Comment by Mickey Mortimer

Etymology

"Mongolian evil lizard", Ada being evil in Mongolian mythology.

Holotype

(GI 100/20) (2.5 m) (old adult) incomplete skull and incompleteskeleton 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 processstrngly notched anteriorly; posterodorsal edge of postacetabular processvery thick; distal ischium curved strongly posteriorly; metatarsal IIreduced in width.

Description

 This genus was first only illustrated and labelled as Adasaurus in a shortpaper by Barsbold (1977), it was later described extremely briefly byBarsbold (1983).The holotype specimen was approximately 2.5 meters long ifthe ilium is scaled from Deinonychus.It was from an old individual, sothis is probably close to the maximum size Adasaurus got. Because thespecimen 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 thehigh, relatively large skull.From this, we may assume that Adasaurus had ahigher skull than Velociraptor, more similar to Dromaeosaurus andDeinonychus. 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 ischialpeduncle, 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 moreproximally 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 analogousto the anterior edge of dromaeosaurids faces ventrally.The ilium has aposterodorsal 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 footlacks 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 hasa much smaller heel that does not contribute to the phalangeal joint andpedal ungual II is enlarged (232%) with a prominent flexor tubercle. Thereis no particular similarity to Adasaurus.Adasaurus therefore probably belongs to the Dromaeosauridae.

This isconfirmed by Norell and Makovicky (1997) and Perle et al. (1999), who referto it as a dromaeosaurid, and by including it in my analysis, where itgroups with other dromaeosaurids.Since Norell and others actually haveaccess to the entire specimen and are theropod experts, I trust they arecorrect in their familial assignment, especially after my analysis showsidentical results.Answering where within the Dromaeosauridae Adasaurusbelongs is a more difficult task however.

The lack of a large flexortubercle on pedal ungual II is a more primitive feature than otherdromaeosaurids, so perhaps it is basal to the clade.Since the stout pedalphalanx 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, aprimitive feature that would not support such a subfamily (assuming Dromaeosaurus even has this character).

In addition, the opisthopubicpelvis would indicate Adasaurus is more advanced than Achillobator. Thefourth trochantor, slightly anteriorly projecting pubic foot and absence ofa proximodorsal ischial process suggests that this species is basal to theBambiraptor, Sinornithosaurus, Unenlagia group.

So Adasaurus is a rather basal dromaeosaurid, probably close to Achillobatoror Dromaeosaurus, but outside the subgroup containing Bambiraptor,Sinornithosaurus and Unenlagia.If Rahonavis ever ends up in theDromaeosauridae, it will be in this group as well, so it is also notparticularily close to Adasaurus.

The evidence for placing it in asubfamily with Dromaeosaurus is equivocal since the character evidence wouldinclude Achillobator in that subfamily as well, and Achillobator is moreplesiomorphic than Adasaurus in pelvic structure.

References

Barsbold, 1977. On the evolution of the carnivorous dinosaurs. Sovmestnaya Sovetsko-Mongol'skaya Paleontologicheskaya Ekspeditsiya Trudy. 1(4) 48-56.
Barsbold, 1983. Carnivorous dinosaurs from the Cretaceous of Mongolia.Transactions of the Joint Soviet-Mongolian Palaeontological Expedition, 19
117 pages.