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Figure 16.14 Matching fins and legs of the first tetrapods: the pectoral fin of the Devonian sarcopterygian fish Eusthenopteron (a) shows bones that are probable homologs of tetrapod arm bones, such as in the Devonian amphibian Acanthostega (b). Acanthostega had eight fingers and Ichthyostega had seven toes on its hindlimb (c). (d) The early tetrapod Acanthostega. (Courtesy of Mike Coates.)

Reptiles and their descendants made a clean break from the water by producing a kind of egg that did not have to be laid in water.

The cleidoic ("closed") egg, sometimes called the amniotic egg, is enclosed within a tough semipermeable shell, hence its name.

This egg type is seen in all members of the clade Amniota (reptiles, birds, mammals): it is the familiar hen's egg that we eat for breakfast. Primitive mammals, such as the platypus, still lay eggs, but most mammals have suppressed the egg and it "hatches" inside the

Figure 16.15 Fossil amphibians: (a) skull of the Early Triassic temnospondyl Benthosuchus; (b) skeleton of the Early Permian temnospondyl Eryops; and (c) skeleton of the Early Permian reptiliomorph Seymouria. (a, courtesy of Mikhail Shishkin; b, c, based on Gregory 1951/1957.)

Box 16.8 Classification of amphibians

The amphibians are a paraphyletic group as they exclude their descendants, the reptiles. Modern amphibians are clearly distinguishable from the diverse fossil groups.

Superclass TETRAPODA

Subclass BATRACHOMORPHA (Amphibia) Order "TEMNOSPONDYLI"

• Broad-snouted, low-skulled amphibians, showing a range of sizes

• Early Carboniferous to Early Cretaceous

Infraclass LISSAMPHIBIA

• Frogs, salamanders (newts) and caecilians (gymnophionans)

• Early Triassic to Recent

Order NECTRIDEA

• Small slender aquatic forms

• Early Carboniferous to Late Permian

Order MICROSAURIA

• Terrestrial and aquatic long-bodied forms with deep skulls

• Early Carboniferous to Early Permian

Subclass REPTILIOMORPHA Order ANTHRACOSAURIA

• Narrow-skulled, fish-eating amphibians

• Early Carboniferous to Late Permian

Order SEYMOURIAMORPHA

• High-skulled terrestrial amphibians

• Late Carboniferous to Late Permian mother's womb. The amniotic eggshell is usually hard and made from calcite, but some lizards and snakes have leathery eggshells. The shell retains water, preventing evaporation, but allows the passage of gases, oxygen in and carbon dioxide out. The developing embryo is protected from the outside world, and there is no need to lay the eggs in water, nor is there a larval stage in development. Inside the eggshell is a set of membranes that enclose the embryo (the amnion), that collect waste (the allantois) and that line the eggshell (the chorion) (Fig. 16.16). The chorion is the thin papery tissue just inside the eggshell, which you peel off a hard-boiled egg. Food is in the form of yolk, a yellow material rich in protein.

The oldest-known amniote, Hylonomus from the mid-Carboniferous of Canada (Fig. 16.17a, b) is known only from its skeleton; no amniotic eggs are known from the Carboniferous. Hylonomus has been superbly well preserved inside ancient tree stumps, into which it crawled in pursuit of insects and worms, and then was overwhelmed by flood-waters. Hylonomus looks little different from some amphibians of the time, such as the microsaurs, but it shows several clearly amniote characters, a high skull, evidence for

Figure 16.16 The cleidoic egg of amniotes in cross-section, showing the eggshell and extra-embryonic membranes.

(d) Synapsid (e) Diapsid

Figure 16.17 The earliest reptile, and early reptile evolution: (a, b) the mid-Carboniferous reptile Hylonomus, skeleton and skull; (c-e) the three major skull patterns seen in amniotes: anapsid, diapsid and synapsid. (Based on Carroll 1987.)

Quaternary

Quaternary

Figure 16.18 Phylogeny of the major groups of fishes and tetrapods.

additional jaw muscles and an astragalus bone in the ankle. We know Hylonomus laid clei-doic eggs because of the shape of the evolutionary tree: how can that be?

Amniotes radiated during the Late Carboniferous, giving rise to three main clades. These are distinguished by the pattern of openings in the side of the skull, especially the temporal openings behind the eye socket (Fig. 16.17c-e). The primitive state is termed the anapsid ("no arch") skull pattern, since there are no temporal openings. The two other skull patterns seen in amniotes are the synapsid ("same arch"), where there is a lower temporal opening, and the diapsid ("two arch") pattern, where there are two temporal openings. These temporal openings correspond to low-stress areas of the skull, and the edges serve as attachment sites for jaw muscles.

These three skull patterns diagnose the key clades among amniotes (Fig. 16.18; Box 16.9). The Anapsida include various early forms such as Hylonomus, as well as some Permian and Triassic reptiles, and the turtles. The Syn-apsida include the "mammal-like reptiles" and the mammals, and the Diapsida includes a number of early groups, as well as the lizards and snakes and the crocodiles, pterosaurs, dinosaurs and birds. All modern amniotes produce the cleidoic egg, and the structure is so similar that we can be sure this egg arose at the base of clade Amniota; so, Hylonomus, clearly located within the evolutionary tree of Amniota, must have shared the cleidoic egg.

Box 16.9 Classification of the reptiles

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