Subphylum Vertebrata – Early Vertebrates and

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Transcript Subphylum Vertebrata – Early Vertebrates and

The Origin and Early Evolution of Tetrapods
 Superclass Tetrapoda: four-limbed vertebrates; a monophyletic group
comprising amphibians, reptiles, birds, and mammals; terrestrial, some
with secondary modifications of limbs and transitions to aquatic/marine
habitats (ex. sea snakes, sea turtles, cetaceans)
 Characteristics of the Terrestrial Environment: oxygen ~20x more
abundant in air than in water; little support against gravity; great
fluctuations in temperature; great variety of habitats
 The Transition from Lobe-finned Fishes to Terrestrial Vertebrates


Romer’s “Gap”: historical paucity of vertebrate fossils between early Devonian
freshwater lobe-finned fishes and first fully terrestrial tetrapods (late
Devonian); gap now filled by several transitional specimens
Tiktaalik (2006): transitional form between ancestral lobe-finned fishes (ex.
Eusthenopteron) and first tetrapods (early forms aquatic, with gill
arches and paddle-shaped tails, ex. Ichthyostega, Acanthostega)
 Transitional Characteristics (Tiktaalik): fish-like scales, fin webbing, and fish-like
tail, but limbs with joints (shoulder, elbow, wrist), flat head with raised eye
sockets, muscular chest and neck lift neck out of water, primitive ribs

Tetrapod lung likely evolved from air-filled cavity in response to low or unstable
levels of dissolved oxygen (typical in swamps and mudflats)
Fig. 25.2
Fig. 25.3
Fig. 25.1
Extant Amphibians – Diversity and Life Cycles
 Amphibian Diversity and Characteristics: ~6,000 species; ectothermic;
strengthened skeletons; most with aquatic larvae (with gills and
lateral lines) and metamorphosis to terrestrial adults (with primitive
lungs and cutaneous respiration)  dependent on standing water;
toxic skin secretions (some extremely venomous)
 Order Apoda: Caecilians (~173 species)
 Elongate, limbless, burrowing; found in tropical rainforests (esp. South America);
terminal anus; sensory tentacles on head; eat worms; internal fertilization via
spermatophores; limbless salamanders
 Order Urodela (Caudata): Salamanders and Newts (~553 species)
 Tails, lateral limbs, carnivorous; diversity greatest in North America; terrestrial
species lack aquatic larvae; most with both gills and lungs during life cycle, but
cutaneous respiration important; some exhibit paedomorphosis, retain gills as
adults (ex. mud puppies, axolotl); many capable of regenerating limbs
 Order Anura: Frogs and Toads (~ 5,283 species)
 Hind legs modified for jumping (anti-predation) and swimming (webbed), tails
absent in adults; metamorphosis and major changes in lifestyles from larvae
(tadpoles) to adults; hibernate during winter in temperate climates; most with
external fertilization
 Families include Ranidae (bullfrogs, leopard frogs), Hylidae (tree frogs), and
Bufonidae (toads, with short legs and thick skin)
Fig.25.4
Fig. 25.8 and Fig. 25.9
Fig. 25.5
Fig. 25.6
Fig. 25.7
Fig. 25.11
and
Fig. 25.12
Fig. 25.25
Fig. 25.13
and
Fig. 25.14
Anurans: Form and Function
 Integument and Coloration: skin thin, loose, and glandular (mucous and serous glands), with
chromatophores (Tyndall scattering from deep layers  blue color filtered by overlying yellow layer  green hues)

Poison-dart frogs (Dendrobatidae) among most venomous animals (tetradotoxin); secretions used on
blowgun darts; warning coloration
 Skeletal and Muscular Systems: force transferred to hind limbs by urostyle (fused trunk
vertebrae); five-rayed hind limb, four-rayed front limb; skull reduced in rear (gill area)


Epaxial (dorsal) muscles: support the head and brace vertebral column
Hypaxial (ventral) muscles: support viscera (more than in fishes)
 Respiration and Vocalizations: gas exchange via skin, mouth, and lungs; frog lung a positivepressure breather (air forced into lung via throat muscles); vocal cords and sacs well
developed in males (species-specific mating calls)
 Circulation: shift to lung breathing required blood circuit to lungs (pulmonary arteries and
veins); partition down center of atria separates pulmonary from systemic flow; two
atria in frogs contract asynchronously  single ventricle has some separation
between oxygenated and deoxygenated blood (aided by spiral valve in conus
arteriosis)
 Feeding and Digestion: adults with varied diet (mainly insects) and protrusible tongue
(attached at anterior end of mouth); tadpoles mainly herbivorous
 Nervous System and Senses: forebrain (olfaction and cerebrum), midbrain (vision), and hindbrain (hearing and balance; cerebellum and medulla); hearing with tympanic
membrane, stapes (single ear bone), and inner ear with semicircular canals; eyes with
rods and cones (color vision) and nictitating membrane
 Reproduction: eggs usually fertilized externally during amplexus and attached to vegetation;
some species tend eggs and/or brood larvae; metamorphosis from tadpoles ranges
from three months to three years
Fig. 25.15
Fig. 25.18
Fig. 25.17
and Fig. 25.19
Fig. 31.22
Fig. 25.20
Fig. 25.21
and Fig. 25.22
Fig. 25.24
Fig. 25.26
Fig. 34.9
Amphibians and Environmental Issues
 Pollution and Sentinel Species
 Sentinel Species: first to show effects from pollution (“canary in
the coal mine”)
 Amphibians exposed to both air and water pollutants during life
cycle; vulnerable due to porous skin; developmental
abnormalities noted in recent decades (especially extra and
missing limbs)
 Introduced Species
 Introduced amphibians include African clawed frog and cane
toad (plague level in Australia)
 Endangered Species and Extinction
 Threats include epidemic fungal pathogen, climate change (ex.
golden frog), and introduced predators/competitors (ex.
rainbow trout)