Tetrapod Evolution

Tetrapoda

includes lissamphibia and amniota

amniota

includes synapsida+mammals and reptilia; sister to lissamphibia

reptilia

includes testudines and diapsida; Includes all other amniotes to be monophyletic (Without birds is paraphyletic); many obscure synamoporphies like phi keratins in skin and scales

diapsida

includes lepidosauria + testudines, crcodylia and aves

lepidosaura

includes squamata and rhynchocephalia

outgroup to tetrapoda

3: actinopterygii, actinistia, dipnoi

actinopterygii

ray finned fish; very diverse, highly derives, fins have extrinsic muscles

sarcopterygii

lobed fined fish; includes actinistia, dipnoi, and tetrapoda; fins supported by bone with intrinsic muscle; teeth w/ enamel

Actinistia

• Coelacanth, Latimeria

• 2 extant species

•  Symmetrical, 3-lobed tail

• “living fossil”

Dipnoi

• Lungfishes

•  3 extant genera

• Gondwanan origin

• Evolved after the split of pangea and only in the southern section (now only parts of the southern hemisphere)

• 1 in S. America, Africa, Au

tetrapoda

• Includes all modern amphibians and amniotes, and their last common ancestor

• Partial shared features (mostly skeletal)

tetrapoda shared features

• Ankle and wrist joints.

• Fully developed pectoral girdle.

• Pectoral girdle free from skull.

• Discrete shaft of humerus.

• Paired bones in the distal part of limbs.

  • Radius and ulna

  • Tibia and fibula

• Digits at end of limbs.

Osteolepiform “fishes”

• Large, flat predatory fish 

• Support on pectoral fins

• Perhaps move out of water

• includes Tiktaalik

  •  Transitional form to tetrapods. → picture

  • Mobile neck; pectoral girdle detached from skull.

  • Still retained gill arches.

Primitive tetrapods (Icthyostegalia)

includes: Acanthostega, Ichtyhostega, Tulerpeton

Acanthostega

• No weight bearing wrists or ankles

• Had 2/3 of 41 tetrapod synapomorphies.

• Limbs like fish but with digits instead of fin rays.

• Short ribs did not enclose body.

Ichtyhostega

• No evidence of gills arches.

• Improved inhalation into lungs.

• Pectoral girdle completely separate from skull.

• Bend in limbs (knees and elbows).

• They are starting to spend time on land

Tulerpeton

First robust shoulder joints.

Slender digits. Still >5 digits.

Superficially similar to crocodile

how did primitive tetrapods support their weight

development of weight-bearing limbs; larger and more differentiated + joints; pectoral girdle dissociated from skull; limb girdles more closely attached to axial skeleton (forces transfer to the center axis)

how did primitive tetrapods move

swimming —> walking; more robust limbs; intrinsic muscles in limbs

how did primitive tetrapods breathe air

lungs (earliest adaptation for terrestrially and appeared in bony fishes); swim bladders evolved from lungs; gills —>gills and lungs —> lungs only

how did primitive tetrapods conserve water

ammonia —> uric acid; use of kidneys; mucous glands, postures, behaviors to limit loss in amphibians

how did primitive tetrapods hear

• Hyomandibula bone of the 1st gill arch gives rise to stapes, an inner ear bone that connects to tympanum.

• Enhances conduction of sound vibrations to the inner ear.

• Loss of lateral line system for vibration detection in water.

  • Still present in amphibian larvae.

how did primitive tetrapods feed

• Loss of suction feeding.

• Increasing use of jaws and tongue

how did primitive tetrapods reproduce

amphibia: largely tied to water; External fertilization in anurans (as with fish); Aquatic eggs; Aquatic larvae

amniotes : amniotic egg

Early amniotes

reptilomorph outgroups to extant amniotes; diverse, terrestrial, some herbivores

Amniote synapomorphies

• Amniotic egg

• Internal fertilization

• Loss of labyrinthodont teeth

• Several other skull and limb characters

amnion

• surrounds embryo

• Forms space filled with amniotic fluid

• permeable to allow Gas exchange

• Protects embryo

  • Desiccation

  • Concussion (cushion from falls)

• Becomes the innermost layer of the placenta

Chorion

• Outermost membrane within the shell

• Forms other/outer layer of placenta in mammals

Allantois

• Sac formed from posterior part of GI

• Collects wastes during development

• Grows as it fills (yolk sac shrinks simultaneously)

• Involved in formation of umbilical cord in placental mammals

Yolk Sac

• Filled with yolk, nutrient source for embryo

• Shrinks as yolk is used

Shell

• Other membranes from outside embryo but are connected to embryo.

• Shell is created by mother.

• Can be calcified or leathery.

• Involved in gas exchange and protection

transition to dry habitats

many amphibians become terrestrial (but still have permeable skins and need water to reproduce); but amniotes lifecycle are independent of water bc of amniotic egg and skin (Increased keratinization, Deposition of lipid in epidermis, Scales, feathers, hair)

3 Major amniote groups

synapsida (mammalia and stem mammals), anapsida (testudines and stem turtles), diapsida (archosauria, lepidosaura, and others)

synapsida

one temporal fenestration (opening without structure through it behind the orbit/eyesocket) most likely ancestral form

anapsid

The presence of ___ skulls in turtles means there was a major change among the diapsid condition widespread in reptiles.

Mammalia Synapomorphies

Lower temporal fenestra

Atlas vertebra

Hair

Mammary glands