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Re: Hairy Hypsilophodontids
--- "Simon M. Clabby" <firstname.lastname@example.org> wrote:
> There may be one at Dinosaur Isle, Isle of Wight, as
> according to the catalogue of dinosaurian material in
> that museum, as published in Martill and Naish's
> "Dinosaurs of the Isle of Wight", there is an entry
> that reads
> "6626 ..... _H.foxii_ Skin".
Neat! Any details? (I suppose I could just look at the book when I get home...)
> When I called psittacosaurids an offshoot I meant it
> in such a way as to mean "psittacosaurids had an
> ornithopod ancestor", not "Psittacosaurids are
But psittacosaurids did not have an ornithopod ancestor. They (and other
marginocephalians) share an ancestor with ornithopods, but this ancestor was
not an ornithopod, at least by current usage of _Ornithopoda_.
Infraordo Ornithopoda was once used to cover pretty much all bipedal or
facultatively bipedal ornithischians, including the most basal forms
(_Lesothosaurus_, _Pisanosaurus_), basal thyreophorans (_Scutellosaurus_),
heterodontosaurids, and non-neoceratopsian marginocephalians
(pachycephalosaurs, psittacosaurids). This is pretty obviously a paraphyletic
assemblage based on a primitive trait, roughly equivalent to non-eurypodan,
non-neoceratopsian _Ornithischia_. It makes about as much sense as creating a
group for all quadrupedal mammals.
Clade _Ornithopoda_ could be defined as something like "all ornithischians
sharing more recent ancestry with _Iguanodon bernissartensis_ than with
_Triceratops horridus_", or some equivalent formulation. It includes
"hypsilophodonts" and iguanodontians (including hadrosaurs). By some
phylogenies it also include heterodontosaurids, but it seems that this opinion
is falling out of favor. Like any clade, _Ornithopoda_ is not ancestral to any
taxon which it does not also include.
Relating this back to the original point, if a basal ornithopod
("hypsilophodont") were found with structures similar to those of the Jehol
_Psittacosaurus_, it would strongly suggest that the trait was present in the
basal cerapod (_Cerapoda_ ~= _Ornithopoda_ + _Marginocephalia_), possibly
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