Many amniote synapomorphies arewidely interpreted as adaptations to the rigors of life on land. Indeed,Amniota owes its name to what may be its most distinctive attribute, alarge "amniotic" egg. While most of us are most familiar with the hard-shelled eggs found in birds, Stewart (1997) showed that the first amniotic eggs probably had a flexible outer membrane, and that a mineralized (but still flexible) outer membrane is a synapomorphy of reptiles. The heavily mineralized, hard shell is a synapomorphy of archosaurs (crocodiles and birds), and it also appeared at least three times in turtles, and a few times in squamates. This probably explains why the oldest known amniotic egg (Coyne, 1999) only dates from the Lower Triassic (220 My), whereas the oldest amniote dates from the Upper Carboniferous (310 My); the eggs of most (if not all) Paleozoic amniotes must have had a flexible, poorly mineralized or unmineralized outer membrane, and thus had a low fossilization potential (Laurin, Reisz & Girondot, 2000).
The amniotic egg possesses a unique set ofmembranes: amnion, chorion, and allantois (Fig. 3). The amnion surroundsthe embryo and creates a fluid-filled cavity in which the embryo develops.The chorion forms a protective membrane around the egg. The allantois isclosely applied against the chorion, where it performs gas exchange andstores metabolic wastes (and becomes the urinary bladder in the adult). Asin other vertebrates, nutrients for the developing embryo are stored in theyolk sac, which is much larger in amniotes than in vertebrates generally.Hatchling amniotes also possess an egg-tooth and horny caruncle on thesnout tip to facilitate exit from their hard-shelled eggs. The amnioticegg, together with a penis for internal fertilization, loss of afree-living larval stage in the life cycle, and the ability to bury theireggs, enabled amniotes to escape the bonds that confined their ancestors'reproductive activities to aquatic environments. It has been suggested that theoriginal function of the extra-embryonic membranes of the amniotic egg was to facilitate interactions between the mother and the embryo (Lombardi, 1994), but this hypothesis is not supported by the distribution of extended embryoretention in vertebrates, according to most proposed phylogenies (Laurin &Girondot, 1999). Some components of theamniotic egg have been variously modified within Amniota. Placentalmammals, for example, have suppressed the egg shell and yolk sac, andelaborated the amniotic membranes to enable nutrients and wastes to passdirectly between mother and embryo.
Figure 3. Development of extraembryonic membranes in an amniote egg(chick). In this early developmental stage, the yolk sac is expanding overthe yolk. The amnion and chorion are expanding over the embryo and willeventually form the amniotic chamber. The allantois is expanding towardthe chorion, with which it will form a respiratory membrane, in addition tostoring metabolic wastes of the embryo. Redrawn from Campbell (1993). Copyright © 1996 Michel Laurin.
The comparative aridity of the terrestrial environment affects all aspectsof amniote biology, and not just their reproductive systems. Thus,amniotes have a relatively impervious skin thatreduces water loss. They also possess horny nails that, among other things,enable them to use their forelimbs to dig burrows into which they canretreat during the heat of the day. The imperative to reduce water loss isequally evident in the density of renal tubules in the metanephric kidneyof amniotes, in the larger size of their water-resorbing large intestines,and in the full differentiation of the Harderian and lacrimal glands in theeye socket whose antibacterial secretions help to moisten and, along with athird eyelid (the nictitans), to further protect the eye from desiccation.The commitment of amniotes to a life on land is also revealed by anextensive system of muscle stretch receptors that enables finercoordination and greater agility during locomotion, their enlarged lungs(which are the only remaining organs of gas exchange owing to the loss ofgills), and the complete loss of the lateral line system other vertebratesuse to detect motion in water.
Many of these features are rarely preserved in fossils, but there are somenovelties in the skeleton that are no less diagnostic of amniotes. Forexample, amniotes have at least two pairs of sacral ribs, instead of justone pair. They also have an astragalus bone in the ankle, instead ofseparate tibiale, intermedium, and proximal centrale bones. Finally, theyhave paired spinal accessory (11th) and hypoglossal (12th) cranial nervesincorporated into the skull, in addition to the ten pairs of cranial nervespresent in amphibians.
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