Size and Locomotion in Teratorns (Aves: Teratornithidae)
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SIZE AND LOCOMOTION IN TERATORNS (AVES: TERATORNITHIDAE) KENNETHE. CAMPBELL,JR. 1 AND EDUARDOP. TONNI2 'NaturalHistory Museum of LosAngeles County, 900 ExpositionBoulevard, Los Angeles, California90007 USA, and Divisi6n PaleontologfaVertebrados, Facultad de CienciasNaturales y Museo,1900---La Plata, Argentina ABSTRACT.--Theextinct family Teratornithidaecontains the world'slargest known flying birds. A new methodof determiningbody weightsof extinctbirds, basedon the size of their tibiotarsi,facilitates the estimationof the wing dimensionsof thesegiant birds. An analysisof the bonesof the teratornwing showsthat they closelyresemble those of condors, suggestingthat teratornsflew in a manner similar to theselarge New World vultures. The bonesof the pelvicgirdle and hindlimbsindicate that teratornswere probablyagile on the ground,though better adapted for walkingand stalkingthan running. We estimatethat the largestteratorn, Argentavismagnificens, weighed 80 kg and had a wingspanof 6• m. It probablybecame airborne by spreadingits hugewings into the strong,continuous, westerly winds that blew across southern South America before the elevation of the Andes Mountains and, once aloft, flew in the manner of condors.Received 26 April 1982, accepted18 October 1982. THE extinct avian family TeratornithidaeL. viduals of T. rnerriarnifrom the asphalt depos- Miller 1909 is presently known from four its of Rancho La Brea, California, where re- species:Teratornis rnerriarni L. Miller 1909, T. mains of raptorsand vultures abound (Howard incredibilisHoward 1952, Cathartornisgracilis L. 1962b). It was assumed that the teratorns were Miller 1910, and Argentavismagnificens Camp- entrapped in oil seepsat RanchoLa Brea while bell and Tonni 1980. The known teratorns feeding on carcassesof other trapped animals. rangedfrom very large to truly giganticin size, Consequently,all reconstructionsof teratoms with A. magnificensbeing the largestknown have pictured them as slightly larger versions flying bird. The single known specimenof A. c•fcondors, usually sitting in a treewaiting for magnificens,an associatedpartial skeletonfrom a trapped animal to die or feeding in groups Huayquerian(late Miocene) deposits of Argen- on large carcassesas vultures are wont to do. tina, is nearly 21/2times as large, in linear di- As discussedherein, teratornsprobably did mensions, as living condors.Argentavis rnag- fly in the mannerof condors,but an analysis nificensis also the oldest known teratorn, and, of the functional morphology of the teratorn with the exceptionof an indeterminate speci- skull (Campbell and Tonni 1981) indicates that men from late Pleistocenedeposits of south- thesebirds were not scavengers.The structure westernEcuador (Campbell 1976, Campbell and of their maxillary rostrum and mandibles and Tonni 1980),it is the only teratornknown from their ability to spreadtheir mandiblesproxi- outside North America. mally, as evidencedby the planeof rotationof Interpretationsof teratornshave alwayspic- their quadrates, indicate that teratorns were tured them as scavengers,flying and feeding predaceouscamivores that consumedtheir prey in a manner like that of living vultures (Fisher whole. They were functionally incapable of 1945; Howard 1950, 1962a; Stock 1956). When feeding by tearing piecesof flesh from car- the first speciesof teratorn, Teratornisrnerriarni, cassesas vultures do. Appropriatelysized prey was described(Miller 1909), it was placed with for Teratornis rnerriarni would have included the New World vultures in the family Vulturi- frogs, lizards, nestlingor fledglingbirds, and dae (= Cathartidaeauct.), probablybecause of rodents.With a skull over 55 cm long and 15 its very elongated, highly vaulted, raptorial- cm wide, however, Argentavismagnificens was appearing, hooked beak and the dose resem- capableof swallowingeven hare-sizedanimals blance of its postcranialskeleton to that of con- whole. dors. These interpretationswere reinforced by Although the postcranialskeleton of tera- the recoveryof the remains of over 100 indi- torns alwaysimpressed previous workers with 39O The Auk 100: 390-403. April 1983 April 1983] TeratornSize and Locomotion 391 its similarity to that of condors,major differ- tween weight and wing area by the equation enceswere alsonoted (Miller 1909, 1910;Fisher 1945).A detailed comparativeanalysis (in pro- W = •z'S •', (1) gressby KEC) of the bones of Teratornismer- where W is weight in g and S is surfacearea riami indicates that, while teratorns are related in cm2. Data were fitted by least squaresto the to New World vultures, they may be just as logarithmic form of this and the following closelyrelated to storks(Ciconiidae). The post- equations.The data for the Accipitriformes(= cranial skeletonof teratornsdisplays a mosaic Falconiformesauct.) yielded the following es- of vulturid, ciconiid, and unique characters. timation equation: The most strikingdifferences among the three families are found in their skulls, the result of W = 0.05338'S •'27• adaptations for very different feeding meth- ods. New inquiries and synthesesof available Similarly,he suggesteda relation,ship between data (Ligon1967, Olson 1978, Rea in press)have wing area (S) and wingspan(b, in ½m), questioned the traditionally perceived rela- tionships of storks and New World vultures, b = c•2'S•2. (2) and an ever-increasingbody of evidence is The values fitted for his passeriform model being accumulatedthat supports the hypoth- were: esis of their common origin. The recognition that teratornsare related to both of thesegroups D = 2.221'$ ø'•. strengthensthis hypothesis. The proposedrelationship between wingspan SIZE (b) and aspectratio (AR) was: Estimation of body mass and dimensions AR = c•'b •3. (3) of extinct animals is often difficult and frus- For the passeriformmodel he obtained the fol- trating. The presentwork is no exception.Dif- lowing estimation equation: ferent methods produce different results, and the lack of critical data for modern birds seri- AR = 4.49'b ø'•8. ously impairs present efforts. Data are partic- ularly sparsefor large birds, especiallythose Using this seriesof equations,each fitted from threatened with extinction, such as condors, empirical data, one can predict the wing-sur- the active collectionof scientificspecimens of face area,wingspan, and aspectratio of a fossil which ceased before the importance of such bird that conformsto the passeriformmodel if simple information as weight data was recog- an accuratefigure for its weight can be ob- nized. tained. Greenewalt (1975a) used previously pub- Fisher (1945) was the first to estimate the lished dimensional data from over 1,400 indi- weight of a teratorn,that of Teratornismerria- vidual birds to analyze the relationshipsbe- mi. He measured the areas of the sternum and tween (a) live body weight and wing area, (b) synsacrumin the Bald Eagle (Haliaeetusleuco- wing areaand wingspan,and (c) wingspanand cephalus)and the California Condor (Gymno- aspectratio. He divided the flying birds (ex- gyps californianus)and comparedthose areas cluding hummingbirds) into three groups: a with the known weights of the individuals passeriformmodel, a shorebird model, and a measuredin an attempt to determine an index duck model. Each group differs substantially to body weight for birds of different sizes but from the others in wing loading (weight sup- with similar body build. His estimatefor T. ported per unit wing area) at a given body merriarniwas "about 50 lbs [23 kg]." weight. The New World vultures fall into his For a bone's dimensions to correlate well with "passeriformmodel," and, becausewing load- body mass,the bone must function to support ing in teratornswas probablyvery similar to the body. In birds, these are the bones of the that in condors (see below), the dimensional wing or the leg. Althoughhigh correlationsbe- relationshipsobserved within New World vul- tween somedimensions of certain wing bones tures probably hold for teratornsas well. and body weight undoubtedlyexist within the Greenewalt expressed the relationship be- numerousmorphofunctional groups of birds, 392 CAMPBELLAND TONNI [Auk, Vol. 100 5.0- 4.5- 4.0- o, ß 0.5 1.0 1.5 2'.0 log Tibiotarsus Circumference {mml Mean St. Dev. Regression Line Res. Meal1 Sq. N=324 X .90334 .37234 X = .38272 •Y+.08727 .00388 COR =.986 Y 2.1845 .95922 Y= 2.5400 *X-.10996 .02574 Fig. 1. Computer-generatedplot demonstratingthe correlationbetween the live bodyweight (g) and the least circumference of the tibiotarsus (mm) in birds. their widely disparateflying abilities preclude correlation coefficient is 0.986 (data from 324 any single wing-bone measurement from individuals of 19 orders and 45 families, in- maintaininga high degreeof correlationto body duding such disparate taxonomic and mor- weight throughoutthe ClassAves. A high cor- phofunctionalgroups as penguins, ostriches, relation, first noted by John Anderson (pers. falcons,shorebirds, owls, hummingbirds, and comm., Anderson et al. 1979), does exist, how- passerines).By using the regressionequation ever, between the least shaft circumference of log Y = 2.54-1ogX - 0.10996, the leg bones of birds and their live body weights (Fig. 1). Betweenthe logarithm of the where Y is live body weight (g) and X is least least shaft circumference of a bird's tibiotarsus shaft circumference (mm) of the tibiotarsus and the logarithmof its live body weight, the (log = commonlogarithm), we can predict a April 1983] TeratornSize and Locomotion 393 TAnrE 1. Predictedmean dimensionsof Teratornismerriami, Argentavis