By the
21st century, vertebrate paleontologists were beginning to adopt phylogenetic taxonomy, in which all groups
are defined in such a way as to be monophyletic; that is, groups include all
descendants of a particular ancestor. The reptiles as historically defined
would be paraphyletic, since they exclude both birds
and mammals, although these also evolved from animals like dinosaurs and early
therapsids that were traditionally called reptiles.[11] Colin Tudge wrote:
Mammals
are a clade, and therefore the cladists are happy to acknowledge the traditional taxon Mammalia; and birds, too, are a clade,
universally ascribed to the formal taxon Aves. Mammalia and Aves are, in fact, subclades within
the grand clade of the
Amniota.
But the traditional class Reptilia is not a clade. It is just a section of the
clade Amniota: the section that is left after the Mammalia and Aves have been
hived off. It cannot be defined by synapomorphies, as is the proper way. It is
instead defined by a combination of the features it has and the features it
lacks: reptiles are the amniotes that lack fur or feathers. At best, the
cladists suggest, we could say that the traditional Reptilia are 'non-avian,
non-mammalian amniotes'.[12]
Despite
the early proposals for replacing the paraphyletic Reptilia for a monophyletic Sauropsida, that term was never adopted
widely or, when it was, applied consistently.[13] When Sauropsida was used, it
often had the same content or even the same definition as Reptilia. In 1988 Jacques Gauthier proposed a cladistic definition of Reptilia as a
monophyletic node-based crown group containing turtles, lizards and
snakes, crocodilians, and birds, their common ancestor and all its descendants.
Because the actual relationship of turtles to other reptiles was not yet well
understood at this time, Gauthier's definition came to be considered
inadequate.[13] A variety of other definitions
were
proposed by other scientists in the years following Gauthier's paper. The first
such new definition, which attempted to adhere to the standards of the PhyloCode, was published by Modesto and
Anderson in 2004. Modesto and Anderson reviewed the many previous definitions,
and proposed a modified definition which they intended to retain most
traditional content of the group while keeping it stable and monophyletic. They
defined Reptilia as all amniotes closer to Lacerta agilis and Crocodylus
niloticus than to Homo sapiens. This stem-based definition is
equivalent to the more common definition of Sauropsida, which Modesto and
Anderson synonymized with Reptilia, since the latter is more well known and
more frequently used. Unlike most previous definitions of Reptilia, however,
Modesto and Anderson's definition includes birds.[13]
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