In the previous post we looked at the feathers and filament-like structures that covered the bodies of coelurosaurian theropods. While basal coelurosaurs – compsognathids and tyrannosauroids – possessed filament-like ‘Stage 1’ structures alone, members of Maniraptora (the coelurosaur clade that includes oviraptorosaurs, therizinosauroids, birds, deinonychosaurs and, probably, alvarezsaurids) possessed indisputable vaned feathers. That is, complex feathers that had a distinct central rachis with vanes on either side composed of parallel barbs. What is surprising is how luxuriant some of this covering seems to have been, and on how much information we now have about the arrangement of feathers on the bodies of these dinosaurs…
Though it has been suggested at times that vaned feathers simply must have evolved for flight, the phylogenetic distribution of these structures currently indicates that they first evolved in flightless maniraptorans and were only later exapted by long-armed maniraptorans for use in locomotion. Of course a well-known minority opinion, best known from the writings of Greg Paul, is that feathered maniraptorans are secondarily flightless and descend from volant bird-like ancestors. While this remains possible it lacks support from the fossil record, though that may or may not mean much. I want to avoid discussion of this area here and will cover it another time [image of sinornithosaur above borrowed from here].
Feathered fingers and hand flags
Exactly how feathers were arranged on the arms and hands of both basal birds and non-avian maniraptorans has long been unclear, and both non-avian maniraptorans and archaeopterygids have conventionally been depicted as possessing unfeathered fingers. However, this just doesn’t work given that the second finger is needed to support the remiges* that we now know were present throughout maniraptorans: this is what it does in living birds. Derek Yalden’s 1985 study was important in showing exactly how the remiges would have grown off of the first and second phalanges of the archaeopterygid second finger (Yalden 1985) [adjacent image shows Yalden’s reconstruction of Archaeopteryx, with a modern magpie’s wing above it]. A check of the literature on archaeopterygids shows that this configuration has been widely recognised (Bohlin 1947, Rietschel 1985, Griffiths 1993, Stephan 1994, Elzanowski 2002), though rarely brought to the attention of artists for some reason.
* Remiges are the large feathers of the forelimbs (singular remex). The large feathers that grow from the tail are termed rectrices (singular rectrix).
Incidentally, there has been some minor historical disagreement over exactly how many remiges were present in archaeopterygids (there were most likely 11 primaries and a tiny distal 12th one, and at least 12 secondaries), and also about how the hand claws were arranged: I agree with Elzanowski (2002) that the claws were directed perpendicularly to the palmar surface in life, and rotated anteriorly in most (but not all) specimens during burial. It has also been suggested on occasion that the fingers of archaeopterygids and other feathered maniraptorans were united in a single fleshy ‘mitten’ as they are in modern birds, and hence unable to be employed in grasping (Martin & Lim 2005). Given that the interphalangeal finger joints of archaeopterygids appear suited for flexion and extension, and that the third finger apparently remained free and flexible in birds more derived than archaeopterygids (Gishlick 2001), this is unlikely to be correct; it’s based on a depression in the sediment that Martin and Lim identified around the bones.
[Image above of excellent Caudipteryx skeleton from here].
Like those of archaeopterygids and modern birds, the remiges of non-avian theropods would also have been attached to the phalanges of the second manual digit as well as to the metacarpus and ulna, and indeed we can see this in the fossils. It’s the case in the sinornithosaur NGMC 91-A and Microraptor (remember that, while we now have more feathered theropods that we did just a few years ago, we still only have soft-tissue preservation in a tiny minority of taxa). Surprisingly, in Caudipteryx the remiges are restricted to the hands alone, and don’t extend from the arm [see image below]. They seem to have formed little ‘hand flags’ that are unlikely to have served any function other than display. Were ‘hand flags’ unique to Caudipteryx, or more widespread? Were all oviraptorosaurs like this?
If you’re wondering: yes, Caudipteryx (currently represented by two species, C. zoui Ji et al., 1998 and C. dongi Zhou & Wang, 2000) is an oviraptorosaur and possesses a suite of characters unique to this group. It is not a member of Aves, despite the efforts of some workers to make it into one. In contrast to later members of Oviraptorosauria, it possessed premaxillary teeth, had proportionally elongate hindlimbs and lacked a claw on its third manual digit. For a previous post on oviraptorosaurs see Luis Rey and the new oviraptorosaur panoply, and for a discussion of tooth function in Caudipteryx and other feathered maniraptorans see The war on parasites: an oviraptorosaur’s eye view.
Given that several maniraptoran lineages were clearly predatory and, given the morphology of their manual claws, fingers and wrists, presumably in the habit of grabbing at prey with their hands, wouldn’t the remiges have interfered with the use of the hands in predation? The short answer is no. The long answer – taken here from a paper Alan Gishlick published on forelimb function in Deinonychus – is ‘feathers on the hands would not have greatly impeded the use of the hands in predation. Because the feathers are attached at an angle roughly perpendicular to the claws, they are oriented tangentially to the prey’s body, regardless of prey size’ (Gishlick 2001, p. 315). It is important to note here that theropod hands appear to have been oriented such that the palms faced medially: that is, they faced inwards, and were not parallel to the ground as used to be imagined [adjacent image of Microraptor wing from here].
However, feathering would have interfered with the ability of the hands to bring a grasped object up toward the mouth given that extension of the maniraptoran wrist would have caused the hand to rotate slightly upwards on its palmar side. If both feathered hands are rotated upwards and inwards at the same time, the remiges from one hand would collide with those of the other. For this reason, maniraptorans with feathered hands could grasp objects, but would probably not be able to carry them with both hands. Senter (2006) has proposed that dromaeosaurids and other maniraptorans may have solved this problem by clutching objects single-handedly to the chest. Feathered hands would also have restricted the ability of the hands to pick objects off of the ground, given that the feathers extend well beyond the ends of the digits.
It remains possible that some maniraptorans lacked remiges on their fingers, but the only evidence we have in fact indicates the contrary. It’s recently been argued that the particularly long second digit of the oviraptorosaur Chirostenotes was used as a probing tool, locating and extracting invertebrates and small mammals and so on from crevices and burrows (I have a post planned on this subject – it’s to be called ‘The probing guild’ – and will talk more about this subject then). It seems highly unlikely that a digit that is regularly thrust into small cavities would have had feathers extending along its length, so either Chirostenotes didn’t probe as proposed, or its second finger was unfeathered, unlike that of Caudipteryx and the other feathered maniraptorans [adjacent Microraptor image from here].
Given the problems that the feathers might have posed for clutching and grabbing prey from the ground, we might also speculate that some of these dinosaurs deliberately removed their own remiges by biting them off. Some modern birds (notably motmots) manipulate their own feathers by biting off some of the barbs, so this is at least conceivable, albeit totally speculative of course.
I wanted to cover hindlimb and tail feathers too, but… another time. Come back soon!
Refs – –
Bohlin, B. 1947. The wing of Archaeornithes. Zoologiska Bidrag 25, 328-334.
Elzanowski, A. 2002. Archaeopterygidae (Upper Jurassic of Germany). In Chiappe, L. M. & Witmer, L. M. (eds) Mesozoic Birds: Above the Heads of Dinosaurs. University of California Press (Berkeley), pp. 129-159.
Gishlick, A. D. 2001. The function of the manus and forelimb of Deinonychus antirrhopus and its importance for the origin of avian flight. In Gauthier, J. & Gall, L. F. (eds) New Perspectives on the Origin and Early Evolution of Birds: Proceedings of the International Symposium in Honor of John H. Ostrom. Peabody Museum of Natural History, Yale University (New Haven), pp. 301-318.
Griffiths, P. J. 1993. The claws and digits of Archaeopteryx lithographica. Geobios 16, 101-106.
Martin, L. D. & Lim, J.-D. 2002. Soft body impression of the hand in Archaeopteryx. Current Science 89, 1089-1090.
Rietschel, S. 1985. Feathers and wings of Archaeopteryx, and the question of her flight ability. In Hecht, M. K., Ostrom, J. H., Viohl, G. & Wellnhofer, P. (eds) The Beginnings of Birds – Proceedings of the International Archaeopteryx Conference, Eichstatt 1984, pp. 251-265.
Senter, P. 2006. Comparison of forelimb function between Deinonychus and Bambiraptor (Theropoda: Dromaeosauridae). Journal of Vertebrate Paleontology 26, 897-906.
Stephan, B. 1994. The orientation of digital claws in birds. Journal fur Ornithologie 135, 1-16.
Yalden, D. W. 1985. Forelimb function in Archaeopteryx. In Hecht, M. K., Ostrom, J. H., Viohl, G. & Wellnhofer, P. (eds) The Beginnings of Birds – Proceedings of the International Archaeopteryx Conference, Eichstatt 1984, pp. 91-97.