Final answer:
The most likely explanation for the absence of many fossils in newer rock layers, when compared to older layers, is that the species represented by those fossils went extinct. Fossil records, which display a progression of evolution and historical biodiversity patterns, strongly support this conclusion.
Step-by-step explanation:
The majority of fossils that appear in lower rock layers are not present in later rock layers because the species went extinct. This pattern is a fundamental concept in understanding the fossil record and the history of life on Earth. The ages of rock layers correspond to different periods of geological time, with the oldest layers at the bottom. These layers and the fossils within them help paleontologists to determine when organisms existed and how evolution progressed over time.
When a significant number of fossils found in one layer are absent in the subsequent layer without any evidence of geographic relocation or rock layer erosion, it is likely that a mass extinction event occurred. This conclusion is drawn from the abrupt disappearance of species which often characterizes mass extinctions found in the fossil record. Therefore, as new evidence shows the evolution of form and the losses in biodiversity, the most plausible explanation for the absence of many species in later rock layers is extinction.