16.1: Evolution of Amniota

Learning Objectives

By the end of this section, you will be able to do the following:

• Describe the main characteristics of amniotes
• Explain the difference between anapsids, synapsids, and diapsids, and give an example of each

 

Characteristics of Amniotes

The reptiles (including dinosaurs and birds) are distinguished from amphibians by their terrestrially adapted egg, which is supported by four extraembryonic membranes: the yolk sac, the amnion, the chorion, and the allantois (Figure 29.22). The amniotic egg is the key characteristic of all amniotes, inlcuding mammals as well. The evolution of the extraembryonic membranes led to less dependence on water for development and thus allowed the amniotes to branch out into drier environments. The chorion and amnion develop from folds in the body wall, and the yolk sac and allantois are extensions of the midgut and hindgut respectively. The amnion forms a fluid-filled cavity that provides the embryo with its own internal aquatic environment. In amniotes that lay eggs, the shell of the egg provides protection for the developing embryo while being permeable enough to allow for the exchange of carbon dioxide and oxygen. The albumin, or egg white, outside of the chorion provides the embryo with water and protein, whereas the fattier egg yolk contained in the yolk sac provides nutrients for the embryo, as is the case with the eggs of many other animals, such as amphibians. Here are the functions of the extraembryonic membranes:

1. Yolk sac: blood vessels in the yolk sac transport yolk nutrients to the circulatory system of the embryo.
2. Chorion: the chorion facilitates exchange of oxygen and carbon dioxide between the embryo and the egg’s external environment.
3. Allantois: the allantois stores nitrogenous wastes produced by the embryo and also facilitates respiration.
4. Amnion: the amnion protects the embryo from mechanical shock and supports hydration.

In addition to these membranes, the eggs of birds, reptiles, and a few mammals have shells. An amniote embryo was then enclosed in the amnion, which was in turn encased in an extra-embryonic coelom contained within the chorion. Between the shell and the chorion was the albumin of the egg, which provided additional fluid and cushioning. This was a significant development that further distinguishes the amniotes from amphibians, which were and continue to be restricted to moist environments due their shell-less eggs. Although the shells of various reptilian amniotic species vary significantly, they all permit the retention of water and nutrients for the developing embryo. The egg shells of birds (avian reptiles) are hardened with calcium carbonate, making them rigid, but fragile. The shells of most nonavian reptile eggs, such as turtles, are leathery and require a moist environment. 

Most mammals do not lay eggs (except for monotremes such as the echidnas and platypuses). In most mammals, the yolk sac is very reduced, but the embryo is still cushioned and enclosed within the amnion. The placenta, which transports nutrients and functions in gas exchange and waste management, is derived from two of the extraembryonic membranes.

Visual Connection

Figure 29.22 An amniotic egg. The key features of an amniotic egg are shown.

 

Additional derived characteristics of amniotes include a waterproof skin, accessory keratinized structures, costal (rib) ventilation of the lungs, and internal fertilization. Alpha keratin is a strong structural protein found in the skin of all amniotes, and it prevents desiccation by preventing water loss through the skin. A number of keratinous epidermal structures have emerged in the descendants of various reptilian lineages, and some have become defining characters for these lineages: scales, claws, nails, horns, feathers, and hair. Compared to the permeable skin of amphibians, this provided amniotes with an improved ability to take advantages of niches farther from water. However, it also means that amniotes cannot respire across the skin, and breathing must be more efficient. Negative pressure ventilation using muscles found within and around the ribcage improved the respiratory efficiency of amniotes. 

Internal fertilization occurs most often in land-based animals, although some aquatic animals also use this method. Internal fertilization has the advantage of protecting the fertilized egg from dehydration on land. The embryo is isolated within the female, which limits predation on the young. Internal fertilization enhances the fertilization of eggs by a specific male. Fewer offspring are produced through this method, but their survival rate is higher than that for external fertilization. There are three ways that offspring are produced following internal fertilization. In oviparity, fertilized eggs are laid outside the parent’s body and develop there, receiving nourishment from the yolk that is a part of the egg. This occurs in most bony fish, many reptiles, some cartilaginous fish, most amphibians, two mammals, and all birds. In ovoviviparity, fertilized eggs are retained in the parent (usually the female), but the embryo obtains its nourishment from the egg’s yolk and the young are fully developed when they are hatched. This occurs in some bony fish (like the guppy Lebistes reticulatus), some sharks, some lizards, some snakes (such as the garter snake Thamnophis sirtalis), some vipers, and some invertebrate animals (like the Madagascar hissing cockroach Gromphadorhina portentosa). In viviparity the young develop within the female, receiving nourishment from the parent’s blood through a placenta. The offspring develops in the female and is born alive. This occurs in most mammals, some cartilaginous fish, and a few reptiles.

 

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Evolution of Amniotes

The first amniotes evolved from tetrapod ancestors approximately 340 million years ago during the Carboniferous period. The early amniotes quickly diverged into two main lines: synapsids and sauropsids. Synapsids included the therapsids, a clade from which mammals evolved. Sauropsids were further divided into anapsids and diapsids. Diapsids gave rise to the reptiles, including the dinosaurs and birds. The key differences between the synapsids, anapsids, and diapsids are the structures of the skull and the number of temporal fenestrae (“windows”) behind each eye (Figure 29.23). Temporal fenestrae are post-orbital openings in the skull that allow muscles to expand and lengthen. Anapsids have no temporal fenestrae, synapsids have one (fused ancestrally from two fenestrae), and diapsids have two (although many diapsids such as birds have highly modified diapsid skulls). Anapsids include extinct organisms and traditionally included turtles. However, more recent molecular and fossil evidence clearly shows that turtles arose within the diapsid line and secondarily lost the temporal fenestrae; thus they appear to be anapsids because modern turtles do not have fenestrae in the temporal bones of the skull. The canonical diapsids include dinosaurs, birds, and all other extinct and living reptiles.

Figure 29.23 Amniote skulls. Compare the skulls and temporal fenestrae of anapsids, synapsids, and diapsids. Anapsids have no openings, synapsids have one opening, and diapsids have two openings.

 

The diapsids in turn diverged into two groups, the Archosauromorpha (“ancient lizard form”) and the Lepidosauromorpha (“scaly lizard form”) during the Mesozoic period (Figure 29.24). The lepidosaurs include modern lizards, snakes, and tuataras. The archosaurs include modern crocodiles and alligators, and the extinct ichthyosaurs (“fish lizards” superficially resembling dolphins), pterosaurs (“winged lizard”), dinosaurs (“terrible lizard”), and birds. (We should note that clade Dinosauria includes birds, which evolved from a branch of maniraptoran theropod dinosaurs in the Mesozoic.)

The evolutionarily derived characteristics of amniotes include the amniotic egg and its four extraembryonic membranes, a thicker and more waterproof skin, and rib ventilation of the lungs (ventilation is performed by drawing air into and out of the lungs by muscles such as the costal rib muscles and the diaphragm).

In the past, the most common division of amniotes has been into the classes Mammalia, Reptilia, and Aves. However, both birds and mammals are descended from different amniote branches: the synapsids giving rise to the therapsids and mammals, and the diapsids giving rise to the lepidosaurs and archosaurs. We will consider both the birds and the mammals as groups distinct from reptiles for the purpose of this discussion with the understanding that this does not accurately reflect phylogenetic history and relationships.

Visual Connection

Figure 29.24 Amniote phylogeny. This chart shows the evolution of amniotes. The placement of Testudines (turtles) is currently still debated.

 

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This page titled 16.1: Evolution of Amniota is a derivative of Biology 2e by OpenStax that is licensed under a CC BY 4.0 license.