American Museum of Natural History

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506 OSBORN, THE REPTILIAN SUBCLASSES DIAPSIDA AND SYNAPSIDA.
CHORISTODERA.-The Choristodera (compare pp. 492, 493) suddenly appear
and leave no known descendants. The natural position of this group seems to
be that of an independent order, bearing relationships to the less specialized
Rhynchocephalia similar to those which the Mosasauria bear to the Lacertilia.
The amount of divergence in the two cases appears to be as great or greater;
that is, Champsosaurus and more especially Simcedosaurus are as far or further
removed from Sphenodon than Mosasaurus is from Varanus.
SUMMARY.
The more conservative general conclusion regarding the classification of
these seven groups, therefore, is that we must await additional evidence. The
Protorosauria, the Pelycosauria, the Choristodera, seem to be entitled to rank as
distinct orders. Future discovery may link the Rhynchosauria and Procolophonia
more closely to the Rhynchocephalia, as suborders; in which case the true
Rhynchocephalians should be distinguished as the suborder Sphenodontina. The
relationships of the Proganosauria are the most uncertain, and will depend upon
our future knowledge of the structure of the skull.
3. PHYLOGENY.
General Phylogeny.-A general phylogenetic conclusion may be expressed
diagrammatically as below:
Subclass Subclass
(SYNAPSIDA DIAPSIDA
Class REPTILIA Diaptosauria
( Cotylosauria
Class AMPHIBIA STEGOCEPHALA
that is, the Stegocephala gave rise to the Cotylosauria or most primitive Synapsida
from which sprang the higher Synapsida, also the Diaptosauria or most primitive
Diapsida; from which, in turn, were partly or wholly derived the higher
Diapsida.
Special Phylogeny.-The more special phylogenetic conclusion, of a very
provisional and hypothetical character, may be expressed as follows:
Sphenodon Recent
Rhynchocephalia (t. 1. a.') Dinosauria (t.) Choristodera (a.) Cretaceous
Jura. Trias.
Rhynchosauria (I.) Protorosauria (t.) Pelycosauria (t.) Proganosauria (a.) .............. Permian and
Lower Trias.
Procolophonia (t.)
Primitive or Uns.pecialized Diaptosauria.............Carboniferous
and Lower
Permian
't. = terrestrial; 1. = littoral; a. = aquatic.