11 Onto the Land and Back to the Sea: The Amniotes
Innovative Eggs
The most fundamental innovation is the evolution of another fluid-filled sac, the
amnion, in which the embryo floats. Amniotic fluid has roughly the same composi-
tion as seawater, so that in a very real sense, the amnion is the continuation of the
original fish or amphibian eggs together with its microenvironment, just as a space
suit contains an astronaut and a fluid that mimics the earth’s atmosphere. All of the
rest of the amniote egg is add-on technology that is also required for life in an alien
environment, and in that sense it corresponds to the rest of the space station with its
food storage, fuel supply, gas exchangers, and sanitary disposal systems.
—Richard Cowen, The History of LifeLike the transition from “fish” to “amphibians,” the transition from “amphibians” to
“reptiles” has long been confused by misunderstandings and inadequate terminology. If we
use the terms “fish,” “amphibians,” and “reptiles” in the traditional sense, they are not natu-
ral monophyletic groups, but “grades” of evolution because they do not include all descen-
dants of a particular group. We have already discussed in chapter 10 how the monophyletic
group Tetrapoda is preferred to the archaic term “amphibian.” The same goes for the grade
we call “reptiles.” If you look at the living turtles, snakes, lizards, and crocodiles, they all
have scaly skins and sluggish metabolisms, and it is easy to lump them together based on
these shared primitive characteristics. But as we will detail in the next chapter, birds are their
descendants, so unless you include the birds within the Reptilia, it is an unnatural paraphy-
letic “wastebasket” group (figs. 5.4 and 11.1). To avoid confusion with this long-standing
misuse of the concept, most modern systematists use the term “Reptilia” to mean the clade
that includes turtles, snakes, lizards, and crocodiles plus birds. The so-called “mammal-like
reptiles” (more properly, the synapsids), on the other hand, branched off from the family tree
before the branching point of true reptiles, so it is incorrect to label synapsids by that obso-
lete term (even though people keep using it). Likewise, there are many primitive tetrapods
that are neither synapsids nor reptiles but more advanced than the creatures we saw in the
previous chapter. To avoid the misleading term “Reptilia” for these animals, most modern
systematists prefer to use the term “amniotes” for all vertebrates that are more advanced
than traditional amphibians. The Amniota thus includes the classical concepts of Reptilia,
Mammalia, and Aves (birds).
The diagnostic character shared by all living amniotes is the land egg, or amniotic egg
(fig. 11.2). Instead of laying hundreds of tiny soft-shelled eggs in the water (as fish and
amphibians do), amniotes lay fewer but larger eggs that can survive out of water. Each egg
is covered by a shell (either leathery like a turtle egg or hard like a chicken egg) that protects
the delicate tissues and embryo inside against predators and prevents it from drying out.