Re: Two new dinosaurs in JVP: Arcusaurus and Haya

[Tim Williams <tijawi@gmail.com>]

Brad McFeeters <archosauromorph2@hotmail.com> wrote:

> Yates, A.M, M.F. Bonnan & J. Neveling, 2011.  A new basal sauropodomorph 
> dinosaur from the Early Jurassic of South Africa.  Journal of Vertebrate 
> Paleontology 31: 610-625.


Although Yates &c erect a new genus (_Arcosaurus_ - "rainbow lizard"),
there is no net increase in new genera, because _Ignavusaurus_ is sunk
into _Massospondylus_.  Things don't look for _Gryponyx_ either (and
so soon after it jumped off the nomen nudum merry-go-round).


The only quibble I have with the _Arcusaurus_ paper is the use of the
name Plateosauria.  Clade Plateosauria is defined by Yates &c as
"_Plateosaurus_ + _Massospondylus_ and all descendants of their most
recent common ancestor."  This definition goes back to Sereno (1998),
where it was clear that Plateosauria was used for a subset of
Prosauropoda.  However, if prosauropods are paraphyletic, and
_Plateosaurus_ and _Massospondylus_ do not belong to a clade to the
exclusion of sauropods, then Plateosauria would also include all
sauropods!  To prevent this from happening, Sereno later (2005)
amended the definition to include _Saltasaurus as a negative taxon
qualifier.  I think this is a good idea: the negative qualifier
ensures that Plateosauria cannot be used if _Plateosaurus_ and
_Massospondylus_ are not more closely related to each other than to
sauropods


Yates &c actually recovers _Massospondylus_ closer to sauropods than
to _Plateosaurus_.  Because there is no negative qualifier in the
definition, the paper uses Plateosauria for the least inclusive clade
that includes _Plateosaurus_, _Massospondylus_ and sauropods.  This
goes against the grain of the intent behind the name Plateosauria -
which is as a clade of non-sauropod sauropodomorphs.  I know it is
handy to have a clade that includes "advanced" prosauropod-grade taxa
(like _Plateosaurus_ and _Massospondylus_) and sauropods, and which
therefore excludes the most basal sauropodomorphs.  But IMHO it would
be better to use a different name for this clade, such as Sereno's
Sauropodiformes (although this is anchored in _Mussaurus_) or a new
name.




Cheers

Tim