Level 1 — Absolute Beginner

A very big rock from space hit Earth 66 million years ago. The dinosaurs died. But crocodiles did not die.

Scientists in London wanted to know why. They looked at more than 7,000 fossils. A fossil is the hard shape of an old animal in rock.

They found that crocodiles always follow warm places. After the rock hit, some places near the middle of Earth stayed warm. Crocodiles could live there.

Today the world is changing again. Crocodiles want to move to new warm places. But now there are roads, farms and cities in the way.

rock

A hard piece of stone.

space

The area beyond the sky, where the stars are.

dinosaur

A very large animal that lived millions of years ago.

scientist

A person who studies the natural world.

fossil

The shape of a very old plant or animal preserved in rock.

warm

A little hot; not cold.

move

To go from one place to another.

farm

Land where people grow food or keep animals.

Level 2 — Elementary

When an asteroid struck the Earth about 66 million years ago, it threw up so much dust that sunlight was blocked, temperatures fell and most large animals disappeared. The dinosaurs went. Crocodiles stayed. A new study in the journal Science Advances offers an explanation.

Researchers at University College London collected more than 7,000 fossils and records from over 400 living and extinct species, then ran them through climate and geography models covering 245 million years. The pattern was consistent: the crocodile family has always tracked warm climates, moving whenever the warm zones moved.

That habit saved them. The models show that warm pockets near the equator, which the scientists call refugia, stayed liveable after the impact. Those refuges only failed if sunlight dropped by 15 percent, which the authors describe as extreme and unlikely. A fall of 5 to 10 percent left the refuges intact.

The team also looked at hotter periods. Around 50 million years ago the planet was roughly 10 degrees Celsius warmer than today, and about 94 million years ago it was up to 14 degrees warmer. Crocodiles coped by moving. The lead author, Dr Alfio Alessandro Chiarenza, warns that modern roads, farms and cities now block the routes they would need.

asteroid

A large rocky object that travels through space.

dust

Very small dry pieces of earth or rock in the air.

extinct

No longer existing as a species.

species

One particular kind of living thing.

model

A computer simulation used to test how a system behaves.

track

To follow the movement of something.

equator

The imaginary line around the middle of the Earth.

refuge

A safe place that protects you from danger. Plural in science: refugia.

Level 3 — Intermediate

The extinction that ended the age of dinosaurs is usually told as a story about who died. A study published in Science Advances and reported on 30 October turns it around and asks why one group barely noticed. Crocodile-line archosaurs, the branch that includes today's crocodiles, alligators and caimans, passed through the Chicxulub impact 66 million years ago and are still here, essentially unchanged in body plan.

A team at University College London, led by Dr Alfio Alessandro Chiarenza with Professor Philip Mannion as a senior author, assembled more than 7,000 fossil occurrences from over 400 living and extinct species and tested them against palaeoclimate and palaeogeography reconstructions spanning 245 million years. One signal dominated the record: wherever the warm zones of the planet sat, the crocodile line sat there too. Their history is less a story of adaptation than of relocation.

The modelling then asked whether such warm refuges could have survived the impact winter. They could. Equatorial refugia remained habitable under a 5 to 10 percent reduction in sunlight, and only collapsed when the researchers forced a 15 percent reduction, a scenario the authors regard as extreme and improbable. Notably, raising atmospheric carbon dioxide in the simulations did not change the amount of available habitat, which suggests temperature and light, not carbon chemistry, set the limits.

The group has also lived through the opposite extreme. During the Early Eocene, about 50 million years ago, global temperatures ran roughly 10 degrees Celsius above today's, and at the Cretaceous Thermal Maximum around 94 million years ago they reached up to 14 degrees above modern levels. Crocodiles answered both by migrating, and some grew to remarkable sizes along the way, including Sarcosuchus at up to 12 metres. The authors' warning is therefore pointed: the strategy that has carried the lineage through 245 million years requires open ground to move across, and roads, farmland and cities have now closed most of it.

extinction

The disappearance of a species or group of species.

body plan

The basic structural layout shared by a group of animals.

occurrence

A recorded instance of a species found at a place and time.

palaeoclimate

The climate of the distant geological past.

reconstruction

A scientific rebuilding of past conditions from available evidence.

relocation

Moving from one place to another to live.

habitable

Able to support living things.

lineage

A line of descent; a family of related species over time.

Level 4 — Advanced

Survivorship in mass extinctions is rarely a matter of toughness, and the crocodile line is the clearest illustration of that principle in the fossil record. A paper published in Science Advances and reported on 30 October proposes that crocodile-line archosaurs persisted through the Chicxulub impact not because they endured the post-impact winter but because a thin band of the planet never fully experienced it. The work, led by Dr Alfio Alessandro Chiarenza of University College London with Professor Philip Mannion among the senior authors, assembles more than 7,000 fossil occurrences from over 400 extant and extinct species and projects them onto palaeoclimate and palaeogeographic reconstructions spanning 245 million years.

The resulting signal is unusually clean. Across that interval, the distribution of the clade tracks the distribution of thermally suitable habitat with a fidelity that makes the group something close to a biological thermometer. Their evolutionary conservatism, long treated as evidence of a successful general-purpose design, reads differently in this framing: the body plan did not need to change because the animals moved instead, and the planet, for most of the Mesozoic and early Cenozoic, obliged by offering somewhere warm to move to.

The impact-winter modelling supplies the decisive test. Equatorial refugia remained habitable under sunlight reductions of 5 to 10 percent, and collapsed only when the authors imposed a 15 percent reduction, a forcing they characterise as extreme and improbable given current estimates of aerosol loading. Elevating atmospheric carbon dioxide in the simulations did not expand or contract available habitat, implicating insolation and temperature rather than carbon chemistry as the binding constraints. The lineage's tolerance at the other end of the range is equally instructive: it passed through the Early Eocene, roughly 10 degrees Celsius warmer than the present, and the Cretaceous Thermal Maximum near 94 million years ago, up to 14 degrees above modern values, producing giants such as the twelve-metre Sarcosuchus along the way.

The conservation inference is where the paper stops being a study of the past. If the mechanism that carried crocodilians through the single most severe biotic crisis of the Phanerozoic was latitudinal and regional relocation, then the relevant question for the coming century is not whether the animals can tolerate warming, which the record suggests they can, but whether they can still reach the places that will be suitable. Roads, agricultural conversion, levees and urban sprawl have fragmented exactly the low-lying wetland corridors that migration requires. A species can be physiologically robust and ecologically doomed at the same time, and the authors' argument is that the oldest surviving archosaur lineage may now be discovering the difference.

survivorship

The pattern of which groups live through an event and which do not.

clade

A group of organisms that share one common ancestor and all its descendants.