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Flapper, or Glider? A Reply to Speakman & Thomson on Archaeopteryx
The Volant Urvogel: A Reply to Speakman and Thompson.
As in all matters pertaining to the urvogel, in which contention is legion, there remains a vivid debate as to just what the aerial faculties of Archaeopteryx were. Classically, Archaeopteryx has been regarded as a glider at best, incapable of powered flight (e.g., Heptonstall 1971). However, in the past 20 years new data from reexamination of the urvogel material, and the spectacular discovery of new fossils attributable to Archaeopterygidae (Wellnhofer 1993) have called into question classical assumptions about the aerial capabilities of this marvelous animal. Increasingly, the argument that Archaeopteryx represents a strictly or largely gliding stage in avian evolution is at odds with reality, and one must question the validity of these assertions, which for years seem largely to have been accepted as a matter of convention, substantiation or lack thereof notwithstanding.
The landmark International Archaeopteryx Conference in Eichstatt, held in 1984, reached the consensus that the urvogel was indeed capable of active flight, though it was likely not a skilled aerialist nor was it capable of long-distance migratory flight (Hecht et al 1985). This so called “Eichstatt Scenario” is solidly supported by the data at hand, and curiously, has been further corroborated by the discovery of the seventh urvogel skeleton, attributed to Archaeopteryx bavarica by Wellnhofer (1993).
These developments notwithstanding, there remains steady dissent to the conclusion that the urvogel was capable of flapping flight (Tarsitano 1985, Rayner 1991, Speakman 1993, Speakman & Thompson 1994, Bock & Buhler 1995). In particular, the studies of Speakman (1993) and Speakman & Thompson (1994) have reached the conclusion that the urvogel was incapable of any more demanding aerial activity than passing gliding, and these researchers have gone straight to the heart of the issue: the remigial asymmetry values observed in Archaeopteryx.
Feduccia & Tordoff (1979) and Feduccia (1996) have argued vociferously that the asymmetrical remiges observed in Archaeopteryx are prima facie verification that the urvogel was in fact capable of active, flapping flight not unlike modern birds. However, Speakman & Thomson’s arguments are found wanting in substantiation, as is the assertion that the key to the volant abilities of Archaeopteryx lies in the asymmetry of the remiges.
There is a strong temptation to equate the form of the remiges in Archaeopteryx with definitive evidence establishing whether or not this animal was capable of active flight. After all, there is a biophysical correlate between an asymmetric remex and the ability of an airfoil to support active flight, a correlate underscored by the symmetry observed in the remiges of neoflightless birds (Feduccia & Tordoff 1979, Feduccia 1996).
Contrary to the assertion of Speakman & Thompson (1994), the remiges of Archaeopteryx are clearly asymmetrical, as is observed in the isolated fossil remex and HMN 1880 (most vividly seen on the counterslab). In this regard, the emphasis on the asymmetry values as a diagnostic tool for aerial capacity, which Feduccia & Tordoff (1979) and Speakman & Thompson (1994) have placed on these data, is warranted. However, the strictly quantitative analysis of the asymmetry values offered by Speakman & Thomson (1994) is misleading in that it imputes to the figures these researchers arrived at, greater credibility than they merit. Quite simply, while the remiges are preserved well enough (particularly in HMN 1880) to verify their asymmetrical vane, they are not preserved well enough to permit precise numerical calculation of the asymmetry values of the remiges (Elzanowski in Chiappe & Witmer 2002). Similar emphasis on quantitative analysis of the remigial asymmetry values (e.g., Norberg 1995) is equally baseless.
It thus becomes clear that (contra Feduccia & Tordoff 1979, Feduccia 1996) we must seek more definitive evidence for the volant faculties of Archaeopterygidae amongst other data. Considering the difficulties in quantitative analysis of the remiges outlined above, Speakman & Thompson’s insistence on using the asymmetry values of the flight feathers is inexplicable.
The Beginnings of a Volant Pectoral Architecture
While testimony to the effect that in flight performance Archaeopteryx was essentially a modern avian (e.g., Feduccia 1996, 1999) are clearly at odds with the data available, the fact remains that in the osteology of the urvogel we witness the earliest stage in the drastic modification of the pectoral girdle and thoracic limb to accommodate flight functions.
The traits of the pectoral girdle which most strongly suggest that Archaeopteryx was capable of active flight include: a) a broad, hypertrophied furcula, b) a robust preglenoid process of the coracoid (=biceps tubercle?), and the acute scapulocoracoidal angle (Feduccia & Olson 1979, Feduccia 1996, 1999, Elzanowski in Chiappe & Witmer 2002, Paul 2002).
The morphology of the archaeopterygid furcula is particularly illuminating. As demonstrated by Feduccia & Olson (1979) and further commented on by Feduccia in the 1996 edition of his tome, The Origin and Evolution of Birds, the furcula and adjacent coracoclavicular membrane serve as principal sites of origin for the pectoralis major, the wing-depressor musculature in birds which powers the downstroke. Concomitantly then, expansion or hypertrophy of the furcula is a correlate of increased mass of the pectorales, indicating robust musculature affording a powerful downstroke. These data are difficult to account for if the urvogel was strictly limited to gliding, as there is no need for a robust M. Pectoralis major in a gliding animal.
The robust craniodorsal process of the coracoid, identified by Elzanowski (in Chiappe & Witmer 2002) as the “preglenoid process,” of uncertain homology, is on the contrary, almost assuredly homologous to the biceps tubercle (Ostrom 1974, Chatterjee 1997, Paul 2002). That this structure should be vastly enlarged in the urvogel further testifies to the ability of the pectoral architecture in Archaeopterygidae to support active flight. While the biceps tubercle and the dorsal migration seen in Archaeopteryx most likely had their principal effect on the deflection of the tendinal insertion of the supracoracoideus towards the as yet un-enclosed triosseal foramen, this “preglenoid process” may also have been a site of origin for the coracobrachialis. This muscle may have been the principal wing-depressor powering the downstroke (Elzanowski in Chiappe & Witmer 2002), although from the morphology of the furcula, it seems more probable that the coracobrachialis augmented the pectoralis major.
Lastly, the scapulocoracoidal angle of the pectoral girdle observed in Archaeopteryx falls within the range of 60-65 degrees, comparable to that observed in flighted birds (Feduccia 1996, Elzanowski in Chiappe & Witmer 2002).
Worth examining is Chatterjee’s (1997) curious assertion that as Archaeopteryx lacked a supracoracoideus pulley system, and thus could not effect supinatory rotation by which a wing flip could be executed (Ostrom et al 1999), permitting take off from the ground, it could therefore not elevate its wings dorsally. Chatterjee thus concluded that the flight path in Archaeopteryx would have been undulating, with the urvogel needing to gain altitude between each downstroke, in order to affect another downstroke, as it lacked the physical ability to power the recovery stroke. However, the supracoracoideus is not essential for elevating the wing for the recovery stroke, and the deltoideus major, or other dorsal elevators can serve in its stead (Olson & Feduccia 1979, Feduccia 1996, Elzanowski in Chiappe & Witmer 2002). There is thus no data to support Chatterjee’s conclusion that Archaeopteryx could not power a recovery stroke.
Aspect Ratio and Wing Loading
Aspect ratio refers to the relation of wing length to wing width and reflects the modification of the airfoil to accommodate a variety of flight profiles, from gliding, to rapid maneuvering. Wing loading is the relation between the total area of the airfoil and body mass, in other words, the number of grams any square centimeter of the airfoil must support. As is the case for aspect ratio, wing loading data indicate the aerodynamic qualities of the airfoil in question.
Where does Archaeopteryx fall out in a data plot of aspect ratio/wing loading figures of extinct and extant birds? Curiously, the urvogel plots roughly along the median line in both aspect ratio and wing loading (Norberg 1985, Pennycuick 1989, Feduccia 1996, Elzanowski in Chiappe & Witmer 2002). The wing is elliptical, with moderate wing loading and low aspect ratio, and most closely resembles the wing of galliform, columbiform, and passerine birds suggesting a similar flight profile, albeit at a far cruder level (Van Tyne & Berger 1976, Feduccia 1996). Curiously, the urvogel lacks the exaggerated high aspect ratio wings of such consummate gliding forms as Diomedea spp.
The data provided by the wing loading and aspect ratio figures for the archaeopterygid wing are not compatible with the view that Archaeopteryx was strictly or primarily a gliding form.
The totality of the data from osteological, aerodynamic, and morphological research suggests that contrary to the impression of some, the urvogel was indeed capable of active, flapping flight, albeit on a crude level. In a similar vein, the ambiguity presented in several recent accounts, (e.g. Paul 2002), as to the relative skill with which Archaeopteryx flew is unfounded—the volant faculties of the urvogel are readily apparent.
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