Corresponding authors: Ingmar Werneburg (
Academic editor: Irina Ruf
Therapsids covered the entire spectrum of terrestrial locomotion from sprawling to parasagittal. Switching between sprawling and more erect locomotion may have been possible in earlier taxa. First, the axial skeleton shows little regionalization and allows lateral undulation, evolving then increasingly towards regionalization enabling dorsoventral swinging. During terrestrial locomotion, every step invokes a ground reaction force and functional loadings which the musculoskeletal system needs to accomodate. First insights into the functional loading regime of the fore and hindlimb skeleton and the body stem of therapsids presented herein are based on the assessment and preliminary measurements of the historical collection of therapsids exhibited in the Paleontological Collection of Eberhard Karls Universität Tübingen, Germany. The specimens included are the archosaur
Therapsid locomotion and its evolution has been subject of various studies (e.g.,
We intend to establish a first overview on certain morphological and myological patterns of the body stem across
In contrast to terrestrial vertebrates (Fig.
Phylogenetic overview.
Terrestrial tetrapods (Fig.
Recent reptiles and salamanders keep their upper arms and thighs held laterally in an approximately horizontal position (=abducted or sprawling) (Fig.
In each stance phase of a lepidosaur or crocodilian (Fig.
The scapular blade of mammals swings (
Larger qadrupedal mammals walk on adducted extremities and their limbs are moved in a parasagittal plane (Fig.
Speed of walking is v = stride length (l) * frequency (F), whereby the frequency depends on the pendulum period (T): F = T = 2 * π * square root from l/G (
According to e.g.,
Synapsids show a variety of limb postures and associated changes of the locomotor system. Therapsids such as the dicynodont
The humeral heads of the here studied therapsids seem to have been expanded in dorsoventral direction and strongly compressed anteroposteriorly. The proximal articulation surfaces are relatively rough and the joint congruency in the glenoid is relatively low, suggesting elaborate cartilaginous caps. There is little doubt that extensive humeral abduction and adduction were possible. The flattened shape of the humeral head inhibits long axis rotation, which is necessary for excursions of the anterior zeugopodium. In all synapsids, radius and ulna are well developed. The humerus is held in a sprawling position, the zeugopodium can be pronated so that the digits point forward (e.g.,
The femoral heads of the studied therapsids have a somewhat oval shape with a longer diameter in approximately anteroposterior direction than in dorsoventral direction. A moderate abduction of the thigh is possible, similar but to a lesser extend than the humerus. Femur abductors (e.g., m. iliofemoralis) are muscles, which insert into the proximal femur, while the femoral head is angled from the shaft. This means that the femoral head is proximal and medial to the trochanter major. The knee joint of synapsids is directed forward. Therefore, the zeugopodium of the hindlimb does not need to be pronated, the bony elements maintain their position during the entire walking cycle (e.g.,
Yet, studies on functional loading of skeletal elements are rare for Therapsida:
During therapsid evolution, their tails became lighter and shortened (
To our knowledge, studies focussing on the functional loading of the fore and hindlimbs in combination with the axial skeleton, combining functional morphology with more technical observations, have not been conducted on therapsids. For our review, here we present first insights into the available data based on the historical collection of Friedrich von Huene (1875–1969) at the
An acceleration (in e.g., rapid flight, jumping etc.) imposes a higher functional loading onto the hindlimbs than onto the forelimbs. Therefore, a direct force transmission from the hindlimb onto the axial skeleton via a rather rigid sacral region is advantageous over a hypothetical rigid connection between the pectoral girdle and the axial skeleton. In slowing down or stopping, a tetrapod can afford minor delays in the transmission of force between the trunk and the anterior extremity. The reasons for this are a physical principle (illustrated in Fig.
The morphology of the girdles and the associated myology differ between crocodilians, lepidosaurs, birds, and mammals (Fig.
In the pelvic girdle of reptiles, associated with a posterodorsally expanding ilium (Fig.
The pectoral and pelvic girdles are very similarly functionally loaded: On a transverse section, weight is approximately evenly distributed on all four limbs while standing (Fig.
In the skull, mass inertia and weight combine to form a resultant running in lateroventral direction, which is in line with the sprawled position of the forelimbs (Fig.
Multiplication of a transverse force at a given place by its lever arm results in the bending moment, which, if plotted along the trunk, unites to form a curvilinear function (Fig.
During walking and trotting, body weight is supported in phases by only one limb of a pair, while the other swings foreward. In addition to the transverse forces and the bending moment, the trunk has to resist a torsional moment as well (Fig.
The skeleton of the pelvic girdle is expanded at its ventral side in anteroposterior direction as well as in lateromedial direction. This corresponds with the pattern of compressive forces in lateral (Fig.
The ribs of early therapsids differ from those of mammals and are similar to those of diapsid reptiles: proximally, they have two articular surfaces, the head and the tuberculum. This vlike shape provides stability and inhibits that the ribs are bend outward or inward. The force that induces outward or inward bending of the ribs is given in sprawling tetrapods when the stylopodia are abducted. In contrast, cranial or caudal bending of the ribs about an axis which connects both joints is easily taking place. Torsional moments are greater in reptiles than in mammals, because their ground reaction forces have longer lever arms. As a consequence, the trunk in extant reptiles is more rounded, or tubelike than in mammals that have a more laterally compressed trunk at the level of the pectoral and the pelvic girdles. Doubleheaded ribs are suited to sustain various loads (Fig.
1. The compressive components of torsion, which lead to bending of the rib outward (Fig.
2. The force of the pulling and weightcarrying m. serratus (Fig.
3. Downward directed pull of the ventral body wall by gravity: The tension of the body wall leads to more vertically oriented ribs in the most posterior part of the dorsal vertebral column again.
4. Carrying the body weight in resting or sprawling bellydragging (sensu
It can be expected that the ribs are inclined to average angles mediating between the various functional loadings, which may vary dependending on the lifestyle. The independance from the most ventral part of the ribs, close to the sternum is secured by the not ossified and therefore mobile (see above) cartilaginous parts (Fig.
Therapsida evolved a relatively heavy head with a short neck and a short, or at least not heavy tail. These characteristics shifted the center of mass of the body forward, towards the forelimbs (
Most recent mammals walk with both limb pairs moved in a parasagittal plane. The swinging scapula provides an additional leg segment that adds to stride length. The necessity for strong leg adductors to maintain the sprawling posture becomes superfluous. Instead, recent mammals have a slender sternum which is craniocaudally oriented (Fig.
If the tail is reduced, it cannot provide a sufficiently stable insertion for the retractors of the femur. Instead postcoxal processes of the pelvis (synsacrum and ischium in birds, ischia and pubis in mammals) offer areas into which the retractors may insert (Fig.
An array of ribs, in especially the anterior part of the trunk is a characteristic of all
In the most cranial ribs the m. serratus pulls dorsally and in the more posterior ribs the muscular body wall (m. rectus abdominis, m. obliquus abdominis externus, m. obliquus abdominis internus, m. transversus abdominis) pulls ventrally. The m. serratus suspends the ribs and carries the body (
Parasagittal locomotion evolved convergently in
Limb segments of the more extended parasagittal position can become longer without an increase of energy consumption. Many synapsids were of medium size [(e.g.,
Possible evolutionary scenarios for the development of parasagittal locomotion are:
1. Small mammals hold their legs horizontally only during part of the locomotory cycle (roughly during one third) and save some energy this way (e.g.,
2. The horizontal components of the ground reaction forces perpendicular to locomotion (F_{y} in Fig.
3. For a digitigrade limb posture, an additional limb segment is formed by the tarsus and metapodium. This additional limb segment elongates the free limb and therefore step length (e.g.,
4. During sprawling locomotion of synapsids, humeral long axis rotation may be limited, based on the oval humeral head surface observable in limbs (
5. A swinging scapula and an accordingly restructured thorax add to stride length by basically adding another additional limb segment to the leg (
Therapsid heads have been rather large and presumably heavy. Lateral movements of the heavy head require a broad support from the forelimbs, especially when performed rapidly. Hindlimbs are not subject to this condition, especially, if the tail is never exposed to statical loading or lateral accelerations. A reason for a more parasagittal posture of the hindlimb in therapsids earlier in evolution than in the forelimbs may be that the reduced, lighter, and/or shortened tail did not constrain the hindlimb to a sprawling position earlier in locomotion than the forelimb was subjected to.











rib 9 (left)  n.m.  n.m.  0.6  1.0  0.3  1.1  2.6  1.7  1.5  n.m. 
rib 11 (left and right)  n.m.  n.m.  0.6 and 0.6  1.0 and 1.1  0.4 and 0.5  1.0 and 1.0  Broken and 3.7  1.6 and 2.7  Broken and 2.1  n.m. 
rib 16 (left)  n.m.  n.m.  0.7  1.1  0.7  1.0  2.3  1.6  1.8  n.m. 
rib 19 (left)  n.m.  n.m.  0.5  0.8  2.3  1.4  1.8  n.m. 











rib 1  n.m.  n.m.  0.6  1.1  0.5  0.6  1.9  1.7  1.0  0.3 
rib 2  n.m.  n.m.  0.7  1.2  n.m.  n.m.  n.m.  2.1  n.m.  n.m. 
rib 3  n.m.  n.m.  0.9  1.1  0.7  0.8  n.m.  n.m.  n.m.  n.m. 
rib 8  n.m.  n.m.  1.2  1.8  0.8  0.7  n.m.  n.m.  n.m.  n.m. 
rib 9  n.m.  n.m.  1.2  1.8  0.9  0.7  n.m.  n.m.  n.m.  n.m. 
rib 12  n.m.  n.m.  n.m.  n.m.  0.8  0.9  3.5  2.5  2.8  2.3 
rib 13  n.m.  n.m.  0.9  0.8  0.9  0.8  2.9  3.3  2.8  2.2 
rib 16  n.m.  n.m.  0.8  1.3  n.m.  n.m.  n.m.  n.m.  n.m.  n.m. 




second last presacral  3.6  3.0  3.8 
last presacral  3.5  3.2  3.7 
1. sacral  3.5  3.2  2.8 
2. sacral  3.2  3.2  2.9 
3. sacral  2.6  2.5  2.8 
1. caudal  3.0  2.5  3.0 
2. caudal  2.4  2.7  3.0 
3. caudal  2.7  3.0  2.5 
4. caudal  2.6  2.7  2.6 
5. caudal  2.8  2.4  2.6 
6. caudal  2.7  2.7  2.4 
7. caudal  2.5  2.7  2.6 
8. caudal  2.7  3.0  2.2 
9. caudal  2.1  1.8  2.4 
10. caudal  2.0  1.8  2.0 
11. caudal  1.6  1.6  2.1 
12. caudal  1.6  1.4  1.8 
13. caudal  1.4  1.3  1.8 
14. caudal  1.4  1.4  1.8 











rib 1 (left)  n.m.  5.5  0.4  1.2  0.4  0.9  2.0  n.m.  n.m.  n.m. 
rib 2 (right)  n.m.  5.5  0.5  0.6  0.4  0.5  2.0  1.5  1.6  0.9 
rib 3 (right)  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  2.4  2.1  1.2  1.0 
rib 4 (left)  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  3.1  2.5  1.6  1.1 
rib 7 (right)  15.0  15.5  0.5  1.3  0.5  1.0  3.2  2.5  3.4  0.4 
rib 9 (right)  20.0  16.0  0.8  1.6  0.7  1.3  3.2  2.9  2.4  0.5 
rib 10 (right)  20.6  17.0  0.7  1.1  0.8  1.1  2.9  2.4  1.7  n.m. 
rib 11 (right)  20.9  16.0  1.0  1.5  1.0  1.0  n.m.  n.m.  n.m.  n.m. 
rib 12 (left)  17.0  16.6  0.6  1.2  0.7  1.2  2.9  n.m.  n.m.  n.m. 
rib 13 (right)  20.7  18.3  0.8  0.9  0.6  1.3  n.m.  n.m.  n.m.  n.m. 
rib 14 (left)  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  n.m. 
rib 18 (right)  21.5  20.4  0.7  0.8  0.5  0.9  n.m.  n.m.  n.m.  n.m. 
rib 19 (right)  17.7  15.8  0.5  0.8  0.5  1.1  n.m.  n.m.  n.m.  n.m. 
rib 20 (left)  23.0  16.4  0.6  0.8  0.4  0.8  n.m.  n.m.  n.m.  n.m. 




second last presacral  3.0  2.4  3.3 
last presacral  2.5  2.8  2.5 
1. sacral  2.8  2.3  3.5 
2. sacral  2.9  2.5  3.4 
1. caudal  2.0  2.4  2.6 
2. caudal  2.3  2.3  2.4 
3. caudal  2.0  2.2  2.3 
4. caudal  1.8  2.4  2.1 
5. caudal  2.0  2.2  2.0 
6. caudal  1.5  2.0  1.8 
7. caudal  1.2  1.8  1.8 
8. caudal  1.0  1.6  2.0 
9. caudal  1.1  2.0  1.7 











rib 12 (left and right)  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  13 and 16  7 and 7  12 and 13  n.m. 
rib 13 (left and right)  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  15 and 15  6 and 7  12 and 13  n.m. 
rib 14 (right)  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  15  7  12  n.m. 
rib 15 (right)  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  13  5.5  12  n.m. 




second last presacral  9.0  7.2  4.5 
last presacral  10.0  8.7  4.6 
1. sacral  10.0  10.2  5.5 
2. sacral  7.5  11.5  5.5 
3. sacral  8.3  9.3  6.0 
1. caudal  8.5  8.8  5.5 











rib 1  65 cm  65  6.82  9.75  5.8  7.61  21.27  14.8  11.4  3.1 
rib 2  85  85  6.9  7.68  5.3  6.15  24.85  12.96  17.87  3.55 
rib 3  115  110  7.07  8.67  7.2  7.38  23,2  15.28  11.54  3.34 
rib 4  135  130  5.8  8.4  4.66  7.5  27.2  13.7  22.1  3.18 
rib 5  150  135  7.32  11  7.3  8.45  32.12  13,5  18.3  3.55 
rib 6  140  130  5.33  9  4.45  6.87  n.m.  n.m.  n.m.  n.m. 
rib 7  155  135  6.3  9.6  7.36  7.76  n.m.  n.m.  n.m.  3 
rib 8  180  135  6.84  7  5.75  7.09  21.7  14.3  17.34  2.8 
rib 9  170  145  5.67  8.81  5,97  7.48  n.m.  n.m.  n.m.  n.m. 
rib 10  190  165  4.22  10.4  5.42  5.5  n.m.  n.m.  n.m.  n.m. 
rib 11  210  185  6.2  7.58  6.25  6.35  21.89  16.22  16.2  
rib 12  190  170  7.12  7.52  6.82  6.41  33.41  14.56  26  3.5 
rib 13  180  145  5.87  8.54  6.01  7.61  34  17.5  25  1.2 
rib 14  175  140  7.47  7.95  6.91  6.14  36.02  15.61  25.11  2.9 
rib 15  170  135  9.42  7.64  7.63  4.55  32.53  14.34  25.11  4.3 
rib 16  160  130  5.69  6.68  4.94  7.1  28.6  13.73  22.9  3.43 
rib 17  130  110  8.33  9.12  6.43  8.48  n.m.  12.7  n.m.  4 
rib 18  110  100  5.43  8.56  5.69  6.26  29.8  12.23  28.05  6 
rib 19  100  80  5.73  7.2  4.42  4.75  20.4  13.43  19.7  1.85 
rib 20  85  65  5.68  9.84  5.21  7.55  19.92  11  14.3  1 
rib 21  90  70  5.86  8.25  5.62  9.11  18.2  10.2  17.4  1 




second last presacral  20.4  19  25 
last presacral  18.7  18  15 
1. sacral  36  20  18 
2. sacral  23.7  18  19 
3. sacral  21.4  16  17 
1. caudal  25,4  22  22 
2. caudal  23.3  12  19 
3. caudal  24.2  14  22 
4. caudal  22  12  14 
5. caudal  16.2  14  14 
6. caudal  19.20  12  10 
7. caudal  18.2  14  16 











rib 8 (left complete)  55  47.5  2.7  2.6  1.8  2.4  13.6  10.9  5.6  1.3 
rib 9 (left)  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  14.0  10.7  6.6  1.4 
rib 12 (right)  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  10.6  7.6  7.0  n. m. 
rib 14 (left)  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  8.1  4.8  5.4  n. m. 
rib 15 (left and right)  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  6.5 and 6.6  3.9 and 4.8  5.1 and 5.2  n. m. and 0.3 
rib 16 (left and right complete)  110.5  76.5  3.8  4.5  3.7  4.4  7.5 and 7.2  4.7 and 4.3  4.4 and 4.6  n. m. and 0.2 
rib 17 (right)  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  6.2  4.2  3.7  n. m. 
rib 18 (right)  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  6.7  4.5  5.3  n. m. 
rib 19 (right)  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  5.8  4.1  3.6  n. m. 
rib 20 (left and right)  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  6.1 and 6.2  4.0 and 4.2  3.7 and 4.6  n. m. 
rib 23 (left)  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  8.7  5.1  3.9  n. m. 
rib 24 (left)  n.m.  n.m.  n.m.  n.m.  n.m.  n.m.  8.5  6.0  4.5  n. m. 




second last presacral  —  —   
last presacral  9.2  8.9  5.4 
1. sacral  9.4  11.6  42.4 
2. sacral  n.m.  n.m.  n.m. 
3. sacral  n.m.  n.m.  n.m. 
4. sacral  n.m.  n.m.  n.m. 
5. sacral  n.m.  n.m.  n.m. 
6. sacral  n.m.  n.m.  n.m. 
7. sacral  n.m.  n.m.  n.m. 
8. sacral  6.2  5.6  n.m. 
1. caudal  n.m.  n.m.  n.m. 
2. caudal  7.5  8.6  4.0 
3. caudal  8.4  8.0  4.1 
4. caudal  7.8  6.5  4.5 
5. caudal  7.0  6.4  4.8 
6. caudal  7.1  8.2  4.2 
7. caudal  7.3  4  3.4 
Segments of body stem. The remains of the vertebral columns of









7  21  3  33–34  2  0  7  
6  19  2  35–40  6  0  Impossible to determine  von Huene (1939–42)  
unknown  27 presacrals ( 
3 ( 
> 20 in e.g. the specimen described by 
all  0  Impossible to determine 
DFGgrant WE 5440/61 to Ingmar Werneburg.
The authors have declared no competing interests.
HP wishes to express that this article is based to a large extent on the work done by former colleagues of him in alphabetical order: Wolfgang Maier, Carsten Niemitz, Adolf Seilacher, Frank Westphal, Ulrich Witzel, and coworkers in alphabetical order: Andreas Christian, Norman Creel, Brigitte Demes, Martin Fritz, Michael Günther, Stephan Recknagel, Hartmut Witte. Although some do not show up in the reference list, all of them helped to solve problems, often with the aid of mathematics, often with FEtechniques. They also kept a watchful eye on my tendency to consider things as selfevident and insisted on stepbystep thinking. The same kind of influence was exerted from many graduate and PhD students who were working at times in Functional Morphology. HP owes many thanks to all of them! Further, the authors would like to thank the reviewers, Jörg Fröbisch and one anonymous reviewer, for there many helpful comments that led to an improved manuscript.
The material we examined includes the skeletons of the archosaur
1.
2. The mounted skeleton of
3.
4.
No cervical ribs have been preserved. Only two ribs, ribs 8 (left side) and 16 (right side), are complete (von Huene 1935–
5.
6.
7.