<oo/oggiral Journal oj-the Linnean Society (1984), 82: 177-188. With 2 figures

What size was Archaeopteryx?

L). W. YALDEN

Departmen1 q f ~ o o l o g y , Victoria Unirlersi!~ OJ .lPanchesLei, Manchestrr IM13 9PL

KPuiued April 1983, accepted,for publication J.4. 198.7

‘l’hc s i x (mass) of Archneopte!yx, primarill 1 1 i i . R ( ~ l i n sprcimcn. has h e m rstimatcd in two ways. i\ ncw three-dimensional reconstruction sugges~s a mass of no more than 271 g. Application of alhmetric e q u a h i s derivtd from both birds :itid nianirnals to mrious h r a r chtnmsions yields a rarige of rstimatrs from 112 to 2269 g, 1 J u L all t h c more plausiblc rstirnatcs liir in the ranxe 220-330 g.

KEY LVOKDS: Archneopterq.~ - allrimetry - ,Jur:issic r lvrs - L’omnpsogna~hus

~:o”l’l~;”l’s

Introduction . . . . . . . . . . . . . . . . . . 177 .I new r r c o n s t r u c h i . . . . . . . . . . . . . . . . . I79 Extra~xilations ti-om all~imctrv . . . . . . . . . . . . . . . 18 1 Kcsults . . . . . . . . . . . . . . . . . . . . 185 Discussion . . . . . . . . . . . . . . . . . . . . 185 Zlckii~ii~lrdgrtneii~s . . . . . . . . . . . . . . . . . 187 KCf(~rcl1ccs. . . . . . . . . . . . . . . . . . . 187

l ~ ’ l ’ R ~ ~ ) l ~ ~ ’ c ~ ’ l 10s Linear measuremcxits of the fossils of .-lrchaeoflteryi Eithogrczphica have been

a\.ailable, and have regularly been uscd to oRer approximations of the original sizc of the animal, since Owen’s original descriptions of the Loridon specimen (Owen, 1863). Most often, A r c h a e o ~ ~ / e ~ i ~ z is said to be “about the size of’a ra\;en’‘ (Swinton, 1958), “the size o f a crow” (Swinton, 196011, or the “siac o f a magpie Yicn pica” (de Beer, 1964). Rather morc precise statements of it:; rnass include those ofJerison (1968, 500 g ) , Heptonstall (1970, 500 g ) , Yalden (1971a, 200 gj, and Hopson (1977, 300 9 ) . Other measures of size, wing area and wingspan wer? not, apparently, estimated until about 14. years ago (Heptc;nstall, 1970 - wing area 373 cm2, excluding the body strip; Yalden, 197 1 a- w i n g area 479 c m 2 , including the body strip, wingspan 58.8 cm).

T h e variation in the estimates derives in part from the difkrent analogues which have been used-a raven, Coruus corax, weighs over 100 g, a carrion crow, C,’OTZJUS corone, about 500 g, and a magpie, Pica pica, about 200 g [Coomhs, 1978). Heptonstall (1970, 1971 and pers. comm.) says that he reached his estimate in part by comparing Heilman’s ( 1926) well known reconstruction of Archaeopteryx

:c, 1984 Tlir Liiincan Socirry or London 177 0024 -4082/84/090177 + 12 003.00/0

178 D. W. YALDEN

WHAT SIZE WAS ARCHAEOPTERYX? 179

with pigeons, Columbia livia, and in part by comparison of the lengths of various bones with various skeletons; they matched those of a crow, Corvus corone, fairly well, and he concluded that a mass of 500 g was appropriate. 'Yalden (1971aj arrived at his figure of 200 g by comparison both with birds of similar wing span to Archaeopteryx and by comparison with mammals of similar head-and-body length. The precise basis for other comparisons seems not to have been given.

It seems desirable to try to resolve these discrepancies. A factor of 2 or more difference in mass affects any discussion of the flying abilities of .4rchaeopteryx, or lack of them (Heptonstall, 1970, 1971; Yalden, 1971a, b, c). Relative brain size (Jerison, 1968; Hopson, 1977) also requires an accurate assessment of body size, and if McNab (1978) is correct in arguing that the evolution of endothermy requires a reduction in size, the size of Archaeopteryx may be relevant to that argument as well.

Clearly it is unsatisfactory to base estimates of body mass on some single analogue selected from the range available. Two possibilities present themselves. One is to attempt a better reconstruction than that offered by Heilmann (1926); in the light of recent evidence, especially on the shape of the pelvis (Ostrom, 1976a; Wellnhofer, 1974), the shape of the body needs modification. If the shape can be properly reconstructed, its volume and therefore its mass can be estimated. This was done very successfully for Pteranodon by Bramwell & Whitefield (1974), but our knowledge of the three-dimensional anatomy of Archaeopteryx does not allow such precision as they achieved. The :second method uses the various allometric equations that have recently been published (or specially calculated for this study) relating various dimensions to body mass in both birds and mammals (Greenewalt, 1975; Radinsky, 1978; Maloiy et al., 1979; Alexander et al., 1979).

In view of Professor Kermack's earlier appointment as Lecturer in Biological Statistics at University College, and of his early paper on allometry (Kermack & Haldane, 1950), this seems an appropriate application of his earlier study to his later field.

A NEW RECONSTRUCTION

Most of the available linear measurements for all five Archaeopteryx skeletons are presented by Wellnhofer (1974). The Maxberg and Haarlem specimens are too incomplete to be used in the present analysis, and are not consj.dered further. Of the three major specimens, the Berlin example presents far more information on the plumage than the others, and has been used as the basis for this reconstruction. Some aspects are only preserved in the London specimen; the

Figure 1 . Scale reconstruction of Archaeopteryx lifhographica, Berlin specimen, in pl:m view. T h e lengths of bones and feathers are as accurate as possible, but the shape of some bones (in particular the ribs and most of the vertebrae) in this view is uncertain. No attempt has tieen made to reconstruct the appearance of the gastralia. Because the humerus is very long in Arc-haeopteyx (by comparison with modern birds), there apprars to he a large gap between the tiody and the innermost secondary. I n modern birds, thc tertiary feathers, attaching to the humeru:,, fill this gap, and presumably did so in Archaeopteryx. The area of the body strip between the wings, including this gap, was calculated from the mean chord of the wings (8.85 em) and a width of 11.5 cm, to be 101.8 cm2, The area ofone wing, distal to the dotted line, has a n area of 188.6 cm72 The dotted line marks the boundary of the body strip; its chord seems rather short and the est imatd area, hence also the estimate of the total wing area, is somewhat conservativr. A chord of 11.5 cni, and a body strip of 132 cm2, might be more likely.

180 L). U'. YALDEN

axial skeleton of the Berlin specimen (length of pubis, of scapula, and of pre- caudal vertebrae) is approximately 92% of the size of the London one, and measurements derived from the latter have been scaled appropriately. The relative positions of the bones in lateral view are much influenced by the Eichstatt specimen, as depicted by Wellnhofer (1974), while Ostrom's reconstruction of the pectoral girdle (Ostrom, 1976b) has also been very useful.

A plan view of Archaeopler_yx is shown in Fig. 1 . The body axis as shown is maximal, being the length of all the vertebral sections and the skull length (as given by Wellnhofer, 1974) drawn in a straight line; in life, there was presumably some curvature of the vertebral column, as seen from the side, which would have foreshortened the body axis. The most controversial aspects of the reconstruction, which therefore deserve some explanation, are the width of the body and the shape of the wings.

The London specimen gives us four indications of the width of the body. ( 1 ) The furcula has a width of 30 mm across the arms, so the Berlin furcula

should have been 27.6 mm across. The width between the scapulae of the Berlin specimen is only 21 or 22 mm (Yalden, 197 1 a ) , so has suffered some 4 mm of lateral compression.

(2) The coracoid (as shown by Ostrom 1976b: fig. 4) has a projected width from glenoid to sternal border of 17.8 mm, which scales to the Berlin specimen as 16.4 mm; twice this width implies a thorax 33 mm across.

( 3 ) The pelvis is shown in anterior view (de Beer, 1954; text fig. 6). Appropriately scaled, this implies a width of 24 mm across the outside of the acetabula of the Berlin specimen, and a pelvic canal 17.7 mm wide.

(4) There is a rib shown in anterior view on pl. 9, fig. 3, of de Beer (1954). A plausible graphic reconstruction of a vertebra, with its transverse processes, and the ribs suggests a thorax width of 36 mm; this width applies if the ribs extended directly laterally, rather than being swept back somewhat posterolaterally, and may be regarded as a maximum value.

Wing shape was reconstructed by Yalden (1971a) with the bends of only 20" at the elbow and wrist. Since making that reconstruction, radiographs of a fresh dead magpie, Pica pica, and blackbird, Turdus merula, fixed with their wings fully extended, show that the bend at the elbow, particularly, was likely to be rather greater. In Fig. 1, the bends at elbow and wrist are 70" and 40" respectively. This reduces the wingspan from 588 mm (Yalden, 1971a) to 545 mm; the maximum possible wingspan, with all elements drawn in a straight line, would be 599 mm, but that is obviously a purely notional 'wingspan'. The wing area (including body strip) is unaffected by this change in planform, at 479 cm* (Yalden, 1971a). The tail, scaled down from the measurements given by de Beer (1954) for the Loridon specimen, has an area of 159 cm2.

The lateral view of Archaeopteyx (Fig. 2 ) looks much 'squarer', less streamlined, in the body than Heilmann's version, but the length of the body seems to have been rather exaggerated in his reconstruction, even with the pubis directed posteroventrally (Yalden, 197 1 c). With the pubis pointing approximately at right angles to the axis of the backbone, the body must have been about 61 mm deep at that point." The depth of the thorax derives from

*'l'he paper hy Walkcr (1980) arrived after the above sertion was completed, hut hefore the figures were finalized. A modified reconstruction, following his ideas, appcars as Fig. 2B. 'l'his alters the side area of the body, but, in view of the narrow taper that the postrriorly p l a rd pubis would give to the body, would probably not affcct its volumc.

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WHA'I' SIZF. \ V h S ARCHAEOPTERYX? I83

Table 1. Weight of body components in the magpie, Pica pica

Body, subtoral 36.68 g 5.04 2.67

It 2 26.43 2'.7!1 '15.26

11.89 29.73 16.22 2.26

Extremities. subtotal 60.84 g

Components (total) 174.41 10.02

t,'rc.;h dead wcight I ! l :Lf; 10.3

Each figurc is the mean value from eight individuals. T h e difference between the win of component wciglits and frcsh dead weight is due t o fluid 105s ~ii i r luding bloodj and gut contents (from 'l'atner, 1980).

appropriate one is difficult. The various values are presented in Table 3. Notes on their source and appropriatcncss arc given below. The measurements of honcs of Archaeopteryx which have txcn p u t into the equations are all taken from \.liellnhofer (1974: table 6.2) and given here as Table 2. The Ekhstiitt, Berlin and London specimens differ somewhat in size, and separate estimates can be given for each one when the estimates are based on bone size; only in the Berlin specimen is the plumage adequately known for that to be used for extrapolation.

Wing area. Greenewalt (1975) gives a number of allometric equations rclating to modern birds, but only that relating wing area to mass is useful in the present context. He gives thrce equations for this relationship, for three morphological models which he terms 'passeriform', 'shore-bird' and 'duck' models--these are not strictly taxonomic groupings. Of these, thc first is clearly the most appropriate for Archaeo/,~teg~x, and is used in Table 3.

Cursorial birds. Maloiy et al. (1979) discuss the allometric relationships of bones, muscles and tendons in a sample of eight cursorial birds ranging in size from a 75 g quail, Coturnix coturnix, to a 41.5 kg ostrich, Strztthio camelus. Ostrom ( 1976a) has argued both that Archaeofiteryx evolved from bipedal cursorial dinosaurs, and that it was a cursorial bipedal predator itself. Comparison with cursorial birds might therefore be most appropriate. Allometric Iequations, and appropriate measurements on Archaeopteryx, are available for lengths of femur, tibio-tarsus and tarsometatarsus, and for sagittal mid-shaft diameters of femur and tibiotarsus. Wellnhofer ( 1974) does not give the tarsometatarsus diameter of Archaeopteryx, and in view of the less complete fusion, comparic,on with birds would be perhaps inappropriate.

Birds in general. Maloiy et al. (1979) do not consider the forelirrtb (wing). M y reference collection includes skeletons of 26 individual birds whc'se weight was known, and which weighed between 100 and 1000 g. These belong to 16 species and eight orders. Lengths and breadths (sagittal plane, at midhaft) have been

I84 D. W. YALDEN

Table 2. Dimensions of Archaeopteryx specimens substituted in allometric equations (from Wellnhofer, 1974, and this paper).

Berlin London Eichstatt ~ ~

Humerus length (mm) 63.5 75 41.5 Ulna length (mm) 55 67.5 36.5 Femur length ( m m ) 52.6 60.5 37.5 Tibiotarsus length (mm) 68.5 80.5 52.5

Humerus diameter (nim) 3.7 4.0 2.7 Femur diameter ( m m ) 3.5 3.8 3.3

Head-and-hody length ( m m ) t 218 148 Wing arca (cm') 479 ~ ~

M'ing+lail area (cm') 638 -

Wing span (Yalden, 1971a) (mm) 588 ~

387.5 Wing span (this paper) ( m m )

'larsometatarsus length (nim) 37.0 44.0 30.2

Tibiotarsus diameter ( m m ) 3.0 3.5 2.5 Head-and-body length (mm) * 232 154 -

- 545

*Maximum length, summing all skeletal elements in a straight line. t Measured from cast, fbllowing curves.

measured for the humerus and ulna of these, and the allometric equations relating these to body mass calculated.

The hind limb skeleton of these specimens was also measured, and Professor R. McN. Alexander was kind enough to send me the original data used by Maloiy et al . (1979) and Alexander (1983). Thus it is possible to recalculate the allometric equations for the hind limb elements. There seems no value in recalculating all of them, and I confined myself to recalculating the allometric relationships of body mass to femur diameter and to tibiotarsus diameter. Since my interest is to estimate the mass ( m ) of Archaeopteryx from the known diameters ( d ) of its bones, the equations are presented in the form m = adb, rather than as d = a . mb which they used; to compare the exponents therefore, one must take the reciprocal 1 /b , but the constants a are also necessarily different. Cognisant of the criticism of regression coefficients made by Kermack & Haldane (1 950), the exponent b used here is the geometric mean of the exponents for m on d and d on m.

Mammal limb bones. An obvious criticism of the use of modern bird comparisons is that .4rchaeopte~x was much less specialized in its skeleton, and that it might therefore have been rather heavier than modern birds of similar linear dimensions. The obvious solution is to make comparisons also with other vertebrates. Alexander el al. i 1979) give allometric equations relating limb bone lengths and diameters to body mass for a wide range of mammals; 37 species from 17 families and seven orders, ranging from a 3 g pigmy shrew, Sorex minutus, to a 2.5 t African elephant, Lo.xodonta africana.

The results imply that mammals have shorter limb bones for a given mass than birds, but the bipedal spring-hare, Pedetes capensis, has relatively long hind limbs, and one could well argue that comparisons of Archaeopteryx with bipedal mammals would be more appropriate than with mammals generally. Professor Alexander was kind enough to supply measurements for three bipedal mammal

WHAT S I Z E K'AS A K C HA EO P' l E R Y X? 185

species, rangingfrom a kangaroo-rat, Dipussp. , at 102 g to a kangaroo, hrlacropussp., at 13 kg.

Mammal head-and-body length. The standard measurement of mammals for taxonomic and other purposes is head-and-body length; Yalderi (1971aj used a limited sample of such data to estimate the mass of Archneol?teryx. Radinsky (1978) has assembled data for 54 carnivore species and 59 ungulates, presenting allometric equations for these two sets of data separately, and for the 113 species combined. The two sets of species gave rather different equations, with smaller ungulates being lighter than carnivores of the same body length, and the equation for the combined data scems likely to be most appropriate. One reservation concerning the use of this set of data is that the species concerned range from 25 to 300 cm in head-and-body length, that is, the range was entirely above the size of Archaeupqy.x. Inclusion of smaller species [especially insectivores and rodents) would perhaps improve the predictive value of these mammal data, but in view of the results actually obtained, the problem seems less serious in practice than in theory.

K ES u urs

Application of all these equations to the Berlin ,4rchaeopteryx yields 26 estimates of its mass ranging from 112 g to 2269 g (Table 3 ) ; there are also 22 estimates for the Eichstatt Archaeophyx and 19 for the London :,pecimen. Wing area is not available for the Eichstatt or London specimens, and 1.t is not possible to measure the head-and-body length of the London specimen either.

The four highest estimates for each specimen come from extrapolating from the lengths of mammalian limb bones: mammals of a given weight evidently have much shorter legs than birds of the same weight, a conclusion reinforced by the extrapolations fi-om bipedal mammals which are roughly half those from mammals in general. I t is obvious that lengths of bones can vary considerably between animals of the same weight. Bone diameters can vary rather less, for the bone must be strong enough to support the weight in life, and the strength of a bone depends very much on its diameter. Extrapolations of mass from the diameters of the hind limb bones of the Berlin Archaeopteryx all fall between 185 and 333 g, whether from bird or mammal models. The extr,apolation from mammal body lengths falls, at 256 ,g> in the middle of this range.

DISCUSSION

There are no clear criteria for preferring one estimate over another in the large number of estimates in Table 3. It seems clear that extrapolations from the lengths of quadrupedal mammal bones give gross overestimates. Most estimates for the mass of the Berlin Archaeopteryx, whether from diameters or lengths of bones, fall in the range 200-300 g. I t is encouraging that the estimate obtained from the new reconstruction also falls in this range.

The range of estimates emphasizes how different in its proportions Archaeopteryx was from modern birds-that its proportions differ from those of modern mammals is less surprising. The peculiar proportions of Archaeopleryx may go some way to explaining the discrepancies between th,e estimates of previous authors. The humerus is very long in proportion to the other elements,

186 D. W. YALDEN

Table 3. Estimates of the body mass ( 9 ) of Archaeopter_yx

Extrapolation based on

Berlin rxample

London Eichstatl examplc rxample

Aliometric equation*

it‘ing area (Grccnewalt, 197.5) 146

Cursorial birds (Maloiy el d., 1979) Diameter fcmur 254 Diameter tibiotarsus 185 Lcngth frmur 269 Length tibiotarsus 298 Length tarsometatarsus 202

All birds (own da ta+R. McN. Alexander data) Diameter femur 252 Diameter tibiotarsus 230 Diameter humerus 223 Lrngth humerus 363 Length ulna 228

Mammal (Alcxandrr et a/.. 1979) Diameter fcmur 333 Diameter tibia 27 1 Diameter humerus 477 Length femur 606 Lrngth tibia 786 Length humerus 2269 Length ulna 717

306 222 28 I 113 443 80 436 161 288 134

306 220 338 145 27 1 101 497 163 352 96

418 283 416 27 1 586 208 894 237

1301 342 2918 564 1266 230

Mammal (Radinsky, 1978) (Two HB lengths in Table 2 give pairs of estimates) Body length, ungulate 140/112 34/29 Body length, carnivore 427/361 ~ 140/125 Body length, combined 309/256 ~ 90/80

Bipedal mammal (R. MtN. Alcxandcr data) Length femur 329 484 129 Length tibia 314 54 1 128

W’ = 0.05935.9’ 2 7

M = 0.0027B3 45

M = 0.088’ ’ ’ M = 0.024B3 ’

W = 3.326L2 ” N’ = 0.472L’.38

*In equations, L = length of bonc ( m m ) ; D = diameter of bone (mm); W = mass (kg); S = wing area (cm?); M = mass (g); B = head-and-body length (cm)

and indeed to its own diameter; the humerus is nearly as long as that of a carrion crow, while the ulna, by contrast, matches that of a magpie in length. Similarly, in the hind limb, the femur is the same length as that of a carrion crow, but the tibiotarsus matches a magpie’s. Thus one could easily conclude, taking humerus or femur length, that the Berlin Archaeopteryx weighed 500 g like a crow, or, taking the ulna or tibiotarsus lengths as a guide, that it weighed 200 g like a magpie.

The problems of making such comparisons with single specimens, perhaps chosen rather arbitrarily, are the reason for using, instead, allometric equations drawn from a wide range of material. However, that still leaves us with a confusingly wide range of estimates. On mechanical grounds, the diameters of the hind limb bones are likely to give the best estimates; I favour the figures of

LVHA'I' SIZE 1VAS ARCHAEOPI'ERYX? 187

230 or 252 g based on diameters offemur and tibiotarsus, and feel reassured that the figure of 256 g from body length, extrapolated as a mammal, is close to these. O n the other hand, the rather small diameter of these bones is less easy to measure accurately than the lengths of bones. The average based on the lengths of humerus, ulna, femur, tibiotarsus and tarsometatarsus, exti-apolating from bird allometry, is 272 g.

If a figure of 250 g is taken as the weight of a Berlin-sized Archneoptecyx, it follows that the wing loading was rather high, since Greeiiewalt's (1975) equation relating wing area to mass suggests that a modern bird with a wing area of 479 cm2 should only weigh 146 g. O n the other hand, -4ri.haeopteryx had a very large tail area, of 159 cm2; while i t is not strictly justifiable as a procedure, if one adds that area to the true wing area and recalculates the mass, a more plausible figure of 216 g results.

One final caution over estimating the size of Archaeopteryx may be made. The difference in size of the three specimens, which Wellnholkr (1974) has thoroughly documented, is of course approximately cubed when linear measurements are translated into masses. Thus, if one takes the diameter of the femur as the guide, and the Berlin specimen to have weighed 252 g, the London example weighed 306 g and the Eichstiitt specimen only 220 g.

McNab (1978) has argued that endothermy in mammals evolved as mammalian ancestors became smaller while retaining the rate of metabolism of thcir larger therapsid ancestors. It is certainly true that the earliest mammals were much smaller than their ancestors. McNab's argumenl should apply equally to birds. Ostrom (1978) statcs that Compsognathus, a contemporary of ArchaeopLer_yx, is the smallest known dinosaur. He does not give figures for bone diameters, but does quote, inler d i n , lengths for the femur and tibiotarsus. The allometric equations which suggest that the Berlin 4rchaeopferyx weighed 269 or 298 g give estimates €or Compsognathus of 638 or 532 g respectively. Archaeopteryx certainly was smaller than its probable dinosaur ancestors, as far as we know them.

.\C:KNO~Vl,E~I)GthlEN 1 5

I am grateful to Dr Paul Tatner for permission to quote his da ta on the weight of magpie components, and to Professor R. McN. Alexander for very kindly supplying the original data for the bird skeletons which he and his colleagues have used. I also thank Dr Lawrence Cook for computational help, and Mrs Patricia Yalden for typing this paper.

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I88 D. W. YALDEN

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