A History of Vertebrate Success: Fish, Dinosaurs, Horses, Whales, and Bats

Biologists worry that the fossil record is woefully incomplete in representing biological diversity, ecology, and physiology of the past on the scale of life around us. Paleontologists, at the same time, are overwhelmed by the incredibly rich record of life represented by fossils. The fossil record is important because the history of the earth and life are so long that much has changed in the course of time. Fossils enable the history of earth and life to be studied together, providing independent environmental and temporal evidence for understanding life on one hand, and at the same time providing key indices for global correlation and environmental interpretation of the earth. Paleontologists generally choose selectively the best groups to study while focusing on a limited set of important questions: history of diversity through time, phylogenetic relationships of major groups, evolutionary history in relation to process (‘microevolution'), and evolutionary history in relation to innovation (‘macroevolution').

The vertebrate record as a whole shows successive waves of diversification in relation to evolutionary innovation. Diversification of fishes started early in the Paleozoic, with later innovation enabling tetrapods to cross the frontier from aquatic to terrestrial adaptive zones. Dinosaurs and aquatic reptiles dominated Mesozoic lands and seas, and birds colonized the air. Dinosaurs as dinosaurs became extinct at the end of the Mesozoic, but birds and mammals evolved rapidly to become modern orders in the Cenozoic, typified by horses with a dense fossil record from virtually the beginning of the Eocene. Whales crossed the aquatic-terrestrial frontier a second time, this time backward, and bats crossed the frontier from land to air. Both are represented by Eocene skeletons intermediate in age and form that record the transition, Vertebrate success in the past involved innovation and diversification, and vertebrate fossils today illuminate important questions in the history of life.