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Re: birds and/or/with dino's

Mike Taylor <mike@indexdata.com> wrote:

> But why would you follow Sereno (2005)? 

For the sole reason that his Saurischia is defined such that it *must* include 
sauropods, as well as theropods.  This is in line with Seeley's original 
intent, when he came up with the entire Saurischia-Ornithischia dichotomy.

> When Seeley (1888:170) first
> proposed the names Saurischia and Ornithischia, he did so
> with the explicit intention that they should form a partitioning of
> Dinosauria, an intent that has subsequently been universally understood
> and which in PN terms is reepresented by defining Dinosauria as a
> node-stem triplet.  

However, Seeley did more than just partition the dinosaurs into two groups.  He 
also explicitly included theropods and sauropods in the Saurischia (the 
"lizard-hipped" dinosaurs).  Why do I mention this?  Read on...

> If instead of following Sereno (2005), you follow the
> definitions in The Dinosauria, 2nd ed., as define
> Saurischia and Ornithischia as the reflexive branches (Tyrannosaurus not
> Triceratops) and (Triceratops not Tyrannosaurus), then your problem goes
> away.

But Mike, what happens if we come up with a phylogeny that recovers a 
Sauropodomorpha+Ornithischia clade that excludes Theropoda?  (Hey, it could 
happen!)  Using your definitions (from The Dinosauria), sauropods would be put 
inside Ornithischia, and Saurischia would be limited to theropods only.  This 
not only goes against the grain of Seeley's original intent (as reflected in 
his choice of names), but also the entire subsequent history of both groups.

Everybody (from Seeley onwards) regards Saurischia as the group that includes 
*both* theropods and sauropods.  Therefore, I think the definition of 
Saurischia should be linked to the phylogenetic hypothesis that theropods and 
sauropods form a clade to the exclusion of ornithischians.  

The corollary of this, is that if theropods and sauropods *do not* form a clade 
to the exclusion of ornithischians (i.e., we recover a 
Sauropodamorpha+Ornithischia clade), then I think Saurischia should cease to 
exist as a clade.  This seems entirely reasonable to me (and I'm sure Seeley 
would agree, though I won't press this point).  But the definitions you cite do 
not cater for this possibility.  I think they should; so I prefer defining 
Saurischia such that it must include both theropods and sauropods, and defining 
Ornithischia such that it must exclude sauropods (and theropods).

> So this never was a problem of encoding a phylogenetic
> hypothesis in a name.

I guess I didn't get my point across - sorry, I'll try and explain it better.  
What I was saying is that certain names *should* encode a phylogenetic 
hypothesis.  Saurischia and Ornithischia are prime examples.  Thus, in keeping 
with traditional usage, the respective definitions should enshrine Saurischia 
as a Theropoda+Sauropodomorpha clade, and Ornithischia as a clade that can 
never include either theropods or sauropods.

Finally, I wrote (in a previous message): 

> Wouldn't Saurischia have priority over Theropoda, given that Saurischia 
> was named first?  In the event that sauropodomorphs are closer to 
> ornithischians,  Saurischia would quash Theropoda - if we let priority
> alone decide.

Actually, I was wrong here: Theropoda was named first.  So if sauropods are 
found to be closer to ornithischians, and Saurischia is consequently limited to 
theropods only, then Theropoda would have nomenclatural priority over