Yinlong - a new link between marginocephalians and heterodontosaurs.

[Tim Williams <twilliams_alpha@hotmail.com>]

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