Re: Long-necked stegosaur, head tail mimicry?

[Tim Williams <tijawi@yahoo.com>]

Mike Taylor wrote:


> I should think so!  It would be disastrous to
> resurrect that name to
> denote a completely different clade from the one that it
> has been universally understood to mean.


Mike, I disagree completely and utterly with you here.  Now that I've gotten 
that out of the way, here's why...

I don't think it's important in this case to permanently link the name 
Euhelopodidae to what it *used to* include?  If _Euhelopus_ is demonstrated to 
form a clade with one or more other taxa (such as  _Erketu_), then why not 
apply the name Euhelopodidae to this clade?  The name is available, after all.  
Further, AFAIK the name Euhelopodidae has never been phylogenetically defined, 
so it's a clean slate (phylogenetically speaking).  


As for what Euhelopodidae was "universally understood to mean"... I  disagree 
here too.  I don't think Euhelopodidae sensu Upchurch really is "universally 
understood to mean" a clade containing _Mamenchisaurus_, _Omeisaurus_ and 
_Euhelopus_ at all.  This concept only came about fairly recently (c.1995), and 
was not accepted by everyone.  Upchurch himself later abandoned his own 
Euhelopodidae, less than 10 years after.  Euhelopodidae sensu Upchurch also 
differs slightly from Romer's original composition of the group (as 
Euhelopodinae), which also included _Tienshanosaurus_ and _Chiayusaurus_.  


Changes in the composition of other venerable groups happen all the time.  For 
example, Spinosauridae once included all tall-spined theropods, such as 
_Acrocanthosaurus_, _Altispinax_ and _Metriacanthosaurus_.  These taxa are no 
longer in the Spinosauridae, which now includes a whole new raft of taxa - 
_Baryonyx_, _Suchomimus_, _Irritator_, and the like (as well as _Spinosaurus_ 
of course).  Both Ceratosauria and Ceratosauridae have also gone through 
convulsions in content, with the inclusion of _Ceratosaurus_ being the only 
common denominator.  


If you want to look at sauropods, then what about Cetiosauridae, which for most 
of its history was a dumping ground for "primitive" or poorly known Jurassic 
sauropods.  But this nomenclatural history shouldn't preclude Cetiosauridae 
from being phylogenetically defined to include _Cetiosaurus_ and its closest 
relatives - as it was in fact used by Upchurch et al. (2004).  


In summary, the fact that suprageneric groups can undergo significant changes 
in content shouldn't mean that we abandon a name every time there is such a 
change in content.  If a new Euhelopodidae excludes taxa like _Omeisaurus_ and 
_Mamenchisaurus_, but includes _Erketu_, then so be it.  It just underscores 
how our perception of _Euhelopus_'s relationships has changed.


> By the way, note the (rather poor) phylogenetic definition
> of Mamenchisauridae in Naish and Martill (2007:498) --
> 
>     Mamenchisauridae Young & Chao, 1972, a clade that we here
>     diagnose as all those sauropods closer to Mamenchisaurus
>     constructus Young, 1954 than to Saltasaurus loricatus
>     Bonaparte & Powell, 1980 ...


Yes, the definition could be improved (amended) by the addition of further 
external/negative specifiers.  But apart from that, it seems fine to me.


> At least FOUR times -- in Mamenchisauridae, Diplodocidae
> (the neck of
> Supersaurus is best estimated at 15m, longer than any
> other),
> Brachiosauridae sensu stricto and once or more within the
> huge,
> ill-defined cloud that is Titanosauria.  


True.  But I was referring specifically to the anatomical trait of increasing 
neck length by wholesale recruitment of dorsals into the cervical column.  As 
you know, diplodocids and brachiosaurids lengthen the neck principally by 
elongation of pre-existing cervicals.  _Euhelopus_ and mamenchisaurids go one 
better, and add extra vertebrae into the neck, at the expense of the anterior 
dorsals.  The stegosaur _Miragaia_ does this too - which incidentally is how 
this thread originally started.


Cheers

Tim