Biology Reference
In-Depth Information
Amniotic Membrane
Two Pairs of Legs/Limbs
r
r
a
a
Amphibia
Ophidia
Mammalia
Amphibia
Ophidia
Mammalia
Fig. 10.
(a) Synapomorphy of the amnion character in Ophidia and Mammalia
relative to
r
, but symplesiomorphy of the amnion in Ophidia and Mammalia relative
to
a
. (b) Symplesiomorphy of the character “two pairs of legs/limbs” in Amphibia
and Mammalia relative to
r
, but autapomorphy of the character in Ophidia (snakes
have no apparent limbs).
(Fig. 10). An autapomorphy is an apomorphy proper to a single taxon
(Greek
autos
(“self ”)).
The two examples in Fig. 10 illustrate these concepts. In both den-
drograms, we are looking at present-day taxa Amphibia (frogs and
newts), Mammalia (humans, etc.), and Ophidia (snakes).
The left dendrogram is a graphic depiction of the evolution of the
amnion (the innermost membrane enclosing the fetus). Its presence is
denoted by “
”, with mammals and snakes sharing an amniotic ancestor
(“a”). The cenancestor “r” of all three terminal taxa was anamniotic
(“0”), however, just like present-day frogs. We therefore have a synapo-
morphy of the amnion character in Ophidia and Mammalia relative to
“r”, but a symplesiomorphy of the amnion in Ophidia and Mammalia
relative to their cenancestor “a”.
The right dendrogram is a depiction of the Ophidian autapomorphy.
Snakes have lost the two pairs of legs/limbs (presence denoted by “
+
”) that
are a characteristic of all Tetrapoda. There is thus a symplesiomorphy of the
character in Amphibia and Mammalia relative to “r”, while the Ophidia
taxon presents an autapomorphy in its absence of legs/limbs. Note that,
phyletically, snakes still belong to the Tetrapoda phylon since the cenances-
tor “r” of all three terminal taxa was a tetrapod (endowed with four limbs).
+
