The Life Style of Archaeopteryx (Aves)

AsociaciónPaleontológicaArgentina.PublicaciónEspecial7 ISSN0328-347X VIIInternationalSymposiumon MesozoicTerrestrialEcosystems:91-99. BuenosAires,30-6-2001 The life style of Archaeopteryx (Aves) Andrzej ELZANOWSKP Abstract. The lack of modem flight adaptations and flightmaneuverabilitymakesboth ground-up take- offand arborealforagingofArchaeopteryx extremelyimprobable(evenif treeswere present in a part of its habitat).A combinationof heights-down take-off and ground foragingnecessitateda swift terrestriales- cape to a launchingsite and probablyclimbingelevatedobjects.Archaeopteryx does not show any distinc- tive cursorialspecializationand the leg intramembralratiossuggest a slow-pace to multimode (i.e., using various gaits) forager,similar in behavior to today's tinamousand most galliforms.Archaeopteryx was an escaperunner,not a cursorialpredator.The limbsofArchaeopteryx and (non-avian) theropods revealsub- stantial functionaldifferences,which make their similaritieseven more likelyto be synapomorphic. Keywords. Archaeopteryx. Birds. Paleobiological reconstruction. Flight. Locomotion. Foraging. Jurassic. Introduction ground-dwellíng birds (Gatesy et al., 1999) and the limb allometry of theropods is closer to the pattern Starting with Heilmann's (1926) portrayal of observed in mammals than in birds (Christensen, Archaeopteryx von Meyer 1861 as an arboreal animal, 1999). a11major paleobiological reconstructions of the urvo- While the scenario of using the wings as insect gel have been biased by their authors' views on the nets (or fly swatters) met with deserved skepticism, origins of birds and avian flight. Current views on the vision of Archaeopteryx's legs being both raptori- the life style of Archaeopteryx are heavily influenced al and cursorial has been broadly accepted with the by Ostrom's (1974) known attempt to interpret the help of Paul's (1988) suggestive representations of most primitive bird as a cursorial forager by arguing Archaeopteryx as a miniature Deinonychus Ostrom, for the conservation of function of the limbs across 1969 with the raptorial second pedal digit. At first the bird/theropod transition, which is implausible, glance, the second digit may look similar to that of because the bird/ theropod transition entails the Deinonychus in the Eichstatt specimen, because it is emergence of flight as a major locomotor type, which rotated upside down due to a preservational artefact, had far-reaching consequences for the action of hind but other specimens belie the tale of the raptorial foot limbs (Jones et al., 2000) as well as forelimbs. of Archaeopteryx (Elzanowski and Pasko, 1999), Accordingly, the limb skeleton of Archaeopteryx dif- which is perpetuated in semipopular literature fers from that of the Dromaeosauridae (widely be- (Shipman, 1998). lieved to be birds' closest known relatives) in the Also important is the size difference between structure of the shoulder girdle including the ster- Archaeopteryx (0.17-0.47kg, depending on specimen) num, conformation of humeral ends, relative length and typical (nonavian) theropods, e.g., Deinonychus of the forelimb which functioned as a wing, propor- (60-75 kg). Sma11vertebrates tend to be more versa- tions and details of the pelvis, morphology of the fe- tile in locomotor habits than are large vertebrates mur, and intramembral proportions of the legoSuch and many rodents and lizards are at ease both on the differences obviously translate into different relative ground and in trees. Lu11 (1929) ca11ed them ter- positions and attachment areas of appendicular mus- restrio-arboreal forms and noted that their climbing cles in Archaeopteryx and theropods and thus cannot adaptations are not well-rnarked. If Deinonychus was be biomechanically neutral. Not unexpectedly, the the size of Archaeopteryx, it would be difficult to rule movements and positions of the theropod leg seg- out the use of its claws for climbing (Naish, 2000) ments were markedIy different from those of modern and baby maniraptorans may have climbed rocks and banks (if not trees), as do young crocodiles. Conservation of function is a murky subject, 'Institute of Zoology, University of Wroclaw, VI. Sienkiewicza 21, 50335 Wroclaw, Poland. E-mail: [email protected]. poorly addressed in the literature and complicated ©AsociaciónPaleontológicaArgentina 0328-347X/01$OO.OO+50 92 A. Elzanowski Figure 1. A-B, skeletal reconstruetions of Archaeopteryx; A, the complete skeleton with proportions of the Berlin specimen and pubis oriented as in the Munich speeimen. The wing is shown in a hypothetical, maximally folded position involving a distal shift of the ra- dius relative to the ulna (Elzanowski and Paceko, 1999). B, the thoracie girdle with details based on the London specimen. Abbreviations: gl. glenoid; st, bony sternum (whieh may have been eaudally extended by a eartilaginous part). Seale bars equal10 mm in A and 20 mm inB. by the notoriously imprecise usage of the term function between Archaeopteryx and theropods has been tion in biology. There is a laudable tendency to re- adopted in the context of cladistic reconstruction of strict the term function to the action of a structure or bird origins (Gauthier and Padian, 1985;Padian and the way a structure operates (Lauder, 1999),which is Chiappe, 1998), although it has little to do with inseparable from the structure itself within "the cladistics and, in fact, does not help the purpose of form-function complex" (Bock and von Wahlert, sealing the theropod origins of birds because func- 1965);and to distinguish thus defined function sensu tional differences between avian and theropod limbs stricto (which is the meaning adopted henceforth) make their similarities more likely to be synapomor- from the biological role (Bock and von Wahlert, phic than do identical or similar functions, which 1965),which is properly assigned to the form-func- could raise the possibility of homoplasy. While some tion complex. Conservation of function is equivalent functional similarities of the limbs of Archaeopteryx to conservation of the form-function complex and and theropods are demonstrated by shared biome- thus implies maintaining the details of adaptation, chanical properties (such as the sweeping movement which seems to be a commonplace between closely of the hand driven by the semilunate carpal), no oth- related populations and species but becomes less and er similarities should be claimed just because of a less common with the increasing divergence of taxa. known cladistic relationship. This relationship is by A case of actual conservation of function is equiva- definition relative and it, does not say anything lent to the lack of adaptive evolution and thus falls about the absolute evolutionary distance; thus it is ir- under the heading of evolutionary stasis if it persists relevant for functional predictions. Synapomorphic over a long stretch of geologic time. homology alone may or may not imply a functional Curiously enough, the conservation of limb func- similarity. The limb function is determined by the A.P.A. PublicaciónEspecial7,2001 Archaeopteryx life style 93 limb structure, physics of the substrate, size and the passive pronation of the manus during the pow- shape of the entire animal, and the neural control of er stroke (Vázquez, 1992),and coordínate the flexion behavior, which may be responsible for substantial and extension of the forearm and manus (Vázquez, functional differences despite nearly identical mor- 1994). phology (Lauder, 1995). Because of the evident dif- The prevailing opinion among experts in flight ferences in limb anatomy and proportions as well as mechanics is that Archaeopteryx was capable of active in the adult body mass and shape, no limb function flight in its simplest form (Norberg, 1985; Rayner, can be assumed to be conserved between the 1991).The wings, as well as the tail (Gatesy and Dial, theropods and Archaeopteryx. 1996), lacked the maneuverability used by modern Another meaning to conservation of function is a birds for landing, takeoffs, and flight between obsta- persistent relationship between a form-function com- des (such as tree branches). Archaeopteryx was poor- plex and a biological roleoIn the course of phylogeny, ly if at all adapted for flight at low speeds, which is the form-function complexes and thus structures are biomechanically complex. Flight at low speeds ne- constantly reassigned or co-opted to new biological cessitates controlled changes of the pitch of the entire roles (see, e.g., Raff, 1996)and lose the old ones while wing relative to the body and of the plane of the still maintaíning some similarity. Probably the most manus relative to the wing. It involves upstroke important condition for the conservation of a single supination and downstroke pronation of the structure/role relationship is the conservation of oth- humerus and flicking of the manus from the plane of er structure/role relationships, because the set of bi- the wing toward the body in the upstroke to avoid ological roles to fill remains constant within each ma- excessive drag (Rayner, 1991).The minimum power jor type of organisms (such as the tetrapods). The as- speed (at which the least work has to be done) was signment of biological roles to locomotor modes and around 8 mis for the Berlin specimen (Yalden, 1971b; structures is likely to remain constant, e.g., withín Rayner, 1985). Some modern birds may have the uniform genera or families of birds, where species power output less dependent on flight speed (Dial et differ in the

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