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 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." One fossil of a prehistoric marine reptile, now on view in a German museum, shows the animal giving birth.
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. 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.
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 bone-crushing 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.
LIFE SCIENCE
Sea Monsters
Pages 4-5 1100L
Critical Thinking
* Which modern animals are viviparous? (mammals) Which are not? (birds, amphibians, insects)
* Many prehistoric marine reptiles resembled modern aquatic animals in some ways. What does that suggest about evolution? (Animals of different eras adapt to the same environments in similar ways.)
National Science Education Standards
* Diversity and adaptation of organisms: evolution, prehistoric life
Page 3
Sea Monsters
1. D, 2. G, 3. F, 4. E, 5. C, 6. A, 7. B, 8. H
Sea Monsters (Page 4)
Vocabulary Challenge
Match each numbered word or phrase with its correct description. Write the letter of the description in the space provided.
__ 1. ectothermic
__ 2. mosasaurs
__ 3. viviparous
__ 4. ichthyosaurs
__ 5. herbivore
__ 6. plesiosaurs
__ 7. Mesozoic
__ 8. pliosaurs
A. prehistoric marine reptiles that moved like turtles
B. a prehistoric era
C. an organism that eats plants
D. cold-blooded
E. prehistoric marine reptiles that had long, supple bodies and may have rippled through the water like eels
F. produces live offspring
G. prehistoric marine lizards that had long heads, short necks, and long, sinuous tails
H. prehistoric marine reptiles that had short necks, bulky bodies, and heads like those of crocodiles
Source Citation
Fraser, Stephen. "Sea Monsters: a new wave of fossils reveals the oceans' prehistoric giants." Current Science, a Weekly Reader publication 16 Apr. 2010: 4+. Academic OneFile. Web. 28 Apr. 2010.
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