Lecture 23: tetrapods, amphibians, amniotes

What are tetrapods?

– Four footed animals

What groups are tetrapods? (4)

-amphibians

-reptiles

-birds

-mammals

When did vertebrates move onto land?

400 million years ago

What are some traits of early tetrapods?

-freshwater

-Modified fleshy fins to ambulate in the shallow water

-Sometimes went on land

Ancestral tetrapods

– Aquatic as juveniles

– Terrestrial as adults

– Lateral line present

Summary:

Tetrapod (4)

– Limbs, pelvis and pectoral girdle are more ossified

– fusion of bones

– Larger articulations of vertebrae to avoid sagging

– Modern amphibians are an intermediate transition between water and land but highly specialised

What group did modern amphibians arise from?

labyrinthodonts

Transition of the modern and amphibians (5)

-late Devonian., About 365 mya

-radial fin rays supporting a tail

-defined limbs with 8 digits

-weight-bearing girdles

-internal gills

When did the digits of land tetrapod arise?

-before the transition to land

Which two groups of tetrapod lived around the same time?

-Acanthostega

-Coelacanth

When did modern amphibians arise?

– Jurassic

– Roughly 200,000,000 years ago

What are the three extant orders of amphibians?

-order Anura

-order Urodela/Caudata

-order Gymnophiona/Apoda

Order Anura

-amphibians

-include frogs and toads

Order Urodela/Caudata

-include salamanders and newts

Order Gymnophiona/Apoda

-include caecilians

Summary:

caecilians (3)

-legless

-blind

-can be aquatic or subterranean

What amphibian is not found in Australia

-caecilians

(Order Gynophiona/Apoda)

Which modern amphibian most closely resembles ancestral amphibians?

– Salamanders

What are some ancestral characteristics that modern amphibians retained?

– Need for aquatic or habitats

- Need for higher temperatures; tropical and temperate

What happens when aquatic larvae metamorphose into terrestrial adults? (3)

-lose lateral line system

-lungs replace gills

-develop legs

Roughly how many species of amphibians have been discovered?

Around 3000 species

Who did ancestral amphibians evolve from?

-lobe fin fishes → scales (Caecilians retain vestigial scales)

stratum corneum

-modern amphibians have a very thin one

-Limited protection from abrasions and dehydration because of cutaneous respiration

-blood supply is very close to skin surface

Cutaneous respiration

-gas exchange

– Capillary beds are in epidermis (the only known instance in vertebrates)

– Large surface area to body mass ratio

Amphibians:

vertebral adaptations

– Articular processes interlock to provide more rigidity and support due to the lack of water buoyancy

Amphibians:

Pectoral girdle adaptations

– Not joined to the skull

– Development of cervical vertebrae

- Free movement of head

Amphibian

Pelvic girdle adaptations

– Presence of ilium, ischium, and pubis

Part of sacral region articulates with pelvic girdle

How many digits do amphibians have on their hindlimbs?

5

How many digits do amphibians have on their forelimbs?

4

What are four adaptations to the amphibian integument?

– Mucus glands

– Granular glands

– Chromatophores

- Eyelids and tear glands

Integument Adaptations:

Mucus gland

– Continuous secretion

– Prevents dehydration

– Limited role in pathogen entry prevention

Integument Adaptations:

Granular glands

– Poison glands: milky, distasteful secretion

– Toxic: Neuro, toxicity, hallucinations, blood poisoning

Granular glands:

Poison dart frogs

Secrete alkaloids in their poison

– Do not synthesize alkaloids; ingest them from insects and keep them in the glands

Integument Adaptations:

Chromatophores

– Found within the dermis

– May be present within the epidermis

– noxious skin secretions often coupled with striking color

chromatophores:

Poison dart frogs

-aposematic coloration: warning to predators

Integument Adaptations:

Eyelids and tear glands

-developed at metamorphosis

Amphibians:

Reproduction

– Reproduce in or near water

-amplexus (mating posture)

-female releases eggs, male sprays sperm over them

-many species of frogs where males carry tadpoles on their back (some species only carry one, some carry many)

Definition:

Amplexus

– Mating position of frogs where male clasps onto back of female

caecilians:

Reproduction

– Internal fertilization

– Males and females press cloacae together and sperm is transferred into female

Salamander:

Reproduction

– Males produces spermatophore

-females pick up spermatophore with cloaca

Definition:

Spermatophore

-a protein capsule containing a mass of sperm

Amphibians:

Brain

-little development from that fishes

– Lateral line present in aquatic larva, but lost in terrestrial adults

Definition:

Lateral line system (fish and aquatic juvenile amphibians)

-sensory system used to detect water movements and pressure gradients

What are the three amniotic groups?

Reptiles

Birds

Mammals

What was the key to the vertebrates’ success on land?

Evolution of amniotic egg

What are the four most important parts of the amniotic egg?

-amnion

-Allantois

-chorion

-yolk sac

amnion (3)

– Encloses the embryo

– Fluid filled

– Protects the embryo from mechanical shock

allantois (2)

-contains and separates fetal urine from the embryo

-contributes to placenta in eutharian mammals

chorion (2)

-outer envelope

-contributes to placenta in eutharian mammals

Yolk sac

– Source of nutrition for embryo

Amniote egg:

Birds and reptiles

– Embryo and associated membranes are enclosed by a calcareous or leathery shell

-prevents desiccation and allows terrestrial reproduction

Amniotic egg:

eutherian and marsupial mammals

-lost the shell

Eutherians: form a placenta

Marsupials: deliver altricial young

Amniotic egg:

Monotremes (ie. platypus)

-retained egg shell

What is the basal group of amniotes?

-Cotylosauria

“stem reptiles”

What are the two major languages of amniotes?

-Sauropsida

-Synapsida

When did Sauropsida & Synapsida diverge?

-Carboniferous period (~300 mya)

Sauropsids

-dinosaur

-modern reptiles

-birds

Synapsids

-mammals

What is one way to identify amniotes?

Skull fenestration

Definition:

Fenestration

-a natural or surgically added opening in a surface

What are the three skull types (based on fenestration)?

-anapsids

-diapsids

-synapsids

Definition:

Anapsid

-no fenestrae

-ancestral amniotes

-modern turtles and tortoises

Definition:

diapsids

-2 fenestrae

-dinosaurs, lizards, snakes, crocodiles, tuatara, birds

Definition:

synapsids

-1 fenestrae

-mammals

what is the noted structure?

amnion

what is the noted structure?

allantois

what is the noted structure?

chorion

what is the noted structure?

yolk sac

what is the noted structure?

yolk & nutrients

what is the noted structure?

albumin

what is the noted structure?

shell

what is the noted structure?

amniotic cavity w amniotic fluid

what is the noted structure?

embryo