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

Re: Unorthodoxies in Reptilian Phylogeny [Romer 1971]



 *Limnoscelis* is considered as the closest relative of Diadectidae
 according to Laurin and Reisz (1995), Ruta et al. (2003) and a number
 of other works.

Almost -- *Tseajaia* occupies that position:

Diadectomorpha
 |--*Limnoscelis*
 `--+--*Tseajaia*
    `--Diadectidae
         |--*Ambedus* (or: How Not to Form a Latin Noun From a Verb)
         `--+--*Orobates*
            `--+--*Desmatodon*
               |--*Diasparactus*
               `--*Diadectes*

(I think the trichotomy is resolved, I just forgot how.)

It's just annoying that not only the reconstructions by Moss (1972) and Berman et al. (1992) -- the only two papers ever written on *Tseajaia* -- contradict each other, but that their _photos_ of the _same skull_ do so, too! It's incredible what you can do with slightly different lighting, a slightly different camera angle, and different amounts of ink to trace the sutures (...something that would nowadays be done in the photo, digitally, instead of _on the skull itself_... <headdesk>). Does *Tseajaia* possess fused postparietals (a diadectomorph autapomorphy)? The photo by Berman et al. (1992) says yes. The photo by Moss (1972) says no. I suspect the former is the correct answer, but I don't have any scientifically defensible argument for this, and nobody is going to finance me a trip to New Mexico anytime soon.

 Both forms the clade Diadectomorpha which is one of the possible
 sister group of Amniota (the relationships of Lepospondyli,
 Diadectomorpha, and *Westlothiana* with Amniota are still disputed).

It's dead obvious that Amniota and Diadectomorpha are sister-groups*, so obvious that Romer agreed in his later years (and declared *Diadectes* a "labyrinthodont amphibian"). The next outgroup is probably Lepospondyli**, probably followed by Seymouriamorpha, though these two might reverse positions one day, weakly supported as that part of the tree is (pers. obs., manuscript in prep.). *Westlothiana* is a fairly obvious lepospondyl, judging from the latest (and so far most detailed) description (Paton et al. 1999) -- especially from the specimen drawings as opposed to the reconstructions, but even if the reconstructions are taken at face value (as Ruta & Coates 2007 apparently did); however, I'm told _three_ teams are currently reexamining the publicly owned and perhaps even the privately owned fossils.

* Except that the name Amniota has a node-based definition, so the future discovery of even closer relatives than Diadectomorpha is possible. ** Which becomes a junior synonym of Amphibia if the lissamphibians are lepospondyls.

Diadectomorph monophyly is fairly well supported, though it's not rock-solid either. Few things are.

*Westlothiana* consistently comes out as the basalmost lepospondyl. The rest of lepospondyl phylogeny is a horrible mess (pers. obs., manuscript in prep.).

> "Macrocnemus now appears to be definitely a lepidosaur" True or
> false?

Like the rest of the "Protorosauria"/"Prolacertiformes" mess, it's fairly clearly an archosauromorph, rather than the lepidosauromorph Romer thought it was (if we impose modern nomenclature on him). The most important point is that its open lower temporal arch is plesiomorphic for crown diapsids, not an autapomorphy of Lepidosauromorpha. It was, for example, independently closed in Archosauriformes and Rhynchosauridae.