Re: Ankylosauria and Scelidosaurus

[Tim Williams <twilliams_alpha@hotmail.com>]

Nick Gardner wrote:

> With all due respect to the ornithischian workers onlist (I'm sure
> there must be some out there, somewhere!), perhaps it's time to
> declare a moratorium on defining and naming clades within the group
> until their relationships are better understood.

Ah, we could be waiting till the cows come home on that one.  :-)  We have 
to face the unpleasant fact that the majority of nodes are going to remain 
unstable for a very long time to come.

> Taking into
> consideration published analyses like Butler's, and various
> unpublished analyses by myself and others, I really believe that
> overall our current understanding of ornithischian relationships is
> flawed, and that attempting to define the clades given the current
> circumstances may be a disaster waiting to happen.

One possible solution is to emend the definitions to include multiple 
external specifiers.  That way, if there are seismic upheavals in 
ornithischian phylogeny, then many clade names will disappear rather than 
persist with vastly different contents.

For example, Eurypoda could be changed to "the least inclusive clade 
including _Stegosaurus stenops_ and _Ankylosaurus_ magniventris_, BUT NOT 
_Iguanodon bernissartenis_ or _Triceratops horridus_".  That way, if either 
stegosaurs or ankylosaurs are shown to be more closer to ornithopods and/or 
ceratopsians, then Eurypoda will be invalidated.  This also means that 
Eurypoda will always remain faithful to the concept of a 
Stegosauria-Ankylosauria clade; so Eurypoda lasts only so long as stegosaurs 
and ankylosaurs are demonstrated to be most closely related to one another.  
(According to Galton [2006], there is possible evidence of stegosaurs in the 
Late Triassic, which would support a very early divergence for this group.)

But I agree with you, Nick.  The basal relationships of most ornithischian 
clades are so murky that special attention is required when framing 
definitions.  The heterodontosaurs in particular leap to mind.

Nick Pharris wrote:

> Is Eurypoda a node-based taxon anchored on _Ankylosaurus_ and 
> _Stegosaurus_, or is _Scelidosaurus_ explicitly excluded?

Eurypoda is defined as the least inclusive clade including _Stegosaurus_ and 
_Ankylosaurus_; _Scelidosaurus_ is not mentioned.

http://www.taxonsearch.org/dev/taxon_edit.php?Action=View&tax_id=140

Cheers

Tim