Similar Fossil Trick
This trick involves showing a fossil from hundreds of millions of years ago, and then showing a modern animal that looks the same, and claiming that this disproves evolution.
Evolution affords change over time, but it does not require it. If the form of the animal remains the best fit for the current environment, there is no reason that a life form has to change.
Background
I see this argument regularly.
- mosquito stuck in amber estimated at 100 million years and it looks quite similar to a modern mosquito.
- a 56 million years old fossil bee looking exactly like a modern bee.
- The coelacanth was known from 400 million year old fossils, thought to be extinct until a live one was caught in 1938.
- Modern crocodiles are known to look very much like 200 million year old fossils of crocodiles.
No Requirement to Change
The argument appears to be that evolution requires a species to change. That is simply a misunderstanding of evolution.
Evolution is about genes in a gene pool changing in response to environmental pressures. If a climate gets colder, or if a population of animals move into a colder climate, the population will over time tend to have thicker fur (for mammals). It is the increase in coldness that favors the thicker fur, allowing those animals more opportunities to reproduce and pass on genes for thicker fur.
However if the niche that the animal occupies remains stable such that it does not select for different genes, then the population will remain very much the same for long periods of time. Crocodiles have been doing fine for 200 million years and no environmental change has required a change in shape.
Ecological Niches
Any student of speciation knows that an ecosystem will push the shapes of animals into ecological niches making them look similar. A famous example is the hyena which looks very much like a dog, and it hunts and behaves much like a dog, but it actually more closely related to cats than dogs. It seems that in southern Africa dogs were missing, and hyenas were cultivated to fit the ecological niche that dogs normally fit.
Ecological niches can also cause what we call recurrent evolution where in different parts of the world, and even different times, different animals tend to assume particular niches in their own ecosystem, and tend towards looking similar. An example of this is the crab-like body plan which several different lines of animals have evolved to. Another is the hummingbird and the hummingbird moth, the latter getting the name because they look and act so similarly.
Superficial Comparison
The nature of fossils usually only record the exterior shape of the animal or plant. While that is quite indicative of the animal, there are many internal changes that might have taken place not visible in the external shape of the animal. The fauna that crocodiles ate 200 million years ago might be different than what is available today; it is possible that internal adaptations were made because of change in diet which are not visible. Over the 200 million years, there could be many internal changes that are not visible.
Possible Confusion
It is possible that the creationist is seeing the ancient animal and thinking it not just like a modern animal, but that it actually is a modern animal, and this is showing a modern animal in a layer of rock that should only contain ancient animals. A modern animal appearing in old rock would be a way to disprove evolution. But there are many cases of animal species that are present across many million years and they fit into the evolutionary phylogenetic tree without any problem.
Summary
A gene pool is that: a pool of individuals with different but overall substantially similar genes. A species is a pool where all individuals can interbreed. It is environmental pressures that select the genes that survive. If the environmental pressure continue to select for the same body form, then the species will remain looking the same on the average. There is no reason that species could not remain the same for 400 million years or longer, if the environmental pressures or the ecological niches remains constant.