What Derived Trait Arose First After Amphibians And Other Animals Diverged?

Identify characteristics of amniotes

Amniotes are a clade of tetrapod vertebrates comprising the reptiles, birds, and mammals. Amniotes are characterized past having an egg equipped with an amnion, an adaptation to lay eggs on land or retain the fertilized egg within the female parent.

Amniote embryos, whether laid as eggs or carried by the female person, are protected and aided by several all-encompassing membranes. In eutherian mammals (such every bit 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.

Learning Objectives

Identify the classes of animals that are amniotes
Describe the chief characteristics of amniotes
Discuss the evolution of amniotes

Amniotic Animals

In the past, the near common sectionalization of amniotes has been into the classes Mammalia, Reptilia, and Aves. Birds are directly descended, however, from dinosaurs, so this classical scheme results in groups that are not true clades. Nosotros will consider birds equally a group distinct from reptiles for the purpose of this discussion with the agreement that this does not completely reflect phylogenetic history and relationships. Instead, mod phylogenetics places birds and reptiles into a larger clade together, though birds (not reptiles) are the truthful descendants of dinosaurs.

The amniotes—reptiles, birds, and mammals—are distinguished from amphibians by their terrestrially adapted egg, which is protected by amniotic membranes. The development of amniotic membranes meant that the embryos of amniotes were provided with their own aquatic environment, which led to less dependence on h2o for development and thus allowed the amniotes to branch out into drier environments. This was a significant development that distinguished them from amphibians, which were restricted to moist environments due their beat-less eggs. Although the shells of various amniotic species vary significantly, they all let memory of water. The shells of bird eggs are equanimous of calcium carbonate and are hard, but fragile. The shells of reptile eggs are leathery and crave a moist environment. Almost mammals exercise not lay eggs (except for monotremes). Instead, the embryo grows within the mother's body; nonetheless, even with this internal gestation, amniotic membranes are nonetheless present.

Characteristics of Amniotes

The illustration shows an egg with the shell, embryo, yolk, yolk sac, and the extra-embryonic membranes

Figure 1. The central features of an amniotic egg are shown.

The amniotic egg is the key feature of amniotes. In amniotes that lay eggs, the trounce of the egg provides protection for the developing embryo while being permeable enough to allow for the substitution of carbon dioxide and oxygen. The albumin, or egg white, provides the embryo with water and poly peptide, whereas the fattier egg yolk is the energy supply for the embryo, as is the example with the eggs of many other animals, such as amphibians. However, the eggs of amniotes comprise 3 additional extra-embryonic membranes: the chorion, amnion, and allantois (Figure ane).

Actress-embryonic membranes are membranes nowadays in amniotic eggs that are not a part of the trunk of the developing embryo. While the inner amniotic membrane surrounds the embryo itself, the chorionsurrounds the embryo and yolk sac. The chorion facilitates substitution of oxygen and carbon dioxide betwixt the embryo and the egg'south external surround. The amnion protects the embryo from mechanical shock and supports hydration. The allantois stores nitrogenous wastes produced by the embryo and also facilitates respiration. In mammals, membranes that are homologous to the extra-embryonic membranes in eggs are present in the placenta.

Additional derived characteristics of amniotes include waterproof skin, due to the presence of lipids, and costal (rib) ventilation of the lungs.

Practice Question

Which of the following statements nearly the parts of an egg are false?

The allantois stores nitrogenous waste and facilitates respiration.
The chorion facilitates gas commutation.
The yolk provides food for the growing embryo.
The amniotic cavity is filled with albumen.

Show Answer

Statement d is false.

Evolution of Amniotes

The first amniotes evolved from amphibian ancestors approximately 340 meg years agone during the Carboniferous period. The early amniotes diverged into ii main lines soon afterwards the first amniotes arose. The initial separate was into synapsids and sauropsids.Synapsids include all mammals, including extinct mammalian species. Synapsids too include therapsids, which were mammal-similar reptiles from which mammals evolved. Sauropsids include reptiles and birds, and can exist further divided into anapsids and diapsids. The cardinal differences between the synapsids, anapsids, and diapsids are the structures of the skull and the number of temporal fenestrae behind each heart (Figure 2).

The illustration compares three different skull types. All three skulls are elongated and similar in shape; the only difference between them is the number of holes behind the eye. The anapsid skull (left) has no openings. The synapsid skull (middle) has one opening, and the diapsid skull (right) has two openings, one on top of the other.

Effigy 2. Compare the skulls and temporal fenestrae of anapsids, synapsids, and diapsids. Anapsids accept no openings, synapsids accept one opening, and diapsids have ii openings.

Temporal fenestrae are mail-orbital openings in the skull that allow muscles to expand and lengthen. Anapsids have no temporal fenestrae, synapsids have one, and diapsids have two. Anapsids include extinct organisms and may, based on anatomy, include turtles. Yet, this is yet controversial, and turtles are sometimes classified as diapsids based on molecular bear witness. The diapsids include birds and all other living and extinct reptiles.

The diapsids diverged into two groups, the Archosauromorpha ("ancient lizard form") and the Lepidosauromorpha ("scaly lizard course") during the Mesozoic period (Figure three). The lepidosaurs include mod lizards, snakes, and tuataras. The archosaurs include modern crocodiles and alligators, and the extinct pterosaurs ("winged lizard") and dinosaurs ("terrible lizard"). Clade Dinosauria includes birds, which evolved from a branch of dinosaurs.

Figure 3. This chart shows the development of amniotes. The placement of Testudines (turtles) is currently nonetheless debated.

Practice Question

Members of the order Testudines have an anapsid-like skull with ane opening. Still, molecular studies signal that turtles descended from a diapsid ancestor. Why might this exist the case?

Bear witness Respond

The antecedent of modern Testudines may at in one case have had a 2d opening in the skull, simply over time this might have been lost.

Check Your Understanding

Answer the question(s) below to see how well you understand the topics covered in the previous section. This brusk quiz doesnon count toward your grade in the class, and you can retake it an unlimited number of times.

Utilise this quiz to check your understanding and make up one's mind whether to (1) written report the previous section further or (2) move on to the next section.

Source: https://courses.lumenlearning.com/suny-wmopen-biology2/chapter/amniotes/

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