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Agrosaurus, Lesothosaurus



In response to a couple of queries...

< Date: Thu, 12 Feb 1998 07:47:22 +1100
< Reply-To: dannj@alphalink.com.au
< While I've got the subject raised, does anyone know whether Agrosaurus
< is anchisaurid? Is there enough material to be anywhere certain?
<       Dann Pigdon

Agrosaurus seems very probably an anchisaurid prosauropod, as explained by
Peter Galton some years ago (sorry, I don't have reference right to hand).
It's currently being re-examined by Tom Rich and his colleagues, and I
suspect they'll be publishing something in the near future.

                                                        AND

< Date: Tue, 10 Feb 1998 09:40:12 -0600 (CST)
< What's the consensus (if any), obturator process in _Lesothosaurus_ or
< not?
< Thanks in advance.
< jack

"Lesothosaurus".  I revisited the whole Fabrosaurus/Lesothosaurus tangle in
1992 (Geobios, vol. 25: 283-292 - where you'll find all the evidence and
opinions).  Paraphrasing from that paper (p. 290):
               Paul Sereno (1984: 223) stated that the ischium of
Lesothosaurus possessed an obturator process.  But in a second paper (Sereno
1986: 247) he maintained that "contrary to the original account (Thulborn
1972), a distinct obturtaor process is clearly absent in Lesothosaurus as
shown by the appropriate portion of the right ischium of the type specimen."
Largely on that basis Sereno advocated exclusion of fabrosaurids (=
Lesothosaurus) from the suborder Ornithopoda sensu stricto.  In fact, the
left and right ischia of the type specimen are different:  the left ischium
has a perfectly normal obturator process, while the mirror-image right
ischium (less complete) seems to have no trace of the process.  [Note, the
two ischia are mirror-images and identical in preservation; I prepared them
out from the matrix myself, and there's no doubt that they belong to a
single individual animal.]    I assumed (1972) that the right ischium was
imperfect, or pathological, and illustrated the better-preserved left
ischium... which matches all other ornithopod ischia in having the process.
(Seems less likely that a single ischium of a single animal would invent an
obturator process independently of all other ornithopods!)  The obturator
process is also present on the fabrosaurid ischia described by Santa Luca
(1984: 160-161).  
        Basically, the idea that fabrosaurids lacked the obnturator process
derives from a single (pathological?) ischium in a single animal; all other
fabrosaurid ischia have the process.  A few years ago Alan Charig and I
re-examined the material together in the British Museum... and his
conclusions were exactly the same: two ischia from a single animal, one with
the process and one without it - presumably pathological.

        Incidentally, I'd maintain that the critter's name is Fabrosaurus,
not Lesothosaurus... but not everyone would agree (another long story, which
you'll find in Geobios).

Refs: -

Santa Luca, A.P.  1984.  Palaeontologia Africana, 25: 151-180.
Sereno, P.C.  1984.  In Reif, W.-E. & Westphal, F. Third symposium on
Mesozoic terrestrial ecosystems, Short papers, Attempto Verlag, Tubingen:
219-226.
Sereno, P.C.  1986.  National Geographic Research, 2: 234-256.
Thulborn, R.A.  1972.  Palaeontology, 15: 29-60.
Thulborn, R.A.  1992.  Geobios, 25(2): 283-292.

Back to the Swamp...

Tony Thulborn