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Re: Archosaur Origins...was:MESENOSAURUS ERRATA.
George Olshevsky (Dinogeorge@aol.com) wrote:
<Specifically applying this method to the tetrapods, we get (for example):
Aves: all animals more closely related to modern birds than to modern
Crocodylia: all animals more closely related to modern crocodiles than to
Archosauria: all animals more closely related to Aves and Crocodylia than to
Lepidosauria: all animals more closely related to modern lizards and snakes
than to Archosauria
Diapsida: all animals more closely related to Archosauria and Lepidosauria
than to modern
turtles (if turtles really are "anapsids" and not reversed diapsids/archosaurs)
Chelonia: all animals more closely related to modern turtles than to Diapsida
(unless as noted
above they're actually >in< Diapsida)
Reptilia: all animals more closely related to Diapsida and Chelonia than to
Mammalia: all animals more closely related to modern mammals than to Reptilia
Amniota: all animals more closely related to Mammalia and Reptilia than to
Amphibia: all animals more closely related to modern amphibians than to
Tetrapoda: all animals more closely related to Amphibia and Amniota than to
modern bony fishes
Pisces: all animals more closely related to modern bony fishes than to
The problem with the these definitions is that they do not take into account
the fossil record.
Secondarily, the term Pisces is not a formal phylogenetic term, and in most
cases, the historical
usage of Osteichthyes but the more recent and correct usage of Actinopterygii
is used for this
group, the most exclusive group of osteichthyans not including Dipnoi
Tetrapodomorpha (*Eusthenopteron*, etc...).
Defining crown groups, one uses the diversity of the crown to define the
anchors. Thus, instead
of two anchors, each being a group, one takes an end taxon, as a genus for
instance, and uses
those to root the taxon name. It is also best to define crowns as the content
of their inclusion,
i.e. a node, and not as a stem-group. Some taxa work best as a stem, based on
membership (Aves, Crocodylia, etc.) whereas others work best as nodes (Amniota,
Mammalia) because of the inclusion and classic usage of various groups. The
problem is that a
crown group should not use any fossil taxa in it's structure, or it negates the
use of a crown
group [living membership].
Jaime A. Headden
Where the Wind Comes Sweeping Down the Pampas!!!!
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