the life style of archaeopteryx (aves)

AsociaciónPaleontológicaArgentina.PublicaciónEspecial7VIIInternationalSymposiumon MesozoicTerrestrialEcosystems:91-99. BuenosAires,30-6-2001

ISSN0328-347X

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 itshabitat).A combinationof heights-down take-off and ground foragingnecessitateda swift terrestrial es-cape to a launchingsite and probablyclimbingelevatedobjects.Archaeopteryx doesnot show any distinc-tive cursorial specializationand the leg intramembralratiossuggest a slow-pace to multimode (i.e., usingvarious gaits) forager,similar in behavior to today's tinamousand most galliforms.Archaeopteryx was anescape runner,not a cursorialpredator.The limbsofArchaeopteryx and (non-avian) theropods reveal sub-stantial functionaldifferences,which make their similaritieseven more likelyto be synapomorphic.

Keywords. Archaeopteryx. Birds. Paleobiological reconstruction. Flight. Locomotion. Foraging. Jurassic.

Introduction

Starting with Heilmann's (1926) portrayal ofArchaeopteryx von Meyer 1861 as an arboreal animal,a11major paleobiological reconstructions of the urvo-gel have been biased by their authors' views on theorigins of birds and avian flight. Current views onthe life style of Archaeopteryx are heavily influencedby Ostrom's (1974) known attempt to interpret themost primitive bird as a cursorial forager by arguingfor the conservation of function of the limbs acrossthe bird/theropod transition, which is implausible,because the bird/ theropod transition entails theemergence of flight as a major locomotor type, whichhad far-reaching consequences for the action of hindlimbs (Jones et al., 2000) as well as forelimbs.Accordingly, the limb skeleton of Archaeopteryx dif-fers from that of the Dromaeosauridae (widely be-lieved to be birds' closest known relatives) in thestructure of the shoulder girdle including the ster-num, conformation of humeral ends, relative lengthof the forelimb which functioned as a wing, propor-tions and details of the pelvis, morphology of the fe-mur, and intramembral proportions of the legoSuchdifferences obviously translate into different relativepositions and attachment areas of appendicular mus-cles in Archaeopteryx and theropods and thus cannotbe biomechanically neutral. Not unexpectedly, themovements and positions of the theropod leg seg-ments were markedIy different from those of modern

'Institute of Zoology, University of Wroclaw, VI. Sienkiewicza 21,50335 Wroclaw, Poland. E-mail: [email protected].

©AsociaciónPaleontológicaArgentina

ground-dwellíng birds (Gatesy et al., 1999) and thelimb allometry of theropods is closer to the patternobserved in mammals than in birds (Christensen,1999).

While the scenario of using the wings as insectnets (or fly swatters) met with deserved skepticism,the vision of Archaeopteryx's legs being both raptori-al and cursorial has been broadly accepted with thehelp of Paul's (1988) suggestive representations ofArchaeopteryx as a miniature Deinonychus Ostrom,1969 with the raptorial second pedal digit. At firstglance, the second digit may look similar to that ofDeinonychus in the Eichstatt specimen, because it isrotated upside down due to a preservational artefact,but other specimens belie the tale of the raptorial footof Archaeopteryx (Elzanowski and Pasko, 1999),which is perpetuated in semipopular literature(Shipman, 1998).

Also important is the size difference betweenArchaeopteryx (0.17-0.47kg, depending on specimen)and typical (nonavian) theropods, e.g., Deinonychus(60-75 kg). Sma11vertebrates tend to be more versa-tile in locomotor habits than are large vertebratesand many rodents and lizards are at ease both on theground and in trees. Lu11 (1929) ca11ed them ter-restrio-arboreal forms and noted that their climbingadaptations are not well-rnarked. If Deinonychus wasthe size of Archaeopteryx, it would be difficult to ruleout the use of its claws for climbing (Naish, 2000)and baby maniraptorans may have climbed rocksand banks (if not trees), as do young crocodiles.

Conservation of function is a murky subject,poorly addressed in the literature and complicated

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92 A. Elzanowski

Figure 1. A-B, skeletal reconstruetions of Archaeopteryx; A, the complete skeleton with proportions of the Berlin specimen and pubisoriented 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 mminB.

by the notoriously imprecise usage of the term func-tion in biology. There is a laudable tendency to re-strict the term function to the action of a structure orthe way a structure operates (Lauder, 1999),which isinseparable from the structure itself within "theform-function complex" (Bock and von Wahlert,1965);and to distinguish thus defined function sensustricto (which is the meaning adopted henceforth)from the biological role (Bock and von Wahlert,1965),which is properly assigned to the form-func-tion complex. Conservation of function is equivalentto conservation of the form-function complex andthus implies maintaining the details of adaptation,which seems to be a commonplace between closelyrelated populations and species but becomes less andless common with the increasing divergence of taxa.A case of actual conservation of function is equiva-lent to the lack of adaptive evolution and thus fallsunder the heading of evolutionary stasis if it persistsover a long stretch of geologic time.

Curiously enough, the conservation of limb func-

A.P.A. PublicaciónEspecial7,2001

tion between Archaeopteryx and theropods has beenadopted in the context of cladistic reconstruction ofbird origins (Gauthier and Padian, 1985;Padian andChiappe, 1998), although it has little to do withcladistics and, in fact, does not help the purpose ofsealing the theropod origins of birds because func-tional differences between avian and theropod limbsmake their similarities more likely to be synapomor-phic than do identical or similar functions, whichcould raise the possibility of homoplasy. While somefunctional similarities of the limbs of Archaeopteryxand theropods are demonstrated by shared biome-chanical properties (such as the sweeping movementof the hand driven by the semilunate carpal), no oth-er similarities should be claimed just because of aknown cladistic relationship. This relationship is bydefinition relative and it, does not say anythingabout the absolute evolutionary distance; thus it is ir-relevant for functional predictions. Synapomorphichomology alone may or may not imply a functionalsimilarity. The limb function is determined by the

Archaeopteryx life style

limb structure, physics of the substrate, size andshape of the entire animal, and the neural control ofbehavior, which may be responsible for substantialfunctional differences despite nearly identical mor-phology (Lauder, 1995). Because of the evident dif-ferences in limb anatomy and proportions as well asin the adult body mass and shape, no limb functioncan be assumed to be conserved between thetheropods and Archaeopteryx.

Another meaning to conservation of function is apersistent relationship between a form-function com-plex and a biological roleoIn the course of phylogeny,the form-function complexes and thus structures areconstantly reassigned or co-opted to new biologicalroles (see, e.g., Raff, 1996)and lose the old ones whilestill maintaíning some similarity. Probably the mostimportant condition for the conservation of a singlestructure/role relationship is the conservation of oth-er structure/role relationships, because the set of bi-ological roles to fill remains constant within each ma-jor type of organisms (such as the tetrapods). The as-signment of biological roles to locomotor modes andstructures is likely to remain constant, e.g., withínuniform genera or families of birds, where speciesdiffer in the ways their homologous adaptationswork (i.e., in the homologous form-function com-plexes) rather than in the kind of tasks they perform.In contrast, the emergence of a major organizationaltype usually entails a redistribution of tasks betweenform-function complexes and the theropod/birdtransition must have been accompanied by the reas-signment of biological roles to locomotor modes be-cause of the emergence of a novel locomotor mode(flight), which evolved to fill one of the existing bio-logical roles (probably defense).

Flight

The wings and tail of Archaeopteryx are distinctiveflight adaptations, which leave no doubt that it couldfly. However, considerable differences of opinionpersist as to whether and to what extent Archaeop-teryx was capable of active (= flapping) flight(Vazquez, 1992; Bock and Bühler, 1995; Feduccia,1999).A scale reconstruction of the skeleton (figurel.A) reveals that the pectoral girdle is remarkablysmall and the rib cage below the glenoid very shal-low, which limits the mass and fiber length of thewing depressors and thus the arch over which theycan operate (Jenkins, 1993).In addition, Archaeopteryxlacks the fused carpometacarpus, carpal trochlea, U-shaped ulnare (cuneiform), and polyhedral radialcarpal (scapholunar), which are essential for pow-ered flight inasmuch as they help withstand the tor-sional and shearing stresses of the power stroke (Sy,1936; Swartz et al., 1992), particularly to counteract

93

the passive pronation of the manus during the pow-er stroke (Vázquez, 1992),and coordínate the flexionand extension of the forearm and manus (Vázquez,1994).

The prevailing opinion among experts in flightmechanics is that Archaeopteryx was capable of activeflight in its simplest form (Norberg, 1985; Rayner,1991).The wings, as well as the tail (Gatesy and Dial,1996), lacked the maneuverability used by modernbirds for landing, takeoffs, and flight between obsta-des (such as tree branches). Archaeopteryx was poor-ly if at all adapted for flight at low speeds, which isbiomechanically complex. Flight at low speeds ne-cessitates controlled changes of the pitch of the entirewing relative to the body and of the plane of themanus relative to the wing. It involves upstrokesupination and downstroke pronation of thehumerus and flicking of the manus from the plane ofthe wing toward the body in the upstroke to avoidexcessive drag (Rayner, 1991).The minimum powerspeed (at which the least work has to be done) wasaround 8mis for the Berlin specimen (Yalden, 1971b;Rayner, 1985). Some modern birds may have thepower output less dependent on flight speed (Dial etal., 1997),but this may have been achieved throughthe advanced flight adaptations of modern birds andthus may not apply to basal birds.

Takeoff

Since the maximum running speed of the BerlínArchaeopteryx is 2 mi s and the stalling speed about 6mis (Yalden, 1971a; Rayner, 1985), it could take offeither from the heights or run into the wind (Rayner,1991). Archaeopteryx may have or may have not beenable to use the wind inasmuch as maneuvering in thewind after takeoff may have surpassed its steeringcapabilities. But even if some urvogel mastered thewind start, it is extremely improbable that this wastheir only way to start because the escape tactics of aspecies that relies on an appropriate wind from anappropriate direction would be terribly ineffective.

None of the recent attempts to show thatArchaeopteryx was capable of a ground-up take-offwere successful. Ruben (1991)assumed that the flightmusde of Archaeopteryx made up at least 7% of itsbody mass (which is testable with a rigorous skeletalreconstruction) and was reptilian in its metabolismand fiber composition (which is untestable) and thusArchaeopteryx was capable of anaerobic "burst level"power output of today's lizards and snakes (assumedto be 450 WIkg), which enabled it to a "ground up-ward takeoff from a standstill". Ruben's power out-put assumptions were couniered by Speakman(1993)and the proposal remains in the crowded lim-bo of speculations (see also Padian and Chiappe,

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94 A.Elzanowski

Foraging1998¡ Shipman, 1998). Marden (1994) accepted theflight muscle mass to be 7%of the body mass and cal-culated that a 200 g Archaeopteryx with the wings ofthe Berlin specimen would be just capable of aground-up take-off assuming the avian anaerobicburst power output (225W/kg), but realized that thelift/power ratio assigned to Archaeopteryx by his re-gressions is higher than in any living bird and thusunrealistic. In fact, the lift/power ratio ofArchaeopteryx must have been substantially lowerthan in modem birds because of the visibly pooranatomical adaptation of the wing for active flíght,which cannot be compensated by slightly more gen-erous allowances for the flight muscle mass (contraShipman, 1998). In addition, the Berlin specimenweighed about 270 g (Elzanowski, in press) and thelift recalculated on the same assumptions would notsuffice even for 250 g (Shipman, 1998). Shipman(1998)tried to save the ground-up takeoff by assum-ing a much higher estimate of the wing area, whichis, however, based on a highly inaccurate reconstruc-tion of the Archaeopteryx wing, with the forearm andthe set of secondaries at least 1.5 times longer thanpermitted by the wing skeleton (Elzanowski, inpress).

Avowedly inspired by a phylogenetic bias towardthe cursoríal origins of avian flight, Burgers andChiappe (1999)proposed that avian powered flightoriginated as a by-product of running and rowingwith the wings to genera te additional thrust on theground. Aside from the optimistic assumption thatArchaeopteryx was well adapted for powered flight,there are at least two fatal flaws in Burgers andChiappe's model. The generation of additional thrustin running Archaeopteryx assumes an extensive rota-tion of the humerus about its long axis from a heavi-ly hyperpronated to a horizontal flight position. Infact, an extensive rotational movement is impossiblein the shoulder joint of Archaeopteryx, because boththe glenoid and the humeral head are elongated, thatis, extended approximately cranio-caudally (figureLB).

Burgers and Chiappe's central claim is that flap-ping the wings to speed up a run generated a "resid-ual" lift that, at a critical speed, enabled Archaeopteryxto take off. Even if Archaeopteryx were adapted wellenough to try it, the increasing lift would result in aloss of traction, which continues to be one of the ma-jor difficulties of all cursorial models of the origin offlight (Feduccia, 1999).A simultaneous use of feet andwings for the takeoff in waterbirds, which Burgersand Chiappe cite as an example, does not resolve theloss-of-traction problem, because a starting waterbirdholds its feet perpendicular to the water surface anduses them as paddles. In conclusion, the cursorialtakeoff of Archaeopteryx remains as unlikely as ever.

A.P.A.PublicaciónEspecial7,2001

The teeth of Archaeopteryx suggest broadly de-fined insectivory, that is, feeding on small animals,primarily arthropods. Archaeopteryx may have for-aged in the interior of the German and Bohemianland masses (Wellnhofer, 1995) between mostlybushy to arborescent conifers (mostly araucarias),bennettites, ginkgos and seed fems; on the shores ofpools and streams (Thulbom and Harnley, 1985) in-habited by numerous water insects; as well as onbeaches (Viohl, 1985) if launching sites for rapidemergency takeoffs were available nearby. Indeed,since beaches are among the richest feeding groundsand birds are the only small-to-medium-size verte-brates that extensively forage on them (Dyck, 1985),there is a good possibility that avian evolution start-ed in the shore habitat, as suggested by the remark-ably early appearance of highly specialized waterbirds that must have descended from shore foragers

T

100

M F10 2Q 30 40 50 60 70 80 90

••0 +A7

,~/- -- ~~~,,:::.:.' ~ es e·'ES•eg

+e•+eH•

Figure 2. Triple ratios of femur (F) to tibiotarsus (T) to tar-sometatarsus (M) in the basal birds (+), cursorial foragers (dottedline), terrestrial multimode foragers (continuous line), arborealcuckoos (broken line). Each species is represented by a uniquecombination of the percentage length values for F, T and M (Table1) on the three corresponding axes of a temary diagram. The scat-ter plot is magnified below the temary diagram. Abbreviations:+A1 Archaeopteryx lithographica, +A5 Eichstatt Archaeopteryx,+A7 Archaeopteryx bauarica, Bv Burhinus uermicul atus, +CConfuciusornis sanctus, +CH Changchengornis hengdaoziensis,Cg Clamator glandarius, Cs Centropus sinensis, Cv Colinus vir-ginianus, Da Dromas ardeola, Es Eudunamus scolopacea, GcGeococcyx califomianus, Gg Guira guira, Ho Haem aiopus os-tralegus, Lm Lagopus mutus, Nm Nothura maculosa. See Table 1for taxonomic assignments.


(Elzanowski, 1983),and the recent paleoecological re-construction of a basal bird, Confuciusornis Hou et al.1995 (Peters and Ji, 1999) suggests a riparian oraquatic habitat. Wellnhofer (1995)argued against thebeach habitat of Archaeopteryx because of its rarity asa fossil, especially in comparison to the pterosaurs,but the probability of a flying animal's falling into thewater is heavily if not primarily dependent upon itsuse of flight, which was a foraging mode for thepterosaurs but probably only an emergency mode forArchaeopteryx.

There are at least two reasons why foraging intrees is extremely unlikely for Archaeopteryx even ifthey were present in its range. First, Archaeopteryxwas not adapted for the pursuit of small, mobile ani-mals (primarily arthropods) among the branches, be-cause this requires high maneuverability that doesnot seem possible for a bipedal animal without themaneuverability of wings or, as in some mammals, aprehensile tail (Cartmill, 1985). In fact, only passer-ines and a few other avian taxa evolved enough ma-neuverability to forage effectively in trees. Secondly,it does not look like the Solnhofen araucarias har-bored much food easily available to unspecialized ar-boreal insectivorous birds, which today feed, as dosome cuckoos, on caterpillars and similar larvae ofsymphytan hymenopterans. The flighted imagoes ofsuch insects would have been preserved in the richSolnhofen fauna (Barthel et al., 1990) if they wereabundant.

Garner et al. (1999) proposed the PouncingProavis Model according to which birds evolvedfrom predators that perched and pounced on groundprey and obviously implied this mode of life forArchaeopteryx. However, the Pouncing Proavis Modelhas two major problems. First, a pouncing attack re-quires a considerable precision of landing and is usedtoday by advanced skillful fliers, such as the owls(Strigiformes), rollers (Coraciiformes), and shrikes(Laniidae), which cannot run and most of them arepoor walkers. Second, it is hard to see what wouldthe protobirds, which were clumsy fliers, unable totake off back to the perch, gain by poorly controlledpouncing, climbing back, and waiting again insteadof using their strong legs of terrestrial birds to pursuethe prey on the ground, and thus "why a cursorial di-nosaur became a sit-and-wait ambush predator"(Hedenstrom, 2000). A feeding adaptation cannotevolve unless it brings energetic gains and this seemsextremely unlikely for the initial stages of avian flightno matter what sequence of behavioral adaptationsled to its origins. Avian flight is a costly adaptationand its origins involved a high energetic cost ofbuilding up in ontogeny and maintaining throughlife a second, in addition to the strong legs, locomo-tor apparatus for climbing and flying. In contrast to

95bats and pterosaurs, flight does not replace but ratherfunctions in addition to terrestrial locomotion (orswimming) in all primitive birds. In the majority ofmodem birds and the main flight muscles (pectoralisand supracoracoideus) alone make up 15%-25% ofthe body mass. Such a costly adaptation could haveevolved only in response to predation since only sur-vival that leads to reproduction (and, of course, re-production itself) is maximized at any cost, whileother functions are optimized in terms of lowestcost/benefit ratios.

Terrestriallocomotion

Many reconstructions of the terrestrial locomo-tion in fossil vertebrates are muddled by the notori-ous imprecision of the term "cursorial" (Stein andCasinos, 1997;Carrano, 1999).With reference to ter-restrial birds, it is proposed here to distinguish be-tween escape runners, such as today's tinamous andmany galliforms, which run only in emergency to es-cape a predator, and cursorial foragers, such as somecharadriiforms, which run almost constantly in pur-suit of food. A cursorial forager has to be so welladapted for running as to make it profitable in termsof energy gains whereas an escape runner does not.

Archaeopteryx has a strong but unspecialized hindlimb (figure 1.A) with a full-length fibula and anasymmetrical anisodactyl foot with digit IV muchlonger than digit Il. However, the pes of theSolnhofen specimen (to be classified in a separategenus) is more symmetrical and thus suggests some-what more cursorial habits. The opposable halluxand laterally compressed pedal claws indicate perch-ing ability (Yalden, 1985, 1997) and the preservationof right foot of the Munich specimen in a graspingposition, with the claws of hallux and fourth toe su-perimposed, support this conclusion (Wellnhofer,1995).But Archaeopteryx was certainly at ease on theground, as attested by its strong legs, with the tibialonger than the femur, and a slightly elevated, shorthallux that did not interfere with terrestrial locomo-tion. However, not a single feature of Archaeopteryxrepresents the cursorial end of the spectrum of am-bulatory adaptations (Carrano, 1999) and modemground birds seem to have the hind limb, pelvis andvertebral column (especially synsacrum) betteradapted for cursorial locomotion than Archaeopteryx(Bock,1986).

Traditionally, the intramembralleg proportions ofArchaeopteryx have been compared to those of birdsthat are similar to Archaeopteryx in overall shape(body, limbs and tail), such as cuckoos and touracos(Engels, 1938; Brodkorb, 1971), which are arborealand arboreo-terrestrial foragers (table 1). However,the leg proportions of Archaeopteryx show the best


96 A. Elzanowski

Table 1. Leg intramernbral triple ratíos of fémur (Fe) to tíbíotarsus (Ti)to tarsometatarsus (Me) in selected Mesozoic (?) and modem birdsrepresenting arboreal (A), multimode terrestrial (T), and cursorial (TC) foragers (ordered by the decreasing femur length). CUCuculiformes, GA Galliformes (Phasianidae s. l.), SH shorebirds (Charadriiformes), TI Tmamiformes.

Species Fe + TI + Me = 100%Foragingmode'

Body massin grams'

'iConfuciusomis sanctus'iChangchengomis hengdaoziensisClamator glandarius (CU)'iArchaeopteryx liihographicaLagopus mutus (GA)Eudynamys scolopacea (CU)Colinus virginianus(GA)Eudromia elegans (TI)'iEichstatt Archaeoptenp:Odontophorus guttatus (GA)Noihura maculosa (TI)'iArchaeopteryx bavaricaCentropus sinensis(CU)Guira guira (CU)Geococcyx californianus (CU)Haemaiopus ostralegus (SH)Burhinus vermiculatus (SH)Dromas ardeola (SH)

? 37.0 + 42.6 + 20.4336.8 + 40.6 + 22.6333.0 + 40.3 + 26.732.6 + 43.9 + 23.532.6 + 44.4 + 23.032.0 + 42.1 + 25.931.4 + 43.1 + 25.531.2 + 42.9 + 25.930.8 + 44.1 + 25.130.2 + 42.6 + 27.229.5 + 43.4 + 27.129.3 + 45.1 + 25.628.1 + 42.4 + 29.528.0 + 43.0 + 29.026.8 + 42.0 + 31.224.6 + 45.6 + 29.820.5 + 42.8 + 36.718.3 + 41.4 + 40.3

?AtbdTATT

tbdTT

tbdTTTCTCTCTC

138-169451-468est.

422238178660

161-178est.294-314

300208-225est.

236-268136-168

376526320325

'The categorization of Cuculidae based on Payne (1997).'Body mass estimates of Archaeopteryx are derived from the femur diameters using the regression calculated by Campbell and Marcus(1992) and those for extant taxa are from Dunning (1993). 3Median values calculated from Chiappe et al. (1999).

match with phasianid galliforms and tinamous (table1; figure 2), which are typical escape runners andslow-pace to multimode foragers. The phasianid gal-liforms have been suggested before as ecomorpho-logical analogs of the urvogel (Wellnhofer,1995). Thecomparison of leg intramembral ratios amongst birdsof comparable size demonstrates that Archaeopteryxwas anything but a cursorial forager.

Gatesy and Middleton (1997) showed a similarityof leg proportions between Archaeopteryx andConfuciusornis, Sinornis, and Cathayornis, which wereonce thought to be arboreal. However, these resultsare based in part on erroneous measurements takenfrom the literature. In fact, the length ratio of the fe-mur to tibiotarsus to tarsometatarsus based on cor-rect measurements of Confuciusomis (Chiappe et al.,1999) is unlike that in Archaeopteryx (table 1; figure 2)and suggest a very different function of the legoThelife style of Confuciusomis may have been aerialand/or aquatic but does seem to have been arboreal(Peters and Ji, 1999). The correct length measure-ments for the femur, tibiotarsus, and tarsometatarsusin Sinornis are, respectively, 19.0, 25.5, and 14.0 mm(pers. obs.), and their ratio of 32.5+43.6+23.9 = 100%is even closer than reported to that in the much larg-er London specimen ofArchaeopteryx (table 1)but dif-ferent from the ratios in sparrow-sized arborealpasserines (Palmgren, 1937). The length figures usedby Gatesy and Middleton (1997) for the femur andtibiotarsus of Cathayornis yandica specimen #9769 arecorrect but that for the tarsometatarsus is at best un-


certain, because the bone cannot be measured withany precision (pers. obs.). The functional meaning isof the similarity in leg proportions between the crow-sized London Archaeopteryx on one hand and thesparrow-sized Sinornis and only slightly largerCathayornis on the other is heavily obscured by theirconsiderable size differences inasmuch as the simi-larity of limb proportions in animals from differentsize categories does not directly imply the similarityof leg function (Alexander, 1997).

The comparisons of leg proportions (table 1; fig-ure 2) agree with what is implied by the combinationof ground foraging, inability to take off on the spot,and presence of Compsognathus Wagner, 1861 and(unless Archaeopteryx was endemic to the Solnhofenarea) other theropods, which were threatening atleast as competitors and probably also as predatorsof the young: Archaeopteryx most probably was an es-cape runner (figures 3 and 4), whether it ran to takeoff to a nearest launching site ("perch"), or less likelybut possibly, to leap up and glide (Norberg, 1990) orto take off against the wind (Rayner, 1991).Archaeopteryx ran only as far as needed to take off,most probably to launching sites, such as rocks, ar-borescent bushes, or conifer stem succulents (up to 3meters tall) or simply use a break in topography, asproposed by Peters (1985). This scenario does not re-quire a habitat replete with launching sites such ascliffs (contra Gauthier and Padian, 1985): in emer-gency,Archaeopteryx was certainly able to use even asingle launching "perch" as today's lizards and

Archaeopteryx life style 97

GROUND FORAGING

NONMANEUVERABLEFLlGHT

ESCAPE RUNNING,CLlMBING, AND FLYING

HEIGHTS-DOWN TAKEOFF

Figure 3. A reconstruction of the main constraints to the locomotion and foraging of Archaeopteryx as a bird. Each arrow means "con-strained Archaeopteryx to".

snakes (with probably lesser cognitive abilities) areable to remember and use their single preferredrefuges (Greene, 1994; Stone et al., 2000; MarkPaulissen, pers. comm.).

Running, climbing, and flying were used primar-ily or exclusively for escape (figure 4), which makesplausible the use of the manual claws for climbing asadvocated by Yalden (1985,1997):if used only occa-

LAUNCHING 'PERCf~"

opposed arboreal and cursorial interpretations ofArchaeopteryx. Ostrom (1974) and his followers areright in part insofar as Archaeopteryx was a groundforager and thus predominantly terrestrial, but so areBock (1986), Bock and Bühler (1995), and Yalden(1985, 1997) insofar as Archaeopteryx climbed andused its claws to do it. However, the present paleobi-ological reconstruction of Archaeopteryx clearly

SAFE HEIGHTS

Figure 4. The escape behavior of Archaeopteryx as reconstructed from the locomotor adaptations and functional constraints (figure 3). Theterm "perch" stand s for any structural feature of the habitat that enabled Archaeopteryx to use of gravity for takeoff. The launching"perch" is not necessarily predator-safe and used only for rapid takeoff.

sionally in emergency, the likely damage to the wingfeathers was tolerable because, in terms of Darwinianfitness, if a feature is necessary for survival, it pays tomaintain it even if it moderately interferes with oth-er functions.

Conclusion

The present reconstrucrion of Archaeopteryx as aleisurely forager and an escape runner, climber andflier (figures 3 and 4) integrates the two, heretofore

speaks against the cursorial origin of avian flight.

Acknowledgments

1 thank M.A. Paulissen (McNeese Sta te University, LakeCharles, Lousiana) and f. Paceko and R. Macelak (both Universityof Wroclaw) for, respectively, help in the preparation of figures andinformation on the reptilian defensive behavior; K. Padian(University of California, Berkeley), P. Wellnhofer (BayerischeStaatssammlung für Palaontologie und historische Geologie,Munich), and 1. Chiappe (Los Angeles County Museum) for re-views; and Poland's State Committee for Scientific Research (KBN)for support by grant # 6 P04C 059 17.


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Accepted: April d", 2001.


the life style of archaeopteryx (aves)

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