Re: Are dinosaurs really reptiles? (2.1)

[David Peters <davidpeters@att.net>]

With regard to listing close sister taxa to Petrolacosaurus D. Marjanovic wrote:

<There was, in most cases (the absence of lepospondyls being the most glaring 
exception), no opportunity to provide closer relatives.>

Look again. An entire lineage of closer relatives from an unlikely source.


<Enough are known to provide most of the topology of the tree, yes. But not 
enough are known that we could watch the evolution of every character.>

Look again. You're not using the best inclusion set of taxa.

re: scales and synapsids.
Given that ichthyostegids were scaled like other osteolepids, scales
may have been present on the earliest amniotes, which go back almost
as far. Not the same sort of scales, but then, what the hey.

<Nope. These scales are bone plates in the dermis (and present not just in 
*Ichthyostega* -- there are probably no other "ichthyostegids" --, but also 
present all over the body in almost all tetrapods except lissamphibians, 
diadectomorphs and amniotes; I'm not sure about seymouriamorphs); amniotes 
plesiomorphically retain the ventral scales -- that's what the gastralia are. 
Sauropsid scales are thickenings of the epidermis, consisting of nothing but 
keratin. Calling both "scales" is highly misleading.>

Evolution changed one form of scale to another. The loss of scales is another 
branch on the tree. 

Westlothiana is in the ingroup. Nests with Paleothyris which has
similar supratemporals and tabulars and lots of other synapomorphies.

<Nope (Vallin & Laurin 2004, Ruta & Coates 2007).>

Both shortchanged their taxon inclusion sets to their detriment. Expand the list
expand the possibilities.

re: Diadectomorphs as stem-reptiles.
Way too derived. Way too different from any known pre-amniote.

So they are too derived to be a sister-group?
<Don't take it personal, but you have once again confused "ancestor" and 
"sister-group". NOTHING is too derived to be the sister-group of ANYTHING. 
That's because everything has the right to accumulate autapomorphies. 
Diadectomorpha is a branch of its own, it has its own history, during which 
THINGS HAPPENED.>

Just line up the "usual suspects" and you'll see immediately that some prior 
placements just don't make sense. There are better, more gradual sequences that 
include more synapomorphies. Spoiler: Diadectormopha is part of a continuum 
with living ancestors. Not an evolutionary dead end.

<Hey, you're in good company. People tell me Carroll thinks like you. Nobody 
else does anymore, though.>

I have great respect for this particular hypothesis of Carroll's. It's the only 
one that makes sense and is supported by cladistic analysis.



re: Spinoaequalis
Excellent! Now you're getting somewhere!  Find more like this one and
you've got your lineage. Credit where credit is due.

<What lineage?>

The original question was all about Petrolacosaurus. Remember?



re:     Next question: What were the five to ten sister genera
leading up to Herrerrasaurus? No suprageneric taxa please.  DM wrote:
Translation: if we tell you that the fossil record isn't complete
enough for that, you will simply refuse to accept that answer.
Methinks you haven't thought things through.
So, you're telling me out of hundreds of known taxa, you can't find
sister taxa for the most basal dino/dinomorph? Where do dinos come from?
You can start with Euparkeria. Then what?

<We can arrange all these taxa in a tree. It just so happens that some of the 
side branches have diversified on their own, forming a suprageneric taxon. They 
are there, dude. You can't deny them.>

The problem with nearly all prior cladistic analyses is there use of 
suprageneric taxa. Suprageneric taxa, whether you realize it or not, are made 
up of individual taxa with body parts. Just pick from the genus category and 
the confusion of suprageneric chimaeras will dissipate and you'll see clearly. 
It's actually less work because you no longer get to cherry-pick characters 
from a number of specimens to suit your cladogram. No more decisions to make. 
"Is you is or is you ain't." 

<If our sampling of the fossil record were 10 times denser, we'd probably have 
a few individual species branching off along the internode between Saurischia 
and Herrerasauridae, which seems to be what you were asking for (it's always 
difficult to figure out what you mean). But we don't. >

What I'm looking for is this sequence: Euparkeria ?  ?  ?  ?  Herrerrasaurus. 
Then we'll compare notes.
 
David Peters
davidpeters@att.net