Chapter 9: - Fish and Land Animals

Summary

The Cambrian unleashed the forces of evolution, giving rise to new anatomical adaptations that still form parts of modern animal anatomy. The evolution of the mandible, hyoid cartilage, cricoid cartilage, and styloid ligament from the gill arches of a hagfish-like ancestor is one of the most consequential adaptations. In this time period we also see the evolution of the bony cranium of vertebrates, first seen in armored predators such as the large placoderm, Dunkleosteus. During the Devonian we see the evolution of vertebrate fish diversity and, importantly for land mammals, the appearance of the lobe-finned fishes and the first land animals with a full shoulder and pelvic girdle. Although this chapter provides a broad overview of animal evolution (from fish to amphibians to dinosaurs to mammals) over several hundred million years, the major message is that large adaptive radiations are often only possible in places where there is no competition, either because there is a new open adaptive space (as with amphibians) or because another previous group has suffered a major extinction. Life on earth, once fully established, is a zero-sum game, so for there to be winners there must be losers.