What we've been getting wrong about dinosaurs

Updated 5:47 AM ET, Tue September 21, 2021

Science has transformed our understanding of dinosaurs in the past two decades. Get caught up with what's new in this five-part CNN series.

(CNN)Defined by their disappearance dinosaurs might appear to be evolutionary failures. Not so.

Dinosaurs survived and thrived for 165 million years -- far longer than the roughly 300,000 years modern humans have so far roamed the planet.
They lived on every continent, munched on plants, snapped their jaws at insects, itched from fleas, suffered from disease, got into fights, snoozed, performed elaborate courtship rituals and looked after their young. The creatures were much more diverse -- and downright bizarre -- than what we might recall from childhood books.
Were it not for an asteroid strike 66 million years ago, the ancient creatures still might have dominated our world. And they still are here, in the form of birds we see around us today.
Scientists have discovered more in the past two decades than they had in the prior 200 years about how dinosaurs behaved and evolved. Here's what's new and different about what is known of dinosaurs.

How many dinosaurs were there?

The short answer: Lots.
Take T. Rex, the predator with banana-sized teeth that is perhaps the best studied dinosaur. Scientists believe that each T. rex generation was 20,000 individuals, and this adds up to a total of 2.5 billion during the 2.4 million years they are thought to have lived.
While it's only an estimate and relies on lots of assumptions, it's a good reminder that the fossil record only captures a tiny fraction of ancient life. The same team of researchers purports that for every 80 million adult T. rexs, there is only one clearly identifiable specimen in a museum.
Scientists have definitively identified around 900 dinosaur species -- although there are plenty more potential species for which paleontologists don't quite have enough bones or the fossils aren't well preserved enough to truly designate them as such. And there are about 50 new dinosaurs discovered each year, inspiring many scientists to think we're experiencing a golden age of paleontology.
Many, many more species existed -- one estimate suggests that there were between 50,000 and 500,000, but we might never find their fossil remains.
So many species could exist because they were highly specialized, meaning different types of dinosaurs had different sources of food and could live in the same habitats without competing. For example, with unusually large eyes and hair-trigger hearing, Shuuvia deserti, a tiny desert-dwelling dinosaur evolved to hunt at night, while Mononykus had perplexingly stunted forelimbs, each of which had only one functional finger and claw -- perhaps to eat ants or termites.
It's worth pointing out, of course, that many of the dinosaurs you might be familiar with did not live together as one community. Stegosaurus and T. rex never co-existed, separated by 80 million years of evolution. In fact, the time separating Stegosaurus and Tyrannosaurus is greater than the time separating T. rex and you.

What did they look like?

The first dinosaur discoveries, the earliest more than 150 years ago, focused on the sensational: The big bones and skulls we know from museum atriums.
But dinosaurs came in all shapes and sizes. In fact, some of the most exciting finds in recent years have been tiny. In 2016, a tail belonging to a sparrow-sized creature could have danced in the palm of your hand was found preserved in three dimensions in a chunk of amber.