Final answer:
The substantial disappearance of organisms from the fossil record often indicates periods of mass extinction, such as the event at the Cretaceous-Paleogene boundary, which caused an abrupt loss of biodiversity.
Step-by-step explanation:
A very substantial disappearance of organisms from the geologic fossil record provides evidence for periods of mass extinction. Mass extinctions are significant events in Earth's history and are characterized by the abrupt loss of a large number of species in a relatively short period of time. This is evidenced by the stark transitions between different geological periods, reflected in the changes observed in the fossil record. Often occurring at the boundary of these periods, these events can result in a dramatic loss of biodiversity. For instance, the mass extinction at the Cretaceous-Paleogene boundary led to a substantial reduction in the number and variety of organisms, such as a decrease in the shapes of pollen grains present, indicating a loss of diversity.
Among the causes of such extinctions are various global environmental changes, like climate shifts, meteor impacts, volcanic eruptions, or more speculative causes like gamma-ray bursts from a nearby supernova. An example of such an extinction is the end of the Permian period, which saw a catastrophic loss of around 95 percent of existing species, wiping out certain phyla like the trilobites. However, these mass extinctions also open up ecological niches for rapid speciation and evolution of new species, illustrating the dynamic and complex nature of life's history on Earth.