THE ANATOMICAL RECORD 293:2044–2055 (2010)
Calcified Cartilage Shape in Archosaur
Long Bones Reflects Overlying Joint Shape
in Stress-Bearing Elements: Implications
for Nonavian Dinosaur Locomotion
MATTHEW F. BONNAN,1* JENNIFER L. SANDRIK,1 TAKAHIKO NISHIWAKI,1
D. RAY WILHITE,2 RUTH M. ELSEY,3 AND CHRISTOPHER VITTORE4
1
Functional Morphology and Evolutionary Anatomy (FMEA) Working Group,
Department of Biological Sciences, Western Illinois University, Macomb, Illinois
2
School of Veterinary Medicine, Auburn University, Auburn, Alabama
3
Louisiana Department of Wildlife and Fisheries, Rockefeller Wildlife Refuge,
5476 Grand Chenier Hwy, Grand Chenier, Louisiana
4
Rockford Memorial Hospital, 2400 Rockton Avenue, Rockford, Illinois
ABSTRACT
In nonavian dinosaur long bones, the once-living chondroepiphysis (joint
surface) overlay a now-fossilized calcified cartilage zone. Although the shape
of this zone is used to infer nonavian dinosaur locomotion, it remains unclear
how much it reflects chondroepiphysis shape. We tested the hypothesis that
calcified cartilage shape reflects the overlying chondroepiphysis in extant
archosaurs. Long bones with intact epiphyses from American alligators (Alligator mississippiensis), helmeted guinea fowl (Numida meleagris), and juvenile ostriches (Struthio camelus) were measured and digitized for geometric
morphometric (GM) analyses before and after chondroepiphysis removal. Removal of the chondroepiphysis resulted in significant element truncation in all
examined taxa, but the amount of truncation decreased with increasing size.
GM analyses revealed that Alligator show significant differences between
chondroepiphysis shape and the calcified cartilage zone in the humerus, but
display nonsignificant differences in femora of large individuals. In Numida,
GM analysis shows significant shape differences in juvenile humeri, but
humeri of adults and the femora of all guinea fowl show no significant shape
difference. The juvenile Struthio sample showed significant differences in both
long bones, which diminish with increasing size, a pattern confirmed with
magnetic resonance imaging scans in an adult. Our data suggest that differences in extant archosaur long bone shape are greater in elements not utilized in
locomotion and related stress-inducing activities. Based on our data, we propose tentative ranges of error for nonavian dinosaur long bone dimensional
measurements. We also predict that calcified cartilage shape in adult, stressbearing nonavian dinosaur long bones grossly reflects chondroepiphysis shape.
C 2010 Wiley-Liss, Inc.
Anat Rec, 293:2044–2055, 2010. V
Key words: cartilage; morphometrics; alligator; bird; dinosaur;
locomotion
Grant sponsor: Western Illinois University URC (University
Research Council); Grant number: 3-30185; Grant sponsor:
College of Arts and Sciences Graduate Student grant.
*Correspondence to: Matthew F. Bonnan, Department of Biological Sciences, Western Illinois University, Macomb, IL 61455.
E-mail: [email protected]
C 2010 WILEY-LISS, INC.
V
Received 3 March 2010; Accepted 4 August 2010
DOI 10.1002/ar.21266
Published online 2 November 2010 in Wiley Online Library
(wileyonlinelibrary.com).
JOINT SHAPE IN ARCHOSAURS
Given their large average body size (>1 metric ton)
(e.g., Farlow et al., 1995), many studies of nonavian
dinosaurs have focused on locomotion and weight-support (e.g., Christiansen, 1997, 1999a,b; Carrano, 2001;
Hutchinson and Garcia, 2002; Gatesy et al., 2009).
Unfortunately, the epiphyseal joint cartilage (chondroepiphysis) that capped the ends of dinosaur long bones is
lost during fossilization (Chinsamy-Turan, 2005). These
missing data are problematic given that all models concerning dinosaur locomotion are ultimately derived from
inferences of limb bone articulations. How much size
and shape data are lost, and how these factors influence
dinosaur locomotor models, remains poorly understood.
Nonavian dinosaur long bones grew like those of their
closest living archosaur relatives, crocodylians and birds,
wherein the epiphyses remained cartilaginous throughout life, forming no secondary ossification center as in
mammals and lizards (Haines, 1969; Carter and Beaupré, 2001; Horner et al., 2001; Chinsamy-Turan, 2005).
As archosaur long bones grow in length, the chondroepiphysis grows by addition of new chondrocytes whereas
older cells accumulate and calcify, forming a calcified
cartilage zone (Chinsamy-Turan, 2005). It is this calcified cartilage atop the bone shaft that is utilized in lieu
of the chondroepiphysis to infer locomotion and range of
motion in nonavian dinosaurs (Chinsamy-Turan, 2005).
Previous preliminary data (Holliday et al., 2001)
showed that chondroepiphysis removal from alligator
and bird long bones significantly altered their linear
dimensions: original length was truncated (range,
6%–10%) and marked shape changes occurred (Holliday
et al., 2001). More recently, a well-preserved humerus
from the sauropod dinosaur Cetiosauriscus shows thick
and extensive fossilized epiphyseal cartilage on its distal
end (Schwartz et al., 2007). However, although long
bone size may change dramatically after chondroepiphysis removal, it is not clear whether the same trend holds
quantitatively for calcified cartilage shape: whereas long
bone dimensions will truncate after chondroepiphysis removal, calcified cartilage shape may remain unaffected.
For nonavian dinosaurs, the preserved calcified cartilage
might still retain significant information on their articular surfaces to act as a reliable proxy for joint shape.
Here, we test the null hypothesis that calcified cartilage shape in extant archosaur long bones will not differ
significantly from the overlying chondroepiphysis. An
ontogenetic series of long bones with intact epiphyses
from American alligators, helmeted guinea fowl, and a
sample of hatchling and juvenile ostriches were measured and digitized for geometric morphometric (GM)
shape analyses before and after chondroepiphysis removal. Magnetic resonance imaging (MRI) scans of an
adult ostrich were used to constrain patterns observed
in the hatchlings and juveniles.
MATERIALS AND METHODS
Specimens
To test our hypothesis, we selected taxa that comprise
the extant phylogenetic bracket (EPB) of nonavian dinosaurs, Crocodylia and Aves (Brochu, 2001; Hutchinson,
2006). For Crocodylia, we selected Alligator mississippiensis due to its availability, its nonendangered status,
and the fact that the anatomy of this crocodylian is generalized (Meers, 2003). Thirty-six wild American alliga-
2045
tor (Alligator mississippiensis) specimens (femur length:
range, 37–160 mm) were collected and euthanized by
Louisiana Department of Wildlife and Fisheries biology
staff under general scientific collection permits on the
state-owned Rockefeller Wildlife Refuge (RWR; Grand
Chenier, Louisiana) as part of an annual harvest. Specimens used in this study were salvage from other
studies.
Given that Alligator mississippiensis grows indeterminately, identification of adult and juvenile specimens
is exceptionally difficult. We addressed this difficulty by
dividing the Alligator sample into a ‘‘small’’ and ‘‘large’’
group using the median length of the femur. Given our
sample size (n ¼ 36; range ¼ 37–160 mm), the median
was calculated to be 86 mm: those specimens under the
median were categorized as small, and those above were
categorized as large. Although not ideal, this avoided
problems associated with determining whether an individual had become sexually mature (data not readily
available to us from these salvaged specimens) at a
smaller or larger size. We are also following a precedent
set in other studies (e.g., Bonnan et al., 2008) where,
because of similar constraints, specimens were grouped
by size rather than sexual maturity.
For Aves representatives, we selected one volant species (Numida meleagris) and one nonvolant species
(Struthio camelus). Numida were selected because they
are readily available and spend time both walking on
the ground and flying, and because they utilize their
forelimbs in vigorous activity. Twenty-nine deceased
domestic, free-range guinea fowl (Numida meleagris)
specimens from hatchling to adult size were donated as
salvage from Tim Piper of Macomb, Illinois. For our nonvolant representative, we selected Struthio camelus
because it bears significant weight on its hindlimbs, and
its forelimbs are used for display only. We purchased 25
dead hatchling to juvenile ostriches and 1 adult (Struthio camelus) as salvage from Freedom Sausage Ostrich
Farm in Earlville, Illinois. All specimens were curated
at Western Illinois University under a United States
Federal Salvage Permit to MFB.
Ideally, a large and varied set of archosaurs would be
examined and compared with one another, but a number
of difficulties prevented such an approach. First, among
crocodylians, we had access to a large number of salvaged Alligator mississippiensis specimens, and this
made this taxon the most viable option for obtaining a
reasonably sized sample of specimens for statistical analysis. Second, certain domestic birds, such as Gallus
gallus or Meleagris gallopavo, have been bred for human
consumption and many such birds spend their lives relatively immobilized. Moreover, although available commercially in great numbers, the aforementioned species
are usually culled before maturity, and therefore complete ontogenetic series are difficult to come by. Third, a
sample of ostrich chicks and juveniles, rather than a
complete ontogenetic series, was the most practical
approach for obtaining data from a free range terrestrial
bird and provided us with a statistically meaningful
sample that could be compared with those of Numida
and Alligator. Although a sample which included adult
and subadult Struthio would certainly have been ideal,
obtaining a large enough sample size, and controlling
the maceration of the bones (see below) would have been
impractical. However, recognizing that a sample of
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BONNAN ET AL.
Fig. 1. Measurement and qualitative analysis of long bone shape
before and after chondroepiphysis removal in selected archosaurs. Measurement scheme adapted from Bonnan (2004, 2007) and Bonnan et al.
(2008). Not illustrated: craniocaudal measurement of greater trochanter
width on bird femora. Laser scans of humeri (cranial view) and femora
(caudal view) from Alligator, Numida, and Struthio (juvenile only) show
shape differences before and after chondroepiphysis removal. Numbered
points indicate digitized landmarks (detailed in Table 1).
hatchling and juvenile Struthio specimens would provide
results skewed toward smaller individuals, we utilized
MRI scans of an adult ostrich to confirm the trends suggested by our Struthio sample for larger individuals.
saurs (Farlow et al., 2005; Bonnan et al., 2008) and are
used here as a proxy for body size.
The shoulder, elbow, hip, and knee joints of each specimen were dissected, measured (Fig. 1), and photographed before and after chondroepiphysis removal.
Photographs were taken of each limb element before and
after chondroepiphysis removal for GM shape analysis.
Given that hyaline cartilage is a water-rich tissue, exposure to air will result in its dehydration, shrinkage, and
thus artificial distortion of dimensional and shape data.
To avoid data distortion, we developed a method that
allowed for controlled maceration of muscles and hydration of the chondroepiphysis.
First, frozen limbs were thawed and dissected at the
joints, and dimensional measures were made with digital calipers as the cartilage was exposed. We note that
freezing of epiphyseal cartilage does not result in significant cell destruction and is in fact less damaging to cartilaginous tissues than chemical fixation (e.g., Hunziker
Measurements and Data Collection
We chose to focus on shape changes in the chondroepiphysis of the humerus and femur because these elements are usually the most common, least distorted, and
most often compared bones in dinosaur studies. These
two limb elements provide much of the mechanical support to the forelimbs and hindlimbs, and many of the
largest muscles, which control locomotor movements of
the entire limb, insert or originate on the humerus or femur (Bonnan, 2007). Moreover, humerus and femur
length are highly correlated with body size in mammals
(e.g., Christiansen, 1999a), dinosaurs, and other archo-
2047
JOINT SHAPE IN ARCHOSAURS
TABLE 1. Landmark Points Digitized on Each Element
Element
Humerus
Femur
Number Alligator
Numida
Struthio
Medial extent of humeral head
Lateral extent of humeral head
Lateral constriction of midshaft
Lateral epicondyle
Intersection of medial
and lateral condyles
Medial epicondyle
1
2
3
4
5
Medial extent of humeral head
Lateral extent of humeral head
Deltopectoral crest
Lateral constriction of midshaft
Lateral epicondyle
Medial extent of humeral head
Lateral extent of humeral head
Deltopectoral crest
Lateral constriction of midshaft
Lateral epicondyle
6
Intersection of medial and
lateral condyles
Medial Epicondyle
Medial constriction of midshaft
Medial extent of femoral head
Lateral extent of femoral head
Fourth trochanter
Lateral constriction of midshaft
Lateral epicondyle
Intersection of medial and lateral
epicondyles
Medial epicondyle
Medial constriction of midshaft
Intersection of medial and
lateral condyles
Medial Epicondyle
Medial constriction of midshaft
Femoral head
Greater trochanter
Lateral extent of proximal end
Lateral constriction of midshaft
Lateral epicondyle
Intersection of medial and lateral
epicondyles
Medial epicondyle
Medial constriction of midshaft
7
8
1
2
3
4
5
6
7
8
Medial Constriction of Midshaft
N/A
Femoral head
Greater trochanter
Lateral extent of proximal end
Lateral constriction of midshaft
Lateral epicondyle
Intersection of medial and
lateral epicondyles
Medial epicondyle
Medial constriction of midshaft
See Fig. 1 for illustrations of these landmark points.
et al., 1984). Although freezing may compromise the mechanical abilities of epiphyseal cartilage (e.g., Kennedy
et al., 2007), we are not aware of any published studies
showing that chondroepiphysis shape is adversely
affected. Following our measurements, large muscles
and other obstructing tissues were removed, and the
partially defleshed limbs were submerged in water
heated to 60 C, a temperature at which cartilage shape
has been shown to remain unaltered (Wright et al.,
2005). This water bath allowed for soft tissue removal
and exposure of the chondroepiphysis without damaging
its shape. To ensure that our method did not cause a significant effect, we remeasured the bone dimensions after
24 hr of immersion. After photographing the limb elements in a standardized orientation, water temperature
was increased to 100 C to remove the chondroepiphysis.
Finally, an additional series of post chondroepiphysis removal measurements and photographs were taken.
C desktop laser scanner was used to
A NextEngineV
scan the surfaces of one set of juvenile (small) and adult
(large) Alligator and Numida elements, and one set of
juvenile Struthio elements, before and after chondroepiphysis removal. This allowed us to generate threedimensional models to compare with our statistical data.
Care was taken to moisten the epiphyseal cartilage
between scans with water to prevent dessication and distortion of shape, and we followed the maceration procedure described above.
MRI scans of an adult Struthio camelus were performed at the Rockford Memorial Hospital. We used a
1.5 Tesla (Signa; General Electric, Milwaukee, WI) scanner to obtain proton density-weighted images with frequency specific, fat-suppression (see Novelline, 2004).
This type of imaging sequence accentuates the signal intensity from hyaline cartilage. The hindlimb of the adult
ostrich specimen was scanned intact, but was dissected
from the body wall just before freezing and transport to
the MRI facility. Given that frozen tissues are nearly
opaque to MRI scans, the ostrich hindlimb was thawed
before its analysis. Although it is conceivable that our
dissection of the limb may have caused some distortion
of the cartilage tissues in the adult ostrich, we also
scanned dismembered specimens of a juvenile ostrich, a
juvenile and adult guinea fowl, and an adult alligator. In
these other specimens, MRI scans showed our predicted
match-up between the underlying calcified cartilage and
the chondroepiphysis. In other words, we saw nothing
different in MRI scans of these taxa that we did not already observe in our morphometric data and surface
laser scans.
Morphometric Analyses
SPSS software (v. 16) was used for statistical analysis
of all linear data and the MANOVA and principal components analysis (PCA) of the partial warps data generated
from GM analyses. All linear measurements of bone
dimensions were made with digital calipers, log10 transformed to normalize their distribution (Zar, 1999), and
tested for normality using the Kolmogorov–Smirnov test.
Repeated measures ANOVAs tested for significant linear
differences both in the method and after chondroepiphysis removal.
For GM analysis, we used two-dimensional thin-plate
splines (TPS) because this technique is ideal for analyzing a set of objects (limb bones) that are similar in overall morphology and where the detection of more subtle
shape differences is desired (Zelditch et al., 2004; Slice,
2005). In a TPS analysis, homologous landmark coordinates of all specimens are aligned, rotated, and scaled
into a grand mean reference form via generalized least
squares Procrustes superimposition (Zelditch et al.,
2004; Slice, 2005). Measuring the sum of squared Procrustes distances in the homologous landmark coordinates of each specimen against the reference form
reveals shape differences, which can be analyzed mathematically and visualized as a deformation grid or TPS
(Bookstein, 1991). Normalized shape coefficients generated from the sum of squared Procrustes distances (partial warps) are correlated, dependent variables that
collectively describe shape and are analyzed with
2048
BONNAN ET AL.
TABLE 2. Test of Normality and Percentage of
Dimensional Truncation after Chondroepiphysis
Removal in Alligator mississippiensis
Element
Measurement
Humerus Length
(n ¼ 35)
Proximal breadth
Deltopectoral crest
Distal breadth
Femur
Length
(n ¼ 36)
Proximal breadth
Fourth trochanter
Distal breadth
TABLE 3. Test of Normality and Percentage of
Dimensional Truncation after Chondroepiphysis
Removal in Numida meleagris
KS
% of remaining
normality after truncation
Test (P)
all/small/large
0.200
93/92/94
0.200
0.200
0.200
0.200
91/90/91
90/88/93
90/90/90
94/93/95
0.200
0.200
0.200
92/92/92
91/89/93
96/92/94
Komolgorov–Smirnov (KS) tests of normality are reported
for each measurement.
standard multivariate statistics (Zelditch et al., 2004;
Slice, 2005).
Limb bones were digitized and analyzed using the
TPS program suite developed by Rohlf (TPSUtil,
TPSDig2, TPSRelw; 2008). The landmarks selected for
digitization followed standard landmarks detailed elsewhere (Bonnan, 2004, 2007; Bonnan et al. 2008) (Fig. 1).
Changes in limb bone morphology associated with landmarks are indicated by numbers in parentheses in the
text. Sliding semilandmarks were also digitized to capture the outline of the articular surfaces (see Zelditch
et al., 2004).
As partial warp scores are dependent variables that
together describe shape, a MANOVA of these variables
was used to detect significant differences in limb bone
shape before and after chondroepiphysis removal. A PCA
of the partial warps calculated so-called relative warps
or components of maximum shape variation (Zelditch
et al., 2004; Slice, 2005). Deformation grids of the significant principal components of shape (PRINs) were generated by the tpsRelw program (Rohlf, 2008) and used to
visualize these shape changes. See Table 1 for landmark
descriptions.
A potential limitation of this study is the compression
of three-dimensional long bone shapes into two-dimensional coordinates. Although three-dimensional TPS
applications are available (Zelditch et al., 2004), the
time required to capture such coordinate data was difficult given that long-term exposure of the chondroepiphysis results in its dehydration. For this reason, rapid,
two-dimensional photography was preferred. Certainly,
we recognize the value in capturing three-dimensional
long bone shape data and methods for digitizing threedimensional shape data from wet bones are currently
being explored.
RESULTS
Linear Data
Linear data from all taxa except Numida showed normal distributions (Tables 2–4). For the Numida sample,
the non-normal signal (Komolgorov-Smirnov P < 0.05
for all measurements) is explained by the fact that the
smallest juveniles and full-grown adults cluster closely
together at the lower and upper ends of the size range,
Element
Measurement
Humerus Length
(n ¼ 29)
Proximal breadth
Deltopectoral crest
Distal breadth
Femur
Length
(n ¼ 29)
Proximal breadth
Greater trochanter
Distal breadth
KS
% of remaining
normality after truncation
test (P)
all/juveniles/
all/juveniles
adults
0.004/0.032
92/86/98
0.014/0.114
0.008/0.132
0.004/0.069
0.021/0.105
86/78/94
86/79/91
84/75/93
93/90/98
0.030/0.141
0.027/0.200
0.008/0.104
88/76/99
80/70/97
86/77/96
Komolgorov–Smirnov (KS) tests of normality are reported
for each measurement.
TABLE 4. Test of Normality and Percentage of
Dimensional Truncation after Chondroepiphysis
Removal in Struthio camelus (juveniles)
Element
Measurement
Humerus Length
(n ¼ 25)
Proximal Breadth
Deltopectoral Crest
Distal Breadth
Femur
Length
(n ¼ 25)
Proximal Breadth
Greater Trochanter
Distal Breadth
KS
% of remaining
normality after truncation
test (P) all/smallest/largest
0.200
85/84/86
0.200
0.073
0.200
0.200
58/52/55
77/75/79
53/51/55
86/85/88
0.053
0.200
0.200
80/76/83
80/77/81
72/69/75
Komolgorov–Smirnov (KS) tests of normality are reported
for each measurement.
respectively. This was established statistically. When
only Numida juveniles were tested, a normal distribution was reported for all variables except humerus
length (Kolmogorov-Smirnov P > 0.05; Table 3).
Repeated measures ANOVA tests show no significant
effect of the method on long bones of Alligator and adult
Numida (Table 5). Significant effects did occur with the
juvenile Numida and Struthio samples, but these are
due to swelling of the chondroepiphysis, an effect that
did not appear to alter our shape data (see below).
Removal of the chondroepiphysis causes significant
truncation of all humerus and femur dimensions in Alligator and juvenile birds (Table 5). For adult Numida,
significant truncation of all humerus dimensions occurs,
but only the greater trochanter width is significantly
shortened on the femur (Table 5). On average, Alligator
long bone dimensions are truncated from 4%–10% of the
original length (Table 2), those of Numida are truncated
from 7%–20% (Table 3), and Struthio juveniles show
nearly 50% truncation in some dimensions (Table 4).
Although these data agree generally with previous preliminary data (Holliday et al., 2001) on alligators and
birds, the averages mask an intriguing but intuitive trend:
in both Alligator and the bird taxa, larger individuals
show less truncation than smaller individuals (Fig. 2).
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JOINT SHAPE IN ARCHOSAURS
TABLE 5. Test of Maceration Method and Effect of Chondroepiphysis Removal Using Repeated Measures
ANOVA
Taxon
Alligator
Numida (All)
Maceration of humerus
Maceration of femur
None
All
None
Length, greater
trochanter width,
distal breadth
Distal Breadth
None
Numida (Juveniles)
Numida (Adults)
Distal breadth
None
Struthio (Juveniles)
Distal Breadth
Length, proximal
breadth, greater
trochanter width
Chondroepiphysis
removal: humerus
Chondroepiphysis
removal: femur
All
All
All
All
All
All
All
Greater trochanter
width
All
All
Measurements, which were significantly different (P < 0.05) after maceration and/or chondroepiphysis removal, are
denoted in bold. All indicates every measurement was significantly effected either by the maceration method or chondroepiphysis removal.
For Alligator the trend is subtle, with large individuals
showing about 1%–5% less truncation in their long
bones after chondroepiphysis removal than small specimens (Table 2 and Fig. 2). For Numida the trend is
stark: on average, adults show at least 8% less difference in truncation compared with juveniles, and in some
cases the difference between adult and juvenile dimensional loss is much greater (>10%; Table 3 and Fig. 2).
Struthio juveniles show greater differences between
smaller and larger individuals (range, 2%–7%) than Alligator and are missing the most significant amount of
dimensional data from the proximal and distal breadth of
the humerus at any size (Table 4). However, these major
losses in data are probably due to the immature stage of
these animals and do not reflect a trend that continues
into adulthood. Bivariate plots of the humerus and especially the femur of Struthio show a clear trend toward
diminishing epiphyseal cartilage thickness as size
increases (Fig. 2). Moreover, for the femur, Struthio specimens show the steepest, most positive slope of the three
examined taxa, indicating that even small changes in
body size have a significant, negative effect on cartilage
thickness. In fact, MRI scans of the Struthio adult show
that as in Numida adults only a thin layer of epiphyseal
cartilage is present on the humerus and femur (Fig. 4).
It should be noted that the truncation in length does
not appear to happen equally at the proximal and distal
ends of the long bones in these archosaur taxa. For example, truncation is greater for the distance measured
between the deltopectoral crest and humeral head than
for overall humerus length (Tables 2–4), a result explained
by a thicker proximal chondroepiphysis. If both proximal
and distal epiphyses were of equal thickness, little or no
difference in this measurement would be predicted.
GM Data
GM shape comparisons via TPS of the humerus and
femur before and after chondroepiphysis removal
revealed several patterns. For the humerus of Alligator
and both elements in Numida, it is the second principal
component (PRIN) 2, rather than PRIN 1 that describes
epiphyseal shape. Because shape PRINs record the maximum amount of variation, PRIN 1 often picks up individual variation. For both Alligator and Numida,
individual nuances in deltopectoral crest orientation in
the humerus accounted for the most shape variation in
the sample. This is not unprecedented, as a similar
trend appears in dinosaur taxa for the same landmark
(Bonnan, 2007). For the femur of Numida, differences in
shaft bending among individuals accounted for most of
the shape variation in PRIN 1. Examination of shape
changes associated with PRIN 2 in these taxa and for
these elements clearly showed that chondroepiphysis
shape was linked strongly with this component.
In Alligator, humerus shape changes significantly
across the sample as might be expected (Table 6), and deformation grids show a loss in condyle distinctness (landmarks 5–7 and their associated semilandmarks) and
humeral head curvature (landmarks 1–2 and their associated semilandmarks; Fig. 3). However, although small Alligator show significant femur shape differences (again
associated with reduced condyle prominence; landmarks
range 5–7 and their associated semilandmarks), large Alligator femora show none (Table 6). For Numida, juveniles
show significant humerus shape differences after chondroepiphysis removal (flattened condyles; landmarks range
5–7 and their associated semilandmarks), but surprisingly
no significant difference is reported for adults (Table 6).
Moreover, both juvenile and adult Numida individuals
show no significant difference in femur shape (Table 6). As
expected, the sample of hatchling and juvenile Struthio
show significant shape changes in all humerus and femur
landmarks (Table 6).
When the principal components (PRINs) of shape
change are plotted, a general size-associated trend is discernable for all taxa. For Alligator, humerus and femur
shape differences diminish before and after chondroepiphysis removal with increasing size (Fig. 3), a trend also
apparent in both bird taxa (Fig. 3). These data show that
with the exception of the significant result for the Alligator humerus, as size increases the shape of the underlying
calcified cartilage more closely approximates that of the
overlying chondroepiphysis. Although we did not have a
complete ontogenetic series of Struthio, MRI imaging of
an adult specimen confirm that this pattern of shape convergence continues into adulthood for this taxon (Fig. 4).
DISCUSSION
Summary of Results and Implications for
Extant Archosaurs
The results of our linear data support previous analyses in general (e.g., Holliday et al., 2001): size and linear
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BONNAN ET AL.
Fig. 2. Bivariate plots of element length against the percent of the
original element length after truncation for (A) the humerus and (B) the
femur in Alligator, Numida, and Struthio. In the graphs, Alligator are
represented by open circles, Numida by X symbols and Stuthio by þ
signs. Note that all taxa show trends toward less element truncation
with increasing specimen size. For both the humerus and femur, Alligator shows a gradual trend toward a decreasing amount of truncation. In contrast, the bird taxa show both a greater range of
dimensional loss and a more rapid trend toward decreasing truncation
with increasing size. In particular, note the steep slope for Struthio
juveniles with increasing femur size. All data were measured in millimeters and log10 transformed.
dimensions of the humerus and femur are significantly
truncated after chondroepiphysis removal. However, our
data also show a negative correlation between size and
chondroepiphysis thickness. Without exception, less
truncation occurs in humerus and femur dimensions for
larger individuals than in smaller specimens for Alligator, Numida, and Struthio. Our GM results show that a
surprising amount of shape information is retained in
the calcified cartilage, especially in larger individuals.
Overall, two general trends can be discerned from our
shape data. First, significant differences in shape
between the chondroepiphysis and underlying calcified
cartilage occur most often in juveniles, especially in the
humerus. Second, shape differences tend not to be significant in adults, especially in the femur.
We find it significant that shape differences before and
after chondroepiphysis removal diminish as specimen
size increases. Notably, the femur, which is weight-bearing and the main locomotor element in most archosaurs,
shows nonsignificant shape differences in large Alligator
and across all Numida. Even in juvenile Struthio femora, a clear trend is observed toward a closer approximation of calcified cartilage with the chondroepiphysis as
size increases. These shape data complement our
reported linear trend of diminishing relative chondroepiphysis thickness with increasing size: the calcified cartilage and chondroepiphysis become more intimately
associated in larger individual archosaurs in our sample.
For the humerus, only adult Numida show a nonsignificant difference between the calcified cartilage and chondroepiphysis shape, whereas the effects in Alligator and
juvenile birds are significant.
Our linear and shape data suggest collectively that
long bones experiencing the primary stresses associated
with locomotion or wing-beating will have thinner epiphyseal cartilage and a calcified cartilage shape, which
approximates that of the overlying chondroepiphysis.
Conversely, our data suggest that long bones, which are
not primarily involved in locomotion or flight, will have
thicker epiphyseal cartilage and thus show greater differences in shape between the calcified cartilage and
chondroepiphysis. Overall, our linear and shape data
agree with general trends reported for long bones: epiphyseal cartilage is less thick and more closely associated with calcified cartilage in regions of greatest stress
(Frost, 1990a,b,c,d; Carter and Beaupré, 2001).
An apparent exception to the trend we report occurs
in Alligator humeri. We note that the humerus, which is
used extensively for locomotion, shows significant shape
differences in even the largest specimens. This result is
perhaps explained by a combination of tail drag and
semiaquatic habits in these archosaurs. In crocodylians,
drag force from the heavy tail exerts a significant pull
during locomotion, and the hindlimb experiences greater
stress than the forelimb as it overcomes locomotor and
tail drag forces (Willey et al., 2004). Essentially, the center of mass in these archosaurs is shifted toward the
hindlimbs. It is also significant that older crocodylians
spend more time in water where their hindlimbs and
tail play a greater role in locomotion than their forelimbs (Ross and Garnett, 1989). These ecological factors
could help explain why the humerus, although loaded
and stressed during locomotion, still shows a significant
difference in shape between the calcified cartilage and
chondroepiphysis into the largest Alligator specimens in
our sample.
We also acknowledge that avian long bones grow rapidly and cease growth when adulthood is reached, in
contrast to the slow, indeterminate growth of Alligator
(Horner et al., 2001; Chinsamy-Turan, 2005). These different growth patterns could contribute to the signals
we see in our data. For example, cessation of chondroepiphyseal growth in adult Numida will result in more
ossification and calcification underneath this region, a
pattern somewhat reminiscent of determinate long bone
growth reported for mammals (Carter and Beaupré,
2001). However, a nonsignificant shape difference was
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JOINT SHAPE IN ARCHOSAURS
TABLE 6. PCA and MANOVA of Partial Warp Scores to Test for a Significant Difference in Shape before
and after Chondroepiphysis Removal in Selected Archosaur Taxa
Taxon
Element
Alligator
Humerus
Femur
Humerus
Femur
Humerus
Femur
Numida
Struthio
Shape PRIN (%)
PRIN
PRIN
PRIN
PRIN
PRIN
PRIN
2
1
2
2
1
1
(25%)
(37%)
(20%)
(31%)
(50%)
(47%)
F-Statistic
P (Juveniles)
P (Adults)
15.978
4.440
1.604
2.587
20.541
34.829
0.019
0.050
0.026
0.550
<0.0001
<0.0001
0.004
0.217
0.673
0.806
–
–
The principal component (PRIN) that best accounts for shape change at the epiphysis is indicated, along with the percentage of total shape variation it accounts for in the sample (%). After chondroepiphysis removal, humerus shape does not
change significantly in adult Numida, and femur shape does not change significantly in adult Alligator and across Numida.
Nonsignificant differences are bolded. For Alligator, Juvenile should be read as small, Adult as large.
also reported for large Alligator and juvenile Numida femora samples that should both show significant differences
if growth cessation were the primary factor contributing
to our results. Moreover, the chondroepiphyses of juvenile
birds experienced some swelling (Nishiwaki, unpublished
data) due to our method effect, yet no significant difference in femur shape was reported for juvenile Numida.
Therefore, although growth patterns must contribute to
the signals in our data, they cannot negate the effect of
stress on calcified cartilage shape. Despite differences in
growth rate and its cessation, our data suggest that stress
is a significant factor in determining how closely the
underlying calcified cartilage resembles the overlying
chondroepiphysis shape in archosaur long bones.
Implications for Nonavian Dinosaur
Locomotion
Our linear data on extant archosaur long bones show
that the dimensions of the humerus and femur are truncated significantly after chondroepiphysis removal. In
adult Numida and large Alligator specimens, long bone
elements are truncated by at most 9% of their original
dimensions, whereas juveniles of these taxa and the juvenile sample of Struthio show a substantial loss of
dimensional data. Our data, although limited in certain
aspects, provide a preliminary range of long bone truncation parameters for nonavian dinosaurs. Given our
results, we propose a simple dichotomy consisting of conservative (best-case) and liberal (worst-case) ranges of
constraints (error) associated with the measurement of
nonavian dinosaur humeri and femora.
For both the humerus and the femur, length is the
least truncated measurement, whereas measures of distal breadth or the distance to the anatomical landmarks
(deltopectoral crest, fourth trochanter) are the most
truncated. In all cases, humeral dimensions are more
truncated than those of the femur. Under conservative
constraints, the femur shows at minimum a 3.5% truncation in length and at most a 5% truncation in distal
breadth (Table 7). For the humerus, a 4% truncation in
length would occur, with up to an 8.5% truncation in distal breadth (Table 7). If applicable to nonavian dinosaurs, these data would suggest that measures of overall
length are the most reliable, with a loss of 3.5%–4% of
the original bone length, whereas measures related to
distal breadth are missing almost 10% of their original
dimensions. The picture worsens significantly if nonavian dinosaur bones lost as much dimensional data as
occurs in juvenile or small Alligator, Numida, and Stru-
thio, our liberal range of constraints. Here, humerus and
femur length fare the best but still show a loss of 10%–
13% of their original size (Table 7). All other dimensions
show losses ranging from just over 17% to nearly 30% of
their original measures (Table 7)! Clearly, this would
represent a substantial and potentially detrimental loss
of dimensional data, and it suggests that we are probably missing critical information in small and juvenile
nonavian dinosaurs.
Given our linear data, observations of shape loss in
laser and MRI scans, and our preliminary ‘‘range of
error’’ calculations, we suggest that the linear dimensions of repeatedly stressed humeri and femora of larger
and/or adult nonavian dinosaurs should provide reasonable approximations for calculating aspects of interest to
paleobiologists such as intermembral ratios and estimated mechanical advantage via muscle insertion landmark locations. We reach this conclusion because in all
cases in our sample, large and adult specimens without
exception showed a thin chondroepiphysis closely associated with the underlying calcified cartilage. Therefore,
our conservative ranges of error should be good approximations for truncation in the linear dimensions of large
and full-grown nonavian dinosaurs. However, caution is
warranted for calculating dimension-based metrics from
juvenile long bones or those of adults that were not significantly stressed (e.g., obligate or habitual bipedal dinosaur forelimbs). For the moment, we propose the
following hypothesis: truncation of nonavian dinosaur
long bones dimensions will be substantial in juveniles or
in elements which are not exposed to repeated-stress
activities, but dimensional loss will be reduced in adult
elements and in those involved in locomotion or other
stress-inducing activities of similar magnitude.
Our GM data on extant archosaur long bones shows
that for adult, stress-bearing elements: (1) the chondroepiphysis is thinner and more closely associated with the
underlying calcified cartilage; and (2) the general shape
of the chondroepiphysis is mirrored in the calcified cartilage. In particular, condyles and other articular features
of the chondroepiphysis are retained as distinct, unambiguous features in the underlying calcified cartilage. In
juveniles and in elements of adults that are not stressbearing: (1) the chondroepiphysis is thicker and less
closely associated with the underlying calcified cartilage;
and (2) much of the chondroepiphysis shape is not
retained in the calcified cartilage. In fact, the remaining
calcified cartilage is often flattened and any hint of condyles or other articular features are muted or ambiguous
at best.
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BONNAN ET AL.
Fig. 3. Bivariate plots of principal shape components (PRINs) of partial
warps against log10-transformed humerus and femur length. Filled circles
represent long bone shape with intact chondroepiphysis; open circles represent long bone shape after chondroepiphysis removal. Deformation grids
depicting landmark changes are inset next to each graph: specimen shape
approaches the depicted shape the closer it plots toward one axis of the
PRIN (positive or negative). Data (humerus, femur) are shown for Alligator
(A and B), Numida (C and D), and juvenile Struthio (E and F). Decreasing
distance between filled and open circles indicate fewer shape differences
between the chondroepiphysis and calcified cartilage.
Given these patterns of dimensional loss and shape
change in both branches of the EPB for nonavian dinosaurs, we hypothesize that fossilized calcified cartilage
shape in nonavian dinosaurs will be well-developed in
elements directly involved in weight-support, locomotion,
and other repeated-stress activities in adults or sexually
mature individuals. Our shape hypothesis predicts, in
particular, that distinct, unambiguous condyles and
2053
JOINT SHAPE IN ARCHOSAURS
TABLE 7. Preliminary Estimated Ranges of Truncation for Various Dimensions of the Humerus and
Femur in Nonavian Dinosaurs Based on Extant Archosaur Data
Element
Dimension
Truncation
(adult/significant
locomotor stress)a (%)
Humerus
Length
Proximal breadth
Deltopectoral crest length
Distal breadth
Length
Proximal breadth
Distance: fourth trochanter
Distal breadth
4
7.5
8
8.5
3.5
4.5
5
5
Femur
Truncation
(juvenile/nonlocomotor,
nonsignificant stress)b (%)
12.33
24.67
18.67
27.33
10.33
17.33
20.33
19.67
The most conservative loss (best-case scenario) would involve adult elements that experience significant locomotor (or
related) stresses. A much more precipitous loss in linear dimensions (worst-case scenario) would occur in juvenile elements
or those that remain relatively unstressed throughout development into adulthood. Truncation in adults was calculated
from the average loss reported for large Alligator and adult Numida specimens only. Truncation in juveniles/nonstressed
elements was calculated from the average loss reported for small Alligator and juvenile bird taxa.
a
Best-case scenario.
b
Worst-case scenario.
Fig. 4. Magnetic Resonance Imaging (MRI) of adult Struthio camelus
forelimb and hindlimb: A, Proximal end of humerus (lateral view); B,
elbow (medial view); C, proximal end of femur (caudal view); D, knee joint
(cranial view). In (D), the knee is maximally flexed and a slice through the
distal condyles of the femur is shown in articulation with the more vertical
tibia and fibula. Interpretive line drawings are included next to each MRI
scan to facilitate distinguishing epiphyseal cartilage from the underlying
calcified cartilage and bone. Note that the MRIs of this Struthio adult
specimen shows in all cases a close approximation of chondroepiphysis
shape to calcified cartilage as for adult Alligator and Numida. Abbreviations: cal, calcified cartilage; ep, epiphyseal cartilage; f, femur; fh, femoral
head; fib, fibula; gtr, ‘‘greater trochanter’’ of femur; h, humerus; hh, humeral head; latc, lateral condyle of femur; lcl, lateral collateral ligament;
medc, medial condyle of femur; ole, olecranon process of ulna; t, tibia.
other articular features will be present in the calcified
cartilage of adult, nonavian dinosaur long bones, which
were directly involved in locomotion. Such well-defined
features would suggest that the unpreserved epiphyseal
cartilage was relatively thin and closely associated with
the calcified cartilage in these elements. In contrast, our
shape hypothesis also predicts that those elements that
were not involved in stress-bearing activities will
2054
BONNAN ET AL.
typically show calcified cartilage surfaces that are flattened or ambiguous, with no distinct condyles or other
articular features. Here, such observations would suggest the chondroepiphysis was more extensive and less
intimate with the underlying calcified cartilage.
Our findings for extant archosaurs have provided a
foundation for a future investigation and test of our linear and shape hypotheses on nonavian dinosaurs. Such
analyses could best be carried out across taxa for which
an appropriate ontogenetic or size spectrum is available.
In such samples, we predict that GM analyses of long
bone morphology regressed against linear dimensions
will show significant shape allometry in elements utilized for locomotion or related repeated-stress activities.
Such quantitative analyses of selected nonavian dinosaurs are currently in preparation.
SUMMARY AND CONCLUSIONS
Overall, our data from Alligator and two bird species
provide preliminary but significant information on size
truncation and shape retention in the humerus and femur. For our linear data, we find that length is the most
reliable measurement in stress-bearing elements, but
that other measurements have greater margins of error.
Shape data suggest that in stress-bearing elements in
large or adult archosaurs, the calcified cartilage does not
differ significantly in shape from the overlying chondroepiphysis. Further studies on other crocodylians, birds,
and turtles [which have similar long bone growth strategies to archosaurs (Haines, 1969)] should refine our
preliminary analysis and may clarify the patterns we
show here.
Our results, if applicable to nonavian dinosaurs, suggest that long bone size and dimensions are truncated
significantly in these fossil archosaurs. We suggest that
caution be used in drawing inferences from linear morphometric studies of nonavian dinosaurs, and we plan to
undertake future studies and reanalysis of previously
published linear data on nonavian dinosaurs, which
incorporate a margin of error statistic. Our shape data
suggest that the calcified cartilage preserved on the
ends of nonavian dinosaur long bones should remain a
good proxy for the shape of the once-living chondroepiphysis, but only if the elements are from adults and
were stress-bearing during life. We suggest that
although size information is truncated, the shape of calcified cartilage in stress-bearing nonavian dinosaur long
bones is adequate to infer gross details about locomotion
and biomechanics, especially in adults. Ultimately, we
plan to quantitatively test our long bone hypotheses
against selected nonavian dinosaur taxa.
ACKNOWLEDGMENTS
The authors thank Phillip L. ‘‘Scooter’’ Trosclair and
Dwayne LeJeune of the Louisiana Department of Wildlife and Fisheries for alligator salvage; Tim Piper of
Macomb, Illinois, for donation of Numida specimens;
Freedom Sausage Ostrich Farm, Earlville, Illinois, for
ostrich salvage; and Ryan Stegmann at Rockford Memorial Hospital for producing the MRI scans. The authors
thank two anonymous reviewers for the helpful comments and insights that improved this manuscript. A
WIU Provost Travel Award funded travel to present this
work at the Society of Vertebrate Paleontology meeting
in Bristol, UK, September 2009.
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