Amniota is an extensive group of vertebrates, comprising both reptiles and mammals alike. This includes most species of reptile, birds, crocodiles, and turtles.
They share a common ancestor, yet have split off into distinct major clades with distinct physiological and morphological characteristics. These lineages exhibit varied traits and behaviors.
Origins
Amniota, also referred to as “amniotic vertebrates,” are extant reptiles, birds and mammals that produce live young through an amniotic egg. This evolutionary innovation–which protects developing embryos or fetuses from damage or squishing–occurred during the Late Carboniferous Period.
Amniotes are an intriguing group of vertebrates, comprising extant squamates, turtles, crocodiles and birds. These creatures evolved from the first reptiles with shelled eggs that could survive in dry environments.
Amniotes defined by the presence of an amnion during embryonic or fetal development, which lines the cavity and provides protection from physical harm. This tough extraembryonic layer originates from trophoblast cells within the egg. Once mature, this membrane fuses with the chorion to form an amniotic sac.
The development of the amnion during embryonic or early fetal stage is highly intricate and involves numerous intricate processes, such as neurulation. The amnion plays a significant role in this period as it serves as the connecting link between trophoblast cells of the egg and zygote.
To achieve this goal, an amnion created during embryonic or fetal development to accommodate the developing embryo. This process, known as neurulation, takes place over at least four stages.
These processes involve elevation of the neural folds and bending of the neural plate. Once neurulation has taken place, the amnion sealed into a neural tube, a tubular structure containing bundles of nerves.
Some amniotes, such as whales and some marine turtles, do not develop an amnion during their embryonic stage, allowing them to live entirely underwater. On the other hand, many terrestrial amniotes that lay eggs on land require the amniotic membrane for survival.
Amniotes can divide into three major groups according to the number of temporal fenestrae (openings behind each eye) in their skull roofs, extraembryonic membranes and morphology. Understanding an amniote’s evolutionary history requires consideration of this feature.
Amniota Adaptations
Amniota are a group of limbed vertebrates that includes all living reptiles (Class Reptilia), birds (Class Aves) and mammals (Class Mammalia), as well as their extinct relatives and ancestors.
Amniotes stand out from other tetrapods by developing within a series of protective extra-embryonic membranes known as the chorion, amnion and allantois. These membranes form one of the most distinguishing characteristics of amniotes.
Chorion and amnion further safeguarded by leathery or hard calcareous shells in birds and most live-bearing reptiles. These membranes form the placenta, which supplies nutrients and oxygen to developing fetuses during their early development period.
Amniotes are air-breathing, ectothermic vertebrates with tough waterproof scales on their skin. This adaptation has made them well suited for life on land.
Another remarkable adaptation of amniotes is their amniotic egg, which protects embryos during their early weeks of development and makes them completely independent from water for reproduction. This enables them to thrive in drier habitats which are essential for their survival.
Thirdly, tetrapods evolved the ability to have rib cage ventilation, which permits efficient air-breathing. This evolutionary change enabled them to settle on land.
Amniotes have developed an entirely unique method for eliminating waste, which differs from that of their sauropsid ancestors. Instead of excreting uric acid, they excrete urea with small bits of water – especially helpful in dry environments where uric acid can quickly become toxic for tetrapods.
Amniotes have evolved through an adaptive strategy that enabled them to form some of the most diverse and successful lineages among terrestrial vertebrates. Their success can attribute to a number of key traits that allowed them to survive in a variety of environments for 400 million years.
One of the most remarkable adaptive traits is facial evolution. Amniotes’ faces have been the subject of much research due to their remarkable level of change over 150 million years, and a better understanding of their anatomy and developmental biology could explain this remarkable range of facial variation.
Amniota Life Cycles
Amniota has undergone various life cycles throughout its evolution. The most notable ones are the oviparous and placental cycles, which involve birds, mammals, and reptiles (see below).
Amniotic eggs contain extra-embryonic membranes that provide nutrients, waste disposal and gas exchange during development to the embryo. Bird and reptile eggs also feature a yolk sac which supplies nitrogen for the growing embryo.
The amnion, unique to Amniota, is a membrane that separates the early embryo from the large mass of yolk. It also prevents the embryo from adhering to eggshell membranes and minimizes mechanical damage during development.
Comparatively, in birds, reptiles and mammals the embryo encased in a leathery or hard-shelled egg. Hemiphractid frogs however, have their embryos housed within multinucleated oocytes that have enhanced with transcripts during multinucleated oogenesis.
It is crucial for these amniotes to develop from large eggs, which provide them with an advantage over hemiphractid tadpole oocytes which contain only minimal transcripts.
Amniotes have also been known to increase their egg size through meroblastic cleavage, which allows the development of extra-embryonic structures like the amnion and chorioallantois (see above). These organs allow an embryo to receive all necessary nutrients without needing a watery environment like land.
These adaptations have allowed amniotes to spread from ocean predators onto land, where they developed new reproductive strategies in order to sustain their large populations and ability to flourish on land.
The major clades of amniotes have developed unique strategies to cope with the changes they have endured. These distinctions can see in their gross morphology as well as strategies for maintaining population size.
One of the most noteworthy characteristics of marsupials is their much slower rate of reproduction than placentals, leading to smaller offspring than any other amniote group. This explains why marsupials typically produce smaller litters and produce less biomass when of similar size as placentals.
Amniota Evolution
Amniota are mainly terrestrial vertebrate families, such as reptiles, birds and mammals. The amniotic egg is what sets them apart from their amphibian ancestors and marks an important step in the evolution of tetrapods.
In addition to the amniotic egg, amniotes possess several other features that enable them to survive and reproduce on land. These include a hard shell that retains water (versus amphibians’ soft, waterproof shells) as well as membranes separating the egg from its body; these membranes also prevent eggs from sticking onto extraembryonic materials like placenta or uterus.
These membranes also shield the embryo from predators. In times past, amphibians had to compete with other species for food and shelter; thus, these traits proved essential in their rise over their aquatic counterparts.
Another significant aspect of an amniotic egg is that it provides an early stage of development for the embryo. This has proved essential in protecting young and has contributed to the evolution of many complex and adaptive traits found among amniotes.
This developmental stage may have evolved for various reasons, including the necessity to delay the deposition of an egg with an embryo. Indeed, it shares many convergent evolutionary relationships with other vertebrate groups that share similar developmental requirements as amniotes such as fishes and crocodiles.
An evolutionary trait of amniotic eggs is their development of chorioallantois, which serves as a temporary embryonic exchange organ, supporting development and transporting nutrients from ectoderm to endoderm. This organ has observed in many extant bird species and may have evolved as a means to nurture embryos during their early stages.
Chorioallantois may provide a mechanism for embryo migration from the ectoderm to its endoderm, as well as aiding fertilization and release of proteins essential for development. This trait has long observed among amniotes, along with their amniotic membrane.
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