Final answer:
Horseshoe crabs are not useful as an index fossil due to their long existence with little evolutionary change, making them unsuitable for precisely dating geological strata. Index fossils typically represent organisms that lived for shorter, well-defined time periods.
Step-by-step explanation:
Horseshoe crabs would not be ideal as an index fossil because they have existed for about 200 million years with very little change. Index fossils are typically organisms that lived for a relatively short geological time frame. They are used by paleontologists to identify and date the strata in which they are found. Since horseshoe crabs have such a long temporal range, they cannot provide precise temporal markers for geologic formations.
Conversely, organisms like Trilobites, which lived for a more defined period of time before going extinct, are considered excellent index fossils. Similarly, the detailed fossil records of horses and whales demonstrate evolution through distinct transitional forms, indicating that for more concise geological timeframes, other species would serve as better index fossils than horseshoe crabs.