[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Archosauromorph classification & Thecodonts

I don't remember if I ever posted this updated version of my thecodont classification and discussion on this list, but here it is. It should noted that I plan on moving Trilophosauridae so that it branches off after the rhynchosaurs and prolacertiform families (or perhaps between those two groupings.
And Timothy, if Longisquamidae and Sharovipterygidae don't belong in this order, where would you place them? If they aren't one of the many Triassic thecodont families that went extinct, I would certainly like to know what else they could be.
Thanks, Ken Kinman
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
Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com