What if dinosaurs still lived today?

Long, long ago, dinosaurs walked on Earth. Some were very, very big!

Then most of them died out. What if they had not? What if dinosaurs lived next door today?

Dinosaurs lived on land for millions of years. Then, 66 million years ago, an asteroid more than 6 miles wide hit Earth, and most dinosaurs died out. But not all of them: scientists say birds are the only dinosaurs left.

So what if the other dinosaurs, like the giant plant-eaters, had survived too? This lab is a thought experiment. No one can know for sure, but fossils tell us a lot about how dinosaurs lived. Choose a path to explore.

The end-Cretaceous (K-Pg) extinction 66 million years ago, triggered by an asteroid more than 6 miles across striking what is now the Yucatán Peninsula, ended the non-avian dinosaurs. Beaked birds were the only dinosaur lineage to survive.

This thought experiment imagines a world in which non-avian dinosaurs also survived to the present. It cannot give predictions, only reasoned ideas based on what fossils reveal about dinosaur feeding, climate tolerance and ecology.

Look at a chicken, a duck or a big ostrich. Scientists say these birds are dinosaurs!

The very first birds came from small dinosaurs with feathers.

A pigeon or a penguin doesn't look much like a giant dinosaur, but the connection goes all the way down to the bones. About 150 million years ago, the first birds evolved from small, feathery dinosaurs that looked a bit like raptors.

That is why paleontologists call birds 'avian dinosaurs' and all the others, from Stegosaurus to Brontosaurus, 'non-avian dinosaurs.' So dinosaurs do still live today: you might see one at a bird feeder.

Most paleontologists familiar with vertebrate phylogeny regard birds as dinosaurs: properly, avian dinosaurs, with all others non-avian dinosaurs. The evidence strongly supports descent from a maniraptoran dinosaur. Birds first evolved about 150 million years ago, in the Jurassic, from small, feathery, raptor-like theropods; Archaeopteryx, about 150 million years old, still had teeth.

For more than 80 million years birds diversified alongside other dinosaurs. At the K-Pg boundary only beaked birds survived, and the term 'non-avian dinosaurs' marks that split.

Imagine big plant-eating dinosaurs munching in a park! They would need lots and lots of plants.

People would have to share the land carefully, the way we share it with elephants today.

Big plant-eating dinosaurs called sauropods were so huge that scientists have wondered how several kinds could share the same plants. If they lived today, they might need very large wild areas full of plants, a bit like the protected parks where elephants live. That is an idea, not a fact.

Scientists also know that Earth was warmer when the last dinosaurs lived. In Denali, Alaska, where dinosaur footprints have been found, the average temperature was about 51 °F then and is about 28 °F now.

Upper Jurassic North America supported four or five coexisting sauropod species; UC Berkeley's Museum of Paleontology notes their immense size raises the question of how they partitioned food, and tooth microwear suggests some selective feeding and browsing at different heights. A modern world with sauropods would face the same problem, plus competition with today's herbivores and people: an idea, not a prediction.

Climate matters too. The Cretaceous was warmer overall: Denali's mean annual temperature was about 51 °F then, versus 28 °F today, with mild, wet winters at a paleolatitude of 65–70° N.

Scientists learn about dinosaurs from bones and footprints in rocks, called fossils.

Fossils tell us a lot, but not everything. So we use our imagination carefully.

Everything we know about the non-avian dinosaurs comes from fossils: bones, teeth and footprints. Teeth show what they ate; footprints show where they walked. In Denali, a university geology class found the first dinosaur footprints in 2005, and thousands more have turned up since.

But fossils can't tell us how dinosaurs would behave today, among cars, cities and modern animals. That is why every 'what would happen' in this lab is an idea. The facts are about the past and about birds.

Inferences about non-avian dinosaurs rest on fossil evidence: skeletons, dental microwear, trackways and the plants and sediments around them. Denali's Cantwell Formation, whose basin began filling about 80 million years ago, has yielded thousands of tracks since a University of Alaska Fairbanks field class found the first in 2005.

None of this can tell us how 66 million years of further evolution would have changed them, or how they would interact with modern ecosystems. Any claim about a present-day world with non-avian dinosaurs is therefore speculative; this lab labels such claims as ideas.

You can meet a dinosaur today! Look at the birds in your yard or at a farm.

Ostriches, cassowaries and chickens are all birds, so they are living dinosaurs too.

The best way to meet a dinosaur today is to watch birds. Ostriches, cassowaries, chickens, owls and sparrows are all living dinosaurs, according to paleontologists.

Birds survived the asteroid when the other dinosaurs did not. Scientists think beaks may have helped: after the disaster, birds with toothless beaks could eat seeds and nuts. Today, people protect birds and their homes, which means protecting the last dinosaurs on Earth.

Every living bird, from ostriches to hummingbirds, is an avian dinosaur. Paleontologists are still debating why only beaked birds survived the K-Pg extinction; one idea is that toothless beaks let them eat seeds and nuts in the aftermath, when other food was scarce.

Studying birds alongside fossils helps scientists test ideas about extinct dinosaurs. Conserving birds and their habitats is, quite literally, conserving the dinosaur lineage.