If 99.9 percent of animals that once lived are now extinct what animals are not extinct that lived hundreds of millions of years ago?

Answer 1

There are only a few animals that survived including snakes, crocodiles, frogs, small mammals, and the Cocelacanth. The largest crocodile, Deinosuchus, downscales, then survived the mass extinction probably because it was underwater. The Cocelacanth was a fish that should have died out before the dinosaurs even existed. But then a female fisherman netted one off the coast of South Africa.

---------------------------------------------------------------------------------------------------------

The estimation that 99.9% of living beings now being extinct refers to the vast majority of species being extinct (which includes all living things, including the plants, fungi, and single-celled organisms). This is not unusual considering that species inevitably change over time, at least at the genetic level if not morphologically, and go extinct all the time, whether during mass extinctions or simply through the process of natural selection and ecological displacement. It is estimated that the average length of time an animal species remains extant is some 10 million years, though with a great deal of variance. It is important to know that the designation species is somewhat arbitrary, as it's often difficult to delineate at which point a certain species begins and another one ends, considering that the process of evolution is gradual and doesn't neatly separate ancestors and descendants. Usually one can only separate them through hindsight, and because intermediate forms have gone extinct in the meantime, thus simplifying the naming affairs. As for which animals are alive that lived millions of years ago, this obviously depends on the context. There are plenty of animal orders that still exist after hundreds of millions of years, and single celled organisms have been around for literally billions of years. For instance, the animals which we now call "fish" have been around since at least the Ordovician (almost 500 million years ago), and the cnidarians (jellyfish etc.) have been on the planet since the Ediacaran period (some 580 million years ago). Sharks have been around since the Silurian (440 million years ago), although the sharks that were around at that time only superficially resembled their modern cousins and descendants. People usually notice morphological similarities (body shape) and those are the features that are preserved in the fossil record.

Often times, animals that resemble their ancient ancestors or relatives are called "living fossils", examples include the living coelacanth genus Latimeria, the horseshoe crabs, the brachiopod Lingula, the Queensland lungfish (Neoceratodus fosteri), the crustacean Triops cancriformis and many others.