Archosauromorph classification

Ken Kinman kinman at HOTMAIL.COM
Mon May 22 16:03:14 CDT 2000

Dear Colleagues:
     Below is an updated version of my recent preliminary classification of
Thecodontiformes (including markers showing the placement of their
descendants---pterosaurs, crocodyliforms, dinosaurs and birds).  It has been
expanded to include the parathecodonts (trilophosaurs, rhynchosaurs, and
prolacertiforms) at the base of the classification.  Thecodontiformes and
descendants form a clade which cladists refer to as Archosauromorpha.
This clade is thought to be a sister group of another large clade called
Lepidosauromorpha (incl. lizards, snakes, etc.).  An initial examination of
dinosaur synapomorphies still convinces me that it is best to regard them as
two separate formal orders, coded as sister groups in an "informal" clade
(but I'm still not convinced they should be "formally" joined as a single
order, since possible convergences in hip and hindlimbs have not been
thoroughly evaluated).
     Admittedly some groups are "wastebaskets", and as one example, I
dismantled the mammalian "Condylarthra" in my 1994 book (yes, even I dislike
many paraphyletic groups, when they are no longer useful).
     However, I definitely do not think this is the case with the
"Thecodontia", and being really tired of hearing strict cladists badmouthing
this taxon as a worthless "wastebasket", I am here presenting a preliminary
classification of Order Thecodontiformes down to family level.  The Kinman
markers {{in double brackets}} show the cladistic placement of those Orders
which have been paraphyetically removed (or anagenetically upgraded, to put
it another way), including pterosaurs, crocodyliforms, and both orders of
dinosaurs (thence to birds as well).
       More information about each family will be added later when the basic
phylogeny is established.  I put Doswelliidae and Elastichosuchidae in this
time, but their placement is extremely uncertain.
  1  Trilophosauridae
  2  Rhynchosauridae
3A  Protorosauridae
  B  Prolacertidae
  C  Megalancosauridae
  D  Tanystropheidae
  ?  Sharovipterygidae
  ?  Longisquamidae
  4  Proterosuchidae
  5  Erythrosuchidae
  6  Proterochampsidae
  7  Euparkeriidae
  ?  Doswelliidae
  ?  Elastichosuchidae
8A  Scleromochlidae
  ?  {{Order Pterosauriformes}}**(see notes below)
  B  Lagerpetonidae
  C  Lagosuchidae
  D  {{Order Saurischiformes}} (thence to birds,
         although this is disputed by Feduccia et
         al. who apparently believe birds evolved
         from an unknown/uncertain thecodont family.
  E  {{Order Ornithischiformes}}
  9  Erpetosuchidae
  ?  Ctenosauriscidae
10  Ornithosuchidae
11  Phytosauridae
12  Prestosuchidae
13  Stagonolepididae (aetosaurs)
14  Rauisuchidae
15  Gracilisuchidae
16  Postosuchidae
17  Poposauridae
18  Sphenosuchidae
19  {{Order Crocodyliformes}}
     For those unfamiliar with the Kinman System (1994), the main cladistic
sequence is numbered, and subsidiary sequences are lettered (8A, B, C, D, E,
show the cladistic relationships within the Ornithodira clade).
    **It should be noted that the placement of Pterosauriformes in the
Ornithodira clade is controversial, the major competing theory being that
pterosaurs instead evolved from prolacertiforms (perhaps as sister group to
Sharovipterygidae, according some workers).
      Thecodontiformes was a perfectly good holophyletic (strictly
"monophyletic") group from the middle of the Permian to the middle of the
Triassic, and shows a lot of diversity (about 30 families).  But at the end
of the Triassic they were replaced by their more successful descendants
(which we call crocodyliforms, pterosaurs, and  dinosaurs & birds).
      With all the controversy and uncertainty how these families are
interrelated (and how they are related to their descendants), this taxon not
only continues to serve a useful purpose, but the phylogenetic uncertainty
(and relatively fragmentary fossil record) is such that it will remain a
useful taxon for some time to come.  Unless, of course, one really prefers
the uninformative and confusing type of classification which the Arizona
Tree of Life is providing.
    Strict cladists tend to put either too much or too little information
into their formal classifications (with a never ending proliferation of
taxa).  A more informal and flexible coding for intermediate taxa (which are
often controversial anyway) can pack in just as much information in a more
stable and less confusing manner.  And it also has that added component of
anagenetic information as well.
                         ---------Kenneth E. Kinman
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