out to be a mosasaur that I named Tylosaurus kansasensis in 2005.”

What remains to be learned about the prehistoric ocean-goers? “Did they have a four-chambered heart like a crocodile or a three-chambered one like a lizard? Did they live together in family groups like whales or porpoises? Did they care for their young? How long did they live?” says Everhart.

“It is an endless list of biological questions,” adds Caldwell.


evolution       e ·vo ·lu ·tion

Advanced Definition


  1. the continuous modification and adaptation of organisms to their environments through selection, hybridization, and the like.
  2. the theory or study that describes this process as the cause of species’ existence and characteristics. (See Darwinian theory.)

the dispute over evolution vs. creation

  1. a gradual process of change and development that something goes through, usu. becoming more complex and sometimes better.

the evolution of electronic weaponry

These are some examples of how the word or forms of the word are used:

  1. Evolution refers to the process by which traits inherent to a species of animal change, and sometimes grow more specialized to the environment.
  1. The whale shark’s Latin name, Rhincodon typus, means “rasp tooth type.” Hueter believes the teeth are vestigial, an evolutionary leftover from the whale shark’s ancestor.
  2. Charles Darwin, the famous biologist who first proposed the scientific theory of evolution, described the coral reef as an oasis in the desert of the ocean.
  3. When the famous naturalist Charles Darwin, who helped develop the theory of evolution, visited the Galapagos Island in the 1830s, he made an interesting discovery about native birds.
  4. “[Evolution is] not a linear process,” says Tattersall, explaining that one species doesn’t necessarily give way to another and another. There are many “evolutionary dead ends”-species that simply die out.
  5. Sexual selection is an example of natural selection. That’s the principle that animals with advantageous traits are more likely to survive and pass those traits to the next generation. Natural selection drives evolution.
  6. Hazen’s background is studying minerals-solid, crystalline materials that form naturally through geological processes and make up rocks. He’s using that knowledge to figure out how ancient minerals might have been involved in the evolution of the first primitive life-forms.
  7. Tiny bacteria used the sulfur from the vents to make food – a process called “chemosynthesis.” Other animals, like worms and shrimp, then ate this bacteria. A whole ecosystem exists there. Finding this life made scientists reconsider the power of evolution.


offspring       off ·spring

Advanced Definition


  1. the child, young, or descendant of a particular parent or ancestor.

As his aunts and uncles had no offspring, he had no cousins.

The offspring of the famous racehorse also became champions.

  1. the product or result of something.

These are some examples of how the word or forms of the word are used:

  1. Where do traits come from? It’s easy to spot certain physical traits that were passed down genetically from parents to offspring.
  2. “There is no parental care in any shark species,” says Hueter. “The offspring are strictly on their own after they are born.”
  3. To feed their young, some seabirds, such as albatross and penguins, eat fish and then regurgitate the partially digested meal into the mouths of their offspring.
  4. Why do giraffes have long necks? Why do rabbits produce so many offspring? Natural selection can help us understand why some species are the way they are.
  5. Our Earth is alive with organisms carrying through their life cycle of birth, reproduction and death. All plants, animals and other living things reproduce, resulting in new offspring or organisms.
  6. It’s a complex lottery in whichoffspring of the first two organisms inherit a combination of their genetic material. The possible variations inherent in recombining the parents’ DNA is very, very broad-larger than the pool of entries in the state lotto jackpot!
  7. Reports from just over a year ago say that thousands of pythons have been making their homes in the Everglades at the expense of the native (natural to the area) species. Pythons and anacondas aren’t normal inhabitants of the Florida ecosystem; the ones that have taken over the Everglades are ex-pets and their offspring.
  8. Unlike most of today’s reptiles, the prehistoric marine reptiles were viviparous-the females produced live offspring instead of eggs. “The reason is simple,” says Mike Caldwell, a paleontologist at the University of Alberta in Canada. “If you give live birth you can live anywhere in oceanic environments and are not bound to come ashore to lay eggs.”
  9. Charles Darwin outlined the idea of natural selection in his 1859 book, On the Origin of Species, where he explained there are certain characteristics that help an animal survive, such as a sharp beak that allows birds in the Galapagos Islands to better find and eat their food. These sharp-beaked birds survived long enough to reproduce, and their offspring had sharp beaks too.
  10. Abraham was able to find food in places none of the other elephants had ever thought to look. Soon Abraham had a mate and baby elephants of his own. One morning the herd was standing on the banks of the Ganges. Abraham watched his youngest offspring use his trunk to shoot his sister with water when he noticed a red and yellow barge motoring slowly upriver.

Name: ___________________________________ Date: _______________
Comprehension Questions
1. How many millions of years ago did the earliest marine reptiles evolve from land reptiles?
A. 240
B. 251
C. 65
D. 200
2. How does the author describe prehistoric marine reptiles, in general?
A. Less threatening than a T. rex
B. Similar to modern-day sea crocodiles
C. Large, scary predators
D. Not as threatening as we once thought
3. Which of the following conclusions about prehistoric marine reptiles is supported by the passage?
A. They evolved and adapted over time.
B. They were large, dangerous predators.
C. They are still mysterious to us in many ways.
D. All of the above
4. Read the following sentences:
“Along with the dinosaurs, the giant marine reptiles became extinct 65 mya. But their fossilized remains are abundant around the world.”
The word remains means
A. Stays in the same place
B. Stays behind
C. On reserve, storing it for future use
D. Traces or parts of something
5. What would be another good title for this passage?
A. T. rex vs. Mosasaurs
B. New Fossil Findings, But No New Information
C. Prehistoric Residents Under the Sea
D. Using Fossils to Uncover the Past
6. How are ichthyosaurs different from plesiosaurs?
7. Why might experts believe mosasaurs were “very aggressive animals, capable of pursuing and killing all kinds of prey”?
8. The question below is an incomplete sentence. Choose the word that best completes the sentence.
It makes sense that prehistoric marine reptiles were viviparous _________ it allowed
them to live anywhere in oceanic environments
A. because
B. on the other hand
C. but
D. however
9. Which animal moves in a more sinuous way?
10. Would you describe the roads and paths of a typical mountain as sinuous? Why or why not?

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