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Re: [dinosaur] Latenivenatrix, new troodontid from Dinosaur Park Formation, Alberta (validity of Troodon)



Ben Creisler <bcreisler@gmail.com> wrote:


> Troodon formosus nomen dubium?


Things aren't looking good for _Troodon_.  It's likely heading the
same way as _Deinodon_ and _Ceratops_ (two nomina dubia also from the
Judith River Formation).

The authors (van der Reest and Currie) come very close to declaring
_Troodon formosus_ a nomen dubium... but don't quite do it.
Nevertheless, they concur with the findings of recent studies (e.g.,
Zanno et al. 2011; Larson and Currie, 2013 - both cited) that the
teeth are not distinguishable (diagnostic) at genus level for a number
of troodontids, including _Troodon formosus_.  Therefore, because the
holotype includes only teeth, _T. formosus_ would technically be a
nomen dubium.

But the authors provide some hope for _T. formosus_, if future
material from the same formation and locality can be referred to this
species...

"Although the validity of _Troodon formosus_ may be contentious,
future discoveries may provide the osteological information required
to sort out true relationships in North American troodontids. For any
specimens that are positively identified as _Troodon formosus_,
however, they must originate from the Judith River Formation in the
region from where the holotype was recovered."

Maybe.  The implication is that new diagnostic material could be
referred to _T. formosus_ to keep it alive as a valid taxon.  But it's
not that simple.  Even if troodontid material is recovered from the
JRF that includes diagnostic bones along with teeth that match _T.
formosus_, it may not be enough to save _T. formosus_.  A neotype
would probably have to be designated for _T. formosus_ based on this
new (topotypic?) material.  This could happen, but there's no
guarantee it will (even if potentially referrable material is
discovered).

A similar situation came up for _Ceratops montanus_, another
time-honored taxon named from non-diagnostic material.  The only hope
of keeping _C. montanus_ alive as a valid genus and species is based
on new/referred material.  The best chance was the diagnostic
chasmosaurine specimen CMN 57081 from the JRF, which matched the _C.
montanus_ type material both osteologically and stratigraphically.
The study that described this material (Mallon et al. 2016
DOI:10.1371/journal.pone.0154218) actually stated "It is possible -
even likely, given their close stratigraphic and geological
association - that '_Ceratops_' and CMN 57081 are the same species".
However, they went on to say "without conclusive evidence for such, it
is preferable to erect a new species for CMN 57081."  So CMN 57081 was
granted its own genus and species _(Spiclypeus shipporum_), rather
than being referred to _Ceratops montanus_.  I reckon the same thing
could happen if a diagnostic JRF troodontid specimen was discovered in
the vicinity of the _T. formosus_ type locality.  Unless a neotype was
nominated (and approved), there would always be doubts over whether it
should be referred to _T. formosus_.

Objectively, there's no good reason *not* to treat _Troodon_ any
differently from _Deinodon_ or _Ceratops_.  Alas, _Troodon_ is a
goner.