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Details on Shanyangosaurus



It's time again for details of a taxon that is recently described, but
poorly understood.  Today's species is Shanyangosaurus niupanggouensis.

Shanyangosaurus Xue, Zhang and Bi 1996
S. niupanggouensis Xue, Zhang and Bi 1996
etymology- "Shanyang lizard from Niupanggou", Shanyang being the basin it
was found in and Niupanggou the region.
Shanyang Formation, Shaanxi, China
middle-late Maastrichtian, Late Cretaceous
holotype- (NWUV 1111) partial sacrum, proximal scapula, humeri, femur,
tibia, metatarsal IV (left and right), partial phalanx, pedal ungual
diagnosis- long triangular cnemial crest on tibia projected forward with
apex above posterior apexes of tibial condyles.
main measurements-
    humerus             116 mm
    femur                  258 mm
    tibia                    327 mm
    metatarsal IV     137 mm
Description-
This is a relatively small theropod dinosaur about the size of Velociraptor
or Oviraptor, that could have measured from 1.7 to 2.6 meters in length,
depending on tail size.  It is described very briefly with little relevent
information written.  Luckily, there are accompanying plates.  The text
states the bones are hollow.
The sacral remains consists of two complete fused vertebrae with the
anterior section of a third.  Both are 32 mm long and shown in ventral view.
The scapula is missing it's distal end and is very poorly preserved.  It
would appear to have a low acromion and there is no forward projecting
acromion process visible.
The humerus is 45% of the femoral length and also very poorly preserved, but
appears to heve a more proximally placed deltopectoral crest than most other
coelurosaurs (apex 22% down the shaft).  The internal tuberosity is not very
prominent.
The femur is in general very similar to that figured as Ingenia in The
Dinosauria, differing in a couple of ways.  It's a bit more robust, has less
of a neck below the head, which is separated from the greater trochantor by
less of a groove.  The lateral condyle is broken off, as are any remains of
the lesser or accessory trochantors, if they existed.  The text states that
the fourth trochantor is absent and that the intercondylar groove is deep
and wide.  Importantly, the head is elevated.
The tibia is 127% of femoral length and fairly well preserved (it's a shame
more tibial features aren't used in coelurosaur analyses).  It's also very
similar to Ingenia's, but differs in a couple of ways as well.  The shaft is
bowed laterally and the cnemial crest is a different shape, being much
longer and more prominent, with the anterodorsal edge almost perpendicular
to the shaft.  It's the one reason this taxon isn't a nomen dubium.  (When I
was first writing this post, I thought the tibia was backwards, as the
cnemial crest is so unique that it looks much more normal if the posterior
tibial condyles are the cnemial crest.  When viewed that way, the "cnemial"
crest is just a bit shorter than Ingenia's proximodistally and with a more
verical anterodorsal edge.  I wouldn't have realized my mistake if not for
later noting how large the inner tibial condyle appeared to be and noticing
Xue et al. wrote "long cnemial crest without furrow on it's distal part" in
the diagnosis/description.  They may have the tibia backwards, but I'm sure
they can tell better than I, as they have the actual three dimensional
specimen and not a photocopy.).  There is a fibular crest and no depression
for the ascending process can be observed.
The fourth metatarsal is 53% of femoral length, fairly robust, more so than
many other coelurosaurs, and is almost certainly not arctometatarsalian
because it's wider transversely than deep.  It is narrower transversely when
viewed proximally than Deinonychus.
There is also the distal section of a pedal(?) phalanx and another bone,
which may be the proximal end of the other fourth metatarsal mentioned as it
is roughly similar in proximal view.
The pedal ungual is not as deep as therizinosaurs, but deeper than
Nedcolbertia.  It's more curved than ornithimimids, but less so than
dromaeosaurids.
Of special note is the statement "ribs with horizontal hooks" that is in the
description and has fueled thoughts of uncinate processes.  Note however,
that no ribs are mentioned in the material list, nor are any shown in the
plates.
Relationships-
Shanyangosaurus was originally identified only to the level of Theropoda and
there it has stayed in many people's minds.  Unfortunately, the remains are
very poorly preserved and described.  Shanyangosaurus is obviously some type
of theropod with an unfused non-arctometatarsalian metatarsus, a low
acromion and an elevated femoral head.  This particular combination of
features is only known in oviraptorids, although several other groups could
have this combination of features, but are too poorly known to determine.
These are Ornitholestes, Caudipteryx, Microvenator, Bagaraatan,
Protarchaeopteryx, Alvarezsaurus, Patagonykus, Unenlagia and Rahonavis.
Shanyangosaurus has a non-alvarezsaurid humerus, so Alvarezsaurus and
Patagonykus are eliminated as possibilities.
The scapula of Shanyangosaurus still has an obvious acromion (and no
apparent anteriorly projecting acromion process), so it's not as bird-like
as Unenlagia or Rahonavis yet.
Both Ornitholestes and Protarchaeopteryx are not well described or figured
enough to compare closely to Shanyangosaurus, although Protarchaeopteryx
bears no real resemblence and Ornitholestes is in a phylogenetic position
where you would expect it to have a plesiomorphically high acromion.
Bagaraatan has a tibia that is wider than deep proximally and a fused
tibiotarsus, unlike Shanyangosaurus.
Caudipteryx has an anteriorly projected acromion process and much more
slender metatarsal IV.
Microvenator is more slender with a weaker cnemial crest, but does share the
absence of a fourth trochantor with Shanyangosaurus, so they may be related.
So Shanyangosaurus compares closest to Microvenator and oviraptorids and is
therefore probably an oviraptorosaur, which is congruent with the horizon
and locality, not to mention the currently problematic presence of uncinate
processes.  For those of you wondering if there may be some relationship to
Nomingia, Shanyangosaurus resembles Ingenia more because Nomingia has a
declined femoral head and laterally projected femoral medal condyle.  BTW,
Nomingia is closer to Caudipteryx than Microvenator or oviraptoroids
according to my latest analysis, although I'm waiting for the description to
be published before I trust this result.
I don't think it's possible to narrow down Shanyangosaurus's relationships
much further, as it and Microvenator are poorly known and oviraptorids have
yet to be described well and sorted out phylogenetically.  There are a few
comparisons I can make however.
The acromion of Shanyangosaurus does seem higher than Oviraptor
philoceratops and it seems to lack the anteriorly projecting process of
Caudipteryx.  The humerus is 45% of femoral length, which is less than 66%
for Microvenator, 61% for Ingenia and ~73% for Oviraptor(?).  So if
Shanyangosaurus is an oviraptorid, it has a much shorter arm than others
(although the basal oviraptorosaur Caudipteryx's ratio is 47%, so such short
arms are not unheard of in this group).  The deltopectoral crest is 22% down
the shaft, compared to 30% in Microvenator, 40% in Ingenia and 42% in
Oviraptor.  Again, less than oviraptorids and Microvenator, but on par with
Caudipteryx (~25%).  On the other hand, the humerus is more robust than
either Caudipteryx or Microvenator and closer to Ingenia in this regard.  As
said above, both Shanyangosaurus, Caudipteryx and Ingenia have inclined
femoral heads, whereas Nomingia does not.  Nomingia has a transversely wider
greater trochantor than either Ingenia or Shanyangosaurus.  Microvenator
lacks a fourth trochantor and Shanyangosaurus is supposed to as well, but it
may still be present, but very reduced as in oviraptorids.  Regarding the
distal femur, Microvenator's is expanded laterally more than Ingenia's,
Shanyangosaurus's or Nomingia's., Nomingia and Ingenia have lateral condyles
than project distally, unlike Microvenator and Shanyangosaurus, and
Shanyangosaurus has the smallest medial expansion, while Nomingia has the
greatest.  If correctly identified (see above), the cnemial crest is much
larger and more proximally placed than any other coelurosaur and the
posterior tibial condyles are more prominent and distally projecting than
oviraptorosaurs.  The metatarsal is more robust than Caudipteryx, comparing
well to Oviraptor, Conchoraptor and Ingenia.  The pedal ungual is not as
narrow or sharp as shown for Conchoraptor and has a smaller proximoventral
heel than Ingenia, although it's rather similar to those of Oviraptor.

I'm sorry I can't provide a reference, as I only have the last part of the
paper.  Perhaps Tracy will enlighten us.

And that's my description of Shanyangosaurus.  I hope you all liked it.  I
plan to do more of these types of descriptions of poorly known dinosaurs in
the future.  Next up is Qinlingosaurus.  As usual, I've scanned the figures
of Shanyangosaurus into jpg format for anyone who wants them.  Just contact
me offlist.  Feel free to send any comments or questions.

Mickey Mortimer