Modified June 1, 20-15 with new data on Mononykus (Perle et al. 1994). Thanks to M. Mortimer for the reference.
Mononykus and Shuvuuia
(Fig. 1) are two odd bird/dinosaurs from the Late Cretaceous of Mongolia. Their forelimbs are reduced to a single digit (#1) with digits 2 and 3 vestiges in Shuvuuia GI 100/975 and other specimens (Chiappe, Norell and Clark 1998) or absent in Mononykus IGM N107/6 (Perle et al. 1993), the larger and more derived of the two.
The question is what are those odd forelimbs used for?
They can’t be traditional vestiges because the olecranon process (elbow) is hyper-developed. The forelimbs look to be very strong. The radius and ulna are essentially fused (but not quite) proximally. The digit 1 ungual is a grappling hook.
In modern birds,
extending the elbow unfolds the tucked wing. In Mononykus and kin the hand (wing) can never be tucked or even rotated. Everything appears to be locked in place except the elbow and shoulder.
suggested the odd forelimbs of Mononykus were used to rip open termite mounds. Unfortunately for this hypothesis these dinosaurs would have to belly up to each mound they ripped open, making them vulnerable to a counterattack by termites under their feathers. Current anteaters are lumbering creatures with long snouts that keep them well away from termite defenders. Mononykids were built for bipedal speed. Anteating is not a good match no matter how it is considered.
Whatever those forelimbs were used for,
they were not used full time.
Anything those birds touched with their tiny forelimbs
they would have to belly up to. So let’s consider the safest substrate available, a tree trunk. Neither of these mononykids has a perching foot for tree branches. If these birds spent half their lives resting/sleeping, then why not do it within the relative safety of elevation above the ground, clinging to a tree trunk (Fig. 1)? The sternum on these creatures was sturdy, larger than in Archaeopteryx, ideally built for strong adduction (clinging). If Mononykus was too-large for tree clinging, then the forelimbs could have been used as props for maintaining balance while resting horizontally. After all, nest building and egg-laying were requirements.
Sisters had big claws and some were clingers
Mononykids descend from basal alvarezsaurids, like Haplocheirus (Early Late Jurassic, Choinere et al. 2010), a theropod dinosaur nesting between ornithomimosaurs and more bird-like dinosaurs like Archaeopteryx, oviraptosaurs and therizinosaurs. So it is within their phylogenetic bracket, and well within their abilities for mononykids to cling to trees and other suitable substrates.
The Sharovipteryx analogy
Another unrelated, but speedy biped with tiny forelimbs is Sharovipteryx (Fig. 2, Late Triassic), a fenestrasaur also capable of clinging to tree trunks, especially in preparation for a glide. Longisquama had a similar morphology.
The odd forelimbs of mononykids
evolved from the prey-catching forelimbs of basal alvarezsauroids, like Hapolcheirus, to enable mononykids to rest vertically on tree trunks in the present hypothesis. I haven’t read all the literature. Has this idea been put forth earlier? Any other ideas out there?
Chiappe LM, Norell MA and Clark JM 1998. The skull of a relative of the stem-group bird Mononykus. Nature, 392(6673): 275-278.
Chiappe LM, Norrell MA and Clark JM 2002. The Cretaceous, Short-Armed Alvarezsauridae: Mononykus and its Kin pp. 87-120 in Chiappe LM and Witmer LM eds, Mesozoic birds: Above the Heads of Dinosaurs. University of California Press. 536 pp.
Choiniere JN, Xu X, Clark JM, Forster CA, Guo Y and Han F 2010. A basal alvarezsauroid theropod from the Early Late Jurassic of Xinjiang, China. Science 327 (5965): 571–574.
Perle A, Norell MA, Chiappe LM and Clark JM 1993. Flightless bird from the Cretaceous of Mongolia. Nature 362:623-626.
Perle A, Chiappe LM, Rinchen B, Clark JM and Norell 1994. Skeletal Morphology of Mononykus olecranus (Theropoda: Avialae) from the Late Cretaceous of Mongolia. American Museum Novitates 3105:1-29.
Senter P 2005. Function in the stunted forelimbs of Mononykus olecranus (Theropoda), a dinosaurian anteater. Paleobiology 31(3):373–381.
Suzuki S, Chiappe L, Dyke G, Watabe M, Barsbold R and Tsogtbaatar K 2002. A new specimen of Shuvuuia deserti Chiappe et al., 1998, from the Mongolian Late Cretaceous with a discussion of the relationships of alvarezsaurids to other theropod dinosaurs. Contributions in Science (Los Angeles), 494: 1-18.