Final answer:
At least 85% of the families of shelled marine invertebrates became extinct at the end of the Cretaceous period during the Cretaceous-Paleogene extinction event.
Step-by-step explanation:
At the end of the Cretaceous period, a significant mass extinction event occurred, commonly referred to as the Cretaceous-Paleogene (K-Pg) extinction event. This event was characterized by the rapid decline and disappearance of a large fraction of Earth's species, including various shelled marine invertebrates. According to studies and estimates based on the fossil record, at least 85% of the families of shelled marine invertebrates became extinct during this time. When we examine such events, we can see their profound impact on biodiversity.
For instance, the end-Permian extinction, which is recognized as the largest extinction event in Earth's history, resulted in the loss of approximately 96% of all marine species and 70% of terrestrial species. The extinction at the K-Pg boundary also had a transformative effect on life on Earth, leading to the decline of the dinosaurs and the subsequent rise of mammals. These periods of mass extinctions underscore that neither terrestrial nor marine biotas are infinitely resilient to environmental stresses and have historically resulted in dramatic collapses in diversity.