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Re: Megalosaurids




"Thomas R. Holtz, Jr." wrote:

> > From: Nicholas Gardner [mailto:n_gardner637@hotmail.com]

> > 8.Is Megalosauridae a valid grouping?  If so, what supports it?

> As the existence of _Megalosaurus_ as a valid group (of bones) is
> questionable, I think you can see why the next step up is even more
> tentative.  I have a contribution forthcoming that does support a grouping
> of _Meg._, _Torvo._, _Poekilo._, _Eustrep._, _Piatnitzky._, _Afrovenator_,
> and someone else (heh, heh, heh...), to the exclusion of all other
> theropods.  One the other hand, if you do NOT consider the non-dentary _M.
> bucklandi_ material as belonging to a single taxon, the monophyly of
> Megalosauridae falls apart relative to other basal tetanurines.  As for what
> supports it: wait for the paper... :-)

It's probably appropriate to mention here what Paul did with the
Megalosauridae back in 1988. Of course, none of this is supported by a
cladistic analysis, but it is still very interesting despite being based
on grade.

He first set up the 'Intertheropoda', a group intermediate between the
Ceratosauria and the 'Avetheropoda' (roughly equivalent to the
Tetanurae). It's pretty clear that the Intertheropoda would not be a
monophyletic group; very likely it's more like a paraphyletic basal
outgroup to the tetanurans. But in any event, he includes 2 families
with 5 subfamilies in this group, claiming that they are roughly
intermediately derived in structure as compared to the more primitive
ceratosaurs (including _Procompsognathus_, the Coelophysoid group [in
which he tentatively includes the Spinosaurids], and the Ceratosaurids)
on the one hand and the more advanced avetheropods (_Compsognathus_,
Coelurids, Allosaurids, Tyrannosaurids, and what we would today call
maniraptors) on the other. Anyway, these are his Intertheropod groups**:

Family Megalosauridae
        Subfamily Megalosaurinae
        Subfamily Abelisaurinae
        Subfamily Noasaurinae
Family Eustreptospondylidae
        Subfamily Eustreptospondylinae
        Subfamily Metriacanthosaurinae

So he believes(d) there is in fact a natural grouping around
_Megalosaurus bucklandi_, that it is closely related to the
abelisaur/noasaur group, and a little less closely related to the
eustreptospondyls and metriacanthosaurs, and that it is intermediately
derived in comparison to the ceratosaurs and tetanurans.

Within the Megalosaurinae, he includes only 2 species. One is _M.
bucklandi_, and the other is currently known as _Torvosaurus tanneri_,
but Paul tentatively sunk it into _Megalosaurus_, ending up with two
megalosaur species: _M. bucklandi_ and _M.? tanneri_. He also
tentatively sank _Poekilopleuron_ into _M. bucklandi_. Here is what he
says in justification of his vision of _Megalosaurus_:

'Much as the Megalosauridae is an often abused family, this is the most
abused theropod genus. It has been used as a convenient grab bag, with
more partial remains from more places and times tossed into it than any
other. Long ago this practice got way out of hand, when the name was
used even for Late Cretaceous theropods. Much of this supposed
megalosaur material was inadequate for proper identification; some is
very different and belongs to other taxa. On the other hand, a couple of
differently named theropods appear to belong in this genus. The ilial
blade and curved ischium of the _Torvosaurus_ hip are very similar to
those of the type of _M. bucklandi_. In turn, the arm of
_Poekilopleuron_ is like that of _Torvosaurus_. Since they all share the
same exceptional design, I am tentatively uniting them in
_Megalosaurus_. If correct, true _Megalosaurus_ ranged from Middle to
Late Jurassic in age.
'This is an unusually robust form, in the vertebrae, arm, and pelvis.
The unbooted pubes are especially short and broad, very unlike any other
post-procompsognathid theropod. As I explained in Chapter 8, this seems
to be a secondary specialization, rather than truly primitive. What
little is known of the skull shows that it is that of a normal big
theropod. ...
'Since French _Poekilopleuron bucklandi_ Eudes-Deslongchamps, 1838 lived
at exactly the same time as _M. bucklandi_ and appears to be a member of
the genus, it is tentatively placed in the same species. Sad to say, the
only specimen was destroyed in the last World War.'

And this, by the by, is what he says in Chapter 8:

'The next more derived group is the intertheropods, which are good-sized
theropods with features that place them between the basal and the
advanced theropods. This group includes the heavily built megalosaurs
and the normally built eustreptospondyls and metriacanthosaurs. These
advanced paleotheropods are more derived than coelophysians and
ceratosaurs because of their still more slender shoulder blades, larger
outer femoral process, vertical pubes with growing boots, and narrower
central cannon bone .... The short-forearmed megalosaurs are rather
strange theropods, especially the South American abelisaurs and
noasaurs. This shows that they had left the mainstream of theropod
evolution. Small _Noasaurus_ paralleled some protobirds by developing a
special killing claw on the second toe. _Megalosaurus_ is also unusual
in that it has broad pubes, like early dinosaurs. This once led me to
believe that it was the most archaic of theropods, and Peter Galton and
James ("Dinosaur") Jensen to conclude that megalosaurs and some other
big theropods evolved completely separately from small theropods. But
megalosaurs share too many characters with other theropods for this to
be true. I now believe the broad hips are a secondary reversal to the
old condition. Eustreptospondylians and metriacanthosaurs are a bit more
advanced than megalosaurs in the above listed characters, and less
aberrant. Theropods like these may have been the ancestors of more
advanced theropods. _Metriacanthosaurus_ is especially important in
helping us ascertain the evolutionary sequence, because this
well-preserved form has a large, expanded pubic tip, or boot, and the
shorter anterior trunk ribs that go with the start of an avian lung
system.'

So, granting that Paul was not even working with a cladistic system but
with a Linnaean one, this is roughly something around the sequence he
saw in the fossils available at the time:

Theropoda
  |--Procompsognathidae
  `--+--+--Coelophysoidea (incl Spinosauridae)
     |  `--Ceratosauridae
     `--+--Megalosauridae (incl Abelisaurinae, Noasaurinae)
        `--+--Eustreptospondylidae (incl Metriacanthosaurinae)
           `--Tetanurae

I am not sure what cladistic analyses have been done to support or
refute this basic concept. I know that Paul's visions of the
Ceratosauria and Avetheropoda (=Tetanurae) have essentially been borne
out and supported by the currently accepted cladistic model (aside from
a few details, like the exact placement of the Spinosauridae and
Tyrannosauridae), but the information I have been able to find on
megalosaurs, abelisaurs/noasaurs, eustreptospondyls, and
metriacanthosaurs, relative to other theropod groups, seems to be spotty
and tentative at best. It is my opinion that a working out of the exact
relationships among these groups and their placement relative to the
main flow of theropod evolution will be probably the most interesting
and exciting development to watch for in the field of theropod
phylogenetics over the next decade or 2, possibly even more than the
sorting out of relationships within the Maniraptora.



**Footnote for those interested in a greater level of detail on the
other groups Paul named as intermediate: This is the full list of
species Paul placed in the Intertheropoda.

Megalosaurines:
  *_Megalosaurus bucklandi_ (syn: _Poekilopleuron bucklandi_?, _P.
poikilopleuron_?, _Scrotum humanum_?)
  *_M.? tanneri_ (syn: _Torvosaurus tanneri_)
Abelisaurines:
  *_Abelisaurus comahuensis_
  *_Carnotaurus sastrei_
  *?_Genyodectes serus_
  *?_Xenotarsosaurus bonapartei_
Noasaurines:
  *_Noasaurus leali_
Eustreptospondylines:
  *_Eustreptospondylus oxoniensis_ (syn: _Megalosaurus cuvieri_,
_Streptospondylus cuvieri_)
  *_Piatnitzkysaurus floresi_
  *_Gasosaurus constructus_
  *_Marshosaurus bicentesimus_
Metriacanthosaurines:
  *_Metriacanthosaurus_? sp. (syn: _Szechuanosaurus campi_)
  *_M.? shangyouensis_ (syn: _Yangchuanosaurus shangyouensis_, _Y.
magnus_)
  *_M. parkeri_ (syn: _Megalosaurus parkeri_)

He also lists some fragmentary _incertae sedis_ theropod material,
conveniently right after the intertheropod section and thus between the
ceratosaurs and avetheropods, but makes no firm claim that any of them
actually belong in an intermediate position, or in any other particular
position for that matter; I might as well list them since it is at least
quite possible some or any of them could end up to be related to the
megalosaurs or eustreptospondyls, or otherwise somewhere between the
ceratosaurs and tetanurans:

*_Megalosaurus? nethercombensis_
*_Megalosaurus? hesperis_
*_Xuanhanosaurus qilixiaensis_
*_Iliosuchus incognitus_
*_Stokesosaurus clevelandi_
*_Segisaurus halli_

-- 
--Sean
http://www.livejournal.com/users/spclsd223/