Biol 108 - Topic 31 - Clade Amniotes

Clade Amniotes includes extant ____, including birds, mammals, and many extinct groups, e.g., dinosaurs.

reptiles

Amniote shared derived traits:

• The amniotic egg, the key trait of amniotes

• Internal fertilization via specialized intromittent organs

  • Internal fertilization is necessary for amniotic egg development; therefore, a mechanism of sperm transfer must have been present in the common ancestor of amniotes

• Amniotes undergo direct development into terrestrial forms, without aquatic larvae or metamorphosis in their life cycle.

amniotic egg

• The embryo is surrounded and protected by four extraembryonic membranes: amnion (protects the embryo from mechanical shock), yolk sac (contains yolk, a nutrient store), allantois (waste-disposal sac and gas-exchange organ), and chorion (O₂/CO₂ exchange from embryo to air across the shell).

• The amniotic egg provides gas exchange (allantois, chorion), nutrients (yolk sac and albumen), waste removal (allantois), and protection from desiccation (shell, amnion, chorion) and mechanical shock (amnion/amniotic fluid).

• Amniotic eggs of most reptiles and some mammals have a porous calcareous or leathery shell, which prevents desiccation (the shell is lost in most mammals).

• The amniotic egg is an adaptation to lay eggs on land rather than in water (reproduction is independent of water).

• Amniotes lay their eggs on land (often buried) or retain fertilized eggs internally.

amnion

protects the embryo from mechanical shock

yolk sac

contains yolk, a nutrient store

allantois

waste-disposal sac and gas-exchange organ

chorion

O₂/CO₂ exchange from embryo to air across the shell

intromittent organ,

external genitalic organ of males that is specialized to deliver sperm during copulation and is characteristic of internal fertilization.

four extraembryonic membranes:

• amnion (protects the embryo from mechanical shock)

• yolk sac (contains yolk, a nutrient store)\

• allantois (waste-disposal sac and gas-exchange organ),

• chorion (O₂/CO₂ exchange from embryo to air across the shell).

The amniotic egg provides gas exchange (two struc)

(allantois, chorion)

The amniotic egg provides nutrients (___ and ____)

yolk sac and albumen

The amniotic egg provides waste removal (_____)

allantois

The amniotic egg provides protection from desiccation (______)

external shell, amnion, chorion

The amniotic egg provides protection mechanical shock (____).

amnion/amniotic fluid

Amniotic eggs of most reptiles and some mammals have a _______, which prevents desiccation (the shell is lost in most mammals).

porous calcareous or leathery shell

• In most mammals, the shell is lost, and the functions of the amniotic membranes are adapted for internal development (e.g. allantois forms the umbilical cord, chorion forms the placenta, and the amniotic sac encloses the embryo)

The amniotic egg is an adaptation to lay eggs on land rather than in water (_________).

sex. reproduction is independent of water

Amniotes lay their eggs on ____ often buried) or retain fertilized eggs _____.

land; internally

Additional amniote adaptations for terrestrial life:

• Thick, layered, and relatively impermeable skin that is less permeable to water loss than the skin of amphibians.

  • Allows for water retention as the layered skin reduces water loss and prevents dehydration

• Lung ventilation powered by axial skeletal muscles: muscles associated with the rib cage ventilate the lungs.

Lung ventilation powered by _______.

axial skeletal muscles: muscles associated with the rib cage ventilate the lungs

axial skeletal muscles:

muscles associated with the rib cage ventilate the lungs

Amphibians and amniotes diverged from a common ancestor about 350 mya, and early amniotes exhibited _____ tolerance to drier terrestrial environments compared to early amphibians.

greater

The earliest amniotes were _____ that were similar in appearance to modern lizards, e.g. Hylonomus.

small predators

Classification of amniotes

• The classification of amniotes is based in part on the number of major openings, known as temporal fenestrae, in the post-eye area of the skull: anapsids (lack temporal fenestrae, e.g. turtles), synapsids (one pair of temporal fenestrae, e.g. mammals), diapsids (two pairs of temporal fenestrae, e.g. reptiles).

fenestra

major openings in the post-eye area of the skull

anapsids

lack temporal fenestrae, e.g. turtles

synapsids

one pair of temporal fenestrae, e.g. mammals

diapsids

two pairs of temporal fenestrae, e.g. reptiles

Includes turtles, crocodilians and birds (the remaining extant groups of Archosaurs), and lepidosaurs (incl. lizards and snakes).

• Some diapsids have lost either one (lizards) or both (snakes and turtles) temporal fenestra, or have a heavily restructured skull (modern birds)

Two clades of amniotes:

1. Diapsids (Reptiles). Includes turtles, crocodilians and birds (the remaining extant groups of Archosaurs), and lepidosaurs (incl. lizards and snakes).

2. Synapsids. Includes mammals and numerous extinct taxa.