Geoscience Reference
In-Depth Information
The Joggins Fossil Cliffs preserve a complete Coal Age ecosystem, including the oldest reptiles in the geological re-
cord.
These early reptiles differed from their amphibian forebears in subtle but significant ways. They had com-
paratively small heads, one-fifth of the trunk length rather than the one-third to one-quarter typical of amphibi-
ans. The head was held higher, and they had stronger jaws. Overall, they had more lightly built skeletons,
which aided their mobility on land. Reptiles also have scaley, waterproof skins that prevent evaporation of wa-
ter, an important adaptation to surviving in the air.
But the most important innovation was their novel reproductive strategy. Amphibians were tied to the water,
as frogs and salamanders are today, by their need to lay eggs there; protected only by a thin membrane, their
eggs would dry out if not immersed in water. Reptiles and their descendants, the birds and mammals, were am-
niotes, meaning that a membrane (the amnion) protects the embryo inside a closed egg. Amniotes laid eggs
with a leathery or hard calcereous shell that was semi-permeable, allowing for the exchange of air and gases.
Inside was a complex of protective membranes and fluid, as well as a food supply in the form of a yolk
sac—all that the developing embryo needed to survive until it hatched. The amphibians also retained a fishlike
form of fertilization by shedding sperm near a mass of freshly laid eggs, a wasteful technique that nevertheless
works well enough in water. Reptiles, however, used internal fertilization, or penetrative sex, a more econom-
ical way to deliver sperm.
