12.1 - PHYLUM CHORDATA bilateral symmetry

1. 12.1 - PHYLUM CHORDATA • bilateral symmetry • dorsal nerve cord protected by a supporting rod (vertebrae) • ventral heart • gill slits, tail and notocord are always part of early embryological stage  betrays common phylogeny • Subphylum Vertebata (see Fig. 4, P. 449) • most chordates (95%) are vertebrates • endoskeleton provides support, attachment for muscles  enables larger size, more complex movement • 2 pairs of appendages • large anterior brain surrounded by bone • closed circulatory system  cells bathed in interstitial fluid • gas exchange by gills or lungs • large coelom contains internal organs • multi-layered dermis (skin) can be modified for glands, scales, feathers, hair, nails, claws, horns, hooves • birds and mammals are endothermic (“warm blooded”, able to regulate their internal temperature), all other vertebrates are ectothermic (“cold blooded”, metabolic rate varies with surroundings

2. Ecological Role of Vertebrates • found in almost every habitat • vital part of many food webs, including humans • amphibians play a vital role in wetland ecology • birds consume large numbers of insects and small mammals, act as pollinators and vectors for seed dispersal • herbivorous mammal consume massive amounts of producers before being consumed by carnivores • many mammals are disease vectors • humans are inordinately influential in the biosphere, usually negatively

4. Cladistics • analysis of phylogeny based on shared characteristics • organisms are arranged according to their degree of sharing recent, derived (apomorphic) characteristics rather than of older, primitive (plesiomorphic) characteristics •  more closely related organisms will share a greater number of unique apomorphic features which were not present in their distant ancestors • cladograms are branching diagrams displaying hypothetical relationships in a “tree” (see Fig. 6, P. 451) • organisms are placed at the end of a line representing the lineage leading to each end group • lines end at nodes which represent a point in time where a common ancestor “splits” to give rise to two separate lineages • i.e. an ancestral species with a vertebral column split, giving rise to lampreys and all other vertebrates •  the jawed ancestor at the next node is not an ancestor to the lamprey but is a more recent organism that is a common ancestor to the bony fishes, reptiles, amphibians, and mammals • turtles and leopards share the most recent common ancestor, followed by salamanders

5. 12.2 - Fish • Class Agnatha (jawless fish) • few surviving representatives • soft, eellike bodies with a notocord and cartilagenous skeleton • 70 species of hagfish (scavengers) and lampreys (parasitic) • hagfish mouth has a row of toothlike plates surrounded by fleshy tentacles • lampreys are significant to the Great Lakes fishery •  spawn in streams, larvae metamorphosize to adults, migrate to lakes •  toothed, suckerlike mouth attaches to prey, tears a hole and sucks out blood and body fluids

6. Class Chondrichthyes (cartilagenous fishes) • marine, include sharks, rays, skates • lateral appendages (fins) for swimming (pectoral, pelvic) and steering (dorsal, caudal) • paired gills (5-7 per side) are efficient for gas exchange • sharks have triangular teeth which are repeatedly shed and replaced • paired nostrils and lateral line allow detection of prey • body covered by placoid scales • Reproduction • gametes develop in testes or ovaries  in sharks, sperm transferred into cloaca, shelled eggs retained internally, offspring born alive (oviparous) and independent • skate and ray bodies are flattened, adapted to living on the ocean bottom • enlarged pectoral fins extend onto the side of the head • mouth is often buried in bottom substrate  water is taken in across gills through spiracles (modified gill slits)

7. Class Osteichthyes (bony fishes) • cartilage in young replaced by bone in adults • accessory scales cover body • paired gills covered by bony operculum which flaps to pump water through the mouth and over gills (an “advance” over sharks) • swim bladders regulate depth (used for gas exchange in lungfish) • reproduction – external fertilization of many shed eggs • 2-chambered heart

8. 12.3 - Class Amphibia • three orders (Fig. 1, P. 455) •  legless caecilians •  tailless frogs and toads • tailed salamanders • freshwater/terrestrial •  first vertebrates to colonize land • early life stages in fresh water  eggs fertilized externally •  larvae (herbivorous aquatic tadpoles)  metamorphosis •  terrestrial carnivorous adult • 3-chambered heart – 2 atria collect blood from tissues and lungs, oxygenated and deoxygenated blood mix in single ventricle (limits sustained muscle activity) • gas exchange through skin, mucous membrane of mouth, and simple lungs • Ecological Role of Amphibians • indicator species in wetland ecosystems as they live in water and on land, have thin skin making them vulnerable to change and contaminants

9. 12.4 - Class Reptilia • three subclassesAnapsida (turtles and tortoises) • (Fig. 1, P. 457)  Lepidosaura (lizards, snakes, tuatara) • mostly terrestrial  Archosaura (crocodiles, alligators, dinosaurs) • adaptations allow for radiation to arid terrestrial habitats •  waterproof shell around eggs •  amniotic eggs fertilized and retained internally until ready to hatch •  four membranes protect and nourish embryo (Fig. 4, P. 458) •  amnion (fluid-filled sac cushions and insulates) •  yolk sac (stored food) •  allantois (gas exchange, holds waste) •  chorion (lines shell, encloses other membranes) •  tough, scaly skin •  limbs support body above ground, forward knee position allowed greater forward push and speed (Fig. 5, P. 459) •  large, internally folded lungs aided by ribmovement (Fig. 2, P. 457) •  efficient kidneys excrete concentrated urine • partially separated ventricles deliver higher oxygenated blood to body tissue, allows for more sustained muscle action (Fig. 3, P. 458) • diverse habitats and feeding behaviour • variations based on niche (i.e. snakes lost functional limbs)

10. Early Reptiles • dinosaurs dominated Earth for 160 MY (vs. our approximate 3 MY!)

11. Turtles • bony dorsal and ventral plates, mostly aquatic, herbivores • Lizards • mostly aquatic, insectivores • Snakes • evolved from burrowing lizards (no appendages, modified scales for locomotion) mostly terrestrial, carnovores • Crocodiles and Alligators • mostly aquatic, carnivores