Looking at Skeletons

©Jaime A. Headden

The skeleton of Achillobator, as we have noted, is not complete. The fact that it was, and still is sometimes, thought to be a chimera is because the associated elements seem to come from all over the maniraptoran family. Were this a whole skeleton it would, because it possesses so many different elements, be a rather interesting living (well, dead) case study in skeletal transitions. Arguably the most recognizable element of the dromaeosaur family that is thought to be present, a second hypothesis places this bone on the hand, is the large "killing" claw of the "raptors". The size and length are inferred based on the heel of the phalanx that is said to represent the claw. Other elements of the skeleton also refer the animal to the maniraptoran family including the maxilla that was recovered.

The maxilla appears to be the end of the snout, thus encompassing the pre-maxilla as well, in some renditions though we can see clearly in the previous illustration that the maxilla only is interpreted as being represented by this piece of the puzzle. The ridges shown in what we would expect to be a widely opened fenestra are notably rare for all theropods, but it is seen in at least one other theropod. This rarity adds to the mystery and further confounds the findings with the teeth. The teeth are consistent with the sizes and ratios of length, width, curvature, etc. found in other dromaeosaurus and even troodontids. Visual comparison of the overall shape of the teeth supports that assertion, though we know just looking at something can be considered weaker evidence than the measurements of the teeth; thankfully we have both.

©Jaime A. Headden
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.

One of the other issues that has been raised with the remains attributed to Achillobator is the odd morphology of the pelvic girdle. Looking at the pelvis, D in Mr. Headden's illustration, we can compare many of the elements to the other pelvises of maniraptorans that are also illustrated up next to it. Of particular note are the pubic boot, the "lowest" part of the pelvis in layman's terms. The shape is clearly different from all of the others in that it is almost completely ventrally flexed and it projects both anteriorly and posteriorly at the same time. This is a carnosaur related trait more often than it is a maniraptoran trait, as we can see above. Compare it to this carnosaur's pelvis.

References

Headden, J. A. (2002). Achillobator: Theropod Puzzle. Retrieved April 13, 2013, from Qilong: http://qilong.8m.com/Achillobator.html