amniotes


357
97 shares, 357 points
« Back to Glossary Index
amniotes (Wikipedia)

Amniotes
Temporal range:
PennsylvanianPresent (Possible Mississippian record)
Amniota.jpg
Example of amniotes: Vulpes vulpes and Dimetrodon grandis (two synapsids) & Haliaeetus leucocephalus and a Dermochelys coriacea (two sauropsids).
Scientific classification e
Kingdom:Animalia
Phylum:Chordata
Superclass:Tetrapoda
Clade:Reptiliomorpha
Clade:Amniota
Haeckel, 1866
Clades

Amniotes (from Greek ἀμνίον amnion, "membrane surrounding the fetus", earlier "bowl in which the blood of sacrificed animals was caught", from ἀμνός amnos, "lamb") are a clade of tetrapod vertebrates comprising the reptiles, birds, and mammals. Amniotes lay their eggs on land or retain the fertilized egg within the mother, and are distinguished from the anamniotes (fishes and amphibians), which typically lay their eggs in water. Older sources, particularly prior to the 20th century, may refer to amniotes as "higher vertebrates" and anamniotes as "lower vertebrates", based on the discredited idea of the evolutionary great chain of being.

Amniotes are tetrapods (descendants of four-limbed and backboned animals) that are characterised by having an egg equipped with an amnion, an adaptation to lay eggs on land rather than in water as the anamniotes (including frogs) typically do. Amniotes include synapsids (mammals along with their extinct kin) and sauropsids (reptiles and birds), as well as their ancestors, back to amphibians. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes. In eutherian mammals (such as humans), these membranes include the amniotic sac that surrounds the fetus. These embryonic membranes and the lack of a larval stage distinguish amniotes from tetrapod amphibians.

The first amniotes, referred to as "basal amniotes", resembled small lizards and evolved from the amphibian reptiliomorphs about 312 million years ago, in the Carboniferous geologic period. Their eggs could survive out of the water, allowing amniotes to branch out into drier environments. The eggs could also "breathe" and cope with wastes, allowing the eggs and the amniotes themselves to evolve into larger forms.

The amniotic egg represents a critical divergence within the vertebrates, one enabling amniotes to reproduce on dry land—free of the need to return to water for reproduction as required of the amphibians. From this point the amniotes spread around the globe, eventually to become the dominant land vertebrates. Very early in their evolutionary history, basal amniotes diverged into two main lines, the synapsids and the sauropsids, both of which persist into the modern era. The oldest known fossil synapsid is Protoclepsydrops from about 312 million years ago, while the oldest known sauropsid is probably Paleothyris, in the order Captorhinida, from the Middle Pennsylvanian epoch (c. 306–312 million years ago).

« Back to Glossary Index

Like it? Share with your friends!

357
97 shares, 357 points
Choose A Format
Personality quiz
Series of questions that intends to reveal something about the personality
Trivia quiz
Series of questions with right and wrong answers that intends to check knowledge
Poll
Voting to make decisions or determine opinions
Story
Formatted Text with Embeds and Visuals
List
The Classic Internet Listicles
Open List
Submit your own item and vote up for the best submission
Ranked List
Upvote or downvote to decide the best list item
Meme
Upload your own images to make custom memes
Video
Youtube, Vimeo or Vine Embeds
Audio
Soundcloud or Mixcloud Embeds
Image
Photo or GIF
Gif
GIF format