Amniotes are a group of tetrapods that include reptiles (including birds) and mammals.
They possess adaptations that facilitate life on land, with the amniotic egg being a key unifying feature.
Amniotic Egg
Definition: The amniotic egg is a defining shared derived trait of clade Amniotes.
Key Features:
Forms four extraembryonic membranes in reptiles and mammals.
Provides environments ideal for land reproduction.
Facilitates gas exchange from the chorion and allantois.
Supplies nutrients via yolk sac and albumen.
Offers protection:
Desiccation: By the amnion, chorion, and external shell (varies in structure between reptiles and mammals).
Mechanical Shock: Cushioned by amniotic fluid.
Manages waste through the allantois.
Reproduction:
Allows for sexual reproduction independent of aquatic environments.
Can involve laying eggs on land or retaining fertilized eggs internally, as seen in many mammals.
Internal Development in Mammals
In some mammals, the eggshell is absent, and functions of amniotic membranes adapt for internal development.
The allantois forms the umbilical cord.
The chorion becomes the placenta.
The amniotic sac encases the embryo.
Internal Fertilization
Amniotes utilize specialized intromittent organs for sperm delivery during copulation, allowing for internal fertilization, critical for amniotic egg development.
Intromittent organs indicate the evolutionary adaptation necessary for reproduction in terrestrial environments.
Development Process
Amniotes exhibit direct development into terrestrial forms, bypassing aquatic larval stages.
Adaptations for Terrestrial Life
Water Retention:
Amniotes have thick, layered skin that reduces water loss.
Efficient Lung Ventilation:
The axial skeletal muscles enhance lung ventilation, differing from the inefficient buccal pump used by amphibians.
Evolution of Early Amniotes
Early amniotes diverged from a common ancestor with amphibians around 350 million years ago.
They displayed features suited for drier environments compared to early amphibians.
One of the earliest known amniotes, Hylonomus, was small and predatory, resembling modern lizards.
Classification of Amniotes
Classification is based on the number of temporal fenestrae (openings) in the skull:
Anapsid: No temporal fenestrae (e.g., turtles).
Synapsid: One pair of temporal fenestrae (e.g., mammals).
Diapsid: Two pairs of temporal fenestrae (e.g., reptiles), though some have lost one or both.
Major Groups
Amniotes are a monophyletic group with two main clades:
Diapsids:
Includes turtles, crocodilians, birds, and lepidosaurs (lizards and snakes).