Chapter Summary

• Deuterostomes vary greatly in adult form, but based on the distinctive patterns of early development they share and on phylogenetic analyses of their gene sequences, they are judged to be monophyletic.

• There are far fewer species of deuterostomes than of protostomes, but many deuterostomes are large and ecologically important.

• The deuterostomes comprise three major clades: the echinoderms, hemichordates, and chordates. Review Figure 32.1, Activity 32.1, Animation 32.1

• Echinoderms and hemichordates, together called ambulacrarians, have bilaterally symmetrical, ciliated larvae. Adult echinoderms have pentaradial symmetry and an oral–aboral body orientation. Review Figure 32.3

• The xenoturbellids and acoels are reduced, soft-bodied wormlike marine animals with few distinct organ systems. Their relationships are uncertain, but recent analyses suggest that they may be the sister group of the ambulacrarians.

• Echinoderms have an internal skeleton of calcified plates and a unique water vascular system connected to extensions called tube feet. Review Figure 32.3

• Hemichordates are bilaterally symmetrical and have a three-part body divided into a proboscis, collar, and trunk. Review Figure 32.6

• Chordates fall into three principal clades: lancelets, tunicates, and vertebrates.

• At some stage in their development, all chordates have a dorsal hollow nerve cord, a post-anal tail, and a notochord. Lancelets have all three key chordate features as adults. Tunicates have these features as larvae but most species lose them as adults. Review Figure 32.7

• The vertebrate body is characterized by an internal skeleton, which is supported by a vertebral column that usually replaces the notochord. It is also characterized by internal organs suspended in a coelom, a ventral heart, and an anterior skull enclosing a large brain. Review Figure 32.9

• From estuarine ancestors, vertebrates diversified into many lineages of marine and freshwater fishes. One of these lineages, the lobe-limbed vertebrates, later radiated into terrestrial environments. Review Figure 32.10

• In the gnathostomes, jaws evolved from gill arches. Jaws enabled these vertebrates to grasp large prey and, together with teeth, allowed them to cut food into small pieces. Review Figure 32.12

• Chondrichthyans have skeletons of cartilage; almost all species are marine. The skeletons of ray-finned fishes are made of bone; these fishes have colonized all aquatic environments.

• Lungs and jointed appendages enabled one lineage of lobe-limbed vertebrates to colonize the land. This lineage gave rise to the tetrapods. Review Figure 32.16

• The earliest split in the tetrapod tree is between the amphibians and the amniotes (reptiles and mammals).

• Most modern amphibians are confined to moist environments because their bodies and their eggs lose water rapidly. Review Figure 32.17, Animation 32.2

• An impermeable skin, efficient kidneys, and an amniote egg that could resist desiccation evolved in the amniotes. Review Focus: Key Figure 32.19, Activity 32.2, Investigating Life: Have Shelled Eggs Re-Evolved among Viviparous Reptile Lineages?

• The major living reptile groups are the lepidosaurs (tuataras, along with the squamates, which include lizards, snakes, and amphisbaenians), the turtles, and the archosaurs (crocodilians and birds). Review Figure 32.20

• Birds evolved from a group of active, predatory dinosaurs known as theropods. Feathers arose among the theropods, originally for insulation and to enhance coloration, but eventually developed into adaptations for flight in birds. Review Figures 32.23, 32.24

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• Mammals are unique among animals in supplying their young with a nutritive fluid (milk) secreted by mammary glands. There are two primary mammalian clades: the prototherians (of which there are only five species) and the species-rich therians. The therian clade is further subdivided into the marsupials and the eutherians. Review Table 32.1

• Mammalian phylogeny is strongly associated with the breakup of the major continents during the Mesozoic. Major lineages of eutherians diversified in Laurasia, Africa, and South America. Review Figure 32.28

• Grasping limbs with opposable digits distinguish primates from other mammals. The wet-nosed primate clade includes the lemurs, lorises, and galagos; the dry-nosed primate clade includes tarsiers, monkeys, and apes. Review Figure 32.30

• The ancestors of hominins were terrestrial apes that developed efficient bipedal locomotion. Review Figure 32.34

• In the lineage leading to Homo, brains became larger as jaws became smaller; the two events appear to be developmentally linked and are an example of evolution via neoteny. Review Figure 32.35