Animals include both vertebrates like mammals, fish, amphibians, reptiles, and birds, and invertebrates like insects, mollusks, crustaceans, jellyfish, and worms. Civilization drove a mass extinction that wiped out over half of all species that existed before, and drastic changes to the climate have driven enormous changes among the survivors. Bacteria and fungi have changed even more quickly, driving even more drastic changes in animal behavior, like bacteria that can metabolize plastic growing among the gut flora of ruminants, driving them to eat plastic waste.
The oceans absorb carbon dioxide from the atmosphere, so as the levels of carbon dioxide increased, the oceans absorbed more of it and became more acidic. This affected shellfish in particular, as they could no longer form shells in such an acidic environment. Krill, which formed such a foundation to the food web in the oceans, went extinct, and with them much of the oceanic ecosystem completely collapsed.
Cephalopods, on the other hand, thrived under the new conditions. While fish became increasingly rare, octopi, squids, and nautili proliferated.
Changes in Body Size
Bergmann's rule predicted that in a hotter world animals would become smaller to better dissipate heat. This certainly has held true among most endothermic (“warm-blooded”) animals, which have become smaller. Ectothermic (“cold-blooded”) animals have often shown a tendency towards gigantothermy, meaning that they could better control their body temperature by becoming larger rather than smaller. More carbon dioxide in the atmosphere has allowed for a greater biomass of plants, which have in turn put more oxygen into the atmosphere, making it easier for many of these animals to oxygenate their blood — which has allowed them to grow quite large, just as it once allowed prehistoric insects and reptiles to grow larger.
As a result, most birds and mammals (including humans) have grown noticeably smaller, while insects and lizards have grown larger.