Final answer:
The bottom layers of sedimentary rock contain the oldest fossils, representing early life forms, and sometimes indicating mass extinctions when there is a sudden change in the types of fossils found.
Step-by-step explanation:
The fossils found in the bottom layers of sedimentary rock are probably from organisms that lived a very long time ago. This is because in geology, the principle of superposition states that in an undeformed sequence of sedimentary rocks, each layer is older than the one above and younger than the one below. Consequently, fossils found in these lower layers represent some of the earliest life forms on Earth.
Paleontologists often identify a layer as correlating to a mass extinction based on the sudden absence of fossils that were previously abundant and the emergence of new species in the layers above. The presence of certain types of trace fossils, such as burrows called ichnofacies, can provide additional information about the environment at the time, which might suggest significant changes leading to or following an extinction event.
When analyzing sedimentary rock layers, especially those deposited prior to significant geological periods like the Permian, and finding fossils such as Paradoxides trilobites or Mucrospirifer brachiopods, paleontologists gather evidence of life from different eras. Known extinct species, like belemnites, which are related to modern squids but found only in the fossil record, also provide insights into the organisms that once existed on Earth.
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