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Details on Santanaraptor



Santanaraptor Kellner 1999
S. placidus Kellner 1999
etymology- "Placido's Santana plunderer", Santana being the formation it was
found in and Placido Cidade Nuvens the founder of the Museo de Santana do
Cariri.
Albian, EC
Romualdo member of Santana Formation, Brazil
holotype- (MN 4802-V) three caudal vertebrae, chevrons, ischia (91 mm),
femora, tibia, fibula, pes, integument, musculature
diagnosis- foremon at medial base of lesser trochantor; well developed
sulcus on posterior femoral head; fibular trochlea of triangular shape and
constricted at base.
Description-
This theropod was known first announced in 1996 and emphasis was placed on
the soft tissue preserved with the specimen.  At that time, it was
identified as a probable maniraptoran theropod.  It was later described and
named in 1999.  It was probably about 1.25 meters long, assuming it
resembled Ornitholestes in proportions, but was a juvenile according to
unfused vertebral sutures.
The caudal vertebrae appear to be mid caudals and have low, posteriorly
oriented neural spines.  The chevrons are said to be half the centrum length
and expand slightly anteroposteriorly.
The ischia closely resemble Ornitholestes, differing only in the slightly
more distally placed obturator process, which is narrower.  Thus, they are
of basal coelurosaur grade with a triangular obturator process and
unexpanded distal tip.  There are no dorsal processes or proximolateral scar
and the shaft is rod-shaped distally.
The proximal femur is figured, but the distal portion is just described.
The shaft diameter is 13 mm and the bone thickness is 2.5 mm, so it's
typically hollow.  The wing-like lesser trochantor is separated from the
greater trochantor, which it does not reach, by a cleft.  The fourth
trochantor is a low crest.  The femur has three features unique to this
species that are noted above in the diagnosis.
The tibia, fibula and pes are photographed (which doesn't show up well in a
photocopy) and not described, although the fibula appears about a third as
wide as the tibia.
The metatarsus is typically theropod and said to be 70% of the femoral
length.  Metatarsals II and IV are said to be subequal and it can be seen
not to be arctometatarsalian.  Digits are also preserved, but no details can
be observed.
Soft tissue is found on various parts of the fossil.  The epidermis is very
thin (~0.04 mm) and formed by irregular quadrangles separated by deep
grooves.  There are no scales or feathers preserved.  Striated muscle fibers
were preserved as calcium phosphate, are polygonal in transverse section and
30-50 micrometers in diameter.  The bone still preserves channels for blood
vessels (diameter 20-25 micrometers) and lacunae for osteocytes (diameter ~5
micrometers).  There are also structures preserved that may either be
mineralizations filling the bone's capillaries or replacements of the blood
vessels.  They are rod-like with a rough outer layer and smooth inner layer.
Relationships-
Kellner places this species in the Coelurosauria based on the triangular
obturator process and suggests it may be a maniraptoriform based on Sereno's
(1999) character "obturator notch U-shaped with slightly divergent sides",
which I find highly variable.
I agree that the triangular obturator process shows this is a coelurosaur,
but the wing-like lesser trochantor excludes it from the Maniraptora
(oviraptorosaurs, segnosaurs, dromaeosaurs, troodontids, alvarezsaurs, birds
and a few more taxa in my analysis, a wing-like lesser trochantor is only
known in Beipiaosaurus).  The lack of a proximolateral ischial scar excludes
it from the Tyrannosauroidea.  That leaves it in the large group of basal
coelurosaurs/maniraptoriforms including compsognathids, Coelurus,
Ornitholestes, Scipionyx, Nedcolbertia, Nqwebasaurus, Gasosaurus,
ornithomimosaurs and the new Santana coelurosaur (Martill et al. 2000).
These groups also all have plesiomorphically low lesser trochantors and
prominent fourth trochantors where known.  These are currently in an
unresolved polytomy with tyrannosaurs and maniraptorans in my analysis, so
all coelurosaurs except for Deltadromeus are in a messy Maniraptoriformes.
Whether Santanaraptor will actually be in the most inclusive group with
ornithomimosaurs and maniraptorans when my cladogram clears up remains to be
seen, so I recommend Santanaraptor be placed as a non-maniraptoran,
non-tyrannosaurian coelurosaur.  It may be noted however that of the basal
coelurosaurs listed above, only Santanaraptor and Ornitholestes lack distal
ischial expansions, so perhaps they are more derived (though my analysis has
yet to show this).

References-
Kellner, A. W. A.  1996.  Fossilized theropod soft-tissue.  Nature 379: 32.
Kellner, A.W.A. and D.A. Campos. 1998. Archosaur soft Tissue from the
Cretaceous of the Araripe Basin, Northeastern Brazil.  Boletim do Museu
Nacional, Geologia No. 42: 1-22.
Kellner, A.W.A. 1999. Short note on a new dinosaur (Theropoda,
Coelurosauria) from the Santana Formation (Romualdo Member, Albian),
Northeastern brazil. Boletim do Museu Nacional (Rio de Janeiro, Brasil),
N.S. No. 49: 8 p.

The figure of the proximal femur and ischium is avaliable for those who
contact me offline.  I'm always open for suggestions as to which dinosaurs I
should write "Details on.." segments on.  Any dinosaur goes.

Mickey Mortimer