Dinosaur time gap

They are the two animals everyone can draw, and they are usually drawn in the same swamp. Stegosaurus was gone by about 150 million years ago. Tyrannosaurus does not appear until roughly 68 million years ago. That is a gap of more than 80 million years.

Tyrannosaurus died out 66 million years ago. So Stegosaurus is further in the past from Tyrannosaurus than Tyrannosaurus is from you — and it is not close. You are the more recent neighbour.

Almost every pairing does this. The word "dinosaur" flattens a stretch of time longer than the gap between the last dinosaur and the invention of the aeroplane, and puts everything in it on one page.

Pterosaurs — the flying ones, including Pteranodon and Quetzalcoatlus — are not dinosaurs. They are close cousins that took to the air on a single enormously long finger.

Plesiosaurs, mosasaurs and ichthyosaurs, the long-necked and the toothy things in the sea, are not dinosaurs either. No dinosaur ever went fully marine.

Dimetrodon, the one with the sail on its back, is the worst offender. It lived around 290 million years ago, died out roughly 40 million years before the first dinosaur appeared, and is more closely related to you than to Tyrannosaurus.

Meanwhile the animal on your bird table is a dinosaur. Birds are the branch that lived through the asteroid, which makes the sentence "dinosaurs are extinct" only three quarters true.

The bone itself is almost never dated. What gets dated is the rock — usually a layer of volcanic ash above the fossil and another below it. Ash contains crystals with a known rate of radioactive decay, so each layer can be given an age, and the animal is squeezed between the two.

That is why dates move. A better ash bed, a more precise instrument, or a recount of an old sample shifts a species by a million years or two, and every chart printed before the recount is quietly out of date. The broad order almost never changes; the exact numbers do.

It is also why some animals have a wide range and others a narrow one. A species known from one quarry gets a tight date. A species found across a continent may have lived for ten million years, and its entry is a span rather than a moment.

The first dinosaurs turn up about 230 million years ago, quite late in the story. Life had already been complex for 300 million years before that: trilobites on the sea floor, forests of tree-sized club mosses laying down the world's coal, sharks, amphibians the size of crocodiles, and the sail-backed synapsids that eventually produced mammals.

Most of it was wiped out 252 million years ago, at the end of the Permian — the largest extinction there has ever been, and considerably worse than the asteroid. Something close to nine in ten marine species disappeared. The dinosaurs inherited the space it left.

It is not the biggest extinction, but it is the best documented and the most cinematic. A rock roughly ten kilometres across hit what is now the Yucatán coast 66 million years ago, and left both a crater and a thin layer of iridium-rich clay that shows up in rock all over the world.

Everything larger than a domestic cat had a very bad time. What survived tended to be small, able to burrow, and not fussy about food — which describes early mammals, and describes the ancestors of birds.

The empty world that followed is the reason mammals got big. For 160 million years they had been living around the edges of somebody else's planet.

Try it as a single year. If the age of the Earth runs from January to December, the first dinosaurs arrive around 13 December, the asteroid hits on Boxing Day, and every human being who has ever lived fits into the last twenty minutes of New Year's Eve.

Our own species is about 300,000 years old. The dinosaurs lasted around 165 million years, which is more than five hundred times longer. By any reasonable standard they were not a failed experiment; they were the normal state of the planet, and we are the brief part.