Re: Yinlong - a new link between marginocephalians and heterodontosaurs.

[fam jansma <fam.jansma@tiscali.nl>]

Pictures:

http://www.gwu.edu/~newsctr/fossilfind/

----- Original Message ----- 
From: "Tim Williams" <twilliams_alpha@hotmail.com>
To: <dinosaur@usc.edu>
Sent: Sunday, May 21, 2006 8:34 PM
Subject: Yinlong - a new link between marginocephalians and
heterodontosaurs.


> At last, maybe some clarity with regards to the interrelationships of
> heterodontosaurids, pachycephalosaurs, and ceratopsians...
>
> Xing Xu, Catherine A. Forster, James M. Clark, and Jinyou Mo (in press).
A
> basal ceratopsian with transitional features from the Late Jurassic of
> northwestern China. Proceedings of the Royal Society B: Biological
Sciences
> (FirstCite Early Online Publishing)
>
> Abstract: "Although the Ceratopsia and Pachycephalosauria, two major
> ornithischian groups, are united as the Marginocephalia, few
synapomorphies
> have been identified due to their highly specialized body-plans.  Several
> studies have linked the Heterodontosauridae with either the Ceratopsia or
> Marginocephalia, but evidence for these relationships is weak, leading
most
> recent studies to consider the Heterodontosauridae as the basal member of
> another major ornithischian radiation, the Ornithopoda.  Here, we report
on
> a new basal ceratopsian dinosaur, _Yinlong downsi_ gen. et. sp. nov. from
> the Late Jurassic upper part of the Shishugou Formation of Xinjiang,
China.
> This new ceratopsian displays a series of features transitional between
more
> derived ceratopsians and other ornithischians, shares numerous derived
> similarities with both the heterodontosaurids and pachycephalosaurians and
> provides strong evidence supporting a monophyletic Marginocephalia and its
> close relationship to the Heterodontosauridae.  Character distributions
> along the marginocephalian lineage reveal that, compared to the bipedal
> Pachycephalosauria, which retained a primitive post-cranial body-plan, the
> dominantly quadrupedal ceratopsians lost many marginocephalian features
and
> evolved their own characters early in their evolution."
>
> _Yinlong_ is a cute little guy, found to be the most basal ceratopsian
> (below _Chaoyangsaurus_ and psittacosaurs).  The holotype is ~120cm long
> (though apparently subadult).  _Yinlong_ was probably bipedal.  The skull
is
> weird - but then again, it is a marginocephalian, and weird skulls are
> pretty standard for this bunch.  The skeleton (including skull) shows a
> hotchpotch of ceratopsian, pachycephalosaurian and heterodontosaurid
> features.
>
> Heterodontosaurids are recovered as the sister taxon to marginocephalians.
> The new taxon Heterodontosauriformes is proposed, and defined as a
> node-based clade that includes the most recent common ancestor of
> _Heterodontosaurus_ and _Triceratops_ and all its descendents.  (IMHO, it
> might be a nice idea to emend this definition to include one or more
> negative/external qualifiers, such as adding "but excluding _Iguanodon_
and
> _Ankylosaurus_", in case the position of heterodontosaurids changes
again.))
>   There is weak support for _Agilisaurus_ as the sister taxon to
> Heterodontosauriformes.
>
> Having Heterodontosauridae as the sister taxon to Marginocephalia spells
> trouble for Ornithopoda, as defined by Sereno (2005): "The least inclusive
> clade containing _Heterodontosaurus tucki_ and _Parasaurolophus walkeri_,
> but excluding _Pachycephalosaurus wyomingensis_, _Triceratops horridus_
and
> _Ankylosaurus magniventris_."  Under this definition, Ornithopoda is
> untenable.
>
> Cheers
>
> Tim
>
>