The initial vertebrate colonization of dry land was facilitated by the evolution of lunglike sacs and jointed limbs. The amniotes also evolved impermeable body coverings, efficient kidneys, and the amniote egg, which resists desiccation.
learning outcomes
You should be able to:
Explain how features of amphibians can make them especially vulnerable to environmental change.
Summarize the evidence showing that birds are indeed reptiles.
Describe how a mass extinction affected the diversification of mammals.
Trace the evolution of endothermy, hair, and feathers across the phylogeny of amniotes.
Describe how major changes in Earth’s geology and climate have affected the evolution of tetrapods.
Amphibians have survived and prospered for millions of years, but today many species are disappearing and populations of others are in serious decline. What features of their life histories might make amphibians especially vulnerable to the kinds of environmental changes now happening on Earth?
Amphibians exchange gases and fluids through their permeable skin, which makes them highly vulnerable to many environmental toxins and some pathogens (especially an introduced chytrid fungus). Many species of amphibians have a biphasic life cycle, so they are vulnerable to habitat degradation and loss of both aquatic and terrestrial environments. Most amphibians do not move long distances, so they do not easily move into new habitats when their local environment is destroyed. For these reasons, they are also sensitive to rapid climate changes. Many species of amphibians have highly specialized habitat requirements and live in very restrictive ranges. Habitat loss or change within these restricted ranges often results in extinction.
How has the diversification of mammals been influenced by mass extinction events and continental drift?
The major mammal lineages rapidly diversified after the mass extinction event at the Cretaceous–Tertiary boundary. The extinction of many large lineages of dinosaurs may have opened up opportunities for mammalian diversification. By this time, the major continents had separated by continental drift, so there were independent radiations of mammals in Laurasia, Africa, South America, Europe, and southeastern Asia.
In the not-
Fossil remains of extinct theropod dinosaurs show that many features once thought to be restricted to birds, such as feathers, actually evolved much earlier among the theropods. Other typical “bird” morphological features, such as air-
Consider the phylogeny of the amniotes and the evolution of endothermy, hair, and feathers. Which of these traits likely evolved convergently in more than one lineage? What is a likely functional relationship among these traits? Why do many paleontologists think that several extinct theropods were endothermic?
Hair evolved in the ancestor of mammals; feathers evolved among theropod dinosaurs (seen today among the birds). Among the living tetrapods, birds and mammals are endothermic. Hair and feathers provide body insulation for mammals and birds, respectively. Without these forms of insulation, the maintenance of metabolic body heat would be difficult. Fossil evidence shows that many extinct theropod dinosaurs also had feathers, so many paleobiologists predict that they were endothermic as well. Endothermy would also be expected in large, active predators—
The biology of one eutherian group—