The amniotes are a group of tetrapod Tetrapods are vertebrate animals having four feet, legs or leglike appendages. Amphibians, reptiles, dinosaurs/birds, and mammals are all tetrapods, and even the limbless snakes are tetrapods by descent. The earliest tetrapods radiated from the Sarcopterygii, or lobe-finned fish, evolving into air-breathing amphibians in the Devonian period vertebrates Vertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. The grouping sometimes includes the hagfish, which have no vertebrae, but are genetically quite closely related to lampreys, which do have vertebrae. For this reason, the sub-phylum is sometimes referred to as "Craniata", as all members do that have a terrestrially adapted egg In most birds and reptiles, an egg is the zygote, resulting from fertilization of the ovum. To enable incubation the egg is usually kept within a favourable temperature range as it nourishes and protects the growing embryo. When the embryo is adequately developed it breaks out of the egg in the process of hatching. Some embryos have a temporary. They include mammals Mammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain, birds Birds are winged, bipedal, endothermic (warm-blooded), vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the 5 cm (2 in) Bee Hummingbird to the 2.7 m (8 ft 10 in) Ostrich and reptiles Reptiles, or members of the class Reptilia, are air-breathing, cold-blooded amniotes that have skin covered in scales or scutes as opposed to hair or feathers. They are tetrapods and lay amniote eggs, whose embryos are surrounded by the amnion membrane. Modern reptiles inhabit every continent with the exception of Antarctica, and four living, as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes. In humans, these membranes include the amniotic sac An artificial rupture of membranes , also known as an amniotomy, may be performed by a midwife or obstetrician. This is usually performed using an amnihook and is intended to induce or accelerate labour that surrounds the fetus. These embryonic membranes, and the lack of a larval stage A larva is a young (juvenile) form of animal with indirect development, going through or undergoing metamorphosis (for example, insects, amphibians, or cnidarians), distinguish amniotes from tetrapod amphibians Amphibians , such as frogs, toads, salamanders, newts, and caecilians, are ectothermic (or cold-blooded) animals that metamorphose from a juvenile, water-breathing form to an adult, air-breathing form. Though amphibians typically have four limbs, the Caecilians are notable for being limbless. Unlike other land animals (amniotes), amphibians lay.^[1]

The first amniotes, which resembled small lizards, probably evolved 340 million years ago (Casineria). Their eggs could survive out of the water, allowing amniotes to branch out into drier environments. The eggs could also "breathe" and cope with waste, allowing the eggs and the amniotes themselves to evolve into larger forms. The amniotes spread across the globe and became the dominant land vertebrates. Very early in their history the amniotes split into two main lines. The oldest known fossil synapsid Synapsids , also known as theropsids ('beast face'), are a class of animals that includes mammals and everything closer to mammals than to other living amniotes. In classical systematics, the non-mammalian members are described as mammal-like reptiles, and are sometimes referred to as "proto-mammals" or "stem-mammals" in is Protoclepsydrops from about 320 million years ago, while the oldest known sauropsid Sauropsida is a group of amniotes that includes (among other things) all existing reptiles, dinosaurs, and birds. The Sauropsida is distinguished from Theropsida ("beast faces"), more commonly called Synapsida, which includes mammals is probably Paleothyris Paleothyris was a small, agile, lizard-like reptile which lived in the Middle Pennsylvanian epoch in Nova Scotia and is the oldest known amniote. Paleothyris had sharp teeth and large eyes, meaning that it was a nocturnal hunter. It was about a foot long. It probably fed on insects and other smaller animals found on the floor of its forest home, in the order Captorhinida, from the Middle Pennsylvanian epoch (ca. 306-312 million years ago).

Description

Amniotes can be characterized in part by embryonic development Embryogenesis is the process by which the embryo is formed and develops. It starts with the fertilization of the ovum (or egg) which, after fertilization, is referred to as a zygote. The zygote undergoes rapid mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading to development of an that includes the formation of several extensive membranes, the amnion The amnion is a membranous sac that surrounds and protects an embryo. It is developed in reptiles, birds, and mammals, which are hence called “Amniota”; but not in amphibians and fish, which are consequently termed “Anamniota”. The primary function of this is the protection of the embryo for its development, chorion The chorion is one of the membranes that exists during pregnancy between the developing fetus and mother. It is formed by extraembryonic mesoderm and the two layers of trophoblast and surrounds the embryo and other membranes. The chorionic villi emerge from the chorion, invade the endometrium, and allow transfer of nutrients from maternal blood to, and allantois Allantois is a part of a developing animal conceptus (which consists of all embryonic and extra-embryonic tissues). It helps the embryo exchange gases and handle liquid waste. Amniotes develop directly into a (typically) terrestrial form with limbs and a thick stratified epithelium, rather than first entering a feeding larval tadpole stage followed by metamorphosis as in amphibians Amphibians , such as frogs, toads, salamanders, newts, and caecilians, are ectothermic (or cold-blooded) animals that metamorphose from a juvenile, water-breathing form to an adult, air-breathing form. Though amphibians typically have four limbs, the Caecilians are notable for being limbless. Unlike other land animals (amniotes), amphibians lay. In amniotes the transition from a two-layered periderm to cornified epithelium is triggered by thyroid hormone during embryonic development, rather than metamorphosis.^[2] The unique embryonic features of amniotes may reflect specializations of eggs to survive drier environments, or the massive size and yolk content of eggs evolved for direct development to a larger size.

Features of amniotes evolved for survival on land include a sturdy but porous leathery or hard eggshell, and an allantois Allantois is a part of a developing animal conceptus (which consists of all embryonic and extra-embryonic tissues). It helps the embryo exchange gases and handle liquid waste evolved to facilitate respiration while providing a reservoir for disposal of wastes. Their kidneys The kidneys are paired organs seen in many types of animals, including vertebrates and some invertebrates. Part of the urinary system, they are responsible for urine production as well as a number of other homeostatic functions. These include regulation of electrolytes, acid-base balance, and blood pressure; excretion of wastes such as urea and and large intestines The large intestine is the last part of the digestive system—the final stage of the alimentary canal—in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body are also well-suited to water retention. Most mammals do not lay eggs, but corresponding structures may be found inside the placenta The placenta is an organ unique to mammals that connects the developing fetus to the uterine wall. The placenta supplies the fetus with oxygen and food, and allows fetal waste to be disposed of via the maternal kidneys. The word placenta comes from the Latin for cake, from Greek plakóenta/plakoúnta, accusative of plakóeis/plakoús - πλακόε.

The first amniotes, such as Casineria kiddi, which lived about 340 million years ago, resembled small lizards. Their eggs were small and covered with a membrane, not a hard shell like most modern amniote eggs. Although some modern amphibians lay eggs on land, with or without significant protection, they all lack advanced traits like an amnion. This kind of egg only became possible with internal fertilization. The outer membrane, a soft shell, evolved as a protection against the harsher environments on land, as species evolved to lay their eggs on land where they were safer than in the water. One can assume the ancestors of the amniotes laid their eggs in moist places, as such modest-sized animals wouldn't have too many difficulties in finding depressions under fallen logs or other suitable places in the ancient forests, and dry conditions were probably not the main reason why the soft shell emerged.^[3]

In fish and amphibians there is only one inner membrane, also called an embryonic membrane. In amniotes the inner anatomy of the egg has evolved further and new structures have developed to take care of the gas exchanges between the embryo and the atmosphere, as well as dealing with the waste problems. In order to grow a thicker and tougher shell new ways to supply the embryo with oxygen had to be developed as diffusion alone was not enough. After the egg had developed these structures, further sophistication allowed the amniotes to lay much bigger eggs in much drier habitats. Bigger eggs allowed for bigger offspring and bigger adults could produce bigger eggs, which meant that the amniotes developed the opportunity to grow bigger than their ancestors. Real growth was not possible however, until they stopped relying on small invertebrates as their main food source and started to eat plants or other vertebrates, or returned to the water. New habits and heavier bodies meant further evolution for the amniotes, both in behavior and anatomy.

There are three main lines of amniotes, which may be distinguished by the structure of the skull The skull is a bony structure found in the head of many animals. The skull supports the structures of the face and protects the head against injury and in particular the number of temporal fenestrae The skull is a bony structure found in the head of many animals. The skull supports the structures of the face and protects the head against injury (openings) behind the eye. In anapsids (turtles) there are none, in synapsids Synapsids , also known as theropsids ('beast face'), are a class of animals that includes mammals and everything closer to mammals than to other living amniotes. In classical systematics, the non-mammalian members are described as mammal-like reptiles, and are sometimes referred to as "proto-mammals" or "stem-mammals" in (mammals and their extinct relatives) there is one, and in most diapsids (non-anapsid reptiles, dinosaurs, and birds) there are two.

The skeletal remains of amniotes can be identified by their having at least two pairs of sacral The sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx . In children, it consists of usually five unfused vertebrae which begin to fuse ribs In vertebrate anatomy, ribs are the long curved bones which form the ribcage. In most vertebrates, ribs surround the chest (Greek:θώραξ, Latin thorax) and protect the lungs, heart, and other internal organs of the thorax. In some animals, especially snakes, ribs may provide support and protection for the entire body, a sternum The sternum is a long flat bone (or, in some models, set of three bones) located in the center of the thorax (chest). It connects to the rib bones via cartilage, forming the anterior section of the rib cage with them, and thus helps to protect the lungs, heart and major blood vessels from physical trauma in the pectoral girdle The pectoral girdle is the set of bones which connect the upper limb to the axial skeleton on each side. It consists of the clavicle and scapula in humans and, in those species with three bones in the pectoral girdle, the coracoid. Some mammalian species have evolved to have only the scapula (some amniotes have lost it) and an astragalus The talus bone or astragalus is a bone in the tarsus of the foot that forms the lower part of the ankle joint through its articulations with the lateral and medial malleoli of the two bones of the lower leg, the tibia and fibula. Within the tarsus, it articulates with the calcaneus below and navicular in front. Through these articulations, it bone in the ankle In human anatomy, the ankle joint is formed where the foot and the leg meet. The ankle, or talocrural joint, is a synovial hinge joint that connects the distal ends of the tibia and fibula in the lower limb with the proximal end of the talus bone in the foot. The articulation between the tibia and the talus bears more weight than between the.

Definition and classification

Amniota was first formally described by embryologist Ernst Haeckel Ernst Heinrich Philipp August Haeckel , also written von Haeckel, was an eminent German biologist, naturalist, philosopher, physician, professor and artist who discovered, described and named thousands of new species, mapped a genealogical tree relating all life forms, and coined many terms in biology, including phylum, phylogeny, ecology and the in 1866 Year 1866 was a common year starting on Monday (link will display the full calendar) of the Gregorian calendar (or a common year starting on Saturday of the 12-day-slower Julian calendar) on the presence of the amnion The amnion is a membranous sac that surrounds and protects an embryo. It is developed in reptiles, birds, and mammals, which are hence called “Amniota”; but not in amphibians and fish, which are consequently termed “Anamniota”. The primary function of this is the protection of the embryo for its development, hence the name. A problem with this definition is that the trait (apomorphy Cladistics, from the ancient Greek κλάδος, klados, "branch", is the hierarchical classification of species based on phylogeny or evolutionary ancestry. The term phylogenetics is often used synonymously with cladistics. Cladistics is distinguished from other taxonomic systems because it focuses on the evolutionary relationships of) in question do not fossilize, and the status of fossil forms has to be interfered from other traits. Thus Jacques Gauthier Jacques Armand Gauthier is a vertebrate paleontologist, comparative morphologist, and systematist, and one of the founders of the use of cladistics in biology and colleagues redefined Amniota in 1988, as "the most recent common ancestor of extant mammals and reptiles, and all its descendants".^[4] Gauthiers definition being node-based, the group under his definition has a slightly different content than than the group defined as biological amniotes (apomorphy-based clade).

Traditional classification

Classifications of the amniotes have traditionally recognised three classes The composition of each class is determined by a taxonomist. Often there is no exact agreement, with different taxonomists taking different positions. There are no hard rules that a taxonomist needs to follow in describing a class, but for well-known animals there is likely to be consensus. For example, dogs are usually assigned to the class, based on major traits and physiology Physiology is the study of the mechanical, physical, and biochemical functions of living organisms. Physiology has traditionally been divided between plant physiology and animal and all living things physiology but the principles of physiology are universal, no matter what particular organism is being studied. For example, what is learned about:

• Class Reptilia Reptiles, or members of the class Reptilia, are air-breathing, cold-blooded amniotes that have skin covered in scales or scutes as opposed to hair or feathers. They are tetrapods and lay amniote eggs, whose embryos are surrounded by the amnion membrane. Modern reptiles inhabit every continent with the exception of Antarctica, and four living (reptiles)
  • Subclass Anapsida While "anapsid reptiles" or "anapsida" are traditionally spoken of as if they were a coherent group, it has been suggested that several groups of reptiles that had anapsid skulls may be only distantly related: scientists still debate the exact relationship between the basal reptiles which first appeared in the late ("proto-reptiles", possibly including turtles Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield. "Turtle" may either refer to the Testudines as a whole, or to particular Testudines which make up a form taxon that is not monophyletic—see also sea turtle, terrapin, tortoise, and the)
  • Subclass Synapsida Synapsids , also known as theropsids ('beast face'), are a class of animals that includes mammals and everything closer to mammals than to other living amniotes. In classical systematics, the non-mammalian members are described as mammal-like reptiles, and are sometimes referred to as "proto-mammals" or "stem-mammals" in (mammal-like reptiles, progenitors of mammals Mammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain)
  • Subclass Diapsida Diapsids are a group of reptiles that developed two holes (temporal fenestra) in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, and tuatara. Under modern classification systems, even birds are considered diapsids, since (majority of reptiles, progenitors of birds Birds are winged, bipedal, endothermic (warm-blooded), vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the 5 cm (2 in) Bee Hummingbird to the 2.7 m (8 ft 10 in) Ostrich)
• Class Aves Birds are winged, bipedal, endothermic (warm-blooded), vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the 5 cm (2 in) Bee Hummingbird to the 2.7 m (8 ft 10 in) Ostrich (birds)
  • Subclass Neornithes Modern birds are characterised by feathers, a beak with no teeth , the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. All birds have forelimbs modified as wings and most can fly, with some exceptions including ratites, penguins, and a number of diverse endemic island species (all modern birds, several extinct subclasses recognised)
• Class Mammalia Mammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain (mammals)
  • Subclass Monotremata (egg-laying mammals)
  • Subclass Theria Theria is a subclass of mammals that give birth to live young without using a shelled egg, including both eutherians (placental mammals) and metatherians (marsupials and their ancestors) (marsupials and placental mammals)

This rather orderly scheme is the one most commonly found in popular and basic scientific works. It has come under critique from cladistics, as the class Reptilia is paraphyletic, that is, it has given rise to two other classes not included in Reptilia.

Phylogenetic classification

With the advent of cladistics, some researchers have attempted to establish new classes, based on phylogeny, but disregarding the physiological and anatomical unity of the groups. One such classification, by Michael Benton, is presented in simplified form below.^[5]

• Series Amniota ("reptiles")
  • Class Synapsida - includes non-mammalian synapsids (formerly mammal-like reptiles)
    • *Order Pelycosauria †
    • Order Therapsida
      • Class Mammalia - Mammals
  • Class Sauropsida
    • Subclass Anapsida
      • Order Testudines - Turtles
    • Subclass Diapsida
      • Order Araeoscelidia
      • Order Younginiformes
      • Infraclass Ichthyosauria †
      • Infraclass Lepidosauromorpha
        • Superorder Sauropterygia
          • Order Placodontia †
          • Order Nothosauroidea †
          • Order Plesiosauria †
        • Superorder Lepidosauria
          • Order Sphenodontida - Tuatara
          • Order Squamata - Lizards & snakes
      • Infraclass Archosauromorpha
        • • • Order Prolacertiformes †
            • Division Archosauria
              • Subdivision Crurotarsi
                • Order Crocodylia - Crocodilians
              • Subdivision Avemetatarsalia
                • Order Pterosauria †
                • Superorder Dinosauria
                  • Order Ornithischia †
                  • Order Saurischia
                    • Class Aves - Birds

Cladogram of Amniotes

The cladogram presented here illustrates the phylogeny (family tree) of amniotes, and follows a simplified version of the relationships found by Laurin & Reisz (1995).^[6] The cladogram covers the group as defines under Gauthier's definition.

References

1. ^ Benton, Michael J. (1997). Vertebrate Palaeontology. London: Chapman & Hall. pp. 105-109. ISBN 0-412-73810-4.
2. ^ www.pnas.org
3. ^ Stewart J. R. (1997): Morphology and evolution of the egg of oviparous amniotes. In: S. Sumida and K. Martin (ed.) Amniote Origins-Completing the Transition to Land (1): 291-326. London: Academic Press.
4. ^ Gauthier, J., Kluge, A.G. and Rowe, T. (1988). "The early evolution of the Amniota." Pp. 103-155 in Benton, M.J. (ed.), The phylogeny and classification of the tetrapods, Volume 1: amphibians, reptiles, birds. Oxford: Clarendon Press.
5. ^ Benton, M.J. (2004). Vertebrate Paleontology. Blackwell Publishers. xii-452. ISBN 0-632-05614-2.
6. ^ Laurin, M. and Reisz, R.R. (1995). "A reevaluation of early amniote phylogeny." Zoological Journal of the Linnean Society, 113: 165-223.

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