Phylum Chordata - Chordates Internal skeleton with muscle attachments on outer surface Allows for greater range of movement and development of large body size Segmented body plan All chordates have (in embryo or adult) notochord dorsal nerve tube post-anal tail pharyngeal gill slits Hollow dorsal nerve tube - just below dorsal surface differentiates into brain and spinal cord in vertebrates Flexible dorsal notochord - located just below nerve cord replaced by vertebral column in vertebrate development persists in some primitive chordates - provides support Pharyngeal gill slits - a clue to the aquatic ancestry of all chordates pharynx connects mouth cavity and esophagus to outside in terrestrial vertebrate embryos slits often form pharyngeal pouches, become eustachian tubes in some vertebrates A Post-anal tail extends beyond the anus present in embryo if not in adult form Other animals with tails have a terminal anus
Phylum Chordata has three Subphyla Subphylum Urochordata - tunicates or sea squirts Subphylum Cephalochordata - lancelets Subphylum Vertebrata - fishes, amphibians, reptiles, birds, mammals Subphylum Urochordata - Tunicates or sea-squirts Most are sessile as adults, may be colonial Possess notochord and nerve cord only in larval stage Adults secrete cellulose tunic around body Adults are filter feeders lack a body cavity and segmentation Create water currents with cilia - water drawn into pharynx food particles trapped in mucus produced by endostyle Many have symbiotic photosynthetic bacteria
Most chordate characteristics are only seen in tunicates in the larval stage Larvae are free-swimming -- after finding a suitable location they settle head-first and transform into the adult form Subphylum Cephalochordata - lancelets Scaleless, fish-like marine organisms No obvious head, eyes, nose or ears Notochord runs entire length of body and persists in adults Bury in substrate, expose anterior end only Segmentation of muscles readily visible Have many more pharyngeal gill slits than fishes Filter-feed through the action of cilia in mouth and pharynx - trap food in pharynx Swim with fish-like undulations of the body
The defining chordate characteristics are preserved in the adult lancelet Subphylum Vertebrata - Vertebrates All vertebrates have a distinct head, with skull enclosing a brain Name derived from bony vertebral segments in spinal column Vertebral column - protects dorsal nerve cord notochord surrounded and replaced by vertebral column Possess a cranium - brain case Group also called craniate chordates Most have a bony skeleton, some use only cartilage All have a closed circulatory system
The vertebral centra replace the notochord Neural spines develop to surround and protect the neural tube behind the head All vertebrates also have a unique set of cells that originate above the neural tube during development - neural crest cells Neural crest cells migrate throughout vertebrate body and contribute to the development of many organs Brief overview of vertebrate evolution First vertebrates were marine, lacking jaws or paired fins (550 mya) Jawed fishes then became dominant creatures in the sea (450 mya) Amphibian ancestors first to invade the land (400 mya) Largely replaced by reptiles more suited to live on land (300 mya) Dinosaurs and other reptiles ruled the earth for 150 million years their extinction (65 mya) allowed mammals to become dominant Most living vertebrate groups have diversified greatly since their origin.
The vertebrates are composed of eight classes Four classes are aquatic fishes Class Myxini - Hagfish Class Cephalaspidomorphi - Lampreys Class Chondrichthyes - Sharks, skates and rays Class Osteichthyes - Bony fishes Four classes are terrestrial tetrapods Class Amphibia - Salamanders, frogs and toads Class Reptilia - Reptiles Class Aves - Birds Class Mammalia - Mammals Characteristics of Fishes Gills - extract dissolved oxygen from water around them water passes over filaments rich in blood vessels blood moves opposite the flow of water - an efficient system Vertebral column Internal skeleton with spine surrounding dorsal nerve cord Skeleton may be made of cartilage or bone Brain fully encased in protective skull
Single-loop blood circulation Blood pumped from heart to gills Oxygenated blood from gills passes to rest of body Heart is series of four chambers that contract in sequence Nutritional deficiencies All vertebrates are unable to synthesize aromatic amino acids All must consume these amino acids in their diet Overview of Fish Evolution First fishes known were jawless with heavy bony armor Internal skeleton was made of cartilage Survived by lampreys and hagfish ( Agnathans - no jaws) First jawed fishes occurred 410 million years ago Early jawed fishes were replaced by more agile sharks and bony fishes
Jaws evolved from modified gill arches the bony area between gill slits Evolved as a means to close mouth, and then to capture prey First gill arch behind jaw became supporting strut Joined rear of lower jaw to rear of skull Allowed mouth to open very wide, an efficient weapon Jawless fishes today Hagfishes are scavengers in cool waters - especially in deep water Lampreys have two life-styles large parasites of other vertebrates small filter-feeders in freshwater streams
Sharks became dominant predators about 300 mya Class Chondrichthyes - sharks, skates, rays Skeleton is made of cartilage Have bony teeth and scales with enamel Have always been large agile predators enlarged pectoral fins allow greater lift and maneuvering during swimming Have internal fertilization young are well-developed when born skates and rays have flattened bodies and are more sedentary Class Osteichthyes - bony fishes Evolved at same time as sharks, but in freshwater Developed heavy skeleton made completely of bone Ossification replaces cartilage with bone External plates and scales also made of bone Added swim bladder for respiration/buoyancy Primitive fishes use swim bladder for respiration - a lung Modern fishes use swim bladder for buoyancy
Modern bony fishes have highly mobile fins, thin scales and symmetrical tails They are an extremely successful group and are dominant in freshwater and the sea Two lineages of bony fishes evolved Ray-finned fishes - most modern fish Internal skeleton of bony rays supports and stiffens each fin No muscles within fins Swim bladder used for buoyancy Lobe-finned fishes - 6 living species 5 lungfishes, and the coelacanth have fins with muscular lobes swim bladder used for respiration early form gave rise to amphibians Important characteristics of bony fishes Lateral line system Series of small sensory organs that project into a canal beneath skin surface - sensitive to water pressure changes allows fish to sense movements in water Similar structures found in all vertebrate ears - for hearing Gill cover Hard plate covering gills called the operculum Helps to pump water over gills even when stationary
Class Amphibia - Amphibians includes frogs, salamanders, caecilians Amphibians were the first tetrapods the first terrestrial vertebrates There are three orders of Amphibians Order Anura - frogs and toads Order Urodela - salamanders Order Apoda - caecilians Characteristics of Amphibians Legs - a key adaptation for terrestrial living Frog and salamanders have two pairs, lost in caecilians Lungs - a modified swim bladder used for terrestrial respiration swallow air by moving floor of mouth Cutaneous respiration - gas exchange at skin supplement lung respiration with a highly vascularized skin Moist skin provides extensive surface area, limits body size
Pulmonary veins Veins return blood from lungs to heart Aerated blood leaves heart at greater pressure than it leaves lungs Partially divided heart partially separated pulmonary and systemic pathways imperfect since there is a single ventricle where oxygenated and deoxygenated blood can mix Origin of Amphibians evolved from lobe-finned fishes - skeletal structure of lobe-finned fishes and early amphibians is very similar Earliest amphibian fossil, Ichthyostega built for terrestrial life with lungs and a strong legs and backbone Most amphibians still must lay their eggs in water to keep them from drying out Because of moist skin, adults are also liable to become dehydrated
Modern Amphibians Order Anura - frogs and toads - tailless amphibians Frogs have smooth, moist skin; long legs; live in or near water Toads have bumpy, dry skin; short legs; are adapted to dry environments All are carnivores, eat a wide variety of invertebrates Most return to water to reproduce Eggs lack water-tight membranes and dry out readily Eggs fertilized externally Eggs hatch into algae-eating, swimming larval tadpoles Tadpoles metamorphose into adults Order Urodela - salamanders - amphibians with tails Have elongated bodies, long tails and smooth moist skin Live in moist places, some live entirely in water Lay eggs in water or moist areas Fertilization is external in most species Juveniles look like adults, do not undergo major metamorphosis Order Apoda - caecilians - legless amphibians Highly specialized group of burrowing amphibians Lack legs, have small eyes, are often blind Eat worms and soil invertebrates Male deposits sperm directly into female young are born alive
Class Reptilia - Reptiles includes turtles, lizards, snakes, alligators Reptile improvements on the amphibian plan Leg design allows better body support Lungs and heart are more efficient Dry skin covered with scales Water-tight eggs Key Characteristics of Reptiles Amniotic egg - Water-tight eggs contain food source (yolk) and four membranes: Chorion, amnion, yolk sac and allantois Each important an embryonic life-support system Chorion allows oxygen to enter, but retains water Amnion encases developing embryo within fluid-filled cavity Yolk sac holds food for embryo - extracted by blood vessels Allantois encloses waste products
Dry skin with scales or armor reduces water loss Scales develop as surface cells fill with protein keratin Keratin is also used for mammal hair and bird feathers Thoracic breathing - increase volume of air inspired Expand and contract rib cage to suck air in and force it out Capacity limited only by volume of lungs Improved leg design - legs placed more ventrally under trunk allows better support of the body and greater mobility allows larger body size Reptile Evolution - Reptiles were the dominant land animals for 250 my Four major lineages evolved in series: Pelycosaurs, Therapsids, Thecodonts, Dinosaurs Pelycosaurs - improved predators synapsid skull with temporal openings allowed better muscle attachment and more powerful bite Dominant terrestrial vertebrates for 50 million years Replaced by Therapsids
Therapsids - Mammal-like reptiles had higher food intake, and probably had higher metabolic rate speculated that these were the first endothermic ( warmblooded ) vertebrates - likely evolved in cooler climate dominant terrestrial vertebrates for 20 million years Therapsids were the ancestors of mammals Replaced by Thecodont reptiles - a different lineage They may have had hair, like their mammal descendants Thecodonts - greater energy efficiency Were cold-blooded ectotherms like amphibians and early reptiles environmental change resulted in warmer temperatures endothermy requires less energy intake for body maintenance Had an improved skull - diapsid - Diapsid reptiles, had two pairs of temporal holes in skull First land vertebrates to walk on hind limbs only Dominant for 15 my Replaced by their direct descendants, Dinosaurs
Dinosaurs - greater agility Legs positioned directly underneath body - allowed larger body Enabled running with speed and agility Dominated land for 150 million years Went extinct 65 million years ago - likely due to asteroid impact Overview of the evolution of amniotes - reptiles, birds, mammals, and their ancestors
Modern Reptiles - Four surviving orders Turtles - ancient survivors Have changed little since before time of dinosaurs Have solid skulls (no temporal openings) like first reptiles Have changed little since before time of dinosaurs Lizards and snakes - most modern reptiles are in this group Also evolved very early - 250 mya Diversified after disappearance of dinosaurs Tuataras - a remnant of a group that was once common Only two species survive on islands near New Zealand Small, sluggish, diapsid reptiles, Common in Jurassic, declined in Cretaceous Unable to compete with lizards, in most environments Crocodiles - the most derived reptiles Descended from same line that produced dinosaurs Little changed in the last 200 million years but with advanced characteristics they share with birds parental care, four-chambered heart, vocalizations for mate attraction Crocodiles and birds more closely related to dinosaurs and each other than they are related to lizards and snakes Crocodiles, Thecodonts, Dinosaurs, and Birds form a monophyletic group - the Archosaurs
Other Important Characteristics of Reptiles Internal fertilization - a requirement with a shelled egg Improved circulatory system - Circulatory system provides more oxygen to body Septum in heart extended from atrium partway into ventricle Decreases mixing of oxygenated and deoxygenated Septum totally divides ventricle in crocodiles and birds (a fourchambered heart) All living reptiles are ectothermic, obtain heat from external sources - regulate body temperature through behaviors - basking or seeking shade Order Chelonia - Turtles and tortoises Turtles generally live in water, tortoises live on land Bodies are encased in a protective shell Carapace covers dorsal surface Plastron covers ventral portion Vertebrae and ribs fused with carapace Lack teeth, but have sharp beak Order Rhynchocephalia - Tuataras Two species makes up entire order found only on islands of New Zealand Has spiny crest running down back Has inconspicuous parietal or third eye on top of head has lens, retina and is connected to brain May function as a thermostat, protect it from overheating
Order Squamata - Lizards and snakes Males have paired copulatory organs lower jaw not joined directly to skull allows flexibility in jaw movements most are predatory carnivores Lizards have limbs, snakes do not Snakes lack movable eyelids and external ears - can t hear Lizards are more ancient group Lizards rely on agility and speed to catch prey and avoid predators Most snakes depend on stealth Many lizards can loose tail to escape predator and regenerate new one Order Crocodilia - Crocodiles and Alligators Primitive-looking reptiles also includes caimans and gavials Similar to 200 million year old fossil forms Live in or near water in tropical or subtropical regions Are aggressive carnivores, bodies adapted for hunting by stealth Eyes and nostrils on top of head, lie submerged in water Enormous mouths with sharp teeth and strong neck Can feed underwater, valve prevents water from entering air passage The only living reptiles that care for their young
Class Aves - Birds Most successful of all terrestrial vertebrates great diversity, 8800 species Key Characteristics of Birds Produce amniotic eggs - like reptiles Have keratin scales on feet and legs - like reptiles Lack teeth - developmental mutants have reptile-like teeth Have very small tail - unlike reptiles Feathers Modified reptilian scales Provide lift for flight and insulate Develop from follicles Has shaft, with barbs, with barbules, with interlocking hooks Flexible, light-weight, and strong Can be replaced, like scales Flight skeleton Bird bones are thin, hollow, and light-weight Many have internal struts to strengthen the bone with little added weight Many bones are fused to each other - reduces number Have fused collarbones (wishbone) for recoil of wings during flight Have keeled breastbone - for attachment of flight muscles
The Evolution of Birds Archaeopteryx, the first bird - fossils date to 150 mya Shares features with small therapod dinosaurs Skull has teeth Very few bones are fused to each other Bones are solid (bird bones are hollow) Has long reptilian tail Has no breastbone Considered a bird because of feathers on wings and tail Modern Birds Beak and feet characterize many bird habits Have unique physiology to cope with high energy demands of flight Efficient respiration - flow-through lungs Gas flows through lungs in only one direction and opposite direction of blood flow - an efficient system No dead-air in bird lungs
Efficient circulation Has four-chambered heart - like mammals but independently evolved Wall dividing ventricle is complete, two circulations do not mix Flight muscles get fully oxygenated blood Endothermy Birds maintain higher body temperatures than most mammals Metabolism in flight muscles proceeds at faster rate Feathers provide insulation to conserve heat Class Mammalia - Mammals The least diverse group of vertebrates - 4100 species Almost all large land vertebrates are mammals Most mammals are not large 3200 species are rodents, bats, shrews, moles Greatest diversity was reached about 15 mya numbers are declining due to relatively cool climate
Key Mammalian Characteristics Hair Helps regulate body temperature, allows life in colder climates Not derived from reptilian scales or feathers Each hair extends like stiff thread from bulb-like hair follicle Composed of dead cells filled with fibrous keratin protein Insulates against heat loss Provides camouflage Whiskers function as sensory structures Serve as defensive weapons as in porcupines and hedgehogs Milk-producing glands All mammals have mammary glands that produce milk Mammary glands are modified sweat glands Milk is rich in fat, sugar, protein with 95% water High calorie food needed to support rapid growth of newborn Evolution of Mammals Arose from Therapsids in Triassic, 220 mya First mammals were small insectivores Had large eyes - they may have been nocturnal Mammal jaw reduced to massive bone with a single joint Allows strong bite Two jaw bones moved to middle ear to make three bone chain that improved hearing - malleus, incus, stapes Were a minor group until the Cretaceous (65 mya) Two groups - Prototheria and Theria Prototheria - egg-laying mammals close relatives of therapsids Small, resembled modern shrews Theria - live-bearing mammals Subdivided into marsupials (pouched mammals) and placental mammals
Characteristics of Modern Mammals Endothermy Allows activity in cooler weather and times of day Allows colonization of harsh environments, deserts to arctic Hair provides insulation to support endothermy Higher metabolic rate required as well Efficient circulation provided by four chambered heart Improved respiration Diaphragm breathing muscle allows greater volume of air to be inspired Placenta Most mammals are placental and viviparous (live bearing) Blood stream of mother and fetus in close contact at placenta Food, water, oxygen pass from mother to child Wastes pass from child to mother, carried away
Teeth Reptiles have homodont dentition - all teeth are the same Mammal dentition is heterodont - variation in teeth in mouth Highly specialized to match food eaten Mammals as a group eat a wide variety of foods but individual species often have highly specialized feeding Digestive systems for eating plants Most mammals are herbivores with help of symbiotic bacteria cellulose is major food Some mammals have four-chambered stomachs first chamber is large, holds most cellulose-digesting bacteria Partially digested plants are regurgitated (cud) and chewed Swallowed again and digested by succeeding stomachs then passes to intestine Other mammals digest plant material in the large intestine Have relatively small stomachs, do not chew a cud Bacteria live in pouch called the caecum, off the large intestine Herbivores must eat great mass to gain sufficient nutrition
Hooves, Horns, and Antlers claws, fingernails, hair, and hooves are made of keratin Hooves cushion and protect toes in running mammals Horns have a core of bone surrounded by keratin sheath not shed, bony core is attached to skull Outer layer is compacted hair-like layers Antlers are made only of bone Male deer grow and shed a set of antlers each year Covered by thin skin layer of velvet while growing Velvet dies and is scraped off when antlers are fully grown Antlers used to combat rival males in fall and winter Shed in spring after breeding season Rhinoceros horn composed of keratinized hair Flight Bats are only mammals capable of powered flight Wings are modified forelimbs leathery membrane of skin stretched over bones of four fingers edges attach to side of body and to hind leg Bats at rest hang upside down from legs Have sonar system to navigate in dark and find insects High frequency pulses emitted through mouth or nose Sound waves reflect off objects, captured by large ears
Modern Mammals nineteen orders Seventeen are placental mammals Two are non-placental mammals : monotremes and marsupials Monotremes - Egg-laying mammals Includes duck-billed platypus and two species of echidna Monotremes retain some reptilian characteristics Lay shelled eggs which they incubate in a nest Shoulder and pelvic bones similar to early reptiles Have a cloaca - a single opening for feces, urine, and eggs More closely related to early extinct mammals Have functioning mammary glands Marsupials - Pouched-mammals Marsupial fertilized egg is amniotic but lacks shell Marsupial egg has much yolk to nourish embryo Embryonic marsupial is born early in development Crawls to marsupial pouch, attaches to nipple, continues to develop for months Nearly all modern marsupials live in Australia and New Guinea Only 20 of 280 total species live elsewhere Marsupials in Australia and New Guinea fill niches that placental mammals fill elsewhere Virginia opossum is the only marsupial in North America
Placental mammals Have well-developed placenta that nourishes embryos for entire development Eggs have little yolk Placenta forms early in course of development Embryos develop in uterus (womb) of mother Fetal placenta formed from chorion and allantois Maternal placenta formed from wall of uterus Young undergo considerable development before birth Many have extensive parental care