Amniotes protect their embryos with extended membranes or by keeping them inside the body. They include mammals, reptiles, birds, and extinct mammal-like dinosaurs. Amniotes emerged 350-310 million years ago, creating large land animals that didn’t depend on water holes for reproduction. The Carboniferous period saw the emergence of the first amniotes, resembling small lizards. The earliest reptiles were Westlothiana, Petrolacosaurus, and Mesosaurus. The Carboniferous was a time when true forests began to grow, providing a food source for animals that could stray from water.
Amniotes are animals that protect their offspring’s embryos by using extended membranes or by keeping them inside the body, unlike most other animals, which lay eggs floating in water. Amniotes include mammals, reptiles, birds and reptiles, and extinct mammal-like dinosaurs (theropsids). Of all 38 animal phyla, only one has amniote members: the chordates, and even then many chordates, which include fish, sharks, rays and amphibians, are not amniotes. However, all non-amphibian tetrapods are amniotes.
Amniotes were nature’s way of creating large land animals that were truly terrestrial, meaning they didn’t depend on water holes as a means for reproduction. Terrestrial animals existed long before amniotes, but mostly as insects. The first amniotes resembled small lizards, and are said to have emerged between 350 and 310 million years ago, during the Carboniferous period. Instead of laying eggs with hard shells, they were surrounded by a hard membrane. These early amniotes were also capable of mating on land, a trait not possessed by amphibians. Prior to this, vertebrate animals, in the form of amphibians, colonized only the shores of ponds, lakes, oceans and rivers, but never went far inland. The emergence of amniotes signaled the eventual decline of amphibians and a flourishing of land-based tetrapod megafauna.
One of the earliest reptiles (reptile-like tetrapods) was Westlothiana, which bore a superficial resemblance to modern lizards. The Westlothiana fossils have been dated to 350 million years ago. Westlothiana exhibits a mix of amphibian and amniote characteristics, and was originally considered a stem amniote, but some scientists have since placed it outside the amniote group. Westlothiana made headlines for being discovered along with other Carboniferous fossils in the walls surrounding a Scottish football pitch. Petrolacosaurus and Mesosaurus are other early tetrapods that have a reptilian appearance, although they were amphibious.
The Carboniferous was a time when true forests finally began to grow in fertile areas of the earth, providing a prodigious food source for any animal evolutionarily gifted enough to stray significant distances from standing water. Competition among amphibians in regions around water would have progressively driven some variants away, until eventually some developed the ability to wrap their eggs in a robust membrane. It all happened incrementally, but within a few tens of millions of years there were animals laying rock-hard eggs in the middle of barren deserts.
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