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The evolution of the mechanical properties of amniote bone
THE EVOLUTION
OF THE MECHANICAL AMNIOTE BONE J. D.
Department
PROPERTIES
OF
CURREY
of Biology. University
of York.
York YO1 5DD,
tJ.K
Abstract-IQ
specimens from 19 species of amniote were tested for various mechanical and physical properties to ascertain whether there were characteristic differences between different groups. All mechanical properties showed very great variation. In general the reptiles were not inferior to the mammals and birds. The histology of living forms was compared to that of fossil forms. to see whether ‘weak’histology was more characteristic of primitive amniotes. The earliest reptiles probably had rather compliant bone. but it was probably tough. Modern types of bone appeared over two hundred million years ago. Very specialised bone. like that of the bullae of whaies and antlers, may have evolved only in the mammals. but the fossit record is not complete enough to assert this confidently.
INTROUUCTIOS
The great Russian-American evolutionary geneticist, Theodosius Dobzhansky, once said: ‘nothing in biology makes sense except in terms of evolution.’ Although it is possible to spend a lifetime of fruitful research into biomechanics without considering the evolution of what is being studied, I am sorry for people who do so; they are surely mis.6ng.a whole dimension, the historical dimension. Gaynor Evans is by first training a zoologist, and a historical and comparative sense underlies his work, not often obviously. but occasionally popping up (e.g. Evans and Krahl. 1945; Evans and Gaff, 1957; Mueller et al., 198 1).This is not to say that an evolutionary perspective is necessarily useful in an operational sense; it is often merely inteIle~tually satisfying. In this paper I wish to consider to what extent, if any, we can say anything about the evolution of the mechanical properties of amniote bone. The amniotes comprise the reptiles, the birds and the mammals, and are considered by zoologists to form a reasonably uniform group of advanced vertebrates. The mechanical properties of the bones of some living animals are fairly well known, and these mechanical properties are to some extent explicable in terms of the bone’s material constituents. The bones of animals that were alive in the past are, of course, not available for testing. How, then, we can obtain any idea of the mechanical properties of these bones? Although fossil bones cannot be tested mechanically, they can be examined histologically. If we have knowledge of the relationship between the histology of modern bone and its mechanical properties, and if the histology of fossil bone is like that of modern bone, we Editor’s note. Submitted special ‘F. Gaynor Evans Biomechanics’. Editors.)
by editorial invitation for the Anniversary Issue on Bone (J. D. Currey, D. R. Carter, A. Viidik. Guest
are in a good position to make inferences about the mechanical properties of the fossil bone. Furthermore, we are on somewhat firmer ground if we can examine the mechanical properties of the bone of animals that are considered to bc in some way ‘primitive’. To a biologist a primitive animal is one that retains many characters that were once widespread in a particular group, but which have been abandoned by some members of that group. For instance, the reptiles were all once ectothermic {cold-blooded) but at least two groups, the birds and the mammals, have evolved from the reptiles and in doing so evolved many new characters, including warm blood (endothcrmy). Present-day reptiles, like the tortoises and the crocodiles, are therefore considered to be primitive in some respects. Ifthe histology of their bone were like that of the bone ofextinct reptiles, and unlike that of the bone of modern birds and mammals, it would be reasonable to suppose that the mechanical properties of the bone of these extinct groups might be like that of these primitive living reptiles rather than like that of the birds and mammals. In this paper I examine several mec~nical properties of bone of various amniotes. and relate these properties to possible explanatory variables, such as mineralization and histology. I also compare the histology ofthese bones with those ofextinct amniotes. Finally I relate the tindings discussed here with other work in the literature, and attempt to make inferences regarding the evolution of the mechanical properties of bone in the amniotes. MATERIALS
AND METHODS
I62 specimens were taken from 34 different bones from 19 species of amniote. The species were chosen to give a good taxonomic spread while at the same time allowing, in almost all cases, the same specimen size and shape to be used. The investigation is concerned with compact bone; no specimen with a porosity of
1036
J. D. CURREY
greater than 40”” was used. and only two specimens had porosities greater than 30”“. Most specimens were taken from the uall of the shaft of long bones, the provenance of the specimens is given in Table I. A few bones had dried out before specimens were prepared from them: all had been deep-frozen at some stage. Both bending and tensile specimens were prepared. The general method of preparation is described by Currey (1987). It involved rough preparation with a bandsaw, and smoothing with increasingly fine grades ofcarborundum paper. Tensile specimens were shaped into a dumbell shape by a milling head guided by a premachined pattern. All specimens were tested wet, at room temperature. Tensile
specimens
The central section, of uniform cross-sectional shape, was 13 mm long. with a square cross-section with 1.8 mm sides. An extensometer was attached to the uniform part of the specimen, which was loaded at a strain rate of about 0.2 s-‘. Output of the load cell and the extensometer went to a storage oscilloscope. Data were reduced from photographs of the screen. The following mechanical properties were determined from the tensile specimens: Young’s modulus of elasticity (E), ultimate stress (rr.,,). ultimate strain (c,,,) and work under the stress-strain curve, which is equivalent to work per unit volume.
Bending
specimens
The specimens tested in bending initially all had the same dimensions (gauge length 30 mm, depth 2 mm, breadth 3.5 mm). They were tested for Young’s modulus of elasticity in bending, using three-point loading. About half the specimens were then broken in threepoint bending, and bending strength calculated. ‘Bending strength’ is calculated stress produced on the outermost tibres of the specimen by the bending moment causing the specimen to fracture. This calculated stress assumes that the specimen behaves linearly elastically until fracture. Burstein et al. (1972) have shown that for specimens of elastic-completely plastic material of rectangular cross-section this calculated stress overestimates the true stress by a factor of more than SO’%. The tension tests and the bending tests showed that the bones tested in this investigation very rarely behaved in a completely brittle way, and indeed nearly all showed some plastic deformation. However, the resulting overestimation of the stress at fracture does not really matter, because all specimens were of nearly the same shape and size, and so the bending strength can be considered to be a normalised measure of the bending moment that caused failure in similarsized specimens, which is probably what would be important for the animals. The other bending specimens were Shaped so that the work of fracture could be determined. Two cuts were taken from a section in the middle of the bone,
leaving a triangular ligament. The specimen was loaded so that the apex of the triangle was loaded in tension. The shape of the ligament was such that. usually, the specimen did not fail catastrophically, as the ordinary bending specimens nearly always did, but failed progressively and stably. The load-deformation curve in these specimens rises to a maximum, and then falls gradually. Because the specimen is always in equilibrium with the forces acting on it. all the work required to fracture the specimen is shown in the load-deformation curve. There is no strain energy locked up in the specimens that is suddenly released, causing the fragments to fly apart (Tattersall and Tappin, 1966). If a specimen did fail catastrophically, the work of fracture was not estimated. Although the work of fracture is not a ‘good’ fracture mechanics property, because it does depend to some extent on the shape of the specimen, this is not important here, because all the specimens were of etfectively the same shape and size. The work of fracture measures the work required to drive a crack slowly through the material. Two physical properties of the specimens were measured quantitatively and, also. some more qualitative assessments were made. The quantitative values determined were the porosity of a cross-section taken just behind the fracture surface, and the mineral content of the specimen. Details of the methods are given in Currey (1987). The calcium content. determined calorimetrically, is expressed as: (mg calcium/g dry defatted bone). Cross-sections and longitudinal sections of many specimens, taken from near the fracture surface, were examined by direct and by polarised light. Qualitative assessments were made of the type of histology present, and of the degree of orientation of the specimens. Bone histology is quite varied. but there are a few fairly readily distinguishable typs. These are described in the section on histology.
Amniote
ecolution
In discussing
the various properties. it will be helpful to have some idea of what might be considered to be the more primitive amniotes. and also to consider how long ago the different groups split apart from each other, and must therefore have been evolving independently since. Figure 1 gives an indication of the evolutionary and temporal relationships of the groups mentioned in this paper. Although now specialised in many ways, there is no doubt that the tortoises probably represent the best we can hope for as a primitive reptile in terms of general physiology etc. They split off from the other reptile lines about 290 million years ago, at about the same time as the clumsy reptiles that would evolve into mammals split off from the rest. The crocodile, the dinosaur and the bird lines split off from each other about 180 million years ago, The wallaby (marsupial) line split from the rest of the
Mechanica)
Table
properties
of amniote
bone
1. Values for the various snecimens discussed in this oaoer
Alligator
(~~~i~afo~missipje~sis)
Whitesided dolphin I Layenorh,vncus acurus) Rib, anterior side Rib. posterior
6.4 6.0
11.9 12.3
174 -
I.540
260 24) 260
23.4 31.4 17.3 20.1
8.8 8.4
88 -
760 -
10.4 9.3
113 -
264
7.5
17.0
271
6.7
253
4.9
271 268
248 250
Femur
side
Jackass penguin i Spheniscus demersus) Radius, humerus, ulna femur and tibia Cow. I yt old (50s f(IIm~.\) Tibia Cow, 9 yr old (Bos ruurus) Femur Sdrus crane (Crtxi utrfiyone) Tarsometatarsus Crocodile (Crocodylus sp.) Frontal Prefrontal Donkey
-
108 -
0.02 1
IS
3 3
-
-
3390 -
77 -
0.016 -.
-
Ill
0.0’9
20
213
2840
-
--
-
20.7 17.2
235 -
2400 -
-
-
135
4.5 5.6
18.7 28.4
179 -
970 -
154
250 240
8.6 7.7
22.3 23.1
252 -
6820 -
218
251 236
6.2 9.0
5.6 7.7
54 91
1470 4760
-
247 251
7.8 5.6
17.6 IS.3
215 -
3190 -
274 274
3.8 3.0
26.8 26.8
260 -
1840 -
131
311 309 315
4.0 2.0 3.6
27.8
55
-
-
34.1 33.6
-49
‘4
27
264
7.8
27.8
259
1427
-
237 255 238 241
9.2 16.4 12.5 14.1
-
3800 7430 -
136 -
0.023 -
19 -
226 229 204
12.3 5.7 13.1 6.0 13.2 3.7 6.2
10.7 15.2 10.4
133 -
5280
152 -
0.034 -
34 -
166
0.04 1 -
47 -
I 2 1 1
268
5.7
22.1
214
1630
-
-.
-
4
267 273
8.7 9.4
18.4 22.4
-
3850 -
-
-
183
227
10.2
11.4
-
2990
-
-
-
I
248 257
5.4 4.0
18.6 19.5
-
6240 2506
-
-
-
3 2
757 __-
-
-
-
0.017
-
7
IO
16
3
0.005
-
3 1
5 2
0.018
-
8 2
I -
-
2 2
-
-
-
II4
0.020
I6
4 2
-
4
(Ey141ts ruhnliu.s) Radius Fallow deer (Dumu damtr) Tibia Fin whale (Bu~~~enupferu ph~sa~f~s) Bulla, white part Bulla, yellow part Flamingo (Phoenicopterus ruber) Tarsometatarsus Galapagos tortoise (G~u~~~~~~e mirfas) Femur Tibia Fibula Humerus Horse (Equus Cuba//us) Femur King penguin (Aptenodyfes porayonica) Humerus, radius and ulna Humerus and ulna Muntjac deer (.tfuntiucus munrjak) Antler Sheep (Otis aries) Metacarpal, posterior Metacarpal, anterior
-
-
-
-
4
0.006
2
0.002
-
4 3 2
-
-
8
-
-
2 4
-
8
0.009
2
4 2
J. D. CURREY
1038
Table 1. (Cod.)
Red deer (Cerrus elaphus) Antler, mature Antler, knobber
-
-
-
195 212 223 213
25.6 10.7 10.8 6.8
3.6 6.4 12.0 10.8
51 -
2530 5560 -
150 264
0.111 0.107
93 158
280 252 174
10.6 4.7 30.7
20.2 19.0 2.2
-
1470 2990
144 -
0.009 -
6 -
273 283 215
4.9 3.0 4.2
16.8 22.1 20.6
-
900 1250
163 -
0.008 -
-
213 211
21.3 21.1
6.2 7.0
86 -
5180 -
93
0.056
28
251
5.3
16.7
168
4540
-
5 13 3 6
Roe deer (Capreolus capreolus)
Femur Antler Red-necked wallaby (Proremnodon rujbgrisea)
Femur Tibia Reindeer (Rangifer larondus)
Antler
7 -
Seal (Halichoerus grypus)
Tibia
-
-
The abbreviations at the top of the columns mean: Ca: calcium in mg g-‘; Por: porosity in “/,; E: Young’s modulus in GPa; ES: bending strength in MPa; WF: work of fracture in Jm -*; udlr:ultimate tensilestrength in MPa; E,,,,;ultimate strain; W work under the stress strain curve inlOsjm-‘N: sample size (in the case of the bending specimens, since bending strength and work of fracture cannot be determined from the same specimen, the sample sizes used to determine these properties are smaller).
Metatheria
Eutheria (Placental
mammals)
Crocodiles
(Marsupials)
lchthyosaurs
Plesiosaurs
Birds
Tortoises
Dinosaurs
Therapsids Mososaurs Pelycosaurs
,
I
I
Fig. 1. Diagram showing the evolutionary relationships of the groups discussed in this paper. The relationships shown here are schematic, showing the time before the present at which dinerent groups began to evolve separately from each other, but not showing how far apart they evolved from each other subsequently. The main point to notice is the extremely long time that most of the groups have been evolving independently.
mammals,
the placenml
mammals, about
90 million
years ago. In some respects the marsupials seem to be somewhat more primitive than the other mammals, for instance they have a lower resting body temperature. Finally, the different groups of placental mammals, that is the whales, the seals. the horses, and the deer, cows
and sheep, have probably
been evolving independently for about 60 million years. In summary, therefore, we might expect to see primitivecharacteristics in the tortoises,crocodilesand alligators, perhaps associated with their low body temperature and slow growth rate; we might expect to
Mechanical properties of amniore bone
see great diRerences between
the mammals
and the
because they have been evoking separately for so long. and we might expect to see more primitive bone in the wallaby than in the other mammals. rest.
RESULTS
Table I and Fig. 2 give the summary results. Usually, different bones are treated separately. though in some cases different bones from the same animal are lumped if they are clearly similar in their properties. Comersely, specimens from diffrrent parts of the same bone are recorded separately. if they are clearly different in some properties. The medians are given here because values for some variables, particularly work of fracture and work under the curve, are rather non-normally distributed. Many of the properties are extremely variable between bones. Calcium content varies from 174 to 309 mg g- ‘, porosity from 2.2 to 30.7x, Young’s modulus from 2.2 to 33.5 GPa, bending strength from 14 to 263 MPa, tensile strength from 27 to 192 MPa and ultimate strain from 0.002 to 0.104. Variation in the extremes of several properties would be much less if the values from the tympanic bulla of the fin whale were excluded. Even if this is done however, the remaining variation is considerable. In looking for
Calcium Porosity Youn ‘s
modu s us
2
X
Bendin strengtR
1039
differences between diKerent taxa. therefore. we have a very variable data set to consider.
The mammals have the bone with the highest calcium (in the whale’s tympanic butla) and the lowest (in antlers). The reptiles have. in general. rather less calcium than the long bones of the birds and the mammals. Porosity
Here the situation is roughly the reverse of that for calcium. Also. the dolphin ribs are very porous; this is characteristic of ribs in general. Youny’s modulus There is great variation. The reptiles, theantl~rs,and the dolphin’s rib have rather low values. 3one with high calcium and low porosity tends to have a high Young’s modulus. This can be tested by a regression anafysis. I use both calcium content and porosity as explanatory variables in the regression equation, and all values are logged. Logged values are appropriate, and moreconvenient, for a variety of reasons discussed by Currey (1987). The regression equation has an RZ of 0.69. that is, 69”/, of the variance in Young’s modulus can be explained using these two explanatory variables.
Tensile
strength
Tensile strain
Work under curve r
X * X
Fig. 2. Summary results. Each symbol represents the median value for the specimens from one bone. expressed as a percentage of the greatest value for that property among all the bones. Open triangles: land reptiles: solid triangles: aquatic reptiles; open lozenges: land birds: solid lozenges: penguins; open circles: land mammals; solid circles: aquatic mammals; crosses:antlers; arrowheads: whale bulla.
J. D. CURREY
I040
Examination of the residuals of the regression will show whether any particular bones stand out. I consider only those whose standardised residuals are greater than 1.96. Only one residual in 20 would be expected to deviate as much as this from the fitted plane. The two crocodile frontals have large negative residuals ( -2.98 and -3.58) indicating that their Young’s moduli are considerably lower than would be expected. Two of the red deer antlers have Iarge negative residuals ( - 2.01 and - 2.57). One flamingo matatarsai specimen has a large positive residual (2.00), and in fact several of the flamingo specimens have rather large positive residuals, that is, these bones stand out from the rest in being rather stiffer than might be expected from their calcium content and porosity.
10
1 * .‘ .
Work of fracture
:
.
, .
. I’”
.
‘ ‘
2.
.
5-
L O-
.
‘, l
..
. .*. . : “. .“s;;y.: ’ . : w..m
.
250
300
Calcium
Fig. 4. Relationship between work of fracture (in kJ m-‘) and calcium (in mg g-‘).
..
300 -
Bending strength
‘_-*
.
9.
The relationship between bending strength and calcium content is shown in Fig. 3. No straightforward reiationshipexists,although high valuesofstrengthare not found in bones with either very high or very low values of calcium. The cloud of points on the lower right are values for the bulla. The other six lowest points are red deer and reindeer antler, a whale rib, an alligator femur and the frontal and prefrontal of the crocodile. The highest points beIong to the metatarsals of the flamingo and crane, a fallow deer tibia and a horse femur. Work of jiracture
The relationship between work of fracture calcium is shown in Fig. 4. The single point on bottom right is a b&a. The highest points belong sheep metacarpal, the Galapagos tortoise and crane metatarsal.
and the to a the
.
200 -
.
Tensile strength loo-
*
l
:**
.*
.“ .
. *
.
. l
.. :a . .. l -,9 . -.=
. .
. .
01
.
’ 200
250
300
Calcium
Fig. 5. Relationship between tensile strength (in MPa) and calcium content (in mg g- I).
are from the antlers of immature red deer (‘knobbers’). The strengths come from the fact that after the specimen has yielded it continues to bear a continuingly higher stress as the strain increases (Fig. 6). in this property it is unique among types of bone I have studied. Ultimate strain
Tensile strength
The relationships for the tensile specimens are based on fewer specimens than for bending. The relationship
between strength and calcium is shown in Fig. 5. Once again the bulla, represented by the two points on the bottom right, is particularly weak. There are four specimens that have spectacularly high strengths. They
There is a very strong relationship between the log of the ultimate strain and calcium content (Fig. 7} R2 is 87%. Bringing volume fraction into the equation does not improve the model. Two specimens having a high positive residual are a specimen of Galapagos tortoise tibia and red deer ‘knobber’. A small negative residual
l ::
.=..
... . : ,=-
:
. I.
Bending strength
l .
.:
.
.
Load
. . .
100
l
L
L .
.
0
.
.
. I .
.”
‘*
.:.
:/
*1
00
250
300
Deformation
Calcium Fig. 3. Relationship between bending strength (in MPa) and calcium content (in mg g-l).
Fig. 6. Load-deformation curve for a tensile specimen of a knobber antler, showing the large amount ofadditional load borne after the yield point.
Mechanical properties of amniote bone
.I . t
.
. :.
..
.$.
.05
Ultimate strain
IOO-
l
. .
.
.
. . . . nm1 . .
.005
.
I
.
.I.
.
9’ 9. .
_.m . .. . .
.
.
.
. .
....
IO-
.. .
..
.01
1041
..
.’
Work .
.
. . . . .. .
. . .
. 0.1 ‘I
. 200
250
300
Calcium Calcium Fig. 7. Relationship between ultimate strain (note scale) and calcium (in mg g-l).
the log
belonged to a specimen of reindeer antler. The two points on the lower right here, and in the diagram for work under the curve, belong to the whale’s bulla. Work under the cwue
Figure 8 shows the relationship between log work and calcium. It is clearly strong, and explains 807; of the variance. Again, adding volume fraction as an explanatory variable does not help. Four specimens stand out as having extreme residuals, three negative: bulla, red deer antler and reindeer antler; one positive: Galapagos tortoise femur. HISl’OLOG1’
OF AMSIOTE
BOSE
The previous sections show that insofar as we are able to assign a degree of ‘modernity’ to bones, the ancient bones are not markedly difl‘erent from the more modern bones. They are similar in two ways: (a) The ancient and the modern bones are equally likely to be responsible for large residuals in the statistical equations; that is to say the bones fit the overall equations equally well. (b) The modern bones provide specimens showing extreme values in the explanarorp variables; that is to say, the ancient bones do not stand out by being particularly highly or poorly calcified, by being bery or hardly porous. or by being structurally highly isotropic or anisotropic. Indeed the most highly mineralised bone is from the tympanic bullae of whales, a thoroughly advanced group. Similarly, the most porous and least mineralised bone is from mammalian antlers. However, if the bullae and antlers are excluded, as being obviously highly specialised bones, then it is true that the highest calcium values are all in the mammals. and the lowest in the reptiles (Fig. 2). These findings pay no regard to the actual histology itself; is it possible that some of the larger residuals w:e do find can be explained by the histology? For Young’s modulus the answer is ‘yes’. The crocodile frontal bones, which had large negative residuals, are very disorganized in structure, with large volumes of material having an orientation at a considerable angle to the long axis of the specimen. And, on the other hand,
Fig. 8. Relationship between IO’J rn-‘: note log scale)
work under the curve (in and calcium (in rneg-I).
the flamingo specimens, which had large positive residuals. had extremely well oriented bone, which appeared virtually dark in cross-section under cross polaroids. indicating that the fibres were oriented along the long axis of the specimen. For bending strength, also, it seems that high strength is associated with well-oriented bone, the which occured at intermediate highest strengths. values ofcalcium, being found in the matatarsals of the two birds, and in the tibia of the cow and the fallow deer tibia. All these bones were well oriented. For the other properties, there is no clear relationship between histology and high or low values. Of course, it has been known for some time that Haversian remodelling is associated with lower strength (Currey, 1959; Reilly and Burstein, 1975; Saha and Hayes, 1976) while on the contrary Martin and Burr have suggested that Haversian remodelling will tend lo protect bone against fatigue failure (1982). However, there is usually a great amount of variation in Haversian remodelling within species, and even within bones. All one can say about this is that, other things being equal, we might expect bone that has undergone less remodelling to be stronger. However, this fact does not allow us to infer much about the evolution of mechanical properties. because Haversian remodelling appeared very early in the amniotes, being present in the inner parts of the cortex of the long bones of the most primitive reptiles (Enlow and Brown, 1957). It is however a less marked feature of the bone of the more primitive groups than of the later ones. If. then, the bones tested in this study truly represented an evolutionary series. we could state that there was no evidence that amniote bone has undergone any significant evolutionary change in mechanical properties. Such differences as there are can be attributed to high or low calcium, high or low porosity, and well or poorly organised tissue. For none of these explanatory properties were primitive bones particularly outstanding. But. living animals are not the same as fossil animals. We cannot measure the calcium content of fossil bone. We have lo rely on histology. The histology of fossil
J. D. CURREI
1041
bone can. in many cases, be examined easily. The study of fossil bone may allow us to answer two questions: ( 1) Is the histology of fossil bone characteristic of present-day bones that are mechani~Ily extreme in some way? (2) Are there types of fossil bone that are not represented in living animals’? If so, are there reasons for thinking that this fossil bone would have mechanical properties ditl’erent from present-day bone‘? ?7re classijication
of bone histology
There is no generally recognized classification of bone histology. However. the classification ofde Riqles (1975) is comprehensive and sensible. It is similar in many ways to that of Enlow and Brown (1956). Many of its subsets will not be discussed here. Cellular and aceMar bone. The bone of the great majority of fish is acellular. However, all amniote bone is, and always has been, cellular. The three main types ofjne structure. At the finestructural level bonecan be considered as either woven, parallel-fibred, or tamellar. Woven bone, characteristic of foetal bone, and of fracture callus, is extremely disorganised, has very fine collagen fibres which are arranged in apparently completely random directions. It is very rarely found in adult amniote bone except as a kind of ‘fossil’, left over from a more youthful stage. Lamellar bone is arranged in lamellae about 5 pm thick. The collagen fibrils nearly all lie within the plane of the lamella, and over quite large distances (scores of microns) all have the same orientation. There is a tendency for the lamellae in adjacent lamallae to lie in rather different directions (see Ascenzi, in this volume). Parallel-fibred bone is intermediate between woven and lamellar bone in its structure. It is more highly calcified than lamellar bone, and its collagen fibres, though generally tending to lie in the same direction, are much less well oriented than thoseof lamellar bone. Parallel-fibred and lamellar bone are both found throughout present-day and fossil amniote bone. Primary and secondary bone. Primary bone is bone that is laid down in space that has not previously been occupied by bone. Secondary bone, on the other hand, replaces bone that has been eroded away previously. For our purposes secondary bone can be considered always to result in the formation of Haversian systems (secondary osteons). A tissue may be crammed with Haversian systems, and is called Haversian bone. It may, on the other hand, just have a scattering of isolated Haversian systems. Primary bone tissue has a number of characteristic appearences, which may grade into each other. f 1) Circumferenfiaf lamellae. These are laid down sub-periosteally, are very well oriented, the fibres tending to have a rather circumferent:al orientation, and blood channels, which are usually longitudinally oriented, are rather sparse. (2) Lamef~ar-tonal tissue. This consists of a series of bands of lamellar bone, often with some layers of
parallel-fibred bone interspersed. Longitudinal blood channels are arranged in neat circumferential rows. This bone often shows very clear arrest lines where the bone has obviously stopped being laid down for a while. (3) Fibrolamellar bone. The paradigmatic form of this histological type is a series of laminae, each one consisting of a two-dimensional anastomosing network of blood channels sandwiched between layers of lamellar bone. which are themselves sandwiched between layers of parallel-fibred bone. The network of blood channels is arranged circumferentially around the cortex of long bones. This arrangement is frequently seen. However, the blood channels are often Iess neatly arranged, having a more three-dimensional arrangement. Even so, fibrolamellar bone always has the topographical arrangement of blood vessels surrounded by lamellar bone, which is itself surrounded by parallel-libred bone. (4) Protoharersian bone. This perhaps unfortunate term, used by Enlow and Brown (1956). refers to bone which is produced by the infilling of large, uniformly oriented, cavities in bone, which have been present since the bone was originally formed. The infilling is by lamellar bone. It is not secondary bone. because it does not replace previously existing bone. It can be distinguished from Haversian bone by the fact that the various systems never encroach on each other. (5) Aoascufar bone. Avascular bone is occasionally seen in parts of reptile bone, and in the bone of small animals such as bats, moles and small passerine birds (Enlow and Brown, 1957, 1958). These types do to some extent grade into each other. For instance, ~rallel-fibred bone may have such fine fibres, so neatly and uniformly arranged, that it looks very like lamellar bone, though it usually can be distinguished from it by the fact that it does not have the characteristic alteration of the predominant direction of the fibres from one lamella to the next. It is possible, nevertheless, with practice. to describe a section of corticaf bone using the terminology above.
Characteristics
offossil
and present-day
bone
Space does not allow a full description of the characteristics of the histology of the bones that have been tested here, and of the bone of extinct animals. It can be said at once however, that both groups are very varied, and there are few types of bone in fossils that have not been tested here. The most important exception is that, because I have not tested small specimens, I have not tested completely avascular bone, Biewener (1982) tested whole bones of various small mammals and birds (which probably had a rather simple histology, even if they were not avascular). He found that the calculated strength properties of the bone material were similar to those of much larger mammals. There are two features of the bone of more ancient, extinct, reptiles that do seem to be different from that of more modern forms. One is the tendency to be
Mechanical properties of amniote bone rather porous. In modern forms porous bone tends to be found in ribs. and also in antlers. However. examination of the photographs in de Riqlk’ works shows that in the more primitive reptiles there was a greater tendency for quite porous bone to be present in the shafts of long bones. The other feature is that the reptiles had a greater tendency to have a great deal of cancellous bone in the lumen of the long bones. Present-day birds and mammals usually have a lumen that is clear of bony tissue. However. this latter feature is not examined here. because I tested only compact bone specimens. not cancellous bone or whole bones. There is a remarkable similarity, which must have evolved separately at least four times. between the bone of the extinct aquatic reptile groups (Mososaurs, Plesiosaurs and Ichthyosaurs. Fig. I) and the whales. In all these groups. both reptile and mammal, the cortical bone tends to consist of primary bone which originally had large and numerous longitudinal vascular canals running through it. After the bone had been laid down, these largecanals filled in with lamellar bone, giving the appearence of Haversian systems, the ‘Protohaversian systems’ of Enlow and Brown. This presumably has some relationship with the way of life of these various animals. and shows how the bones of all amniotes seem to adopt the same structure according to the biology of the animal. rather than its taxonomic position. Dinosaur bone is remarkably like the bone of modern mammals (Reid, 1983). The compact bone is usually fibrolamellar, with the same pattern as seen in present-day large mammals (Currey, 1962) and this is often invaded by Haversian systems, often very densely. In both these features this bone resembles that of large herbivorous mammals. Occasionally there is lamellar-zonal bone in which fine fibred primary lamellae. containing rows of blood vessels, is periodically interrupted by resting lines. Lamellar-zonal bone seems not to be found in eutherian mammals. However. several of the wallaby specimens had bone looking very like lamellar-zonal bone. These specimens were as strong and stilfas the eutherian mammal bone. There is no evidence. from the tests reported here. therefore, that the dinosaurs’ bone would have had mechanical properties different from those of extant mammals. Similarly the bones of the advanced mammal-like reptiles (the therapsids) are usually very similar to those of modern mammals. For instance, de Riqles ( 1979) writes ‘The large ‘herbivorous’ forms. such as the Permotriassic Anomodonts. the artiodactyls and Proboscideans of the tertiary and present day. convergence with the etc.. show a histological sauropod dinosaurs that is sometimes extremely precise .*(my translation). As in present-day mammals, Haversian systems were better developed in the more carnivorous mammal-like reptiles. The ancestors of the mammals passed through a stage when they were quite small, at the very most rabbit-sized (Kemp. 1982). Mammals of this size do not possess the uniform fibrolamellar structure seen in
1043
larger mammals and. usually. undergo rather little Haversian remodelling. This serves to emphasize the fact that particular histological types of bone have evolved independently several times: well-organized fibrolameltar bone. for instance. having evolved at least once in the dinosaur line, and at least twice in the line leading to the mammals. Fibrolamellar bone seems to arise in amniotes vvhenever land animals become large and. probably. herbivorous.
DlSCl SSIO\ Each of the mechanical properties determined here shows a very great range of values. and this is still true even when the values for the tympanic bulla, often the most extreme. are removed. Certain types of bone are characteristically high or low in some values. For instance, the deer antlers tend to have a low Young’s modulus, fairly high values for work of fracture. and high ultimate strains in tension. On the other hand the deer tibia has a high value for Young’s modulus and for bending strength and an intermediate value for work of fracture. Slender limb bones (flamingo, crane, wallaby tibia. fallow deer tibia. cow tibia) have high modulus and high bending strength. The crocodile skull bones have low values for Young’s modulus and for bending strength. but the crocodile prefrontal bone has a very high value for work of fracture. The Galapagos tortoise bones tend to have high values for all the measures concerned with toughness: work of fracture. strain at break, work under the curve. The whale bullae have high stiffness. but are low in all other mechanical properties. There is no indication in the present-day bones tested, that particular animals have consistently low values for all properties tested. It seems much more to be true that bones performing different functions have particular properties. One characteristic related to taxonomy rather than to any obvious function. is that the bones of the living reptiles are rather lower in calcium than most of the mammal and bird bones. except the antlers. It is unfortunately not possible to infer calcium content directly from the histology of the specimens. so we cannot tell the amount of mineralization of extinct groups. However. it does seem unlikely that the extremely well-organized fibro-lamellar bone of the therapsids and dinosaurs was not as well-mineralized as the histologically similar bone of present-day mammals. The picture that emerges from these considerations and observations is this. The very early reptiles had rather porous bone. and it was probably not very highly mineralised. As a result it would have had a rather low Young’s modulus and bending strength. Quite probably. however, it would have been. like the Galapagos tortoise bone. adequately tough. Thereafter, in several different lines, the kinds of histology and mechanical properties characteristic of present-
J. D. CURREY
104-t
day animals arose. and what is more arose repeatedly. and a very long time ago. according to the size of the animal (large animals tend to have fibro-lamellar bone). the habitat of the animal (marine deep-diving animals tend to have rather porous bone with protohavers~an systemsjand the functions of the bones (slender bones are stiff and strong in bending. but not necessarily tough). One strong conclusion to emerge is that in no sense can the compact bone of mammals be taken to be the summit of bone evolution. In the admittedly limited data set examined here the bending strength of some birds’ bones is as strong as that of the strongest mammal, the greatest work of fracture is found in a tortoise, and although mammals’ antlers have the highest tensile strengths and work under the curve, the ordinary long bone of a bird is stronger than the long bone of a mammal, and reptile’s long bone has a greater work under thecurve than mammal long bone. The mammals do have two types of mechanically specialised bone: bulla and antler. The fossil record is not good enough to be sure whether they are unique in this. Acknowfed~etnmrs-1 acknowledge the help. in providing specimens, of the British Antarctic Survey. the British Museum (Natural History), Hvalur H. F. (Iceland), Flamingo Park Zoo, the Zoological Society of London. Dr R. Reid and particularly. for her untiring and generous efforts, Dr Caroline Pond. This work was supported by grant no. GR,DO4076, from the Science and Engineering Research Council.
REFERENCES Biewener, A. A. ( 1982) Bone strength in small mammals and bipedal birds: do safety factors change with body size:’ J. esp. Bid/. 98, 289-301. Burstein, A. H.. Currey. J. D., Frankel, V. H. and Reilly. D. T. ( 1972) The ultimate properties of bone tissue: the effects of yielding. J. &mechanics 5, 35-44.
Currey. J. D. ( 1959) Differences in the tensile strength of bone of different histological types. Q, J. microsc. Sci. 103. 111-133. J. D. (1962) The histology of the bone of a prosauropod dinosaur. Palaeontofogr 5, X8-246. Currey. J. D. (19871 The effect of porosity and mineral
Currey.
content on the Young’s modulus of elasticity of compact bone. J. Biomechanics. Enlow, D. H. and Brown. S. 0. (1956) A comparative histological study of fossil and recent bone tissues. Part 1. Texczs J. Sci. 8, 405443. Enlow. D. H. and Brown, S. 0. (1957) A comparative histological study of fossil and recent bone tissues. Part 2. Texas J. Sci. 9, 186-214. Enlow, D. H. and Brown, S. 0. (1958) A comparative histological study of fossil and recent bone tissues. Part 3. Texas J. Sci. IO. 187-230. Evans, F. G. and Goff, C. W. ( 1957) A comparative study of the primate femur by means of the stresscoat and split-line techniques. Am. J. phys. Anthropol. 15, 59-89. Evans. F. G. and Krahl, V. E. ( 1945) Torsion of the humerus; a phylogenetic survey from fish to man. Am. J. Attar. 3, 229-253. Kemp, T. S. (1982) Mammal-like Repriles and the Origin of Mammals. Academic Press, New York. Martin, R. B. and Burr, D. B. (1982) A hypothetical mechanism for the stimulation of osteonal remodeling by fatigue damage. j. Bjo~c~unics 15, 137-139. Mueller, W. C., Ldsker, G. W. and Et-ans. F. G. (1981) Anthropometric measurements and Darwinian fitness. J. biosac. Sri. 13, 309-3 16. Reid. R. E. H. ( 1984) The histology of dinosaurian bone, and its possible bearing on dinosaurian physiology. The und Evolution of Reptiles; Sfructure. Derekopmehr Synposilrm of the Zoological Sacret? of Londan, Vol. 52 (Edited by Ferguson, M. W. J.). pp. 629-663. Reilly, D. 7. and Burstein. A. Ii. (1975) The elastic and ultimate properties of compact bone tissue. J. Biomechanics 8. 393-405. Riqles, A. J. de (1975) Recherches paleohistologiques sur les OS longs des tttrapodes. VII ( I ) Ann/sPukont.61, 51-129. Riqles. A. J. de (1979) Quelques remarques sur I‘histoire evolutive des tissus squelettiques chez les vertebres et plus particul~~rement chez les tetrapodes. Annls biol. 17, I-35. Saha. S. and Hayes, W. C. ( 1976) Tensile impact properties of human compact bone. J. Biomechanics 9. 243-251. Tattersall. H. Cl. and Tappin. G. (1966) The work of fracture and its measurement in metals. ceramics and other materials, J. Mrtr. Sci. I, 296-301.
OF THE MECHANICAL AMNIOTE BONE J. D.
Department
PROPERTIES
OF
CURREY
of Biology. University
of York.
York YO1 5DD,
tJ.K
Abstract-IQ
specimens from 19 species of amniote were tested for various mechanical and physical properties to ascertain whether there were characteristic differences between different groups. All mechanical properties showed very great variation. In general the reptiles were not inferior to the mammals and birds. The histology of living forms was compared to that of fossil forms. to see whether ‘weak’histology was more characteristic of primitive amniotes. The earliest reptiles probably had rather compliant bone. but it was probably tough. Modern types of bone appeared over two hundred million years ago. Very specialised bone. like that of the bullae of whaies and antlers, may have evolved only in the mammals. but the fossit record is not complete enough to assert this confidently.
INTROUUCTIOS
The great Russian-American evolutionary geneticist, Theodosius Dobzhansky, once said: ‘nothing in biology makes sense except in terms of evolution.’ Although it is possible to spend a lifetime of fruitful research into biomechanics without considering the evolution of what is being studied, I am sorry for people who do so; they are surely mis.6ng.a whole dimension, the historical dimension. Gaynor Evans is by first training a zoologist, and a historical and comparative sense underlies his work, not often obviously. but occasionally popping up (e.g. Evans and Krahl. 1945; Evans and Gaff, 1957; Mueller et al., 198 1).This is not to say that an evolutionary perspective is necessarily useful in an operational sense; it is often merely inteIle~tually satisfying. In this paper I wish to consider to what extent, if any, we can say anything about the evolution of the mechanical properties of amniote bone. The amniotes comprise the reptiles, the birds and the mammals, and are considered by zoologists to form a reasonably uniform group of advanced vertebrates. The mechanical properties of the bones of some living animals are fairly well known, and these mechanical properties are to some extent explicable in terms of the bone’s material constituents. The bones of animals that were alive in the past are, of course, not available for testing. How, then, we can obtain any idea of the mechanical properties of these bones? Although fossil bones cannot be tested mechanically, they can be examined histologically. If we have knowledge of the relationship between the histology of modern bone and its mechanical properties, and if the histology of fossil bone is like that of modern bone, we Editor’s note. Submitted special ‘F. Gaynor Evans Biomechanics’. Editors.)
by editorial invitation for the Anniversary Issue on Bone (J. D. Currey, D. R. Carter, A. Viidik. Guest
are in a good position to make inferences about the mechanical properties of the fossil bone. Furthermore, we are on somewhat firmer ground if we can examine the mechanical properties of the bone of animals that are considered to bc in some way ‘primitive’. To a biologist a primitive animal is one that retains many characters that were once widespread in a particular group, but which have been abandoned by some members of that group. For instance, the reptiles were all once ectothermic {cold-blooded) but at least two groups, the birds and the mammals, have evolved from the reptiles and in doing so evolved many new characters, including warm blood (endothcrmy). Present-day reptiles, like the tortoises and the crocodiles, are therefore considered to be primitive in some respects. Ifthe histology of their bone were like that of the bone ofextinct reptiles, and unlike that of the bone of modern birds and mammals, it would be reasonable to suppose that the mechanical properties of the bone of these extinct groups might be like that of these primitive living reptiles rather than like that of the birds and mammals. In this paper I examine several mec~nical properties of bone of various amniotes. and relate these properties to possible explanatory variables, such as mineralization and histology. I also compare the histology ofthese bones with those ofextinct amniotes. Finally I relate the tindings discussed here with other work in the literature, and attempt to make inferences regarding the evolution of the mechanical properties of bone in the amniotes. MATERIALS
AND METHODS
I62 specimens were taken from 34 different bones from 19 species of amniote. The species were chosen to give a good taxonomic spread while at the same time allowing, in almost all cases, the same specimen size and shape to be used. The investigation is concerned with compact bone; no specimen with a porosity of
1036
J. D. CURREY
greater than 40”” was used. and only two specimens had porosities greater than 30”“. Most specimens were taken from the uall of the shaft of long bones, the provenance of the specimens is given in Table I. A few bones had dried out before specimens were prepared from them: all had been deep-frozen at some stage. Both bending and tensile specimens were prepared. The general method of preparation is described by Currey (1987). It involved rough preparation with a bandsaw, and smoothing with increasingly fine grades ofcarborundum paper. Tensile specimens were shaped into a dumbell shape by a milling head guided by a premachined pattern. All specimens were tested wet, at room temperature. Tensile
specimens
The central section, of uniform cross-sectional shape, was 13 mm long. with a square cross-section with 1.8 mm sides. An extensometer was attached to the uniform part of the specimen, which was loaded at a strain rate of about 0.2 s-‘. Output of the load cell and the extensometer went to a storage oscilloscope. Data were reduced from photographs of the screen. The following mechanical properties were determined from the tensile specimens: Young’s modulus of elasticity (E), ultimate stress (rr.,,). ultimate strain (c,,,) and work under the stress-strain curve, which is equivalent to work per unit volume.
Bending
specimens
The specimens tested in bending initially all had the same dimensions (gauge length 30 mm, depth 2 mm, breadth 3.5 mm). They were tested for Young’s modulus of elasticity in bending, using three-point loading. About half the specimens were then broken in threepoint bending, and bending strength calculated. ‘Bending strength’ is calculated stress produced on the outermost tibres of the specimen by the bending moment causing the specimen to fracture. This calculated stress assumes that the specimen behaves linearly elastically until fracture. Burstein et al. (1972) have shown that for specimens of elastic-completely plastic material of rectangular cross-section this calculated stress overestimates the true stress by a factor of more than SO’%. The tension tests and the bending tests showed that the bones tested in this investigation very rarely behaved in a completely brittle way, and indeed nearly all showed some plastic deformation. However, the resulting overestimation of the stress at fracture does not really matter, because all specimens were of nearly the same shape and size, and so the bending strength can be considered to be a normalised measure of the bending moment that caused failure in similarsized specimens, which is probably what would be important for the animals. The other bending specimens were Shaped so that the work of fracture could be determined. Two cuts were taken from a section in the middle of the bone,
leaving a triangular ligament. The specimen was loaded so that the apex of the triangle was loaded in tension. The shape of the ligament was such that. usually, the specimen did not fail catastrophically, as the ordinary bending specimens nearly always did, but failed progressively and stably. The load-deformation curve in these specimens rises to a maximum, and then falls gradually. Because the specimen is always in equilibrium with the forces acting on it. all the work required to fracture the specimen is shown in the load-deformation curve. There is no strain energy locked up in the specimens that is suddenly released, causing the fragments to fly apart (Tattersall and Tappin, 1966). If a specimen did fail catastrophically, the work of fracture was not estimated. Although the work of fracture is not a ‘good’ fracture mechanics property, because it does depend to some extent on the shape of the specimen, this is not important here, because all the specimens were of etfectively the same shape and size. The work of fracture measures the work required to drive a crack slowly through the material. Two physical properties of the specimens were measured quantitatively and, also. some more qualitative assessments were made. The quantitative values determined were the porosity of a cross-section taken just behind the fracture surface, and the mineral content of the specimen. Details of the methods are given in Currey (1987). The calcium content. determined calorimetrically, is expressed as: (mg calcium/g dry defatted bone). Cross-sections and longitudinal sections of many specimens, taken from near the fracture surface, were examined by direct and by polarised light. Qualitative assessments were made of the type of histology present, and of the degree of orientation of the specimens. Bone histology is quite varied. but there are a few fairly readily distinguishable typs. These are described in the section on histology.
Amniote
ecolution
In discussing
the various properties. it will be helpful to have some idea of what might be considered to be the more primitive amniotes. and also to consider how long ago the different groups split apart from each other, and must therefore have been evolving independently since. Figure 1 gives an indication of the evolutionary and temporal relationships of the groups mentioned in this paper. Although now specialised in many ways, there is no doubt that the tortoises probably represent the best we can hope for as a primitive reptile in terms of general physiology etc. They split off from the other reptile lines about 290 million years ago, at about the same time as the clumsy reptiles that would evolve into mammals split off from the rest. The crocodile, the dinosaur and the bird lines split off from each other about 180 million years ago, The wallaby (marsupial) line split from the rest of the
Mechanica)
Table
properties
of amniote
bone
1. Values for the various snecimens discussed in this oaoer
Alligator
(~~~i~afo~missipje~sis)
Whitesided dolphin I Layenorh,vncus acurus) Rib, anterior side Rib. posterior
6.4 6.0
11.9 12.3
174 -
I.540
260 24) 260
23.4 31.4 17.3 20.1
8.8 8.4
88 -
760 -
10.4 9.3
113 -
264
7.5
17.0
271
6.7
253
4.9
271 268
248 250
Femur
side
Jackass penguin i Spheniscus demersus) Radius, humerus, ulna femur and tibia Cow. I yt old (50s f(IIm~.\) Tibia Cow, 9 yr old (Bos ruurus) Femur Sdrus crane (Crtxi utrfiyone) Tarsometatarsus Crocodile (Crocodylus sp.) Frontal Prefrontal Donkey
-
108 -
0.02 1
IS
3 3
-
-
3390 -
77 -
0.016 -.
-
Ill
0.0’9
20
213
2840
-
--
-
20.7 17.2
235 -
2400 -
-
-
135
4.5 5.6
18.7 28.4
179 -
970 -
154
250 240
8.6 7.7
22.3 23.1
252 -
6820 -
218
251 236
6.2 9.0
5.6 7.7
54 91
1470 4760
-
247 251
7.8 5.6
17.6 IS.3
215 -
3190 -
274 274
3.8 3.0
26.8 26.8
260 -
1840 -
131
311 309 315
4.0 2.0 3.6
27.8
55
-
-
34.1 33.6
-49
‘4
27
264
7.8
27.8
259
1427
-
237 255 238 241
9.2 16.4 12.5 14.1
-
3800 7430 -
136 -
0.023 -
19 -
226 229 204
12.3 5.7 13.1 6.0 13.2 3.7 6.2
10.7 15.2 10.4
133 -
5280
152 -
0.034 -
34 -
166
0.04 1 -
47 -
I 2 1 1
268
5.7
22.1
214
1630
-
-.
-
4
267 273
8.7 9.4
18.4 22.4
-
3850 -
-
-
183
227
10.2
11.4
-
2990
-
-
-
I
248 257
5.4 4.0
18.6 19.5
-
6240 2506
-
-
-
3 2
757 __-
-
-
-
0.017
-
7
IO
16
3
0.005
-
3 1
5 2
0.018
-
8 2
I -
-
2 2
-
-
-
II4
0.020
I6
4 2
-
4
(Ey141ts ruhnliu.s) Radius Fallow deer (Dumu damtr) Tibia Fin whale (Bu~~~enupferu ph~sa~f~s) Bulla, white part Bulla, yellow part Flamingo (Phoenicopterus ruber) Tarsometatarsus Galapagos tortoise (G~u~~~~~~e mirfas) Femur Tibia Fibula Humerus Horse (Equus Cuba//us) Femur King penguin (Aptenodyfes porayonica) Humerus, radius and ulna Humerus and ulna Muntjac deer (.tfuntiucus munrjak) Antler Sheep (Otis aries) Metacarpal, posterior Metacarpal, anterior
-
-
-
-
4
0.006
2
0.002
-
4 3 2
-
-
8
-
-
2 4
-
8
0.009
2
4 2
J. D. CURREY
1038
Table 1. (Cod.)
Red deer (Cerrus elaphus) Antler, mature Antler, knobber
-
-
-
195 212 223 213
25.6 10.7 10.8 6.8
3.6 6.4 12.0 10.8
51 -
2530 5560 -
150 264
0.111 0.107
93 158
280 252 174
10.6 4.7 30.7
20.2 19.0 2.2
-
1470 2990
144 -
0.009 -
6 -
273 283 215
4.9 3.0 4.2
16.8 22.1 20.6
-
900 1250
163 -
0.008 -
-
213 211
21.3 21.1
6.2 7.0
86 -
5180 -
93
0.056
28
251
5.3
16.7
168
4540
-
5 13 3 6
Roe deer (Capreolus capreolus)
Femur Antler Red-necked wallaby (Proremnodon rujbgrisea)
Femur Tibia Reindeer (Rangifer larondus)
Antler
7 -
Seal (Halichoerus grypus)
Tibia
-
-
The abbreviations at the top of the columns mean: Ca: calcium in mg g-‘; Por: porosity in “/,; E: Young’s modulus in GPa; ES: bending strength in MPa; WF: work of fracture in Jm -*; udlr:ultimate tensilestrength in MPa; E,,,,;ultimate strain; W work under the stress strain curve inlOsjm-‘N: sample size (in the case of the bending specimens, since bending strength and work of fracture cannot be determined from the same specimen, the sample sizes used to determine these properties are smaller).
Metatheria
Eutheria (Placental
mammals)
Crocodiles
(Marsupials)
lchthyosaurs
Plesiosaurs
Birds
Tortoises
Dinosaurs
Therapsids Mososaurs Pelycosaurs
,
I
I
Fig. 1. Diagram showing the evolutionary relationships of the groups discussed in this paper. The relationships shown here are schematic, showing the time before the present at which dinerent groups began to evolve separately from each other, but not showing how far apart they evolved from each other subsequently. The main point to notice is the extremely long time that most of the groups have been evolving independently.
mammals,
the placenml
mammals, about
90 million
years ago. In some respects the marsupials seem to be somewhat more primitive than the other mammals, for instance they have a lower resting body temperature. Finally, the different groups of placental mammals, that is the whales, the seals. the horses, and the deer, cows
and sheep, have probably
been evolving independently for about 60 million years. In summary, therefore, we might expect to see primitivecharacteristics in the tortoises,crocodilesand alligators, perhaps associated with their low body temperature and slow growth rate; we might expect to
Mechanical properties of amniore bone
see great diRerences between
the mammals
and the
because they have been evoking separately for so long. and we might expect to see more primitive bone in the wallaby than in the other mammals. rest.
RESULTS
Table I and Fig. 2 give the summary results. Usually, different bones are treated separately. though in some cases different bones from the same animal are lumped if they are clearly similar in their properties. Comersely, specimens from diffrrent parts of the same bone are recorded separately. if they are clearly different in some properties. The medians are given here because values for some variables, particularly work of fracture and work under the curve, are rather non-normally distributed. Many of the properties are extremely variable between bones. Calcium content varies from 174 to 309 mg g- ‘, porosity from 2.2 to 30.7x, Young’s modulus from 2.2 to 33.5 GPa, bending strength from 14 to 263 MPa, tensile strength from 27 to 192 MPa and ultimate strain from 0.002 to 0.104. Variation in the extremes of several properties would be much less if the values from the tympanic bulla of the fin whale were excluded. Even if this is done however, the remaining variation is considerable. In looking for
Calcium Porosity Youn ‘s
modu s us
2
X
Bendin strengtR
1039
differences between diKerent taxa. therefore. we have a very variable data set to consider.
The mammals have the bone with the highest calcium (in the whale’s tympanic butla) and the lowest (in antlers). The reptiles have. in general. rather less calcium than the long bones of the birds and the mammals. Porosity
Here the situation is roughly the reverse of that for calcium. Also. the dolphin ribs are very porous; this is characteristic of ribs in general. Youny’s modulus There is great variation. The reptiles, theantl~rs,and the dolphin’s rib have rather low values. 3one with high calcium and low porosity tends to have a high Young’s modulus. This can be tested by a regression anafysis. I use both calcium content and porosity as explanatory variables in the regression equation, and all values are logged. Logged values are appropriate, and moreconvenient, for a variety of reasons discussed by Currey (1987). The regression equation has an RZ of 0.69. that is, 69”/, of the variance in Young’s modulus can be explained using these two explanatory variables.
Tensile
strength
Tensile strain
Work under curve r
X * X
Fig. 2. Summary results. Each symbol represents the median value for the specimens from one bone. expressed as a percentage of the greatest value for that property among all the bones. Open triangles: land reptiles: solid triangles: aquatic reptiles; open lozenges: land birds: solid lozenges: penguins; open circles: land mammals; solid circles: aquatic mammals; crosses:antlers; arrowheads: whale bulla.
J. D. CURREY
I040
Examination of the residuals of the regression will show whether any particular bones stand out. I consider only those whose standardised residuals are greater than 1.96. Only one residual in 20 would be expected to deviate as much as this from the fitted plane. The two crocodile frontals have large negative residuals ( -2.98 and -3.58) indicating that their Young’s moduli are considerably lower than would be expected. Two of the red deer antlers have Iarge negative residuals ( - 2.01 and - 2.57). One flamingo matatarsai specimen has a large positive residual (2.00), and in fact several of the flamingo specimens have rather large positive residuals, that is, these bones stand out from the rest in being rather stiffer than might be expected from their calcium content and porosity.
10
1 * .‘ .
Work of fracture
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.
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300
Calcium
Fig. 4. Relationship between work of fracture (in kJ m-‘) and calcium (in mg g-‘).
..
300 -
Bending strength
‘_-*
.
9.
The relationship between bending strength and calcium content is shown in Fig. 3. No straightforward reiationshipexists,although high valuesofstrengthare not found in bones with either very high or very low values of calcium. The cloud of points on the lower right are values for the bulla. The other six lowest points are red deer and reindeer antler, a whale rib, an alligator femur and the frontal and prefrontal of the crocodile. The highest points beIong to the metatarsals of the flamingo and crane, a fallow deer tibia and a horse femur. Work of jiracture
The relationship between work of fracture calcium is shown in Fig. 4. The single point on bottom right is a b&a. The highest points belong sheep metacarpal, the Galapagos tortoise and crane metatarsal.
and the to a the
.
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.
Tensile strength loo-
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:**
.*
.“ .
. *
.
. l
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01
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’ 200
250
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Calcium
Fig. 5. Relationship between tensile strength (in MPa) and calcium content (in mg g- I).
are from the antlers of immature red deer (‘knobbers’). The strengths come from the fact that after the specimen has yielded it continues to bear a continuingly higher stress as the strain increases (Fig. 6). in this property it is unique among types of bone I have studied. Ultimate strain
Tensile strength
The relationships for the tensile specimens are based on fewer specimens than for bending. The relationship
between strength and calcium is shown in Fig. 5. Once again the bulla, represented by the two points on the bottom right, is particularly weak. There are four specimens that have spectacularly high strengths. They
There is a very strong relationship between the log of the ultimate strain and calcium content (Fig. 7} R2 is 87%. Bringing volume fraction into the equation does not improve the model. Two specimens having a high positive residual are a specimen of Galapagos tortoise tibia and red deer ‘knobber’. A small negative residual
l ::
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:
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.”
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.:.
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*1
00
250
300
Deformation
Calcium Fig. 3. Relationship between bending strength (in MPa) and calcium content (in mg g-l).
Fig. 6. Load-deformation curve for a tensile specimen of a knobber antler, showing the large amount ofadditional load borne after the yield point.
Mechanical properties of amniote bone
.I . t
.
. :.
..
.$.
.05
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1041
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. 200
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Calcium Calcium Fig. 7. Relationship between ultimate strain (note scale) and calcium (in mg g-l).
the log
belonged to a specimen of reindeer antler. The two points on the lower right here, and in the diagram for work under the curve, belong to the whale’s bulla. Work under the cwue
Figure 8 shows the relationship between log work and calcium. It is clearly strong, and explains 807; of the variance. Again, adding volume fraction as an explanatory variable does not help. Four specimens stand out as having extreme residuals, three negative: bulla, red deer antler and reindeer antler; one positive: Galapagos tortoise femur. HISl’OLOG1’
OF AMSIOTE
BOSE
The previous sections show that insofar as we are able to assign a degree of ‘modernity’ to bones, the ancient bones are not markedly difl‘erent from the more modern bones. They are similar in two ways: (a) The ancient and the modern bones are equally likely to be responsible for large residuals in the statistical equations; that is to say the bones fit the overall equations equally well. (b) The modern bones provide specimens showing extreme values in the explanarorp variables; that is to say, the ancient bones do not stand out by being particularly highly or poorly calcified, by being bery or hardly porous. or by being structurally highly isotropic or anisotropic. Indeed the most highly mineralised bone is from the tympanic bullae of whales, a thoroughly advanced group. Similarly, the most porous and least mineralised bone is from mammalian antlers. However, if the bullae and antlers are excluded, as being obviously highly specialised bones, then it is true that the highest calcium values are all in the mammals. and the lowest in the reptiles (Fig. 2). These findings pay no regard to the actual histology itself; is it possible that some of the larger residuals w:e do find can be explained by the histology? For Young’s modulus the answer is ‘yes’. The crocodile frontal bones, which had large negative residuals, are very disorganized in structure, with large volumes of material having an orientation at a considerable angle to the long axis of the specimen. And, on the other hand,
Fig. 8. Relationship between IO’J rn-‘: note log scale)
work under the curve (in and calcium (in rneg-I).
the flamingo specimens, which had large positive residuals. had extremely well oriented bone, which appeared virtually dark in cross-section under cross polaroids. indicating that the fibres were oriented along the long axis of the specimen. For bending strength, also, it seems that high strength is associated with well-oriented bone, the which occured at intermediate highest strengths. values ofcalcium, being found in the matatarsals of the two birds, and in the tibia of the cow and the fallow deer tibia. All these bones were well oriented. For the other properties, there is no clear relationship between histology and high or low values. Of course, it has been known for some time that Haversian remodelling is associated with lower strength (Currey, 1959; Reilly and Burstein, 1975; Saha and Hayes, 1976) while on the contrary Martin and Burr have suggested that Haversian remodelling will tend lo protect bone against fatigue failure (1982). However, there is usually a great amount of variation in Haversian remodelling within species, and even within bones. All one can say about this is that, other things being equal, we might expect bone that has undergone less remodelling to be stronger. However, this fact does not allow us to infer much about the evolution of mechanical properties. because Haversian remodelling appeared very early in the amniotes, being present in the inner parts of the cortex of the long bones of the most primitive reptiles (Enlow and Brown, 1957). It is however a less marked feature of the bone of the more primitive groups than of the later ones. If. then, the bones tested in this study truly represented an evolutionary series. we could state that there was no evidence that amniote bone has undergone any significant evolutionary change in mechanical properties. Such differences as there are can be attributed to high or low calcium, high or low porosity, and well or poorly organised tissue. For none of these explanatory properties were primitive bones particularly outstanding. But. living animals are not the same as fossil animals. We cannot measure the calcium content of fossil bone. We have lo rely on histology. The histology of fossil
J. D. CURREI
1041
bone can. in many cases, be examined easily. The study of fossil bone may allow us to answer two questions: ( 1) Is the histology of fossil bone characteristic of present-day bones that are mechani~Ily extreme in some way? (2) Are there types of fossil bone that are not represented in living animals’? If so, are there reasons for thinking that this fossil bone would have mechanical properties ditl’erent from present-day bone‘? ?7re classijication
of bone histology
There is no generally recognized classification of bone histology. However. the classification ofde Riqles (1975) is comprehensive and sensible. It is similar in many ways to that of Enlow and Brown (1956). Many of its subsets will not be discussed here. Cellular and aceMar bone. The bone of the great majority of fish is acellular. However, all amniote bone is, and always has been, cellular. The three main types ofjne structure. At the finestructural level bonecan be considered as either woven, parallel-fibred, or tamellar. Woven bone, characteristic of foetal bone, and of fracture callus, is extremely disorganised, has very fine collagen fibres which are arranged in apparently completely random directions. It is very rarely found in adult amniote bone except as a kind of ‘fossil’, left over from a more youthful stage. Lamellar bone is arranged in lamellae about 5 pm thick. The collagen fibrils nearly all lie within the plane of the lamella, and over quite large distances (scores of microns) all have the same orientation. There is a tendency for the lamellae in adjacent lamallae to lie in rather different directions (see Ascenzi, in this volume). Parallel-fibred bone is intermediate between woven and lamellar bone in its structure. It is more highly calcified than lamellar bone, and its collagen fibres, though generally tending to lie in the same direction, are much less well oriented than thoseof lamellar bone. Parallel-fibred and lamellar bone are both found throughout present-day and fossil amniote bone. Primary and secondary bone. Primary bone is bone that is laid down in space that has not previously been occupied by bone. Secondary bone, on the other hand, replaces bone that has been eroded away previously. For our purposes secondary bone can be considered always to result in the formation of Haversian systems (secondary osteons). A tissue may be crammed with Haversian systems, and is called Haversian bone. It may, on the other hand, just have a scattering of isolated Haversian systems. Primary bone tissue has a number of characteristic appearences, which may grade into each other. f 1) Circumferenfiaf lamellae. These are laid down sub-periosteally, are very well oriented, the fibres tending to have a rather circumferent:al orientation, and blood channels, which are usually longitudinally oriented, are rather sparse. (2) Lamef~ar-tonal tissue. This consists of a series of bands of lamellar bone, often with some layers of
parallel-fibred bone interspersed. Longitudinal blood channels are arranged in neat circumferential rows. This bone often shows very clear arrest lines where the bone has obviously stopped being laid down for a while. (3) Fibrolamellar bone. The paradigmatic form of this histological type is a series of laminae, each one consisting of a two-dimensional anastomosing network of blood channels sandwiched between layers of lamellar bone. which are themselves sandwiched between layers of parallel-fibred bone. The network of blood channels is arranged circumferentially around the cortex of long bones. This arrangement is frequently seen. However, the blood channels are often Iess neatly arranged, having a more three-dimensional arrangement. Even so, fibrolamellar bone always has the topographical arrangement of blood vessels surrounded by lamellar bone, which is itself surrounded by parallel-libred bone. (4) Protoharersian bone. This perhaps unfortunate term, used by Enlow and Brown (1956). refers to bone which is produced by the infilling of large, uniformly oriented, cavities in bone, which have been present since the bone was originally formed. The infilling is by lamellar bone. It is not secondary bone. because it does not replace previously existing bone. It can be distinguished from Haversian bone by the fact that the various systems never encroach on each other. (5) Aoascufar bone. Avascular bone is occasionally seen in parts of reptile bone, and in the bone of small animals such as bats, moles and small passerine birds (Enlow and Brown, 1957, 1958). These types do to some extent grade into each other. For instance, ~rallel-fibred bone may have such fine fibres, so neatly and uniformly arranged, that it looks very like lamellar bone, though it usually can be distinguished from it by the fact that it does not have the characteristic alteration of the predominant direction of the fibres from one lamella to the next. It is possible, nevertheless, with practice. to describe a section of corticaf bone using the terminology above.
Characteristics
offossil
and present-day
bone
Space does not allow a full description of the characteristics of the histology of the bones that have been tested here, and of the bone of extinct animals. It can be said at once however, that both groups are very varied, and there are few types of bone in fossils that have not been tested here. The most important exception is that, because I have not tested small specimens, I have not tested completely avascular bone, Biewener (1982) tested whole bones of various small mammals and birds (which probably had a rather simple histology, even if they were not avascular). He found that the calculated strength properties of the bone material were similar to those of much larger mammals. There are two features of the bone of more ancient, extinct, reptiles that do seem to be different from that of more modern forms. One is the tendency to be
Mechanical properties of amniote bone rather porous. In modern forms porous bone tends to be found in ribs. and also in antlers. However. examination of the photographs in de Riqlk’ works shows that in the more primitive reptiles there was a greater tendency for quite porous bone to be present in the shafts of long bones. The other feature is that the reptiles had a greater tendency to have a great deal of cancellous bone in the lumen of the long bones. Present-day birds and mammals usually have a lumen that is clear of bony tissue. However. this latter feature is not examined here. because I tested only compact bone specimens. not cancellous bone or whole bones. There is a remarkable similarity, which must have evolved separately at least four times. between the bone of the extinct aquatic reptile groups (Mososaurs, Plesiosaurs and Ichthyosaurs. Fig. I) and the whales. In all these groups. both reptile and mammal, the cortical bone tends to consist of primary bone which originally had large and numerous longitudinal vascular canals running through it. After the bone had been laid down, these largecanals filled in with lamellar bone, giving the appearence of Haversian systems, the ‘Protohaversian systems’ of Enlow and Brown. This presumably has some relationship with the way of life of these various animals. and shows how the bones of all amniotes seem to adopt the same structure according to the biology of the animal. rather than its taxonomic position. Dinosaur bone is remarkably like the bone of modern mammals (Reid, 1983). The compact bone is usually fibrolamellar, with the same pattern as seen in present-day large mammals (Currey, 1962) and this is often invaded by Haversian systems, often very densely. In both these features this bone resembles that of large herbivorous mammals. Occasionally there is lamellar-zonal bone in which fine fibred primary lamellae. containing rows of blood vessels, is periodically interrupted by resting lines. Lamellar-zonal bone seems not to be found in eutherian mammals. However. several of the wallaby specimens had bone looking very like lamellar-zonal bone. These specimens were as strong and stilfas the eutherian mammal bone. There is no evidence. from the tests reported here. therefore, that the dinosaurs’ bone would have had mechanical properties different from those of extant mammals. Similarly the bones of the advanced mammal-like reptiles (the therapsids) are usually very similar to those of modern mammals. For instance, de Riqles ( 1979) writes ‘The large ‘herbivorous’ forms. such as the Permotriassic Anomodonts. the artiodactyls and Proboscideans of the tertiary and present day. convergence with the etc.. show a histological sauropod dinosaurs that is sometimes extremely precise .*(my translation). As in present-day mammals, Haversian systems were better developed in the more carnivorous mammal-like reptiles. The ancestors of the mammals passed through a stage when they were quite small, at the very most rabbit-sized (Kemp. 1982). Mammals of this size do not possess the uniform fibrolamellar structure seen in
1043
larger mammals and. usually. undergo rather little Haversian remodelling. This serves to emphasize the fact that particular histological types of bone have evolved independently several times: well-organized fibrolameltar bone. for instance. having evolved at least once in the dinosaur line, and at least twice in the line leading to the mammals. Fibrolamellar bone seems to arise in amniotes vvhenever land animals become large and. probably. herbivorous.
DlSCl SSIO\ Each of the mechanical properties determined here shows a very great range of values. and this is still true even when the values for the tympanic bulla, often the most extreme. are removed. Certain types of bone are characteristically high or low in some values. For instance, the deer antlers tend to have a low Young’s modulus, fairly high values for work of fracture. and high ultimate strains in tension. On the other hand the deer tibia has a high value for Young’s modulus and for bending strength and an intermediate value for work of fracture. Slender limb bones (flamingo, crane, wallaby tibia. fallow deer tibia. cow tibia) have high modulus and high bending strength. The crocodile skull bones have low values for Young’s modulus and for bending strength. but the crocodile prefrontal bone has a very high value for work of fracture. The Galapagos tortoise bones tend to have high values for all the measures concerned with toughness: work of fracture. strain at break, work under the curve. The whale bullae have high stiffness. but are low in all other mechanical properties. There is no indication in the present-day bones tested, that particular animals have consistently low values for all properties tested. It seems much more to be true that bones performing different functions have particular properties. One characteristic related to taxonomy rather than to any obvious function. is that the bones of the living reptiles are rather lower in calcium than most of the mammal and bird bones. except the antlers. It is unfortunately not possible to infer calcium content directly from the histology of the specimens. so we cannot tell the amount of mineralization of extinct groups. However. it does seem unlikely that the extremely well-organized fibro-lamellar bone of the therapsids and dinosaurs was not as well-mineralized as the histologically similar bone of present-day mammals. The picture that emerges from these considerations and observations is this. The very early reptiles had rather porous bone. and it was probably not very highly mineralised. As a result it would have had a rather low Young’s modulus and bending strength. Quite probably. however, it would have been. like the Galapagos tortoise bone. adequately tough. Thereafter, in several different lines, the kinds of histology and mechanical properties characteristic of present-
J. D. CURREY
104-t
day animals arose. and what is more arose repeatedly. and a very long time ago. according to the size of the animal (large animals tend to have fibro-lamellar bone). the habitat of the animal (marine deep-diving animals tend to have rather porous bone with protohavers~an systemsjand the functions of the bones (slender bones are stiff and strong in bending. but not necessarily tough). One strong conclusion to emerge is that in no sense can the compact bone of mammals be taken to be the summit of bone evolution. In the admittedly limited data set examined here the bending strength of some birds’ bones is as strong as that of the strongest mammal, the greatest work of fracture is found in a tortoise, and although mammals’ antlers have the highest tensile strengths and work under the curve, the ordinary long bone of a bird is stronger than the long bone of a mammal, and reptile’s long bone has a greater work under thecurve than mammal long bone. The mammals do have two types of mechanically specialised bone: bulla and antler. The fossil record is not good enough to be sure whether they are unique in this. Acknowfed~etnmrs-1 acknowledge the help. in providing specimens, of the British Antarctic Survey. the British Museum (Natural History), Hvalur H. F. (Iceland), Flamingo Park Zoo, the Zoological Society of London. Dr R. Reid and particularly. for her untiring and generous efforts, Dr Caroline Pond. This work was supported by grant no. GR,DO4076, from the Science and Engineering Research Council.
REFERENCES Biewener, A. A. ( 1982) Bone strength in small mammals and bipedal birds: do safety factors change with body size:’ J. esp. Bid/. 98, 289-301. Burstein, A. H.. Currey. J. D., Frankel, V. H. and Reilly. D. T. ( 1972) The ultimate properties of bone tissue: the effects of yielding. J. &mechanics 5, 35-44.
Currey. J. D. ( 1959) Differences in the tensile strength of bone of different histological types. Q, J. microsc. Sci. 103. 111-133. J. D. (1962) The histology of the bone of a prosauropod dinosaur. Palaeontofogr 5, X8-246. Currey. J. D. (19871 The effect of porosity and mineral
Currey.
content on the Young’s modulus of elasticity of compact bone. J. Biomechanics. Enlow, D. H. and Brown. S. 0. (1956) A comparative histological study of fossil and recent bone tissues. Part 1. Texczs J. Sci. 8, 405443. Enlow. D. H. and Brown, S. 0. (1957) A comparative histological study of fossil and recent bone tissues. Part 2. Texas J. Sci. 9, 186-214. Enlow, D. H. and Brown, S. 0. (1958) A comparative histological study of fossil and recent bone tissues. Part 3. Texas J. Sci. IO. 187-230. Evans, F. G. and Goff, C. W. ( 1957) A comparative study of the primate femur by means of the stresscoat and split-line techniques. Am. J. phys. Anthropol. 15, 59-89. Evans. F. G. and Krahl, V. E. ( 1945) Torsion of the humerus; a phylogenetic survey from fish to man. Am. J. Attar. 3, 229-253. Kemp, T. S. (1982) Mammal-like Repriles and the Origin of Mammals. Academic Press, New York. Martin, R. B. and Burr, D. B. (1982) A hypothetical mechanism for the stimulation of osteonal remodeling by fatigue damage. j. Bjo~c~unics 15, 137-139. Mueller, W. C., Ldsker, G. W. and Et-ans. F. G. (1981) Anthropometric measurements and Darwinian fitness. J. biosac. Sri. 13, 309-3 16. Reid. R. E. H. ( 1984) The histology of dinosaurian bone, and its possible bearing on dinosaurian physiology. The und Evolution of Reptiles; Sfructure. Derekopmehr Synposilrm of the Zoological Sacret? of Londan, Vol. 52 (Edited by Ferguson, M. W. J.). pp. 629-663. Reilly, D. 7. and Burstein. A. Ii. (1975) The elastic and ultimate properties of compact bone tissue. J. Biomechanics 8. 393-405. Riqles, A. J. de (1975) Recherches paleohistologiques sur les OS longs des tttrapodes. VII ( I ) Ann/sPukont.61, 51-129. Riqles. A. J. de (1979) Quelques remarques sur I‘histoire evolutive des tissus squelettiques chez les vertebres et plus particul~~rement chez les tetrapodes. Annls biol. 17, I-35. Saha. S. and Hayes, W. C. ( 1976) Tensile impact properties of human compact bone. J. Biomechanics 9. 243-251. Tattersall. H. Cl. and Tappin. G. (1966) The work of fracture and its measurement in metals. ceramics and other materials, J. Mrtr. Sci. I, 296-301.