Chordates & Vertebrates

Chordate Characteristics

Key features present in embryos, sometimes absent in adults.
Notochord:
- Flexible, dorsal rod functioning as an endoskeleton.
- Works with muscles for locomotion.
Dorsal Hollow Nerve Cord:
- A tube formed from the folding of ectoderm dorsal to the notochord.
Pharyngeal Slits/Clefts:
- Ancestral function: filter-feeding and gas exchange.
- Water flow: Seas → mouth → pharynx → slits → sea.
Muscular, Post-Anal Tail:
- Contains multiple muscle segments.
- Enables undulating locomotion (wave-like movements).
- Examples: Slithering snakes, swimming fish.
Endostyle/Thyroid:
- Endostyle: Ciliated groove.

Cephalochordata: Lancelets

Exhibit all key chordate features in the adult stage.
Burrow tail-first and filter-feed.
Cilia in the pharynx generate water currents for filter feeding.

Urochordata: Tunicates (“Sea Squirts”)

Highly derived sessile adults (immobile).
Suspension-feed using pharyngeal slits to filter food from water.
Tough outer tunic contains cellulose (rare in animals, common in plants).
Swimming larva possesses all key chordate features.
Adults lack: notochords, post-anal tails, and dorsal nerve cords.

Vertebrate Characteristics

Possess vertebrae (spine) composed of cartilage and/or bone.
Vertebrae enclose and protect the spinal cord (dorsal nerve cord).
Most vertebrates replace the notochord with a vertebral column for primary body support.
Well-Developed Head:
- Cranium (skull of cartilage or bone) houses the brain.
- Brain coordinates voluntary and involuntary responses.
- Paired sensory organs enhance sensory perception.
Early vertebrates lacked jaws and teeth.
Heart:
- 2-4 chambers with valves.
- Closed circulatory system with hemoglobin within blood cells.
- Efficient oxygen delivery supports an active lifestyle.
Gills:
- Gill arches/rods of cartilage or bone support gill slits.
- Muscle action with rods aids in gill ventilation.
- Allowed early vertebrates to suck in food.

Extant Jawless Vertebrates

Eel-like bodies lacking lateral fins.
Possess a notochord but have spinal cartilages.
Cartilaginous skull but lack jaws.
Keratinous teeth assist in feeding.
Myxini: Hagfish
- Marine scavengers with reduced eyes.
- Known for defensive slime production.
Petromyzontida: Lampreys
- Jawless parasites on fish.

Gnathostomes

Vertebrates with hinged jaws.
Jaws derived from gill slit supports; typically have bony teeth for predation.
Larger forebrain enhances smell & sight.
Mineralized skeleton with two pairs of lateral fins or limbs (pectoral & pelvic).
- Bones composed of calcium phosphate.
Armored placoderm fish existed during the mid-Paleozoic Era.
Lateral line system in aquatic species detects slight pressure waves from prey/enemies.
Filamentous gills in aquatic species.
- Countercurrent exchange maximizes oxygen extraction from water.
Osmoregulation
- Maintains internal osmotic pressure at homeostasis when the external environment changes
- Majority of marine invertebrates are osmoconformers (maintain internal salinity that matches environment).
- Solutes in tissues = solute in seawater
- Tissues change with environment changes
- Gnathostomes are osmoregulators.
- Solutes in tissues are constant even when environment salinity changes
- Marine and freshwater bony fish both have low levels in tissues (less than 1%)
- Marine bony fish drink sea water and excrete salt but a little bit of urine
- Osmotic water loss through gills and other body parts of body surface
- Tissues of marine fish are not that much saltier than freshwater fish.
- Freshwater bony fish make lots of dilute urine and uptake salt ions
- Osmotic water gain through gills and other parts of body surface

Chondrichthyes: Cartilaginous Fish

Possess a skeleton made entirely of cartilage.
Ancestors had mineralized bones.
Bone minerals were lost to reduce density, aiding buoyancy.
Buoyancy enhanced by liver oils and active swimming.
Bony Teeth:
- Derived from tooth-like scales.
Most are active marine predators: possess eyes, nostrils, bony teeth, and a spiral valve in the intestine (increases surface area for digestion and absorption).
Few are filter-feeders; some inhabit freshwater environments (e.g., whale shark).
Reproductive Adaptations
- All with internal fertilization
- Oviparous:
  - Embryo develops within a protective egg-case, nourished by yolk.
- Ovoviviparous:
  - Embryo nourished by egg yolk, retained in the mother, live birth.
- Viviparous:
  - Embryo directly nourished by maternal tissue (umbilical cord), live birth.
Includes: sharks, rays, skates, sawfish.

Osteichthyes: “Bony Fish”

Gnathostomes possessing lungs or lung derivatives (swim bladder).
Includes ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii: coelacanths & lungfish) + descendants.
Early lungs aided fish survival in low-oxygen freshwater.
- Fish swallowed air into the digestive tract.
- Outpocketing of the esophagus increased surface area for gas exchange.
- Later, lungs evolved into swim bladders for buoyancy regulation.
Skeletons remain mostly ossified (mineralized with hydroxyapatite).
Adaptations:
- Flat scales and slime reduce drag.
- Operculum (gill cover) protects gills and helps pump water over them.
- Typically oviparous with external fertilization.
- Unshelled jellied eggs.
Actinopterygii: Ray-Finned Fishes
- Thin fins supported by long, flexible rays.
- Gas-filled swim bladder for buoyancy.
- Most diverse group of all vertebrates.
- Inhabit marine & freshwater environments.
- Exhibit diverse feeding strategies: herbivores, predators, and scavengers.
- Examples: seahorse, lionfish, tuna, and eel.
Lobe-Finned Fish (Sarcopterygii)
- Pelvic and pectoral fins supported by bones and muscles.
- Actinistia: Marine coelacanths with lipid-filled swim bladders.
- Dipnoi: Freshwater lungfishes that gulp air into lungs in stagnant ponds (possess lungs, lack swim bladder).

Tetrapods

Evolution of Tetrapods

Characteristics:

Four limbs with wrists and digits (key feature).
Pelvic girdle fused to the backbone.
Neck allowing independent head movement.
Lung-breathing as adults.
Most have no scales on skin.
Ancestors: lobe-finned and lung-breathing fish.
Fossil freshwater fish: Tiktaalik (375 mya), adapted for shallow pools and potentially some land movement.
Diverse tetrapods from mid-Paleozoic all had close ties to freshwater.
Extant tetrapods have five digits (fingers).

Class Amphibia

Most basal extant tetrapod group.
Thin, moist skin prone to water loss.
Gas exchange with air across moist skin and lungs (gulps air like fish to fill lungs).
Expanding mouth cavity “sucks” air into mouth, but mouth has to push air into lungs (positive pressure).
3-chambered heart for better oxygen supply (dual circulation = higher BP).
Reproduction is closely linked to fresh water.
(Diverse mating behaviors, frogs and toads have external fertilization).
Almost all oviparous (fish-like eggs).
Larval stage common: aquatic with gills, metamorphosis into adult form, eats algae aquatic larvae and worms (tadpole), eats insect and worms (adult).

Three groups (orders):

Salamanders (Urodela)
- Four legs and a tail as adults, moist terrestrial habitats.
- Examples: newts, mudpuppies (paedomorphic adults - retain larval features).
Frogs (Anura)
- Four legs but no tail as adults, moist terrestrial and freshwater habitats.
- “Toads” are terrestrial with thick, bumpy skin and breed in ponds or puddles.
Caecilians (Apoda)
- Snake-like, legless adults, evolutionary convergence with snakes.
- Mostly terrestrial, burrowing lifestyle.

Amniotes

Characteristics:

Amniotic egg allows reproduction independent of water.
- Amnion: Protects embryo within a fluid-filled sac.
- Other extraembryonic membranes: Sustain embryo with minimal water loss.
  - Albumen: Stores water.
  - Allantois: Waste disposal/gas exchange.
  - Chorion: Gas exchange.
  - Yolk Sac: Nutrient supply.
- Porous eggshell protects and reduces water loss while allowing o2/ $CO_2$ exchange (rigidity depends on CaCO3 content).
- Fertilization is internal, occurring before eggshell formation.
- In most mammals, the eggshell is absent, but the membranes function.
Waterproof Skin
- Layer of dead cells with keratin and lipids, little to no gas exchange across the skin
- Lizard: think, dry, scaly skin
- Salamander: thin, moist skin
Water-conserving excretion of nitrogenous wastes (NH2 amino groups).
- Land amniotes convert $NH_3$ to less toxic urea or uric acid which costs energy.
- Kidneys concentrate urine, saving water.
- Aquatic animals (most bony fished) - Ammonia is highly toxic most lost thru gills or skin.
- Mammals (most amphibians, sharks and some bony fishes) - Urea is concentrated by kindest in urine
- Reptiles, birds, insects, land snails- Uric acid save the most water, least toxic but most costly
- Bird dropping (bird shit) is made up of digestive (feces) and metabolic (uric acid) waste
Negative-pressure breathing.
- Expanding rib cage draws air in; more efficient than gulping (amphibians can't do this very well).
- Expanding rib cage → increase V → lower P → “sucks in air”
Improved dual blood circulation.
- Septate completely divided ventricle.
Control Of Body Temperature
- Ectotherms:
  - Rely on environmental sources to warm their bodies (behavioral thermoregulation).
  - Low metabolic rates generate little heat, basking (chillax) behavior in all non-bird reptiles
- Endothermic:
  - High metabolic rate warms body (physiological thermoregulation).
  - Endothermic = high metabolic rate warms body (physiological thermoregulation)
  - Needs insulation (feather, fat, or fur), birds and mammals
    - Would a mouse or a lizard of the same weight have to eat the same amount of food per day to survive?
    - No because mice are endothermic (warm-blooded) so they constantly use energy to stay warm, so they need more food, also has a higher metabolism
    - Lizards on the other hand are ectothermic (cold-blooded) so they just soak in the sun to regulate body heat so they use less energy and food

Amniote Phylogeny

Extant nonbird reptiles share many ancestral features:
- Scaly skin without feathers or fur.
- Low profile, sprawling stance (rocky's frog legs standing).
- Low metabolic rates.
- Ectothermic.
- Eggs with leathery, less calcified eggshells (compared to birds).

Turtles

Distinctive Characteristics:
- Upper (carapace) and lower (plastron) body shells fused to ribs and vertebrae.
- Teeth replaced by keratin beak.
- Adapted to land (tortoises), freshwater (freshwater turtles), marine (sea turtles) environments.
- Diverse diets: vegetation and animals.
- Oviparous: always lay eggs that get buried on land.

Lepidosaurs

Tuataras:
- Remnant, ancient lizard-like reptile.
- In lizard/snake clade but with unusual features
- Light-sensitive pineal gland (“3rd eye”) on top of head, unique dentition, no external ears, prefers cooler temperatures
Lizards and Snakes:
- Have eyelids that close and external ear openings
- Majority of lizards are terrestrial predators, insect-eaters and vegetation-eaters
- Lepidosaurs: Squamates
  - Komodo Dragon is the largest eater, vertebrate-eater
  - Snakes descended from lizards (scarlet kingsnake)
  - No: legs, external ear, eyelid.
  - Lizard belly has many small scales across whereas snakes have broad ventral scales
  - All snakes are predators, not all are venomous
  - Snakes and lizards are diverse in birther, some parthenogenesis, most are oviparous, some are ovoviviparous and some are viviparous

Archosaurs

Crocodilians:
- Crocodiles, alligators, and skin.
- Semi-aquatic predators, extant from the Triassic dinosaur age, both dinosaur and bird-like features.
- Gizzard (grinds food in digestive tract) (bofum)
- 4-chambered heart (bird).
- Oviparous, some nesting care.
- Teeth in sockets (dino).
Birds:
- Origin of birds
  - Diverged form a group of bipedal (2 feet), feathered dinosaurs during late Mesozoic Era
  - Archaeopteryx: Earliest known bird with many dinosaur characteristic, no current birds came from this lineage
  - Evolution
  - Birds descended from dinosaurs
  - Rapid miniaturization (became smaller), facial/skull changes (beak), bipedal locomotion, feather, flight
- Bird skulls are similar to embryonic dinosaurs, small change in gene expression patterns in bird embryos give an elongated facial feature
- Adaptations for Flight:
  - Forelimbs modified into wings with keratin contour feathers that provide lift.
  - Skeleton is lightweight/hollow but strong.
  - Broad keel (sternum) for attachment of pectoral muscles.
  - Stubby, feathered tail for flight maneuvers.
  - No teeth, keratin beak over bones.
  - Muscular gizzard grinds food.
  - Efficient 4-chambered heart.
  - Highly efficient respiration with air sacs (negative-pressure breathing).
  - One way air flow, countercurrent gas exchange
  - No “dead air”
  - Expands ribcage for negative pressure inhalation (like all amniotes).
  - Endothermic with high metabolic rate.
  - Feathers provide insulation by trapping air (fluffy feathers increase thickness of trapped air), feathers instead of fat bc it is lighter
  - Highly developed senses and behaviors
    - Mating behaviors
    - Nest building (for eggs (oviparous) and young)
    - Extended parental care brood (sit to incubate) eggs for warmth, calcified egg so that they can survive brooding, majority feed their babies
    - Intelligents: learning, problem solving, and tool use
- Wing adaptations: speed (hawks, eat birds), soaring (seagulls, eat fish), hovering (hummingbirds, eat nectar and insects), silence (owls, eat small mammals)
- Visual acuity (how well they see) in day (eagle) or night (owl) for predation
- Diversity of feet (perching, grasping claws, paddling, wading)
- Diversity of beaks (to get nectar, to spear fish, to drill wood, to open seeds, sives (strains) the water for fish, scoop-net to scoop fish

Mammals

Mammal Fossil History

Descended from synapsid amniotes with mammal-like traits ( teeth and upright stance).
Synapsid jaw-joint reduced to middle ear bones of modern mammals from 1 → 3 ear bones
First true mammals in jurassic period of mesozoic era, they were small and opossum-like (but some early mammals were huge (3 ft long head!))
Diversification of mammals in cenozoic era, after cretaceous extinctions

Derived Characteristics of Mammals

Unique Traits:
- Mammary glands: secrete milk for offspring.
- Sweat glands: secrete 99% water for evaporative cooling (origin of mammary glands).
- Hair or fur: insulation, camouflage, sensory (whiskers), protection (lashes).
- Made of alpha-keratin whereas feathers are beta-keratin
Ribcage breathing now includes diaphragm (contracts/lowers) to improve negative pressure breathing (np ribs in abdominal area).
Lungs: masses of alveoli (dead-end sacs) – larger surface area than non-bird reptiles.
Teeth vary in size, shape, and number and adapted to different food types.
- Carnivore (lions), herbivore, omnivore (us), grazing herbivore (buffalos), gnawing herbivore (buck tooth beaver), insectivore (ant-eater), filter-feeding carnivore (like whales).
- Most reptile teeth are uniform and conical (cone shape), wolves have varied adaptive teeth
Digestive tract adaptations for their food types
- Herbivores have longer tract than carnivores since it takes longer to digest
- Carnivore diet is high in protein and fat that are easily digested by enzymes
- Herbivores can digest starches (plant energy) really easily but they don't have enzymes that can digest cellulose (in cell wall)
- Cecum can have mutualistic microbes that digest cellulose
- Horses have large cecum with microbes and large colon with microbes
- Ruminants are mutualistic migraine that digest cellulose in the rumen since they eat food and the chew it again
- Cows, sheep, hoats, deer, giraffes, antelopes
Other Derived Traits:
- Endothermic with high metabolic rate (insulate with ur or fat and sweat to cool down).
- 4-chambered heart-convergent trait with birds
- Highly developed brain and complex behavior
- Adapted to all habitats, including flight (bats) and aquatic life (manatee)

Mammal Diversity

Know the 3 big clades monotremes (5 species), marsupials 324 species eutherians 5010 species.
Monotremes
- Egg laying mammals
- Lay shelled eggs (oviparous)
- Young hate and lap milk (no nipple)
- Only echidna (4 species) and platypus (1 species)
Marsupials
- Have maternal puch (viviparous)
- Fetus starters to grow in uterus with placenta
- Fetus is born super early and lives in pouch
- Latch onto nipple to finish development
- Australia and New Zealand: very diverse
- Kangaroos, kolas, wombats, marsupial mouse
- United States: only opossum
Eutherians
- Placental mammals (viviparous)
- Better placenta for longer gestation
- Young are more developed at birth
- Horses and deer start walking real quick, dolphins, we’re rlly slow
- The most diverse and widespread mammals (18 orders!)

Primates

Characteristics (related to arboreal (arbol, in trees) life):
- Hands and feet are adapted to grasp.
- Flat nails on digits (fingers).
- Large brains, eyes forward, and flare face.
Living primate groups
- Lemurs and relatives (loris, bushbaby) and tarsiers (the ones with the giant eyes are at night, smallest primate, 3 inches, in Philippines) are all arboreal
- Anthropoids = monkeys, apes, and humans
Anthropoid groups
- New world monkeys - prehensile (grasping/holding) tails, arboreal
- Spider monkey, white-faced monkey (marcel)
- Old world monkeys - tails can’t grasp and they usually are just on the ground (baboons)
- Apes and humans - no tail, in trees and on ground (gibbons, orangutans, gorillas, chimpanzees (closest to humans), bonobos
Homo sapiens characterized by:
- Bipedal locomotion, ground-dwelling, much larger brian, reduced jaws and canine teeth, language, complex tools
- Earliest hominids/hominids (humans) date back to 6.5 million years ago
- Homo sapiens is the most recent (200,000 yrs ago) and are the only hominin today
- Came to north america at least 22,000 years ago
- Footprints found at an ancient lake bed in new mexico
- Used carbon dating from aquatic plant seeds they found in the footprints!
- All humans are 99% genetically similar
- Race is social, not biological
Skin Color reflect the amount of melanin (pigment produced by melanocytes in the epidermis)
- Melanin = derivative of AA tyrosine
- Tone correlates with geography, relative positive to Earth’s poles
- Closer to north = whiter, closer to equator = darker
- Melanin pigmentation has 2 classes
  - Constitutive skin color genetically programed and based on amount of pigment the skin has without sun exposure
  - Factulative (inducible) skin color Increased pigmentation from tanning in exposed ultraviolet rays
  - Immediate tanning = hyperpigmentation
  - Delineated tanning = increase in melanocytes
    - Some skin burns and peels instead of tanning, meaning it’s prone to skin cancer ince melanocortin-1 receptor (Mc1r) is defective which is necessary for melanin production
    - 2 types of Melanin
      - Eumelanin - blackish brown pigment
      - Pheomelanin - reddish, yellow pigment
      - Fair skin freckling and carrot-red hair from defective Mc1r
      - Tones depend on the ratio of the two types
        Large matter of vitamins
        Folate = B vitamin (essential nutrient)
        DNA replication and cell division
        Levels are influenced by UV light
        Intense sun halve sthe amount of folate
        Affects light skins more
        Folate deficiency → spina bifida and anencephaly
        Dark skins aren’t affected
        Melanin absorbs and dissipates (disappears) UV light as heat
        Vitamin D = fat-soluble vitamin
        Maintains a healthy immune system and builds healthy bones and teeth
        Production need UV light
        Dark skins
        More vulnerable to vitamin D deficiency → Rickets, Preeclampsia
        Light skins
        Less affected
Light skin evolved - helped the body produce vitamin D in sun-poor part of the world
Dark skin evolved - helped protect the body’s folate stores in people who lived in sunny places
Body needs a balance of folate and D!!