Phylum CHORDATA, subphylum VERTEBRATA, class AMPHIBIA

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CHORDATA, subphylum VERTEBRATA, class AMPHIBIA (general info)

- BACKBONE
- ECTOTHERMIC (cold blooded)
- Lay eggs in WATER (no shell - water reliant)
- Water-PERMEABLE skin - respiratory organ with mucous glands(GOBLET cells) to keep it moist.
- NON-amniote

CHORDATA, subphylum VERTEBRATA, class AMPHIBIA (general info)

- EXTERNAL fertilisation - Amplexus (embrace) male holds female with front legs and fertilizes eggs as they are layed.
- Most have an aquatic LARVAE (gill breathing) & TERRESTRIAL adult (lung breathing - poorly folded/developed).
- Some are PAEDOMORPHIC - adults retain juvenile characteristics, don't undergo full metamorphosis, ex. Protean salamanders retain gills.

CHORDATA, subphylum VERTEBRATA, class AMPHIBIA, subclasses:

1. subclass LABYRINTHODONTIA * stem amphibians, fish-like.
2. subclass LEPOSPONDYLI *
3. subclass LISSAMPHIBIA modern amphibians, orders: P,A,U,G.

CHORDATA, subphylum VERTEBRATA, class AMPHIBIA, subclass LISSAMPHIBIA, orders:

1. order PRONURA - protofrog*
2. order ANURA - frogs & toads
3. order URODELA (Caudata) - salamanders & newts
4. order GYMNOPHIONA (Apoda) - legless, worm-like

Amphibian HISTORY

- 370 mya Devonian - 1st amphibans developed from rhipidistian lobe-finned fish (similar to modern coelacanth & lungfish) that evolved multi-jointed leg-like fins to crawl along sea bottom. Size: 1-5m. Never developed ability to live entirely on land - have to return to water to lay eggs.
- Carboniferous & Permian - Moved up food chain, top predators for ~100 million years.

Amphibian HISTORY

- Permian-Triassic extinction: 252 mya - followed by wet then drought periods, water may have become stagnant.
- Triassic period - proto-crocodiles began to compete with amphibians - leading to their reduction in size.
- Jurassic period - Anurans (frogs & toads), Caudata (salamanders), Gymnophiona (caecelians).

Ancestors of amphibians

Rhipidistian fish (subclass of SARCOPTERYGII -bony, lobe-finned fish) with functional lungs & strut-like limbs, wrist structure similar to that of early amphibians.
Living SARCOPTERYGIANS - coelacanths, lungfish, tetrapods.

Amphibian history

- Eusthenopteron, Late Devonian, ~385 mya
- Panderichthys, Devonian, 380 mya
- Tiktaalik, Devonian, 375 mya
- Ichthyostega, Upper Devonian, ~370 mya
- Acanthostega, Upper Devonian, ~365 mya
- Coelacanth, End of Devonian, 360 mya

Intermediate Tiktaalik features

FISH - gills & scales
'FISHAPOD' - half-fish, half-tetrapod ear region & limb bones & joints (functional wrist joint with radiating fish-like fins instead of digits)
TETRAPOD - lungs, rib bones & mobile neck

Ichthyostega - an early amphibian

TERRESTRIAL - adapted for life on land, legs provide structural support for walking, modified pelvic girdle attached to vertebral column.

Adverse effect on amphibians

1. CLIMATE change
2. ENVIRONMENTAL change - human activities causing: acidification, industrial discharge, chemicals & toxins.
3. ULTRA-VIOLET RADIATION - increased UV-B (linked to CFC emissions - kills eggs & larvae)
4. DISEASES - Trematode (cause malformations in frogs) & Iridovirus (necrosis)
5. FUNGAL INFECTIONS - Chitrid (Batrachochytrium) - clogs frogs skin pores preventing them breathing.

Frog respiratory structures

1. SKIN - major site for CO2 elimination, more important in winter (~60% water take up)
2. LUNGS - used more in summer as frog is out of water more
3. BUCCOPHARYNGEAL CAVITY

Evolution...

- Terrestrial vertebrates and tetrapods probably arose from SARCOPTERYGIANS
- Two lineages of ancient amphibians diverged:
1. NONAMNIOTE gave rise to: Anura, Caudata, Gymnophiona
2. AMNIOTE gave rise to: Reptiles, Birds & Mammals...

CHORDATA, subphylum VERTEBRATA, class REPTILIA - adaptions to terrestrial life:

- 1st FULLY terrestrial vertebrates
- WATERPROOF horny interlocking scales (SCUTES)
- TEETH on jaws
- HEAD carried on a relatively long neck
- ATLAS BONES - articulate with a single CONDYLE on the skull
- BRAIN - relatively small with moderately developed CEREBRUM

CHORDATA, subphylum VERTEBRATA, class REPTILIA - adaptions to terrestrial life:

- ECTOTHERMIC (cold blooded) & bask in sunlight
- AMNIOTIC (cleidoic-type eggs)
- Mostly OVIPAROUS (some vivparous)
- Penis allowing for internal fertilisation
- NO DIAPHRAGM separation thorax & peritoneal cavity
- Nitrogen excreted as URIC ACID
- PARAPHYLETIC GROUP
- Age of Reptiles - 360-65 mya

CHORDATA, subphylum VERTEBRATA, class REPTILIA, subclasses:

1. subclass ANAPSIDA
2. subclass DIAPSIDA
3. subclass SYNAPSIDA

CHORDATA, subphylum VERTEBRATA, class REPTILIA, subclass ANAPSIDA:

- order: Mesosauria*
- order CHELONIA (turtles, tortoises)
- Solid roof skull & no temporal openings.

CHORDATA, subphylum VERTEBRATA, class REPTILIA, subclass DIAPSIDA:

- MAIN group - 2 temporal openings, includes:
1. infraclass LEPIDOSAURIOMORPHA - lizards & snakes
2. infraclass ARCHOSAUROMORPHA - dinosaurs* & crocodiles
3. infraclass EURYAPSIDA - plesiosaurs & ichthyosaurs

CHORDATA, subphylum VERTEBRATA, class REPTILIA, subclass DIAPSIDA:

1. infraclass LEPIDOSAURIOMORPHA
2. infraclass ARCHOSAUROMORPHA
3. infraclass EURYAPSIDA

CHORDATA, subphylum VERTEBRATA, class REPTILIA, subclass SYNAPSIDA:

- Mammal-like reptiles, have 1 lower temporal opening either side of skull.
- order: THERAPSIDA* - ancestors of mammals!

5 Major Extinctions

- Late Ordovician
- Late Devonian
- Permian-Triassic
- Late Triassic
- Cretaceous-Tertiary (Chixculub crater 125m diameter)

Reptile Lung

- bidirectional lung (bidirectional flow of air)
- NO diaphragm

Chelonian Ventilation

- EXHALATION (active) transversus & pectoralis contract
- INHALATION - transversus & pectoralis relax, obliquus abdominus contracts

Crocodile Ventilation

- MIDDLE PALATE - separates water in mouth and air they breathe
- INHALATION: rib cage expands, liver pulled back, air into lungs
- EXHALATION: rib cage & forward moving liver compress lungs --> air out

Snake Ventilation

- vestigial left lung (small/absent)
- 1 functional lung - contains a vascularised anterior portion & a posterior portion that does not function in gas exchange.
- posterior 'saccular' lung is used for hydrostatic purposes in aquatic snakes (function unknown in terrestrial snakes).

Lizard ventilation

Inhalation - INNER intercostal muscles pull ribs forwards & out
Exhalation - OUTER intercostal muscles contract + elasticity of lungs

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