Chapter 19: The Evolution of Vertebrate Diversity
19.1 Derived characters define the major clades of chordates
 Unlike tunicates, all chordates have a brain, even if it’s a small one. The head holds the brain, which is at the
anterior end of the dorsal nerve cord. Eyes, sensory organs, and a skull will also be found at the head end.
Craniates (“cranium” meaning skull) are all chordates having a head.
 Vertebrates are distinguished by a more extensive skull and back bone, called vertebral column. These skeletal
elements enclose the main parts of the nervous system. The vertebral column is composed of a series of bones,
vertebrae.
 The origin of jaws opened up new feeding opportunities. The evolution of lungs, or lung-like structures gave the
possibility of life on land. Tetrapods (Greek for four feet) – jawed vertebrates with two pairs of limbs were the
first vertebrates on land, and could support their weight on land.
 The evolution of amniotes – animals, who produce terrestrially adapted eggs, enhanced reproduction on land.
19.2 Hagfish and Lampreys lack hinged jaws
 The most primitive craniates are the hagfish and lampreys. Both maintain the notochord, but in lampreys, other
rudimentary vertebral structures are present.
 Present day hagfish (40 species) scavenge on the seafloor. While nearly blind, they have an excellent sense of
smell. They feed by grasping and tearing flesh with specially designed mouthparts. When threatened they
secrete a slime onto the surface of the body, making them hard to catch and grasp.
 Lampreys represent the oldest lineage of vertebrates. Living in freshwater streams, the larvae are suspension
feeders. As they mature into adults, most lampreys migrate to the sea or lakes. As parasites, they feed by
clamping onto the side of fish, use their tongue to penetrate the skin and suck the victim’s blood.
19.3 Jawed vertebrates with gills and paired fins include sharks, ray finned fishes, and lobe-fins
Three lineages of jawed animals with gills and paired fins are commonly called fish.
 Chondrichthyans have a flexible skeleton made of cartilage. This includes the sharks, rays, and skates. Most
have a lateral line system, a row of sensory organs that are sensitive to changes in water pressure and can detect
minor vibrations caused by animals swimming nearby.
o Sharks are mostly adept predators with stream lined bodies and powerful jaws. They have sharp vision
and a keen sense of smell. On the head are electrosensors that detect small electrical fields made by the
muscles of swimming animals.
 Ray finned fish have a skeleton reinforced with a hard matrix of calcium phosphate. Fins are supported by
flexible skeletal rays. Most have flattened scales covering the skin and secrete a layer of mucus to reduce drag
while swimming. A protective flap covering, the operculum, covers a chamber housing the gills. Movement of
the operculum allows fish to breathe without swimming. They also have a swim bladder to keep them buoyant.
o Examples include tuna, trout, and goldfish.
 Lobe fins have a series of rod-shaped bones in their pectoral and pelvic fins. Originally, these lobes were used
to “walk” underwater. Some lineages of lobe fins survive.
o Coelacanth – deep sea dweller
o Lungfish – inhabit stagnant water and gulp air into the lungs
19.4 New fossil discoveries are filling the gaps of tetrapod evolution
19.5 Amphibians are tetrapods – vertebrates with two pairs of limbs
 Amphibians include salamanders, frogs, and caecilians.
o Some present day salamanders are totally aquatic, but many have adapted to life on land.
o Frogs are more specialized for life on land, with strong powerful legs designed to hop.
o Caecilians are nearly blind and are legless. Most burrow into moist soils of tropical areas.
 “Amphibios” in Greek means “living a double life.” Most amphibians are found in damp habitats that allow
their skin to stay moist and have gas exchange. Not all amphibians live a double life; some are strictly terrestrial
while others are strictly aquatic. Many have poison glands that play a role in defense.
o Frogs spend most of their time on land, but their eggs are laid in water. The larval stage is the legless
tadpole that has gills and feeds on algae.
19.6 Reptiles are amniotes – tetrapods with a terrestrially adapted egg
 Reptiles are amniotes, with amniotic eggs being a major trait of the group. Inside the egg, the embryo develops
within a protective sac called an amnion. The group includes lizards, snakes, turtles, and crocodilians (and in
some classification schemes, birds). Crocodiles and alligators are the largest living reptiles, but some turtles
may be heavier. Crocodilians spend most of their time in the water.
 Reptiles have many adaptations for terrestrial life
o Reptile skin is covered with scales and waterproofed with a protein called keratin
o Reptiles do not breathe through their skin but use lungs to breathe. Lungs are surrounded by a rib cage
to aid ventilation.
 Reptiles are ectothermic (“cold-blooded”) meaning that they get their heat externally rather than generating it.
19.7 Birds are feathered reptiles with adaptations for flight
 Some fossil evidence suggests that birds evolved from reptiles. Several fossils of theropod reptiles have been
unearthed, showing that some of these two legged reptiles had feathers. Early feathers were not believed to be
used in flight, but rather served to insulate the body or were used in courtship.
 Bird bodies are designed for flight and have special adaptations.
o Weight reduction – no teeth, tail is supported by a few hollow vertebrae, feathers have a hollow shaft,
and bones have a honeycomb structure
o Feathers have shapes that provide lift and maneuverability
o Large muscles in breast region for flight, which are anchored to a breastbone
o High metabolism and endothermy to provide energy for flight.
o Highly efficient lungs designed to extract more air
o Excellent vision and large brains for quick responses
19.8 Mammals are amniotes that have hair and produce milk
 Hair and mammary glands that produce milk are the two main characteristics of mammals. Hair is insulation to
maintain body temperature.
 Mammals are endothermic. Efficient respiratory and circulatory systems support a high metabolism. A sheet of
muscle, the diaphragm, helps ventilate the lungs.
 Teeth are adapted specifically for the types of food eaten.
 Mammals generally have a larger brain than other animals and provide a relatively long period of parental care
to offspring.
 Three types of mammals:
 Monotremes – mammals that lay eggs but still feed the offspring milk. Two species exist: the duckbilled platypus and the echidna.
 Marsupials – have brief gestation and give birth to tiny offspring that complete development within a
pouch where they receive nutrition by nursing. Includes kangaroos, koalas, and opossums.
 Eutherians (commonly referred to as placentals) bear fully developed young. Have a complex placenta,
structure that allows nutrients from the mother’s blood to diffuse into the embryo’s blood. Young
complete their development within the mother’s uterus attached to the placenta.
18.23 Humans threaten animal diversity by introducing non-native species