Research explores how crocodiles survived ‘dino apocalypse’

How is it that crocodiles have survived long after the dinosaurs perished?

Researchers believe they may know how the apex predators* survived a mass extinction event 66 million years ago that wiped out three quarters of Earth’s species, including the dinosaurs.

And it has to do with keeping warm.

RESEARCH TRACKS PREHISTORIC CROC MOVEMENT
Crocodilians — today’s crocodiles, alligators, caimans and gharials and their many extinct

relatives — have been a roaring success since coming into existence more than 240 million years ago. And new research identifies one important factor in their success — sticking to warm climates throughout their history.

In fact, the researchers said, this trait may help explain why crocs as a group survived the asteroid strike 66 million years ago that doomed the dinosaurs, along with certain physiological* characteristics that make them one tough bunch of reptiles.

The crocodilian evolutionary lineage*, called Pseudosuchia, arose during the Triassic* Period in the aftermath of Earth’s worst mass extinction event at the end of the Permian*, known as “the Great Dying” (about 250 million years ago).

In fact, early crocs appeared on the scene millions of years before the first dinosaurs and mammals.

The researchers tracked the geographic range for crocs during their entire history, guided by an evolutionary tree of more than 400 extinct and living species. They used data from more than 7,000 fossils and geographic records of modern crocs such as the alligators of Florida, saltwater crocodiles of Australia, gharials of India, caimans of Brazil and more.

They combined this information with 52 global palaeoclimate* reconstructions to determine the climatic conditions under which crocs lived over time. While other groups such as dinosaurs and mammals conquered a variety of climates, the study showed that crocs consistently kept to warm climates.

“Rather than continually adapting to new climatic conditions, they largely tracked suitable environments as these shifted geographically, expanding towards the poles during

warmer periods and retreating towards the equator during cooler ones,” said University College London vertebrate palaeontologist* Alfio Alessandro Chiarenza, lead author of the research published in the journal Science Advances.

For example, during an exceptionally warm moment in Earth’s history around 50 million years ago, Dr Chiarenza said, crocs inhabited high-latitude* regions of North America, Europe and even Antarctica.

“One of the most striking patterns is the repeated expansion* and contraction* of suitable habitat in response to global temperature changes,” Dr Chiarenza said.

DIFFERENT TYPES OF PREHISTORIC CROCS
While living crocs are primarily semiaquatic* ambush predators*, their extinct relatives included a broad variety of forms, University College London palaeontologist and study co-author Philip Mannion said.

“They have occupied almost every conceivable* ecological* role,” Dr Chiarenza said.

There were fully land-dwelling apex predators like 9m long Fasolasuchus, pug-snouted herbivores like Simosuchus, armoured plant-eaters like Aetosaurus, mollusc-eaters like Kuttanacaiman, filter-feeders like Mourasuchus and ocean-going types like Metriorhynchus.

And if you thought modern-day crocodiles were scary enough, during the age of dinosaurs, there also were semiaquatic crocs like Deinosuchus, which measured up to 12m long and may have rivalled the Tyrannosaurus in mass.

“Modern crocodilians are only the surviving remnants of a once extraordinarily diverse evolutionary group,” Dr Chiarenza said.

DINOSAUR APOCALYPSE
The asteroid strike at a site called Chicxulub off the coast of Mexico’s Yucatán Peninsula at the end of the Cretaceous* period unleashed a global environmental cataclysm*. Three-quarters of Earth’s species including the dinosaurs, aside from their bird descendants, vanished.

The crocs survived.

“The Chicxulub impact caused a dramatic reduction in incoming sunlight, producing an impact winter* and widespread environmental disruption. However, our climate simulations

suggest that this cooling did not affect the availability of suitable climatic conditions* equally for all groups,” Dr Chiarenza said.

“Under all but the most extreme impact scenario we tested, tropical and subtropical regions retained climatic conditions suitable” for the crocs, Dr Chiarenza said, explaining that there were likely still warm areas inhabited by crocs after the asteroid strike.

The findings suggest that the longstanding preference of crocs for warm and low-latitude* environments may have placed some surviving populations in areas that were sheltered from the freezing, dark conditions caused by the impact. These regions would have been near the equator and in the subtropics where climatic conditions stayed warm and within the range the crocs were accustomed to, Dr Chiarenza said.

For dinosaurs, however, the impact caused a collapse of suitable habitat.

OTHER REASONS CROCS SURVIVED
Crocs also possess physiological traits beneficial for times of environmental stress. They can enter states of dormancy* to cope with harsh conditions, such as aestivation* to survive extreme heat or drought, and brumation* to withstand extreme cold. Crocs today, for instance, may dig burrows or encase themselves in hardened mud during severe droughts, slowing their heartbeat and breathing.

Dr Chiarenza said the ability to conserve energy using these states of dormancy could have been very beneficial following the Chicxulub impact, when sunlight was scarce and there were less species to feed on.

Crocs are also cold-blooded, while recent research suggests that at least some dinosaurs, such as the Tyrannosaurus, were warm-blooded. Bodily requirements for cold-blooded animals are much lower than for comparably sized warm-blooded animals.

“That can become a significant advantage when food becomes scarce. Crocodilians can survive extended periods without feeding,” Dr Chiarenza said. “But even if an animal can survive prolonged starvation, it still needs environmental conditions that remain within its physiological limits.”

Minor edits have been made to this story for Kids News. Reuters reporting by Will Dunham in Washington; Editing by Daniel Wallis

POLL

GLOSSARY

• apex predators: an animal at the top of a food chain that has no predators hunting it
• physiological: relating to the way the bodies of living things function
• lineage: direct line of descent from a common ancestor
• Triassic: the Triassic period was from about 252 million years ago to 201 million years ago, following the mass extinction event at the end of the Permian called “the Great Dying” which was the largest mass extinction Earth has ever known, and leading to the Jurassic period
• Permian: the Permian period was from about 298.9 million years ago to about 251.9 million years ago and was the final period of the Paleozoic Era. During this period, all of Earth’s major landmasses came together to form the supercontinent Pangaea
• palaeoclimate: the climate during a past geological age, before the invention of weather instruments and meteorological records
• palaeontologist: a scientist who studies the history of life on Earth through the examination of fossils
• high-latitude: regions that are situated far from the equator, stretching from subpolar areas towards the North and South Poles
• expansion: widening
• contraction: shrinking
• semiaquatic: living on both land and in water
• ambush predators: a meat-eating animal that catches prey by using camouflage, patience or luring rather than chasing them over long distances
• conceivable: possible to imagine, think of or believe
• ecological: the relationships between living creatures and their physical environment
• Cretaceous: the final period of the Mesozoic Era (lasting from about 145 to 66 million years ago), in which dinosaurs like the Tyrannosaurus rex and Triceratops roamed the Earth and the supercontinent Pangaea continued to break apart, sending continents toward the positions they are in today while raising global sea levels
• cataclysm: a sudden, violent event that causes great destruction
• impact winter: a potential period of prolonged global cooling that scientists think was caused by the impact of an asteroid or comet striking the Earth
• climatic conditions: the long term weather patterns and atmospheric conditions in a geographical region over a period of time, typically 30 years or more
• low-latitude: the geographic region on Earth that lies closest to the equator, between 0 and 30 degrees north or south latitude. These regions are typically describes as the tropics or subtropics and feature warm to hot weather year-round because the sun hits this part of the Earth consistently
• dormancy: a state of temporary inactivity, reduced metabolic function or slowed growth that helps an animal conserve energy and survive harsh conditions
• aestivation: a state of dormancy in animals to survive extreme heat
• brumation: a state of sluggishness or winter dormancy in animals exhibited by cold-blooded animals during cold weather

EXTRA READING
‘Drop crocs’ roamed Australia
Young dinosaur was crocodile’s last meal
T-rex eye evolved for bigger bite

QUICK QUIZ
1. How long ago did the crocodilian family come into existence?
2. What was the mass extinction event that wiped out the dinosaurs but not the crocodiles?
3. When did this event occur?
4. What is one reason crocodiles are thought to have survived the extinction event?
5. What effect did the mass extinction event have on Earth’s climate?

LISTEN TO THIS STORY

CLASSROOM ACTIVITIES
1. What’s the difference?
Australians will probably be familiar with crocodiles, as we have them here in our country. But we might be less familiar with alligators, gharials and caimans. Research to find out about the four types of crocodilians, then make a one-page ‘cheat sheet’ that will help readers understand the similarities and differences between these creatures. You might include key information such as where they live, their size, what they eat, etc. as well as pictures or diagrams.

Time: allow 45 minutes to complete this activity
Curriculum Links: English; Science

2. Extension
Based on what you now know about why crocodilians have survived for 240 million years while other species have been wiped out, name another animal that you think could survive another asteroid strike like the one 66 million years ago. Explain your reasons for believing this animal could survive.

Time: allow 15 minutes to complete this activity
Curriculum Links: English; Science

VCOP ACTIVITY
Let’s practise taking notes
When note-taking, we want to be able to extract the key pieces of information from the text or presentation. You start with determining the main idea: if you had to explain to someone in a sentence what the article is about, what would you say?

Then back up the main idea with two to four key points to support what you first stated.

What would you put as the main idea and supporting points to back up this article?

Once you have had a go at note-taking.

Share your results with a peer to see if you identified the same information or different.

Discuss the finding if you came up with different information.