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Re: Unorthodoxies in Reptilian Phylogeny [Romer 1971]



 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.