Final answer:
Index fossils help identify geological periods and their main characteristics include quick evolution, wide distribution, short existence in geologic time, and abundance in the fossil record. They are central for dating rock layers and understanding the paleoenvironment. Contrary to index fossils, other organisms have traits that are not as easy to identify or categorize for geological dating.
Step-by-step explanation:
Index fossils are specialized fossils used to identify geologic periods and are characterized by four main features: they evolved rapidly, lived over a wide geographic area, existed for a relatively short geologic timeframe, and are abundant in the fossil record. These organisms, such as Belemnites from the Mesozoic era, are used to date and correlate the age of the rock layers in which they are found. Index fossils help paleontologists and geologists understand the paleoenvironment and evolution of life on Earth.
Despite the robust nature of index fossils, many organisms from the past have delicate skeletons and significantly different body plans from modern animals, which can sometimes make identification and interpretation of living habits challenging. While bryozoans and echinoids are widespread throughout the fossil record, making them less useful as index fossils, certain fossils like rugosa and brachiopods have more limited age ranges, rendering them more valuable for determining the age of rock strata.
The study of fossils not only helps determine the age of different rock layers but also provides insights into the evolutionary history—showing how organisms have adapted to their changing environments, indicative through the fossil record.