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Re: Diapsids

Crown-group diapsids (Archosauromorpha and Lepidosauromorpha) have
one -- they have a mobile, slender stapes that can be used for
hearing, like turtles, frogs, and (with extra complications) both
clades of crown-group mammals. Other diapsids, for example
younginiforms, were as deaf as a salamander.

>>> This query has more than one answer;  1) first it would be
better to be specific re: younginiforms. because some were aquatic.
Youngina is the only one I know that is fully terrestrial that has a
complete skull.


Well, and *Youngina* doesn't have a middle ear.

2) Nevertheless, I'm having a very hard time finding
a stapes in and around that clade anywhere in the literature. Seems
paleontologists are content to not discover the stapes when dealing
with fragile skull, difficult matrix issues in these taxa.


That's true.

3) How slender is slender? Perhaps a benchmark X/Y ratio
would be appropriate to set.

Probably it's best to ask the authors of any of the cited analyses (or of the newest analyses -- Müller, Senter).

http://tolweb.org/Diapsida/ has a longer list of characters:

>>> BTW note that the outgroups are marked question marks.

The whole thing is from 2000 and has never been updated. The Tree of Life Project only lets the most famous people contribute -- the advantage is obvious, but the disadvantage is that these people never have time.

> And they use suprageneric taxa. Very bad.

This doesn't matter at all. The page does not present an analysis, it presents a hand-made supertree.

Besides, using supraspecific taxa in an analysis doesn't automatically spell doom either. If the taxon in question happens to be monophyletic, and if its assumed internal phylogeny isn't too wrong, the outcome of the analysis won't change much.

- "Temporal muscles originating on the dorsolateral surface of the
skull table. In araeoscelidians and younginiforms, the temporal
muscles originate from a fascia attached to the lateral edge of the
skull table and from the ventral surface of the skull table."

>>> Can you tell me more about Sphenodon and Alligator since they
would seem to have a skull more similar to that of Youngina and
Petrolacosaurus (complete lower and upper temporal bars)?


I don't have any good illustrations, AFAIK.

In birds and lizards much of the lateral wall of the temporal region is absence.

No, the character talks about the skull table -- frontals, parietals, postparietals.

- "Prefrontal-nasal suture anterolaterally oriented (Fig. 3D). This
suture is parasagittal in araeoscelidians, Claudiosaurus, and
younginiforms (Fig. 3A-C)."

>>> Also anterolateral in Protorosaurus, Boreopricea, 

Should be -- because these are supposed to be archosauromorphs.

Youngoides UC 1528, 

Seems to be pretty far away from the base of the crown-group.

But parasaggital in Paleorhinus, Chanaresuchus, Proterosuchus. 

Seem to be pretty far within the crown-group.

Frankly this orientation just varies.

Assuming it doesn't vary so much that it contains zero phylogenetic signal, the question becomes what the state of this character was at the very origin of the diapsid crown-group. This is what the list is supposed to be -- not an identification key.

- "Squamosal confined to the dorsal half of the skull, except for a
narrow ventral process supporting the quadrate. The squamosal of
younginiforms, Apsisaurus, and araeoscelidians is broad ventrally."

>>> The squamosal of Sphenodon, proterosuchids, erythrosuchids,
Euparkeria is broad ventrally, as broad as in Youngina. Otherwise
you're talking convergence.

Probably we are talking convergence here, because *Sphenodon* is a derived lepidosauromorph and the others are derived archosauromorphs.

- "Strong, broad contact between the paroccipital process and the
cheek. This contact is weak and often cartilaginous in younginiforms
and araeoscelidians."

>>> I like the weasel word "often" which allows for exceptions.

Then just ignore that sentence. It's merely meant to illustrate the situation for those who aren't familiar with it. What's important is only the preceding sentence: the immediate outgroups of the diapsid crown-group lack the character state, while the basalmost members of the crown-group have it.

This character ties into the above 

Why?

- "Quadrate deeply emarginated posteriorly. The quadrate of saurians
supports a tympanum in its deep posterior emargination. The quadrate
of younginiforms has a very shallow emargination that probably did
not support a tympanum. The quadrate of Apsisaurus and
araeoscelidians is not emarginated." (Arguably correlated to the
stapes characters.)

>>> The quadrate of sphenodontians is not deeply emarginated. Snakes
the same way, but let's call those reversals.

Certainly. Both are secondarily "deaf", as shown by their derived phylogenetic positions.

I am seeing a deeply
emarginated quadrate in Prolacerta > Boreopricea,


Fine.

but I'm not seeing
one in Choristodera, Tropidosuchus, Proterosuchids,
Erythrosuchids.Sure the sq and qj are deeply emarginated, but the
quadrate appears to be straight to shallow, even if it leans.


Then maybe the emargination migrated laterally. Just guessing.

And if homologous in diapsids, does the emargination follow the
same pattern in all forms?


Doesn't matter -- if all can be derived from the same pattern.

- "Dorsal process of stapes with ossified connection to paroccipital
process of opisthotic. In other taxa, this connection is
cartilaginous (or it may consist of a tendon), when it is
present." (See above.)

>>> covered above. 

Not quite. But this is probably only observable in isolated stapedes...

- "Large retroarticular process. This is the insertion point for the
muscles that open the lower jaw. The retroarticular process of
araeoscelidians, Coelurosauravus, and younginiforms is much smaller."

>>> This character seems to vary quite abit. I see a large retro
process on Youngoides RC91, but not much at all on Erythrosuchus, for
example.Hupehsuchus from the outgroup has a large retro process.
Snakes, from the ingroup, not much of one.

As mentioned above, all that matters is the variation around the point in the tree that we're discussing. BTW, Ichthyopterygia and their apparently close relative *Hupehsuchus* tend to be found inside the ingroup lately.

- "Cleithrum absent. The cleithrum is a dermal bone located on the
anterior edge of the scapula, dorsal to the clavicle. It is present
in araeoscelidians, Coelurosauravus, and younginiforms."

>>> I don't see the cleithrum in Youngina, or Thadeosaurus.

I remember having seen it illustrated. I'll try to check as soon as possible... which is the opposite of soon :-( ...

It also disappears in other outgroups, like Icarosaurus.

This one is nowadays found closer to the ingroup than the younginiforms. I guess the cleithrum did disappear before the origin of the crown-group, then.

- "Lateral manual centrale (a bone in the wrist) small or absent. The
lateral and medial centralia are approximately of equal size in
araeoscelidians and in some younginiforms primitively (in
Acerosodontosaurus)."

>>> Also small or absent in Prolacerta + Protorosaurus 

Part of the ingroup.

and in Youngina SAM K7710. 

More interesting, but probably rather far away from the ingroup.

I would venture to say that the pattern of
reduction and disappearance appears to be distinct in archosauriforms
and lepidosauriforms, so the process may be convergent.


Could become interesting.

But note that
the little digger Bipes retains a pair  of large centralia.

Is the lateral centrale one of them? (Anyway -- deep within the ingroup, so most likely doesn't matter.)

- "Fifth distal tarsal absent (this is a small bone in the ankle,
proximal to the fifth toe). Araeoscelidians, Coelurosauravus, and
youngina have five distal tarsals."

>>> Absent or fused? Where do you think the hook comes from? 

I don't know.

You can
see this process in action in Homoeosaurus.


Are you sure?

Convergent fusion also takes place in some
outgroup sauropterygians, such as Endennasaurus.


Probably part of the ingroup.

Youngina SAM K7710 also has this hook. 

Really? *Youngina* is famous for lacking it.

If we count "middle ear" as one character -- this may or may not be a
good idea --, we have here ten characters that support the monophyly
of the diapsid crown-group as traditionally conceived (well, since
1991 anyway).

>>> traditionally conceived because not enough taxa were invited to
the matrix, unfortunately.

Well, you have so many extant taxa in your matrix that you're bound to get all manner of long-branch attraction unless you put, for example, toxicoferan autapomorphies into the matrix. As a primitive example -- if I'd put an eagle into my tiny Mesozoic bird analysis without adding any neornithean autapomorphies, it would most probably come out as an avisaurid, next to *Neuquenornis*, instead of clustering with the duck.

>>> Unfortunately for prior trees, in my tree 217 of
228 characters show some convergence, most multiple times.

Why is this unfortunate? Nobody ever claimed otherwise (since the advent of cladistics anyway). Nobody expects all or a majority of characters to have a CI of 1.

It takes a computer to figure it out. A simple list isn't going to work.

Again, the list is not mentioned as an identification key, it's meant as a list of autapomorphies. All these characters retain the ability to evolve.