Short answers, pulled from the story.
Georges Cuvier made this declaration in 1796 when he stood before a collection of fossilized elephant bones from Siberia. He concluded that the species had died out in catastrophic events, introducing the idea of deep time to the scientific community.
The Greek philosopher Xenophanes observed fossilized shells in the mountains in the 6th century BCE and deduced they were remains of life that had once lived in the sea. This observation suggested a history of environmental change despite the prevailing view championed by Aristotle.
Charles Darwin published On the Origin of Species in 1859 to shift the focus from mere classification to the understanding of evolutionary change. This publication validated the idea that gaps in the fossil record were the result of incomplete preservation.
Mary Anning and her family discovered marine reptiles in the Lyme Regis region of England during the Victorian era. These discoveries included Ichthyosaurus and Plesiosaurus which were geologically older than the mammals studied by Cuvier.
Paleontology underwent a profound transformation in the 1950s and 1960s to shift from a field focused primarily on stratigraphy to one that embraced theoretical analysis. This period saw the rise of paleobiology which integrated concepts from biology, ecology, and geology to study the life of the past.