Re: Unorthodoxies in Reptilian Phylogeny [Romer 1971]

[David Marjanovic <david.marjanovic@gmx.at>]

>  If Diadectomorpha is the sister group to Amniota, which genus in both
>  are sister taxa (or closest to the node)?

Newtonmas present for you: I'll explain the terms that you somehow never 
looked up during the last five or more years.

Cladogenesis: one population splits into two clades, which are by 
definition sister taxa.

--+--A
 `--+--B
    `--C

--B and `--C are sister taxa.

--A and
{`--+--B
   `--C}
are sister taxa.

B and C, without their stems (--), are strictly speaking not sister 
taxa, though this simplification is usually harmless.

A and B are _not_ sister taxa, and neither (and for the exact same 
reason) are A and C.

--+--Amniota
 `--Diadectomorpha

--Amniota and `--Diadectomorpha are sister-groups. Period. No member of 
Amniota is the sister-group of any member of Diadectomorpha; these two 
taxa are clades, so any part of Amniota can only be the sister taxon of 
another part of Amniota.

"Which ones are closest to the node?" Do you mean which diadectomorph is 
the sister-group to all other diadectomorphs as a whole, and which 
amniote is the sister-group to all other amniotes as a whole? The answer 
to the former is *Limnoscelis*, as I showed in the tree; the answer to 
the other is that Theropsida and Sauropsida are sister-groups and 
Amniota has a node-based definition.

(All of "Limnoscel(id)idae" was recently sunk into *Limnoscelis*, and 
even into *L. paludis* if I recall correctly.)

(Disclaimer: I have so far acted as if Diadectomorpha had a branch-based 
definition. I don't know if it has any definition. A branch-based one 
would fit historical and current usage best, however.)

>  Why are Solenodonsaurus and Procolophon not considered close to this
>  group (see below)?

I simply forgot about *Solenodonsaurus*. It's either the sister-group of 
(Amniota + Diadectomorpha) or the sister-group of the whole 
(Seymouriamorpha + (Lepospondyli + (Amniota + Diadectomorpha))) clade 
according to recent analyses; unfortunately all specimens are badly 
preserved (I happened to see the best one in Berlin) and therefore 
difficult to interpret. For instance, there are two possibilities for 
where the dorsal midline of the skull is.

*Procolophon* is an ordinary amniote. More precisely, an unremarkable 
sauropsid (except for its adaptations to herbivory). Even more 
precisely, it's a deeply, deeply nested procolophonoid; more 
plesiomorphic ones like *Owenetta* look like standard basal amniotes. I 
don't understand why you bring it up. Is it the herbivory by 
labiolingually broad teeth, shared with Diadectidae, but even more so 
between the latter and *Trilophosaurus*?

>  If Westlothiana is the basalmost lepospondyl, which genera are its
>  closest sister taxa?

By "basalmost" I mean that the entire rest of Lepospondyli, quite 
possibly including Lissamphibia, _as a whole_ is its sister-group.

Every taxon has _one_ sister taxon, except if it forms part of a hard 
polytomy ( = a case where a population _really_ split into three or more 
clades simultaneously -- soft polytomies are gaps in our knowledge where 
we haven't figured out the exact sequence of dichotomies yet, hard ones 
are what the real ones are called).

>  Why not Paleothyris?

Because it's a standard-issue sauropsid, close to Diapsida. Shares 
nothing with *Westlothiana* except plesiomorphies.

>  If Macrocnemus is an archosaurosaurormorph, which taxon is its
>  closest sister taxon?

No idea.

>  Why not Jesairosaurus?

I didn't say it's not. Nobody knows. There is as yet no phylogenetic 
analysis which includes all relevant taxa.