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Details on Adasaurus



This has been considered a genus of dromaeosaurid, a species of
Dromaeosaurus and something more closely related to therizinosaurs by
various authors.  It also comes courtesy of Mike Keesey's wish for recent
dromaeosaurids, before we get to the 90's species.

Adasaurus Barsbold 1977
A. mongoliensis Barsbold 1983
= Dromaeosaurus mongoliensis Paul 1988
middle Maastrichtian, LC
Nemegt Formation, Mongolia
holotype- (GI 100/20) (2.5 m) (old adult) incomplete skull and incomplete
skeleton including ilium (202 mm), pubis (241 mm) and ischium (118 mm)
referred- (GI 100/51) posterior postcranial skeleton including hindlimbs,
phalanx II-1 (23 mm), phalanx II-2 (14 mm), pedal ungual II (28 mm)
diagnosis- preacetabular processes strongly divergent; preacetabular process
strngly notched anteriorly; posterodorsal edge of postacetabular process
very thick; distal ischium curved strongly posteriorly; metatarsal II
reduced in width.
Description-
This genus was first only illustrated and labelled as Adasaurus in a short
paper by Barsbold (1977), it was later described extremely briefly by
Barsbold (1983).  The holotype specimen was approximately 2.5 meters long if
the ilium is scaled from Deinonychus.  It was from an old individual, so
this is probably close to the maximum size Adasaurus got.  Because the
specimen is pathologic (Norell and Makovicky 1997), some features of it's
skeleton may not be representative of the species.
There are skull remains, but all that is said regarding them is that they
"bear a great similarity to other members of the subfamily, obviating the
need to repeat general skull characters" (Barsbold 1983).  The subfamily
referred to is the Dromaeosaurinae (with Dromaeosaurus, Adasaurus and
Deinonychus), separated from the Velociraptorinae (with Velociraptor) by the
high, relatively large skull.  From this, we may assume that Adasaurus had a
higher skull than Velociraptor, more similar to Dromaeosaurus and
Deinonychus.
Adasaurus also has a strongly opisthopubic pelvis, like Bambiraptor,
Sinornithosaurus, Velociraptor and probably Deinonychus, unlike the slightly
opisthopubic condition in Unenlagia and the slightly propubic condition in
Achillobator.
The ilium is generally similar to other dromaeosaurs.  Both ilia are
separated widely from each other, as in other dromaeosaurids, but the
preacetabular processes are divergent, unlike Sinornithosaurus and
Velociraptor.  The preacetabular process is notched anteriorly, as in
Achillobator, but to a greater degree. Deinonychus and Unenlagia have very
slightly concave anterior edges, while Bambiraptor's is convex.  The
anterior edge is also sloped strongly posteroventrally with an acute
anterodorsal corner, as in Achillobator, Deinonychus, Unenlagia and
(probably) Sinornithosaurus, and unlike Bambiraptor.  The ventral
margin of the pubic peduncle is slightly concave, as in Achillobator,
Sinornithosaurus and Unenlagia, as opposed to the pronounced notch present
in Bambiraptor and Velociraptor.  Adasaurus's ilium is also different from
both Achillobator and Unenlagia in that it was lower and lacks a
posterodorsal tubercle.  Thus, Adasaurus resembles Bambiraptor, Deinonychus,
Velociraptor and possibly Sinornithosaurus in these features.  The
postacetabular process is not a gently curved blade not descending below the
ischial peduncle like Bambiraptor, Deinonychus and Velociraptor.  Instead,
it has a slight concavity on it's posterior edge and lies below the ischial
peduncle, as in Achillobator and Unenlagia.  The postacetabular process also
lies below the ischial peduncle in Sinornithosaurus, although it appears to
only have a very slight concavity if any.
The pubic foot has a slight anterior component, smaller than in Deinonychus
and Velociraptor, unlike the enlarged anterior pubic foot in Achillobator
and
the absent one in Bambiraptor, Sinornithosaurus and Unenlagia.  The
posterior pubic foot is of modest proportions, larger than Sinornithosaurus
and Velociraptor, but smaller than other dromaeosaurids.
Adasaurus lacks any posterior ischial processes, unlike all dromaeosaurids
except Deinonychus and Velociraptor.  Besides the apomorphically posteriorly
curved distal end, the ischium is almost identical to Deinonychus, the only
other differences being a slight dorsal curvature and slightly smaller
obturator process.
Adasaurus still has a fourth trochantor (Perle et al. 1999), like
Achillobator, Velociraptor and some specimens of Deinonychus, as opposed to
Bambiraptor and Unenlagia, where it is absent.  It is positioned more
proximally than in Achillobator, as is Velociraptor's.  As in other
eumaniraptorans, the anterodistal femoral fossa of Adasaurus is absent.
The tibiofemoral ratio is said to be greater than Achillobator, which
indicates it was over 0.97.
Barsbold (1983) noted that metatarsal II of Adasaurus was "considerably
thinned".  Thus, it may have been similar to Tochisaurus in this regard,
while in opposition to Velocisaurus and avisaurids, which reduced their
fourth digit.  As for the pedal elements, only digit two has been figured or
described.  Like other dromaeosaurids and troodontids, but unlike Rahonavis,
phalanx II-2 has a large proximoventral heel participating in the phalangeal
articulation.  The second digit is often compared to Dromaeosaurus because
of the reduced ungual, but the second ungual of Dromaeosaurus is unknown.
We can however compare phalanges II-1 and II-2 of Deinonychus and
Dromaeosaurus, then compare Adasaurus and other dromaeosaurids to see how
they compare.  Comparison indicates that phalanx II-1 is near identical in
the two species.  Phalanx II-2 however is much stouter in Dromaeosaurus than
Deinonychus.  The phalanges of Adasaurus and especially Achillobator are
even more stout, while those of Bambiraptor,
Pyroraptor, Saurornitholestes, Sinornithosaurus, Velociraptor and the
unnamed form from Sudan (Rauhut and Werener 1995) are elongate.  The claw of
Adasaurus is reduced (115% of II-2) compared to most other dromaeosaurs
(Deinonychus- 197%, Bambiraptor- ~205%, Velociraptor- 233%, Pyroraptor
~275%), but rather similar to Achillobator (151%).  It also lacks a large
flexor tubercle, unlike other dromaeosaurids, including Achillobator.
Relationships-
Adasaurus has been allied with three groups in the past- segnosaurs,
dromaeosaurids (and specifically Dromaeosaurus itself) and basal avians such
as Rahonavis.
It is obviously not segnosaurian as it lacks a deep preacetabular process
and has a high tibiofemoral ratio and is excluded from the
Therizinosauroidea based on the unreduced postacetabular process.  The one
non-dromaeosaurid feature that's been used to ally it with segnosaurs is the
deflected preacetabular processes, but these are only present in
therizinosauroid segnosaurs.  Since Adasaurus lacks segnosaurian
synapomorphies, it can hardly belong to a subgroup of them.
As Rahonavis is often said to be dromaeosaurid-like and I wouldn't be
surprised if it ended up within the family in my analyses one day, it also
deserves comparison with Adasaurus.  Rahonavis has a slightly opisthopubic
pelvis. The preacetabular process is convex anteriorly and what is analogous
to the anterior edge of dromaeosaurids faces ventrally.  The ilium has a
posterodorsal tubercle and a postacetabular process that does not extend
below the ischial peduncle.  These all differ from Adasaurus.  The only two
similarities within dromaeosauroid theropods between the two is are the
ventral margin of the pubic peduncle is slightly concave and the
postacetabular process has a slight posterior concavity.  The pubic foot
lacks an anterior component.  The ischium has both a posterodorsal and
mid-dorsal process, a blunt tip and a distally placed obturator process.
The femur lacks a fourth trochantor.  Pedal phalanx II-2 is elongate and has
a much smaller heel that does not contribute to the phalangeal joint and
pedal ungual II is enlarged (232%) with a prominent flexor tubercle.  There
is no particular similarity to Adasaurus.
Adasaurus therefore probably belongs to the Dromaeosauridae.  This is
confirmed by Norell and Makovicky (1997) and Perle et al. (1999), who refer
to it as a dromaeosaurid, and by including it in my analysis, where it
groups with other dromaeosaurids.  Since Norell and others actually have
access to the entire specimen and are theropod experts, I trust they are
correct in their familial assignment, especially after my analysis shows
identical results.  Answering where within the Dromaeosauridae Adasaurus
belongs is a more difficult task however.  The lack of a large flexor
tubercle on pedal ungual II is a more primitive feature than other
dromaeosaurids, so perhaps it is basal to the clade.  Since the stout pedal
phalanx II-2 is not present in dromaeosaurid's immediate outgroups
(Protarchaeopteryx, basal troodontids, avians), it is possible that
Dromaeosaurus, Adasaurus and Achillobator could be united in a monophyletic
Dromaeosaurinae.  The relatively small second pedal ungual is, however, a
primitive feature that would not support such a subfamily (assuming
Dromaeosaurus even has this character).  In addition, the opisthopubic
pelvis would indicate Adasaurus is more advanced than Achillobator.  The
fourth trochantor, slightly anteriorly projecting pubic foot and absence of
a proximodorsal ischial process suggests that this species is basal to the
Bambiraptor, Sinornithosaurus, Unenlagia group.
So Adasaurus is a rather basal dromaeosaurid, probably close to Achillobator
or Dromaeosaurus, but outside the subgroup containing Bambiraptor,
Sinornithosaurus and Unenlagia.  If Rahonavis ever ends up in the
Dromaeosauridae, it will be in this group as well, so it is also not
particularily close to Adasaurus.  The evidence for placing it in a
subfamily with Dromaeosaurus is equivocal since the character evidence would
include Achillobator in that subfamily as well, and Achillobator is more
plesiomorphic than Adasaurus in pelvic structure.

Well, that's my take on Adasaurus.  The pelvis and second pedal digit are
the only figures ever made for this genus.  They are easily available in The
Dinosauria and Glut's Encyclopedia, but if someone would like scans, feel
free to ask.  I think I'll take it easy and do Pyroraptor next, saving the
huge post that describes Achillobator for a while later.

Mickey Mortimer