Qilong - Achillobator - Theropod Puzzle

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Achillobator: Theropod Puzzle

Its about 20 feet long, had a tibia shorter than its femur and a large sickle claw; the counterbalancing tail was slender and long, the hip was huge and the pubis vertical, all the vertebrae were pnuematic, and the shoulder pointed down ... what was it? In the Mongolian locality of Burkhant, this strange animal has come to light with some peculiarities that trouble systematic placement.

Achillobator was described by Perle Altangerel, Jim Clark, and Mark Norell in a publication from the National Museum of Geology. The issue of its publication is that of a manuscript that may have been accidentally submitted to the museum's personal journal, in lieu of the intended source in American Museum Novitates. The name comes from the Greek hero Achilles and the Mongolian word for hero, baatar, also spelled as bator or bataar. The authors originally used the presence of a large pedal claw as evidence that this form was a dromaeosaurid, albeit a basal one, with similar teeth and a unique, mesopubic or vertically-oriented pubis pelvic anatomy; the shape of the ilium is tall and almost rectangular, and is superficially like that of Unenlagia, including a maniraptoran posterior blade (or ala) which curves ventrally, a vertical pubic peduncle, a large, thick ischiadic peduncle, and almost no supra-acetabular crest that capped the femur from above, a final feature seen in birds, alvarezsaurids, segnosaurs, and oviraptorosaurs; other taxa have larger crests.

Figure 1: Achillobator giganticus skeletal reconstruction. Skeleton with scale bar (as given), showing all elements preserved.

Achillobator is a unique theropod from the Bayn Shireh Formation of southern central Mongolia, a site known for Harpymimus, Segnosaurus, and Erlikosaurus, adding yet one more taxon to an already unusual assemblage. This theropod is know by less material than the other three save Erlikosaurus (skull, partial arm, partial foot) as well as the less-well known form Enigmosaurus (based on a partial pelvis) which may or may not be synonymous with Erlikosaurus (however, the scale of the two indicates that considering the hips of other segnosaurs, Enigmosaurus belongs to a larger animal than Erlikosaurus and it is unlikely they belong to the same holotype, despite some comments to suggest it).

Achillobator, like the other theropods of the Bayn Shireh horizon (classically, the Baynshirenskaya svita) is unusual it its combination of features, which only few elements appear to conform to the original referral to the Dromaeosauridae. These include:

1) a switch-blade pedal claw (though the claw referred to the foot is actually smaller than the digit it is referred to, and likely belongs to the hand instead) based on the second phalanx of the second toe, which is very short, about as long as high, and had a large proximal and ventral "heel" that would have caused the bone to articulate at a high angle with the first phalanx and "lift" the claw above the ground;
2) a large maxilla without a distinct maxillary fenestra (rare for theropods, but also found in Erlikosaurus) but instead a set of fossae and ridges;
3) teeth with a distinct mesial and distal denticle ratio as in dromaeosaurs and troodontids;
4) a large coracoid tubercle preserved halfway along the height of the element and with a large coracoid foramen, which is found in maniraptoran theropods;
5) a slight acromion expanse that is roughly trapezoidal rather than rectangular, triangular, or finger-like, and as such is relatively unique;
6) a large pubic boot with the posterior process deeper than the anterior, similar to many carnosaurian theropods;
7) a pubic bone which may be either vertical or slightly oriented posteriorly;
8) an ischium that is only shallowly turned backwards on its length and has an obturator process in the proximal half, which is a non-maniraptoran feature and is seen in Ornitholestes among Maniraptora-like theropods;
9) a femur with a medially turned rectangular head, as in many maniraptorans;
10) a large cnemial process of the tibia elevated above the femoral facet; caudal vertebrae with long prezygapophyses that have become curved, extend for more than the length of the previous vertebra, bear the facet for articulating the postzygapophysis on the inside near the base, and are forked at the very tips of the processes, features seen only in the Dromaeosauridae (sensu stricto) along with likely dromaeosaurids Sinornithosaurus and Microraptor;
11) the dorsal and cervical vertebrae each have two pleuroceols, one in the centrum and one on the neurocentral suture and both progress dorsally below the transverse processes;
12) the cervicals are fairly short, indicating that a neck if mae of the approximate 10-11 vertebrae (including atlas) would be relatively shorter than in many other theropods except the basal forms like Herrerasaurus or the relatively long-necked Eoraptor;
13) the cervicals and anterior dorsals, possibly up to the middle dorsal region, bear small to tongue-like hypapophyses, which in birds and oviraptorosaurs are large, flat, and involve four or so distinct successive elements (outnumbering dromaeosaurs which usually have none, or one or two small triangular knobs, as in basal tetanurans);
14) and possibly finally and most interestingly, a tibia shorter than the femur. Achillobator giganticus measured about 5.5 meters or so (or just around 18 feet), and as such it was about the size of Utahraptor from North America and estimates of Segnosaurus from the Bayn Shireh Formation.

Figure 2: Achillobator giganticus, right maxilla in lateral view (reversed); missing margins dashed in.

Chimaeric Achillobator ?

Could Achillobator be a chimaera? The distribution of features appears to show a generalization among both primitive and derived maniraptoriforms, including features not seen in Maniraptora, as well as those seen in derived maniraptoran and bird-like dromaeosaurs and the oddly primitive-seeming and yet bird-like oviraptorosaurs. It seems peculiar that such highly pneumatic vertebrae, unlike dromaeosaurids or even Archaeopteryx, and such an odd maxilla are preserved in combination with such obvious dromaeosaurid tail and foot features, yet the pelvis is very primitive, even among maniraptorans. Apomorphies for Dromaeosauridae or dromaeosaurids with primitive birds are the short, tall second toe's second phalanx, the long prezygapophyses of the tail, and tall, quadratic ilium; the pubic peduncle is deep and long, with a distinct V-shaped notch in the ventral margin. Apomorphies for a more basal position within Maniraptora are the long, roughly C-shaped ischium, proximal obturator process, mesopubic pelvis, and long cranial pubic boot. The extensive pneumaticity of the vertebrae is like birds, oviraptorosaurs, and segnosaurs, which corroborates the basal Maniraptora position as advocated by the sub-iliac pelvic bones. Other features (no maxillary fenestra, double dorsal pneumatic foramina, short cervical vertebrae) are possibly unique to the specimen depending on its position within or relative to Maniraptora.

Figure 3: Pelvises of eumaniraptorans and basal birds in lateral view, reversed to the left side when only the right elements are known, and dashed lines in broken margins.

Key to pelvises: A, Deinonychus antirrhopus; B, Adasaurus mongoliensis; C, Unenlagia comahuensis; D, Achillobator giganticus; E, Microraptor zhaoianus; F, Velociraptor mongoliensis; G, Sinornithosaurus millenii; H, Rahonavis ostromi; I, Bambiraptor feinbergorum; J, Archaeopteryx lithographica; K, Sinovenator changii.

References:

Perle A.; Norell, M.A; and Clark, J.M. 1999. A new maniraptoran theropod -- Achillobator giganticus (Dromaeosauridae) -- from the Upper Cretaceous of Burkhant, Mongolia. Mongolian Museum of Geology Publication: 1-105.

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