Final answer:
The stability of crocodile and shark forms over millions of years doesn't mean they are not undergoing natural selection, but rather their adaptations continue to be successful in stable environments. Fossils demonstrate evolution by providing evidence of extinct species and suggest species share common ancestors. Arguments against the extinction of fossilized species are countered by the extensive fossil record and scientific research.
Step-by-step explanation:
The fact that crocodiles and sharks have remained relatively unchanged for millions of years does not necessarily mean that they are no longer subject to natural selection.
Organisms that are well-adapted to stable environments may not show significant change over long periods, implying their existing traits continue to be successful. Fossils do, however, provide robust evidence of evolution, showcasing a myriad of organisms that lived long ago and have since gone extinct.
Fossils of creatures like the Archaeopteryx support the concept that modern species share a common ancestor, as it exhibits characteristics of both reptiles and birds, suggesting a transitional form in the evolution of birds. Furthermore, when someone argues that all extinct fossils may still have living counterparts, it's important to consider the extensive scope of paleontological research which strongly indicates many of these species no longer exist.
In essence, while some species like crocodiles and sharks have shown little morphological change and are often referred to as 'living fossils', it does not negate the evidence of evolution nor the continuous process of natural selection.