Amniotes

Reptiles evolved from amphibians during the Carboniferous. Their key innovation was the ability to lay eggs on land. These amniotic eggs are a life support system for the embryo away from water. They have allowed reptiles and their descendants to colonize a wide range of environments not accessible to amphibians.

An amniotic egg develops from a single fertilized cell. It comprises a sac that contains the embryo immersed in amni-otic fluid and its food supply (the yolk). In addition there is a space for storing waste products and an air pocket through which gases exchange with the air outside. The whole package is wrapped in a waterproof membrane that can be rigid or flexible (Fig. 11.6).

Unfortunately, eggs are very rare in the fossil record and the oldest known amniotic egg is Triassic in age. However, all eggs produced by modern amniotes are generated in the same way. It is considered that this complicated series of processes is most likely to have arisen only once in evolution, by definition in the most primitive amniote. Other details of amniote anatomy can therefore be used to infer the appearance of egg layers in the fossil record. A suite of early reptiles is known, any of which could have been ancestral to the group. One possible early amniote is Westlothiana, a small tetrapod found in a Carboniferous volcanic lake deposit from the Midland Valley of Scotland. However, it is now considered that this form slightly predates the evolution of eggs. The earliest well-known reptile is called Hylonomus (Fig. 11.7) and found in the hollow tree stumps of a Carboniferous fossil forest in eastern Canada.

Early reptiles were small in size, and this may be a reflection of their physiology. Modern reptiles are cold blooded, and need to maintain a functional body temperature by behavioral methods. They may bask in the sun, or burrow to escape from excessive heat, for example. Such temperature regulation is much faster for a small animal, with a high surface to volume ratio. In contrast, the early amphibians could be much larger, as they lived mainly in water where temperature control is easier.

During the Carboniferous, reptiles radiated into the three main evolutionary lines, or clades, that have dominated terrestrial environments since. This early radiation was into distinct lineages, which are most characteristically differentiated by their skull structure (Table 11.1). The anapsid lineage is primitive and has no holes in the upper skull apart from the eye and nose apertures. This lineage is represented today by modern turtles and tortoises. It has occasionally been more diverse than at present, but has never rivaled in importance the two lines that evolved from it. The synapsid lineage evolved next, and has a single hole in the upper skull behind the eye. This group evolved into mammal-like reptiles and eventually into mammals. The diapsids evolved later and are characterized by two openings in the skull behind the eye. This group diversified into most modern reptile groups, marine reptiles, dinosaurs, and birds.

Each advanced family of reptiles developed different solutions to the problem of greater mobility on land. Amphibians have limbs that articulate at the knee and elbow. Their pelvic and shoulder girdles are rigid and the backbone between flexes to one side and then the other as they walk. This limits their speed and also the length of time for which they can move. As the spine flexes, it compresses one lung and then the other, making it impossible to breathe and walk at the same time. This limitation also applies to modern reptiles, such as crocodiles, that can lunge at prey, but can not chase a prey animal over any distance. Synapsids and diapsids have at different times solved this problem, known as Carrier's constraint, in a variety of different ways.

Allantois: for-

waste storage and gas exchange

Allantois: for-

waste storage and gas exchange

Semipermeable shell

Yolk sac: providing food for the embryo

Semipermeable shell

Embryo in amniotic cavity

Yolk sac: providing food for the embryo

Fig. 11.7 A reconstruction of Hylonomus.

Table 11.1 The three main clades of reptiles.

Origin

Main radiations

Common examples

Anapsids

Anapsids

No skull opening behind the eye socket Synapsids

One skull opening behind the eye socket Diapsids

Carboniferous origin; this is the most primitive group of reptiles

Carboniferous

Carboniferous

Never very abundant or diverse Greatest diversity of form in the Permian Greatest success after the evolution of the shell in the Triassic

Turtles, tortoises

Pelycosaurs in the early Permian, therapsids in Dimetrodon, kangaroos, horses, humans the late Permain True mammals in the Palaeocene

Archosaurs and marine and flying reptiles in the Triassic Dinosaurs in the Jurassic Birds in the Palaeocene

Ichthyosaurs, plesiosaurs, pterodactyls, Triceratops, Tyrannosaurus, seagulls

Two skull openings behind the eye socket

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