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Re: Serendipaceratops confirmed as Australian Early Cretaceous ceratopsian.

Thomas R. Holtz, Jr. <tholtz@umd.edu> wrote:

> A problem I have with the _Notoceratops_ section is that they primarily 
> compare it to derived Late Cretaceous hadrosaurids and ceratopsians, and not 
> the wealth of other ornithischians. For instance, comparison with 
> rhabdodontids
> and _Muttaburrasaurus_ would be useful.

Yes, _Notoceratops_ seems to have been led in by the chin (so to
speak) to bolster the authors' contention that Southern Hemisphere
ceratopsians did exist.  _Notoceratops_ may in fact be a ceratopsian.
Although the case they put forward in support of ceratopsian
affinities is hardly compelling, it's enough to regard it as
provisionally ceratopsian IMHO.  (Tragically, apparently bones were
left behind in the rock when the original jaw-bone was collected...
these other bones might have provided more clues regarding the
affinities of _Notoceratops_).

Richard Bykowski <rbykowsk@umail.iu.edu> wrote:

>The authors repeat over and over about the midshaft width ratio being derived 
>in ceratopsians with respect to other dinosaurs, but can you really diagnose 
>an ulna?

That's an excellent question.  I would say that (in general) an
individual element can be diagnostic at the genus or species level, if
it has a unique feature (autapomorphy) and/or a unique combination of
characters - including overall proportions.  For example, the
Australian theropod (and putative tyrannosauroid) _Timimus hermani_ is
known only from a femur; but it was regarded as valid by Benson et al.
(2012) because the femur had characters that could be used to diagnose
the species.  This diagnosis comprised the extremely gracile
proportions in combination with the tyrannosauroid-like features of
the femur.  In other words, for a tyrannosauroid, the _TImimus_ femur
is remarkable in being so slender (a potential autapomorphy) .

Technically speaking, the _Serendipaceratops_ ulna was also diagnosed
using a unique combination of characters.  However, these characters
are all ratios, and none of them are especially remarkable.  In fact,
as the study makes clear, the ulna greatly resembles that of
_Leptoceratops_ in overall form and size.  The ulnae of
_Leptoceratops_ show variation in their exact proportions - which the
study also mentions.  This is not helpful to the ulna-based diagnosis
of _Serendipaceratops_.

In general I'm fairly tolerant of taxa established on isolated
elements or fragmentary material, just so long as they can be
diagnosed by at least one autapomorphy or (failing that) a unique
combination of characters - such as _Timimus_ (see above) or
_Genyodectes_ (Rauhut, 2004).  I personally think _Lagosuchus_ might
be a valid taxon (no autapomorphies, but the combination of
plesiomorphic characters might be unique, contra Sereno and Arcucci,
1994).  The type material of _Titanosaurus_ may even be diagnosable at
the genus level, despite being regarded as a nomen dubium (Wilson and
Upchurch, 2003).  But _Serendipaceratops_ strikes me as a bridge too
far.  IMHO having proportions different to those of other ulnae does
not qualify as a diagnosis, because the diagnosis in question does not
readily allow further material (such as other isolated ulnae, or
specimens that include an ulna) being referred to _Serendipaceratops_.