Ch 34: The Origin and Evolution of Vertebrates

• Derived Characters of chordates 

• 1. Notochord 

• 2. Dorsal, hollow nerve cord 

• 3. Pharyngeal slits or clefts 

• 4. Muscular, post-anal tail 

• Cephalochordata

• Lancelets (Amphioxus) are named for their bladelike shape 

• Urochordata 

• Tunicates most resemble chordates during their larval stage 

• Metamorphosis involves the resorption of the tail and notochord and 90º rotation of the remaining organs 

• Derived characters of vertebrates 

• Enclose the spinal cord have taken over the mechanical roles of the notochord 

• The neural crest – cells that appear along the edges of the closing neural tube of an embryo 

• A skeletal system and complex nervous system have allowed efficiency at capturing food and evading predators 

• Myxini and Petromyzontida 

• Fossil evidence shows that the earliest vertebrates lacked jaws 

• Jawless vertebrates that remain today: hagfishes and lampreys 

• Derived characters of Gnathostomes 

• “jaw mouth” → have jaws (hinged structures) with teeth

• Hypothesis: jaws evolved by modification of the skeletal rods that supported the pharyngeal (gill) slits 

• Enlarged forebrain associated with enhanced senses of smell and vision 

• The lateral line system, rows of organs sensitive to vibration that are located along each side of the body 

• The three surviving lineages of jaws vertebrates – chondrichthyans, ray-finned fishes (Actinopterygii), and lobe-fins (Sarcopterygii)

• Chondrichthyans

• (Sharks, Rays, and Their Relatives)

• skeleton composed primarily of cartilage

• Oviparous: Eggs hatch outside 

• Ovoviviparous: Eggs are retained; young hatch in the uterus

• Viviparous: The embryo develops within the uterus nourished through a yolk sac placenta

• Osteichthyes

• vertebrates we informally call fishes

• Actinopterygii and Sarcopterygii

• Actinopterygii 

• Ray-Finned Fishes

• Boney rays that support their fins 

• Tuna, lionfish, sea horse, moray eel 

• Sarcopterygii

• Lobe-Fins

• rod-shaped bones surrounded by a thick layer of muscle in their pelvic and pectoral fins

• Three lineages survive and include coelacanths (Actinistia), lungfishes (Dipnoi), and tetrapods

• Derived Characters of Tetrapods 

• Four limbs and feet with digits

• A neck, which allows separate movement of the head

• Fusion of the pelvic girdle to the backbone

• The absence of gills (except some aquatic species)

• Ears for detecting airborne sounds

• Amphibia 

• Salamanders, Frog, Caecilians

• Amphibian means “both ways of life,” referring to the metamorphosis of an aquatic larva into a terrestrial adult

• Derived Characters of Amniotes 

• the amniotic egg, which contains membranes that protect the embryo

• The amniotic egg was a key adaptation to life on land 

• A key derived character of diapsids is a pair of holes on each side of the skull behind the eye sockets through which muscles pass to attach to the jaw

• Lepidosaurs 

• A lineage of diapsids is represented by two species reptiles called tuataras

• Squamates – snakes, lizards

• Archosaurs 

• A lineage of diapsids are Turtles → lack holes in the skull behind the eye socket

• Crocodilians and birds 

• Birds 

• The major adaptation is wings with keratin feathers

• Weight-saving adaptations improve flight include lack of a urinary bladder, females with only one ovary, small gonads, and loss of teeth

• Early feathers might have evolved for insulation, camouflage, or courtship display

• Archaeopteryx- oldest bird

• Living Birds

• Living birds belong to the clade Neornithes

• The ratites are all flightless birds

• Derived Characters of Mammals 

• Mammary glands, which produce milk

• Hair and a fat layer under the skin for insulation 

• Kidneys, which conserve water from wastes

• Endothermy and a high metabolic rate

• Efficient respiratory and circulatory systems

• A large brain-to-body-size ratio

• Extensive parental care

• Differentiated teeth

• Early Evolution of Mammals

• Synapsids have a single hole behind the eye socket on each side of the skull for attachment of the jaw muscle

• Two bones that made up the jaw joint in early synapsids were incorporated into the mammalian middle ear

• Adaptive radiation after Dinosaurs

• Monotremata, Marsupials, Eutherians 

• Monotremes 

• A small group of egg-laying mammals consisting of echidnas and the platypus

• Females lack nipples and secrete milk from glands on their bellies; the baby sucks milk from the mother’s fur 

• Marsupials 

• Opossums, kangaroos, and koalas

• The embryo develops within the mother’s uterus and is nourished by the placenta 

• Born very early in its development

• completes its embryonic development while nursing in a maternal pouch called a marsupium

• Eutherians 

• Have a more complex placenta

• Primates (Eutherians)

• Lemurs, tarsiers, monkeys, and apes

• Grasping hands with flat nails 

• Large brain, short jaws

• Forward facing eyes 

• Opposable thumb (monkeys and apes)

• Derived Characters of Humans 

• Upright posture and bipedal locomotion

• Larger brains capable of language, symbolic thought, artistic expression, and the manufacture and use of complex tools

• Reduced jawbones and jaw muscles

• Shorter digestive tract

• Humans and chimpanzees differ in the expression of 19 regulatory genes

• The Earliest Hominins

• Extinct species that are more closely related to humans than to chimpanzees

• Homo sapiens 

• All living humans are descended from African ancestors

• In 2004, 18,000-year-old fossils were found in Indonesia, and a new small hominin was named: Homo floresiensis

• In 2015, a new member of the human family was discovered, Homo naledi