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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 
crocodiles
  Crocodylia: all animals more closely related to modern crocodiles than to 
modern birds
  Archosauria: all animals more closely related to Aves and Crocodylia than to 
modern lepidosaurs
  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 
modern mammals
  Mammalia: all animals more closely related to modern mammals than to Reptilia
  Amniota: all animals more closely related to Mammalia and Reptilia than to 
modern amphibians
  Amphibia: all animals more closely related to modern amphibians than to 
Amniota
  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 
Tetrapoda>

  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 
(lungfish) and
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 
basal fossil
membership (Aves, Crocodylia, etc.) whereas others work best as nodes (Amniota, 
Reptilia,
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

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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