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Re: Eucoelophysis (was Alamosaurus as biggest North American sauropod)

On Sun, Dec 11, 2011 at 9:58 PM, Tim Williams <tijawi@gmail.com> wrote:
> David Marjanovic <david.marjanovic@gmx.at> wrote:
>> The story of the silesaurid *Eucoelophysis* is more complicated than that,
>> but I'm not sure I remember the details and can't look them up now.
> The holotype material for _Eucoelophysis baldwini_ (NMMNH P-22298)
> consists of a partial postcranium that was discovered in the Orphan
> Mesa locality in New Mexico.  This locality is believed to be the same
> as the Arroyo Seco locality that yielded the original type material of
> _Coelophysis bauri_ (Sullivan et al., 1996).  _E. baldwini_ was
> originally thought to be a coelophysoid theropod by Sullivan & Lucas
> (1999), based on NMMNH P-22298 and a referred pubis (AMNH 2706).  This
> study also speculated that the original _Coelophysis_ syntypes might
> also belong to _E. baldwini_, but this couldn't be demonstrated due to
> its non-diagnostic nature.  However, Eczurra (2006) subsequently
> showed that NMMNH P-22298 belongs to a non-dinosaur dinosauriform
> similar to _Silesaurus_, and AMNH 2706 comes from a coelophysoid.
> Subsequent phylogenetic analyses have recovered _Eucoleophysis_ as a
> silesaurid (e.g., Nesbitt et al., 2010).

We (Randy Irmis, Sterling Nesbitt, and I) noted this at the 2005 SVP
meeting and in our subsequent 2007 paper we discuss the affinities of
much of Cope's Coelophysis material. Subsequent work has also
recognized that Dromomeron material is also present in the sample (see
Irmis et al, 2007, and Nesbitt et al. 2009).

> The locality that yielded _Eucoelophysis_ and the original
> _Coelophysis_ syntypes represents the Petrified Forest Formation of
> the Chinle Group, whereas the abundant skeletons of _Coelophysis_
> (including the neotype, AMNH 7224) from Whitaker Quarry at Ghost Ranch
> represents the stratigraphically higher (i.e., younger) Rock Point
> Formation.

Although it is certain that the Coelophysis Quarry is in strata (the
siltstone member of the Chinle Formation) higher than the Petrified
Forest Member and thus older than the Hayden and Snyder Quarries as
well as David Baldwin's original collecting sites, the exact
correlations of this unit are still debated. Recent paleomagnetic work
by Kate Zeigler and John Geissman have shown that the siltstone member
has a different magnetic polarity signal than the type Rock Point
Member and thus the units are not correlable (i.e., the Coelophysis
Quarry is not in the Rock Point). I just gave a presentation at SVP
this year demonstrating that based on vertebrate biochronology the
siltstone member could be equivalent to the Owl Rock Member (which is
between the Petrified Forest and Raock Point Members in Arizona, but
purportedly absent in the Ghost Ranch area); however, Zeigler and
Geissman suggested that based on paleopole position it could be a
Moenave Formation equivalent, and thus latest Triassic or even early

> Cheers
> Tim