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What is Saltopus?



How many of you have wondered "what is Saltopus?" at one point in your
paleontological career?  It's been known from a fairly complete skeleton
since 1910, but has yet to be reexamined in recent times.  I'm no
professional and I certainly haven't actually examined the holotype of this
species, but I decided to examine published figures and see exactly where it
could be placed in the Ornithodira.
Unfortunately, the Ornithodira is the one clade that this species obviously
belongs to as it shares five synapomorphies:
- forelimb less than 50% of hindlimb (~39%)
- femoral shaft bowed anteriorly at least 80% of length
- tibia subequal or longer than femur (138%)
- metatarsals II-IV elongate with mt III more than 50% tibial length (63%)
- dorsal body armor absent
It also lacks one synapomorphy (hatchet-shaped deltopectoral crest), so it
could be a basal ornithodiran, but then again, Scleromochlus also lacks
this.
So, Saltopus is obviously an ornithodiran, but it is not decribed well
enough to determine presence or absence of dinosauromorph, dinosauriform or
Pseudolagosuchus+Dinosauria synapomorphies.
Saltopus does lack a brevis shalf and a deltopectoral crest at least 35% of
humeral length (it's crest is about 20%), which place it outside the
Dinosauria.  In addition, Saltopus lacks the theropod synapomorphy of a
humerus less than 50% of femoral length.  Two of these features are related
to the length of the humerus, which is incomplete in this specimen, but
whose minimum length can be determined by the complete radius and ulna,
which are always shorter than the humerus in basal ornithodirans.  Two
characters shared with some dinosaurs are an elongate ilium and more than
two sacral vertebrae, but see below.
So if Saltopus isn't a dinosaur, what is it?  I see two possibilities, both
supported by a few possible synapomorphies.
"Marasuchid"
- triangular dorsal neural spines?
- elongate distal caudals (c23 155% length of c1; 191% in Marasuchus)
- broad scapula?
What may be a neural spine is present to the left of the ninth dorsal
vertebra in Saltopus and if so, it appears triangular like Marasuchus.  The
distal caudal centra are more elongate than other archosaurs (where it's
usually less than 120%), but less so than Marasuchus.  Finally, the scapula
appears broad in Benton and Walker's figure, but is not shown Huene's.
Pterosaur sister group
- more than two sacral vertebrae
- elongate distal caudal centra
- slender preacetabular process equaling or exceeding length of
postacetabular process
Benton and Walker said that Saltopus has three sacral vertebrae, but four
are shown in Huene's figure.  This is more than other ornithodirans besides
neotheropods, most sauropods, ornithischians and pterosaurs.  The distal
caudals are elongate, though not as much as pterosaurs.  The preacetabular
process of Saltopus is more elongate than the postacetabular process, a
feature only known in pterosaurs among basal ornithodirans.  The ilium of
this species is actually very elongate, being 96% of femoral length.  This
compares to 30-51% in basal ornithodirans and basal theropods and 61-105% in
neotheropods.  The only non-dinosaurian ornithodirans with an ilium over 60%
of femoral length are pterosaurs (~70% in Eudimorphodon).  Although
neotheropods also have elongate ilia with more than two sacral vertebrae,
their preacetabular portion is much shorter basally and it would require
four reversals to place Saltopus as a neotheropod, as opposed to one
reversal if it were a pterosaur relative.
So in conclusion, Saltopus is a non-dinosaurian ornithodiran, probably
either a "marasuchid" or a pterosaur relative, with evidence for the latter
a bit better.  This is of course assuming pterosaurs are ornithodirans.....
This is of course just my hypothesizing, but I hope it's at least somewhat
accurate and clarified (as much as possible) the phylogenetic position of
this little known genus.

Mickey Mortimer