How can we tell the forelimb posture of Triceratops?
Shin-ichi Fujiwara
Nagoya University Museum
Email: sifjwr@num.nagoya-u.ac.jp
We would never see the extinct animals alive. Then, how can
we tell how they looked like, and how they stand and walk? There
has been a lot of controversy over the reconstruction of forelimb

Fig. 1. Triceratops reconstructedin (left) upright/sagittal and (right)
sprawling forelimb postures.

posture in extinct tetrapods, mainly due to our poor knowledge on therelationship between the skeletal morphology and the limb postureseven among the modern taxa. Here I introduce some of my studies
that tackled the problems in determining forelimb postures of extinct
taxa, especially focusing on Triceratops (Ceratopsia), one of the most
famous quadrupedal dinosaurs.
Topic 1) Upright/sagittal or sprawled?*1—It has been difficult to tellwhether the extinct animal walked on its forelimb right beneath their
trunk like many mammals do (upright/sagittal), splayed out like lizards
and salamanders do (sprawling), or creeped on bellies like sloths do(creeping) (Fig. 1). Muscle moment arm analyses using 300< modern
taxa found that quadrupeds in upright/sagittal, sprawling, andcreeping postures emphasize moment arms of the respective antigravity muscles of the elbow joint—the elbow extensor, adductor, andFig. 2. Kernel density distributionsof the moment arm ratios for elbow
extensor (Ex/A), flexor (Fl/A), andadductor (Ad/A) in modern (blue)
upright/sagittal, (green) sprawling,and (red) creeping quadrupedal
tetrapods*1.

flexor muscles, respectively (Fig. 2). Therefore, the moment arms of
the elbow muscles can be used to discriminate the forelimb
posture in extinct forms.
Topic 2) Elbow joint angle in supporting the body*2–3—
Modern tetrapods are known to maintain the limb joint angles
during the support phase within the limited range, although
the joint allows larger range of motion. Can we determine thejoint angle of stance phase from the bone geometry?
According to the study on over 30 modern quadrupedal
tetrapod taxa that employ upright/sagittal posture, they wereshown to keep the elbow joint at angle where the momentarm of the anti-gravity muscle maximize (Fig. 3). The
orientation of the muscle attachment from the joint thus could
be a powerful tool to estimate the joint angles in stance of theanimal.

Fig. 3. Relationship between (horizontal)
the elbow joint angle where the extensor
moment arm maximize and (vertical) the
elbow joint angle while supporting thebodyinmodernupright/sagittal
quadrupeds*2.

Topic 3) Scapular position*4–5—The scapulae are not directly connected with the ribcage. Therefore,
determining the positions of the scapulae in relation to the trunk has been most difficult topic in theskeletalreconstructionofextinct

tetrapods.

However,allthemoderntetrapods

thatfacultatively/obligately support their body on forelimbs share the scapular position near the median,
and above the anterior portion of the ribcage. In the support phase of quadrupedal posture, theribcage is suspended between the scapulae via serratus and rhomboid muscles encountering thegravity. Three-dimensional muscle moment analyses on extant quadrupedal taxa revealed that netroll, -yaw, and -pitch moments of the trunk
caused by the contraction of these muscles
and the gravity become negligible, if thescapulae are in the position shared by modern
quadrupeds (Fig. 4). Furthermore, the ribs
beneath the scapulae were shown to haverelatively higher strength against the verticalcompression

among

the

other

ribs

in

quadrupeds. This is consistent with thefunction of the specific ribs to support the bodyagainst the gravity. These features would
together provide a reasonable reconstruction
for the scapular position in extinct quadrupeds.

Fig. 4. Appropriate positions of the scapula to prevent
net-roll (red), -yaw (green), and -pitch (blue) of thetrunk about the hip-joint caused by body weight andcontractile forces of anti-gravity muscles connecting
the trunk and the scapula in cat (Felis silvestris catus).
The animal can stay stable in support on the forelimb,
if the scapular position satisfies the above three
conditions (red, green, and blue zones)*5.

According to the above-mentioned interpretations, the forelimb postures were indicated to vary
among the ceratopsian dinosaurs. Among the ceratopsians, Triceratops likely employed
upright/sagittal forelimb posture, whereas, Protoceratops likely employed sprawling postures,
respectively. The elbow angle of Triceratops is likely maintained in 130–140° in stance. However,both taxa would share the scapular positions with modern quadrupedal tetrapods.

References:*1) Fujiwara S, Hutchinson JR (2012) Elbow joint adductor moment arm as an indicator of forelimb posture in
extinct quadrupedal tetrapods. Proc Roy Soc B 279: 2561–2570. DOI: 10.1098/rspb.2012.0190.
*2) Fujiwara S (2009) Olecranon orientation as an indicator of elbow joint angle in the stance phase, andestimation of forelimb posture in extinct quadruped animals. J Morph 270: 1107–1121. DOI:
10.1002/jmor.10748.
*3) Fujiwara S, Endo H, Hutchinson JR (2011) Topsy-turvy locomotion: biomechanical specializations of theelbow in suspended quadrupeds reflect inverted gravitational constraints. J Anat 219: 176–191. DOI:
10.1111/J.1469-7580.2011.01379.
*4) Fujiwara S, Kuwazuru O, Inuzuka N, Yoshikawa N (2009) Relationship between scapular position andstructural strength of rib cages in quadruped animals. J Morph 270: 1084–1094. DOI: 10.1002/jmor.10744.
*5) Fujiwara S (2018) Fitting unanchored puzzle piece in the skeleton: appropriate scapular position for thequadrupedal support in tetrapods. J Anat 232: 857–869. DOI: 10.1111/joa.12778.