No doubt you have heard the claims from young earth creationists about living fossils. A "living fossil" is an organism that is thought to have been extinct, and then it shows up alive on planet earth.
The claim by young earth creationists is that these living fossils are proof that the earth is young. The reasoning is that since all organisms were alive only 4,300 years ago, before the time that they date the Flood of Noah, then it is reasonable to assume that they are still alive today.
I'm sure everyone is aware of the most famous living fossil, the coelacanth, the fish thought to be extinct 65 million years ago. In 1938, a specimen was caught off the coast of South Africa, and since then several more have been caught and examined. Besides the coelacanth, Answers in Genesis gives several examples in the articles linked above.
Determining an Organism's Life Span
How is a fossil's life range is determined? When an organism is found in the fossil record, paleontologists look for the oldest and youngest specimen to determine it's life range. For example, if the oldest specimen of a species of dinosaur is in a rock formation dated during the Berrasian Age of the early Cretaceous, then the earliest this dinosaur existed is about 140 million years ago (140 Ma). If the youngest example of this same species is from the Turonian Age of the Late Cretaceous, then it died out about 90 million years ago. Thus, the dinosaur is said to have lived from 140 to 90 Ma.
After this date range is published, another sample of this dinosaur is unearthed in a rock bed dated to the Santonian Age of the Late Cretaceous. Does this mean the earlier dates were wrong...obviously. But the wrong dates were not based on guesswork, but on a lack of complete evidence. Now, this dinosaur's lifespan is said to be from 140 to 84 Ma.
Scientists make this kind of adjustment all the time. New fossils are found, extending the known lifespan of the organism because of new fossil discoveries.
There is no trick or deception on the part of the scientist...he/she is merely interpreting the available evidence to reach a conclusion.
What happens when a fossil species is found to be alive, such as the coelacanth? It's known lifespan is extended...that's it! It doesn't' mean the earth is young, it doesn't even provide good evidence for a young earth. For the coelacanth, its lifespan was extended from 400-65 million years ago to 400 Ma to the present. Nothing further can be implied from this data.
In the case of our dinosaur, if it was found alive today, naturally the young earth creationists would claim it is a living fossil. The real scientific world would merely adjust its lifespan to 140 Ma to the present. There would be no panic in the scientific world...they would not wonder if the earth really is only 6,000 years old. The rest of the evidence clearly indicates an old earth, and one living dino would not contradict that.
The presence of a living fossil in no way is evidence for a young earth. Yes, there are fossils that occasionally turn up living, but there are millions more which are long gone, which went extinct millions of years ago, and there is no evidence they are alive today. Therefore, we have millions of extinct species, and a handful of so-called "living fossils" which have a gap of millions of years in the fossil record. The evidence strongly supports millions of years of extinction, and an old earth.
When a "living fossil" is found, YECs naturally jump on it and proclaim it as proof of a young earth. However, when they grasp at this one straw and bring attention to it, they are ignoring the 50 foot tall haystack from where it came from.