The right answer is hard body parts.
A significant change in the environment is a challenge for existing animals; those who until then were perfectly adapted to their environment soon see their existence become more difficult and, natural selection being at work, it is essential that their direct descendants "adapt" to the new conditions under penalty of being eliminated.
Strange animal lineages were identified during this period: some are found in current offspring but most have disappeared without trace: animals that did not give offspring were as well-armed as the others to survive and the others had "found" very ingenious adaptations.
