Know When to Fold ‘em
2011
Of all the issues in the forelimbs of dromaeosaurs the range of motion at the elbow is probably the least controversial. Carpenter, Senter, Sullivan, Gishlick and Ostrom all reconstruct Deinonychus with a maximum flexion at the elbow near 55˚.
However there are differences among the experts regarding just how bird – like the forelimbs were. Carpenter (2002)1 wrote that birds have a gliding mechanism at the elbow that theropods do not have. Logic dictates that by “theropods” he must have meant “non – avian theropods”.
Carpenter apparently saw all non – avian theropods as homogenous in terms of their anatomical similarity to birds. His conclusions about the elbow of dromaeosaurs were drawn from just one taxon: Deinonychus. But Deinonychus is not the only dromaeosaur, and it is possible that there is a diversity of limb function within the family. For example, Senter (2006)2 studied the forelimb of Bambiraptor, and found that it differed from Deinonychus in the hands.
Microraptor, therefore, may have had an elbow anatomy that was more or less like birds than inDeinonychus.
Microraptor Wings 5 - Elbow Mobility
8/3/11
Comments Widget
Middleton and Gatesy (2000)3 noted that, in birds, forelimb folding is most effective when the radius and humerus are of about equal length. This allows the wrist to be adjacent to the shoulder when the wing is folded, thus keeping the folded wing from obstructing head motions. Dromaeosaurs have this essentially avian proportion (see Sinornithosurus, right).
Carpenter critiqued the former authors since they did not analyze joint surfaces in dromaeosaurs to see if such folding was possible.
It is still a salient point either way, however, since the forelimb proportions of ancestral paravians may have been exaptations before tighter forelimb folding developed in avialans.
In demonstrating the automatic folding mechanism of the wing in modern birds Vazquez (1994)4 showed that the radius actually dislocates from the humerus. This apparently occurs whenever flexion exceeds 60˚, and it causes the radius to shift distally, parallel to the ulna. Range of motion studies, such as those by Senter, explicitly assume that no dislocation will happen, and constrain the range of motion accordingly. Obviously, larger ranges of mobility are possible if dislocation is allowed.
Figure 1: NGMC 91-A from Qiang et al. 2001, Nature.
Figure 3D: radial dislocation from Vazquez 1994, Zoomorphology.
Is there any evidence for dislocation or an automatic folding mechanism in Microraptor?
I surveyed published or publicly displayed specimens of Microraptor and its closest relatives within the Dromaeosauridae to gather data regarding the range of motion that was possible in the elbows. I considered the angle between the longest axis of the humerus and the longest axis of the ulna, as preserved in articulated or nearly articulated fossils. Fossils, of course, can always show distorted body positions, and disarticulation would probably be indistinguishable from dislocation in life. I therefore stuck just to the range of folding that I found, and I considered a value for range of motion to be supported if it was found in two or more specimens.
The specimen marked “wikimedia” is presented on the wikipedia page about Microraptor, where it is listed as M. zhaoianus. Apparently the image is a snapshot from a museum display in China.
The specimen listed as NGMC 91 is assigned to Sinornithosaurus and pictured above. It was published in Qiang et al. (2001)5. Sinornithosaurus is found in most phylogenies to be the closest relative of Microraptor, closer than Deinonychus or Bambiraptor, and thus it is likely to better reflect the anatomy of Microraptor. These two specimens considered together, along with other unofficial specimens that can be found in internet searches, suggest that elbow flexion as great as 21˚ was possible in Microraptor.
The wikimedia specimen of Microraptor zhaoianus.
According to Vazquez, in living birds dislocation of the radius occurs whenever the humeroulnar angle goes below 60˚. Did it also occur in Microraptor?
Without examining these specimens in person I would not hazard an opinion. In the next installment I will consider the wrist, a highly studied and heavily debated anatomical feature, and here the presence or absence of an automatic folding mechanism will have important consequences.
The assumption that elbow folding in Microraptor had a maximum value of about 55˚, the same as in Deinonychus, is also found in later work on wing folding, such as Sullivan et al. (2009)6. Sullivan et al. explicitly began with the arm position of Padian (2001)7, Figure 3a, investigating the effects of wrist position on the position of the primary feathers. Yet Padian’s figure assumes much more flexion at both the elbow and wrist than the former authors. In their reconstruction of Microraptor, in Figure 3, they show elbow flexion near 75˚, while in Padian (2001) it is more like 20˚. I will go into this in more detail in the next installment, on the wrist of Microraptor.
Sullivan et al. (2009) state that Microraptor could not keep its primaries out of contact with the ground unless the arm was pronated from the shoulder or strongly retracted. In the figures below I have placed the humerus as Padian did, roughly parallel to the dorsal vertebral column, and flexed the elbow at about 30˚ and the wrist at about 90˚. These values are well within the range of positions preserved in fossils of Microraptor and Sinornithosaurus. The primaries do not drop as far as the distal metatarsal I and, therefore, do not strike the ground.
I conclude that the degree of elbow flexion assumed in such a demonstration is a critical variable.
Other results from my model agree with Sullivan et al. in two other ways that have little to do with the elbow: the hands cannot be brought to the mouth from the above position without the primaries impacting the ground, thus limiting their use in predatory motions. I also agree that pronation of the whole wing at the shoulder is a likely way for the animal to elevate the distal tips of the primaries further out of the way of the ground. See the results of mild pronation below- it can result in bringing the lowest primaries up to the knee, even keeping a 90˚ angle at the wrist.
Poore et al. (1997)8 found that living birds can rotate their humeri around the long axes in a similar manner by up to 80˚ yet, to my knowledge, no study on the range of motion in non – avian maniraptoran forelimbs has investigated this critical degree of freedom.
Model of Microraptor gui with two successive degrees of pronation at the shoulder.
1 Carpenter, K. 2002. "Forelimb biomechanics of nonavian theropod dinosaurs in predation". Senckenbergiana Lethaea 82: 59–76
2 Senter, P. 2006. Comparison of forelimb function between Deinonychus and Bambiraptor (Theropoda: Dromaeosauridae). Journal of Vertebrate Paleontology 26:897-906.
3 Middleton, K.M. and Gatesy, S.M. 2000. Theropod forelimb design and evolution. Zoological Journal of the Linnean Society 128:149-187.
4 Vazquez, R.J. (1994) The Automating Skeletal and Muscular Mechanisms of the Avian Wing (Aves). Zoomorphology 114: 59-71
5 Ji, Qiang; Norell, Mark A.; Gao, Ke-Qin; Ji, Shu-An; Ren, Dong (2001). "The distribution of integumentary structures in a feathered dinosaur". Nature 410 (6832): 1084–1087.
6 Sullivan, C. Hone, D.W.E, Xu, X. Zhang, F. 2010. The asymmetry of the carpal joint and the evolution of wing folding in maniraptoran theropod dinosaurs. Proceedings of the Royal Society B. 3 March 2010.
7 Padian, K. 2001 Stages in the origin of bird flight: beyond the cursorial-arboreal dichotomy. In New perspectives on the origin and early evolution of birds: Proc. Int. Symp. In Honor of John H. Ostrom (eds J. Gauthier & L. F. Gall), pp. 255–272. New Haven, CT: Peabody Museum of Natural History.
8 Poore, S.O.; A. Ashcroft; A. Sanchez-Haiman and G.E. Goslow, Jr. 1997. The contractile properties of the M. supracoracoideus in the pigeon and starling: A case for long-axis rotation of the humerus. J. exp. Biol., 200: 2987-3002.
Amazing news John.
Someone finally did it - you calibrated skeletal ROM reconstructions against ROM data in joints with their connective and other soft tissues. Indeed, Dr. Senter wrote that such work was needed, but unavailable.
I presume you did alligators and birds and probably mammals as well?
-Jason
