Computer programs and digitizer

All CAD tasks (mounting the skeleton, motion range analysis) were conducted using McNeel Associates Inc. “Rhinoceros® NURBS modeling for Windows®” versions 3.0 and 4.0. Digitizing of toy models also took place in Rhinoceros©, using an Immersion™ Microscribe 3D digitizer. The resulting point clouds were meshed in Geomagic Inc. Geomagic Qualify 8.0© (time limited evaluation version).

Methods of CAD virtual skeleton creation and range of motion assessment

The virtual skeleton of GPIT1 was created in a bipedal standing pose (Mallison 2010). For the assessment of mobility, those elements of importance to the joint being investigated (e.g., two consecutive vertebrae or humerus, radius and ulna) were copied into a new Rhinoceros® file. One object was given a different color and made immobile (“locked”), the other one duplicated and moved to an extreme position (e.g., maximum dorsiflexion). Other copies were moved into further extreme position (e.g., extreme lateral flexion) until all possible positions had been created. Combinations of extremes (e.g., maximum lateral flexion at maximal dorsiflexion) were also created. Throughout the process, the articulation of the elements was checked in six axial views and a freely rotatable perspective view. Where necessary, objects were made translucent to allow better judgment of the feasibility of the created articulation. Angles were measured in the CAD program either directly using bone landmarks, or by adding a line to the bone before motions were tested, copying and moving this line with the bone, and then measuring the angle between the two lines. The resulting files for each motion between vertebrae were imported into one new file, to illustrate e.g., maximum dorsiflexion of the entire dorsal vertebral column. Similarly, for the limbs the files from stylopodium, zeugopodium and autopodium were united. Here, all non-redundant positions were combined, to illustrate the total motion range of the limb.

Motion between vertebrae was assumed to be possible if any zygapophysal overlap was retained. This means that in contrast to Stevens and Parrish (1999, 2005a, b), 50% overlap is not required. Stevens and Parrish (1999) use this value, but as they point out (Stevens and Parrish 2005a), giraffes retain only minimal overlap, and avian cervicals retain between 25% and 50% of overlap. Here, minimal overlap is therefore used, and the resulting position then assessed for its feasibility. Overlap of 50% is used when higher values lead to unrealistic results as mentioned below.

Influence of soft tissues on the range of motion

Ligaments, muscles and tendons.—Aside from the bones, soft tissues have a large influence on the motion range of joints. For example, ligaments play an important role in blocking or strictly limiting certain degrees of freedom in many joints (McGinnis 2004; Stevens and Parrish 2005a). Sometimes these limitations are obvious from the bone shapes. For example, the shape of the distal condyles of the human femur indicates that the knee is, as a first approximation, limited to flexion/extension only, even though the bones alone would allow other motions such as antero-posterior sliding. Similarly shaped femora in extinct animals can thus be assumed to have had a similar complement of ligaments, and the knee therefore a similarly limited amount of degrees of freedom. If insertion sites of ligaments can be found on the fossil, and the corresponding ligament in extant taxa reliably identified, the accuracy of a motion range analysis can increase, but in the case of Plateosaurus few traces were found, and no corresponding ligaments could be identified with certainty.

Muscles and their tendons can also limit motions (McGinnis 2004), because they cannot be stretched at will. The greater the distance between a muscle's path and the joint it works on, the lesser the angle the moved body part can cover. The prime example is the caudofemoralis longus muscle, running from the underside of the anterior tail to the fourth trochanter of the femur. Its moment arm on the hip is accordingly large, and the more distal position of the fourth trochanter in dinosaurs compared to lizards and crocodiles means that the angle the femur can cover is smaller. The exact rotation angle, however, depends on many unknown variables, among them the internal structure of the muscle (parallelfibred or pennate?), the relative length of its tendon, and the path both take through other soft tissues, so that the resolution that can be achieved by modeling the muscle is insufficient to be of help for a range of motion study (contra Paul 2005). In such cases evidence from skeletons found in life-like positions can help determine minima for joint deflection angles. Many skeletons of Plateosaurus were found belly down, with strongly flexed hind limbs, e.g., MSF 23 (Fig. 1B) and especially SMNS F33 (Fig. 1A). Although sediment compaction has compressed SMNS F33 to the point where the femur, tibia and metatarsus are in contact with each other along the bone shafts, it is reasonable to assume that Plateosaurus could indeed adopt a resting pose in which the pelvic girdle supported part of the animal's weight. SMNS F33 can be used to reconstruct this position, by attempting to place the digital skeleton into the same pose without disarticulating any joints. Where this is not possible the digital bones must be placed in the closest possible position that is dorsoventrally of greater extent, because SMNS F33 was mainly dorsoventrally compressed. The resulting pose of the digital skeleton (see below) is then the best estimate for the posture SMNS F33 had before sediment compaction. The resulting protraction angle of the hind limb and the flexion angle of the knee and ankle are required minima. In the following, if angles determined using this or similar approaches are smaller than those suggested by bone-bone articulation, the smaller angle is given.

Motion range limitations are especially important and difficult to determine from the bones alone where muscles or tendons cross more than on joint. The maximum length of the muscle may be insufficient to allow full range of motion in all joints, effectively reducing the range of motion, while single-joint muscles rarely do so (McGinnis 2004). For example, Crocodilus porosus (IPFUB OS 13) shows limits of about 13° per joint in the tail for lateral flexion based on the bones alone at 50% zygapophsal overlap and 22° with minimal overlap, but is limited to roughly 6° per joint when the complete tail is flexed to one side, based on measurements of overall curvature of the tail on photographs (Fig. 3A; own unpublished data). In contrast, if the tail is placed in a sinusoidal curve, so that e.g., only ten articulations flex to the left, and the following ten flex to the right, the soft tissues do not reduce the maximum angle as much, allowing 10° of motion per joint (own unpublished data). The osteoderms of crocodiles have an even larger influence blocking flexion and extension of the trunk. Overall, Crocodylus porosus is capable of roughly a 45° extension in the dorsal series (own unpublished data), while the vertebrae alone would allow more than 90°. The influence of soft tissues on the motion range of a joint depends on many factors, and cannot be taken into account in detail here, especially because there is no direct fossil evidence of soft tissues known in Plateosaurus.

Fig. 3.

Examples for the influence of soft tissues on joint motions. A. Outline drawing of caudals 5 and 6 of salt-water crocodile Crocodylus porosus, IPFUB OS 13 in dorsal view. Anterior is up. Caudal 6 is shown in positions with full, 50% and minimal zyapophysal overlap (0°, 10°, 21°, respectively). Width of caudal 5 across transverse processes is 113 mm. B–D. Ulnae of stegosaur Kentrosaurus aethiopicus Hennig, 1915 from the Upper Jurassic Tendaguru Formation of Tanzania, in anterior (B₁–D₁) and lateral (B₂–D₂) views. Right (B, field number St [unknown]) and left (C, field number St 113) ulnae, both part of GPIT 1424 (mounted skeleton). D. Left ulna (part of skeletal mount in MFN) MB.R.4800.33 (length 306 mm) shows cartilage preservation on the distal and especially proximal end, preserving a large olceranon process.

Articular cartilage.—Especially difficult to estimate is the influence of articular cartilage on joint motion. In many joints of dinosaurs, including Plateosaurus, the amount of cartilage was small, comparable to extant mammals and birds. Especially in the digits and along the vertebral column there are well developed bony articulation surfaces, so that good matches exist between the bones. However, the ends of limb bones as well as the articular edges of e.g., the scapulae, coracoids and sternals retained a large amount of cartilage, so that the shape of the preserved bone may not correlate closely to the actual articulation surface. In general, there appears to be a good correlation of the general shape and main features (but not the fine details) in adult archosaurs, but juveniles of all and adults of some selected taxa show massive differences (Bonnan et al. 2009; own unpublished data). Cartilaginous tissues are rarely preserved on fossils, so the thickness of cartilage caps in dinosaurs is unclear. Often, it is claimed that even large dinosaurs had only thin layers of articular cartilage, as seen in extant large mammals, because layers proportional to extant birds would have been too thick to be effectively supplied with nutrients from the synovial fluid. This argument is fallacious, because it assumes that a thick cartilage cap on a dinosaur long bone would have the same internal composition as the thin cap on a mammalian long bone. Mammals have a thin layer of hyaline cartilage only, but in birds the structure is more complex, with the hyaline cartilage underlain by thicker fibrous cartilage pervaded by numerous blood vessels (Graf et al. 1993: 114, fig. 2), so that nutrient transport is effected through blood vessels, not diffusion. This tissue can be scaled up to a thickness of several centimeters without problems.

An impressive example for the size of cartilaginous structures in dinosaurs is the olecranon process in the stegosaur Kentrosaurus aethiopicus Hennig, 1915. In the original description a left ulna (MB.R.4800.33, field number St 461) is figured (Hennig 1915: fig. 5) that shows a large proximal process. However, other ulnae of the same species lack this process, and are thus far less distinct from other dinosaurian ulnae (Fig. 3B, C). The process on MB.R.4800.33 and other parts of its surface have a surface texture that can also be found on other bones of the same individual, and may indicate some form of hyperostosis or another condition that leads to ossification of cartilaginous tissues. Fig. 3B–D compares MB.R.4800.33 and two other ulnae of K. aethiopicus from the IFGT skeletal mount. It is immediately obvious that the normally not fossilized cartilaginous process has a significant influence on the ability to hyperextend the elbow, because it forms a stop to extension. Similarly large cartilaginous structures may have been present on a plethora of bones in any number of dinosaur taxa, so that range of motion analyses like the one presented here are at best cautious approximations.

Methods for comparing the virtual skeleton to previous reconstructions

For comparison of a reconstruction drawing to the virtual skeleton, the drawing was imported into Rhinoceros© as a background image and scaled to fit the virtual skeleton. If possible, a fit was created for the overall length of the dorsal series and the femur length. If this was not possible due to differences in proportion, the dorsal series alone was used. Initial attempts to include the sacral series as well failed, because of different deformations of the sacra of GPIT1 and SMNS 13200, the latter of which is the base for most reconstruction drawings. If there were several views of a reconstruction available, they were imported into one Rhinoceros© file and arranged as background images in the appropriate axial viewports (dorsal view in the “top” viewport, anterior view in the “left” viewport, etc.). The virtual skeleton was then posed to conform to the drawing as well as possible. In case of length differences between bones, the virtual bone was shifted to the proximal/anterior end of the corresponding element in the drawing, creating a visible gap in the next distal articulation. Ribs were ignored in most cases, due to the relatively high degree of deformation in the ribs of GPIT1, and the usually quite schematic style of the ribs in the drawings. The sole exception is the set of drawings by Paul (1987, 1997, 2000). Paul considers his drawings “technical reconstructions”, even when they are based on old photographs of mounts, and uses them for taxonomic investigations (Paul 2008). Clearly, these drawings must thus conform to higher standards than others that are created purely for illustrating general proportions of the animal. In the case of multiple views, the bones were first arranged according to the lateral view, then in the other views. If this required misaligning them in lateral view, a duplicate was created instead. The resulting pose was assessed in all axial and the perspective views.

Skeletal mounts in museums were inspected for correct articulation visually and on photographs. Because several mounts were visited several years ago, before the conception of the work presented here, and have since been dismantled, their assessment was based on photographs alone. Here, and for life-sized 3D models, the problem of edge distortion had to be solved. Where possible, a number of photographs were taken with a 50 mm lens from a fixed distance at a right angle to the long and transverse axes of the mount or model, a 25% frame cut away from them, and the remaining, least distorted central parts combined into a composite image. Alternatively, a 300 mm lens was used to take one picture at a large distance. While these photographs are more distorted than a composite picture, they are better than a wide angle shot from a short distance.

One problem that could not be solved easily unless at least two orthogonal views were available is that of perspective distortion in dynamically posed mounts. If, e.g., the neck or tail curves towards or away from the viewer, they will appear shorter than they really are. This effect is hard to judge in lateral view alone. Where no picture from another perspective was available, I forced the virtual skeleton to fit the lateral view. This may hide articulation errors in the mounts or scaling errors in the models.

The digital 3D models were imported into Rhinoceros© directly, while the toy models were first mechanically digitized using the point cloud digitizing procedure detailed in Mallison at al. (2009), and meshed to create a 3D body in Geomagic Qualify 8.0©. The virtual skeleton was arranged to conform to them in the same way as for multiple view drawings. The 3D models by Gunga et al. (2007) were compared directly to the point cloud scan file of the IFGT GPIT1 skeletal mount, on the basis of which the models were created.

Range of motion

In the following, maximum motion ranges with (where necessary) considerations of forces acting on articulations will be discussed for separate functional body units, such as the hindlimb or the tail. Note that the determined motion ranges are the extremes possible for these parts alone, not for the entire skeleton. In some joints, motions are possible to degrees that are blocked by other body parts given certain articulation angles of other joints. These combined motion ranges are discussed afterwards.

Vertebral column: cervical series.—Plateosaurus has 10 cervical vertebrae (plus a rudimentary proatlas), 15 dorsals and three sacrais (Huene 1926). Huene (1926) also lists 41 preserved and at least 15 missing distal caudals for SMNS 13200. In GPIT1 45 caudals are present, and only a very small number may be missing at the tip of the tail. Atlas and axis of GPIT1 are attached to the skull and were not available for CT scanning. For reference, the cervical column was placed so that the neural canal of the last vertebra was horizontal. The neck was mounted in neutral pose (following Stevens and Parrish [1999], the pose with perfectly overlapping zygapophyses and parallel centra faces is often also termed osteologically neutral pose [ONP]), although there are strong indications that the posture has no special significance for an animal's habitual posture (Christian and Dzemski 2007; Taylor et al. 2009). In lateral view the neck then projects upwards slightly, forming a 7° angle between the horizontal and the line connecting the posterior opening of the neural canal on the last and the anterior one on first preserved cervical. However, there is no marked angling of the centrum-centrum articulations at the base of the neck, nor marked keystoning. Rather, the centra are trapezoidal in lateral aspect, with the anterior face dorsally displaced in relation to the posterior face. This shift is present in all but the first two cervicals, “stepping up” the neck while the intervertebral discs are parallel to each other. The anterior three cervical centra are slightly keystoned, creating a ventrally concave arch that places the anterior surface of the centrum of the epistropheus in a near vertical position. The shape of the atlas and its articulation with the epistropheus result in a downwards inclined position of the skull, as described by Huene (1926: 28).

Any definitive assessment of neck mobility requires knowledge of the shape and potential flexibility of the cervical ribs. These, however, are not fully preserved in GPIT1 today, and have not been prepared separately in other individuals of Plateosaurus. Several articulated necks have been retained in the position they were found in. Especially well preserved are SMNS F33, SMA unnumbered, and MB.R.4429. Of these, SMNS F33 and MB.R.4429 curve strongly, are well articulated, and their cervical ribs are distally slightly bent, indicating some flexibility in vivo. Although the cervical ribs in Plateosaurus overlap the next vertebra, it is unlikely that they blocked intervertebral motion. For the digital analysis presented here I therefore ignored them, instead comparing my results to the articulated necks.

Fig. 4.

Range of motion of prosauropod Plateosaurus engelhardti Meyer, 1837 using the digital skeleton mount of GPIT, from Trossingen, Germany. A. Lateral view of cervicals in neutral articulation, maximal dorsiflexion and maximal ventriflexion. B. Dorsal view of cervicals in neutral articulation and maximal lateral flexion. C–F. Dorsal vertebral column and ribcage in dorsal view in maximal lateral flexion (C), lateral view in maximal ventriflexion (D), lateral view in maximal dorsiflexion (E); air exchange volume determination (F). Pink ribs and dark green volume = exhaled volume, red ribs and translucent green volume = inhaled volume. See text for further explanation. G. Tail in lateral view, showing (top to bottom) dorsiflexion at 10° and at 5° per joint, neutral articulation, maximum ventriflexion. H. Tail in dorsal view, straight and at 10° lateral flexion. Length of cervical series 103 cm, length of dorsal series 137 cm, length of caudal series 261 cm. Anterior to the left in A–C and F–H, to the right in D and E.

Ventral mobility of the neck is limited to a 100° curve, measured as the angle between the anterior face of cervical 2 and the posterior face of cervical 10. Mobility is greatest between the anterior vertebra, and decreases with the reducing length of the vertebrae posteriorly. In the posteriormost three cervicals, ventriflexion is minimal, both because of the dorsal displacement of the zygapophysal articulation, and the trapezoidal shape of the centra. Dorsiflexion is possible to a slightly larger extent (110°), with the posterior half of the neck contributing the largest part. The anterior part is in comparison quite stiff (Fig. 4A).

Lateral motions of the neck cannot be accurately judged because of the imperfect preservation of the cervical ribs. However, a 180° curve appears plausible as a minimum, placing the head 0.6 m laterally and 0.3 m above the neck base, with the snout pointing caudally (Fig. 4B). SMNS F33, MB.R.4429, and MSF 23 do not contradict this interpretation. All are less flexed than appears possible from the virtual mount, but all also appear not to be flexed to the limits of the intervertebral joints. An articulated neck in the SMA (unnumbered) from Frick shows a similar curvature as SMNS F33, although created by both dorsiflexion and lateral flexion. Galton (2001: fig. 1c) figures the skeleton of AMNH 6810 as found based on an unpublished drawing by Friedrich von Huene. Its neck is curved ∼180°, with the largest flexion angles occurring in the middle part of the neck. However, it is not clear whether these vertebrae are still articulated. In all, it seems reasonable to assume that the values given above are possible, but a slightly more restricted range of motion cannot be ruled out. Torsion is almost totally blocked by the medially angled zygapophysal articulations along the entire neck.

Vertebral column: dorsal series.—ONP articulation creates a slight dorsally concave arch in the anterior five dorsals, and a slight ventrally concave curve in dorsals 6 to 10. From dorsal 11 on, the series is straight. As in SMNS 13200, the zygapophysal faces of GPIT1 are medially inclined about 45° between dorsals 1 and 2, measured by placing a rectangle on the articulation surface of each side in the CAD program, and having the program calculate the angle this plane forms with the vertical axis. From dorsal 3 on, the zygapophyses are oriented progressively less steep up to dorsal 6, where the angle is about 18°. Further posteriorly, they are oriented more steeply again, and reach approximately 35° between dorsal 15 and the first sacral. This angle is 10° lower than in SMNS 13200. It is unclear whether this difference is caused by intraspecific variation or deformation. The angles for GPIT1 were determined on the right side only, because the lateral processes and zygapophyses of the left side are all displaced dorsally by postmortem deformation. The zygapophysal alignment indicates a high mobility in dorsoventral direction in the posterior and even more so in the anterior third of the dorsal series. The middle part is somewhat less flexible in dorsoventral direction, and additionally allows strong lateral flexion. Torsion is blocked by steeply oriented zygapophyses, and thus only possible, to a limited degree, in the middle third of the dorsal series. Articulation of the digital files confirms this interpretation. However, lateral flexion of the vertebrae alone is possible to an extent that appears unreasonable, allowing nearly a full circle for the entire dorsal series. Lateral flexion between consecutive vertebrae of over 30°, as possible between, e.g., dorsals 7 and 8, leads to intersection of the ribs. Apparently, lateral motion in vivo is not blocked osteologically, but rather by the maximum compression of the intercostal tissues. Addition of the ribs to the 3D file leads to a maximum lateral curvature of the dorsal series of about 110° (Fig. 4C). Flexion is also blocked by the ribs, not the intervertebral articulations, allowing a 85° curve (Fig. 4D). Extension was limited by the intercostal and ventral soft tissues, the effect of which is difficult to determine. However, a 90° curve for the entire dorsal series seems possible (Fig. 4E). In life, the dorsal mobility of the anterior dorsals was certainly greater, as rib motion was ignored here. This would have increased the motion range of the neck by moving the neck base.

Ribcage.—The ribs of GPIT1 all show the marks of postmortem deformation and distortion. Comparison between contralateral elements shows that this damage is not significant enough to make a reconstruction of the ribcage width impossible. The capitulum can be matched accurately with the parapophysis in most ribs, but the dorsal displacement of the left transverse processes of many dorsal vertebrae results in a dorsal displacement of the diapophysis (term used here sensu Huene [1926] and Wilson [1999], referring to the articulation surface, not the entire transverse process). In some dorsals, the tuberculum cannot be articulated with it at all, in others the resulting position elevates part of the rib shaft as high as the top of the neural arch. Similarly, on the right side some neural arches, and with them the diapophyses, have been displaced ventrally. Here, the respective ribs are tilted medially. In the virtual skeleton an attempt was made to correct for these errors, placing the ribs as symmetrically as possible. This results in a high-oval ribcage cross-section, in which the transverse axis at the widest point (dorsal 6) is slightly shorter than the vertical axis, if the latter is measured from the ventral end of the corresponding rib. The 4th to 9th ribs are almost equal in length, and the cross sections only slightly less wide than in the 6th. Fitting an ellipse into the rib pairs of this region results in a ratio between the transverse and the vertical axis of 0.85. In the first to third rib pairs a greater difference exists between the axes of the ellipse, with the transverse axis roughly 0.6 times the length of the vertical axis. The posterior third of the ribcage is only slightly wider than the anterior third, but the ribs are much shorter. Their distal ends form a posteriorly ascending line that corresponds to the pubes.

While the correct angle between the longitudinal axis of the animal and the rib shafts cannot be determined with ease, it is nevertheless possible to estimate the tidal volume of Plateosaurus on the basis of the reconstructed ribcage. As in tyrannosaurids (Hirasawa 2009), the ribs rotate around an axis defined by the articulations between capitulum and parapophysis, and tuberculum and diapophysis. The orientation of these axes is one of the main factors defining thorax kinematics. The axes of rotation were determined by creating cylindrical bodies that were arranged with one end touching the diapophysis and the other end the parapophysis. Five point objects were created on the medial side of each rib, splitting the medial face of the rib into four segments, and grouped with the rib, so that any motion of the rib also affects the points. All ribs were rotated around their respective articulation axes by +5°, while a copy of each rib (with the attached points) was rotated by -5°, creating a 10° difference between the two rib sets. This angle results in rib kinematics similar to those observed during rest or moderate activity in extant basal birds (Claessens 2009) or reconstructed for tyrannosaurids (Hirasawa 2009), with lateral displacement of the ventral end of the rib always surpassing ventral displacement. Based on the points on each rib, curves were created that in general terms follow the rib shape, forming U-shapes open ventrally. These were closed by creating a straight curve section closing the gap. From each set of curves, a NURBS body with straight sections was lofted, and its volume determined (Fig. 4F). Similar bodies were also created for the neutral rib positions, and for rib rotations of + 10° and -10°. The difference between the volumes for +5° and -5° rotation is almost 1900 cm³, or 19 liters. Values for neutral to +/-10° are ∼1500 cm³ and ∼2500 cm³, so 2000 cm³ appears to be a good average estimate of the tidal volume. While there is no reason to assume that all ribs rotated through the exact same angle during breathing, the approach chosen here is sufficient for a rough estimate of the air exchange volume, because small changes of the angle in the anterior and posterior ribs have only a minuscule influence on the volume. Altering the angle of the ribs in their supposed neutral position has only a minimal effect on the air exchange volume, and a strong posterior angling results in a biomechanically disadvantageous position of the anterior ribs, in which compressive stresses on the pectoral girdle would work to laterally compress the anterior body even further. Also, the flattened shafts in the anterior ribs only form a continuous curve if the ribs are angled close to the position chosen here. Tilting them posteriorly would lead to the attaching musculature creating torsion forces in the rib shafts.

Vertebral column: caudal series.—In GPIT1 the zygapophyses of the caudals are angled between 50° and 60°, blocking torsion of the tail as well as limiting lateral flexion to ∼10° to 12° per intervertebral joint. Due to the large number and short length of the vertebrae, the tail can still cover a large arc (Fig. 4G, H), allowing a lateral divergence of the proximal ten caudals of about 45° form the long axis of the animal even if only a 5° lateroflexion per intervertebral joint is assumed. Flexion is blocked by the haemapophyses at an angle of roughly 7° per joint, rather than by the zygapophses. Except for the first two, the haemapophyses are at least as long as the corresponding vertebra is high, but are strongly posteriorly inclined. In all, the distance between the distal ends of the haemapophyses and the ventral rim of the centra makes up 45% of the height of the tail, emphasizing the relatively greater role played by the caudofemoral musculature in prosauropods compared to crocodiles, where the same distance corresponds to less than 30% of tail height (Crocodylus porosus IPFUB OS 13).The first twenty haemapophyses are distally slightly thickened, and from caudal 15 on they tend to show a slight posterior curvature, but there is no distinct change of shape at any point. Extension to values greater than 10° per intervertebral joint creates large gaps between the centra, but there is no osteological block below 17°, when the neural spines collide. However, at such an angle no room is left for soft tissues. Taking soft tissues into account, a maximum angle of 5° to 7° per articulation appears reasonable (Fig. 4G). Lateral motion was probably limited by soft tissues. The bones alone easily allow for lateral flexion between 7° and 10° per intervertebral joint with 50% zygapophysal overlap (Fig. 4H), which would allow the entire tail to curve over 360°. A sensible assumption is that mobility in vivo was comparable to extant crocodilians, in which the zygapophyses (except for those of the first 5 caudals, which are less steeply oriented) are angled medially at slightly smaller values (40° to 55°; Crocodylus porosus IPFUB OS 13), allowing the tail tip to barely touch the side of the body. This corresponds to joint lateroflexion values of about 6°.

Pectoral girdle.—Both clavicles of GPIT1 were preserved complete and nearly in articulation (Huene 1926: 41). The fact that they as well as the anterior margins of the coracoids were in contact when excavated in both GPIT1 and GPIT2 indicates that this position reflects in vivo articulation, as pointed out by Huene (1926). Other finds show a similar arrangement, with closely spaced coracoids, e.g., MSF 23 and SMNS F 33, as well as several individuals from Halberstadt (Germany, Jaekel 1913–1914). Medially touching clavicles were reported for the closely related prosauropd Massospondylus by Yates and Vasconcelos (2005). Therefore, the pectoral girdle formed a narrow U-shape, allowing little or no rotation of the scapula against the ribcage.

In GPIT1, the left coracoid is badly deformed. The anterior and dorsal margins of the coracoid have been folded medially, but the rest of the bone as well as its association with the scapula appears unremarkable. However, the right coracoid has a much weaker curvature in dorsal view, as does the right compared to the left scapula blade. Other coracoids of Plateosaurus show great variability in both the curvature and the angle they forms with the scapula (see Moser 2003). When constructing the virtual mount I used an average of the bones of GPIT1.

The exact position of the pectoral girdle on the ribcage is impossible to determine, because of several uncertainties. First of all, it is unclear how far cranially the midline of the pectoral girdle projected. Positions below the 8^th cervical to the 2^nd dorsal are theoretically possible. However, very caudal positions require rotating the ribs improbably far back, and very cranial positions create too much room for soft tissues between the scapulae and the ribs, or force an extremely dorsal position of the scapulae. This is problematic, because the dorsal ends of the scapula blades then project nearly as far dorsally as the tips of the neural spines. Only positions with the anterior edges of the coracoids below the last two cervicals appear possible (Figs. 1D; SOM 1 at  http://app.pan.pl/SOM/app55-Mallison_SOM.pdf). Also, the angle between the scalupar blade and the vertebral column cannot be determined exactly, but an angle steeper than 45° for the long axis of the scapula seems likely (see Bonnan and Senter 2007; Remes 2008; Mallison 2010, in press). The resulting position with a far ventrally, anteriorly and steeply angled scapula is similar to that of sauropods (Remes 2008) and even highly derived ornithischians, as evidenced by an articulated Triceratops find (Fujiwara 2009).

Forelimb: shoulder.—The glenoid on both the left and the right scapulacoracoid in GPIT1 is a simple U-shaped trough, despite the massive deformation of the left element. This is also true of all other well preserved specimens that can unequivocally be assigned to the same genus as GPIT1 (GPIT2, SMNS13200, MB .R.4404, MSF 23, see Galton [2000]; Moser [2003]). Therefore, humerus motion was limited to a simple rotation around an axis transverse to the scapula blade when significant forces acted on the glenoid. The narrow U-shape of the pectoral girdle and the curvature of the scapulacoracoid dictate that this transverse axis is angled between 35° and 50° medially compared to the long axis of the animal, depending on the exact arrangement of the pectoral girdle chosen. Similarly, the axis can be tilted medioventrally up to 15°, or run horizontal, depending on the dorsoventral position of the scapulae on the ribcage. In the articulation deemed most probable here (roughly the middle of possible scapular positions in anterior and lateral views), the axis is angled 45° medially in dorsal view, and horizontal in anterior view. Therefore, humerus protraction and retraction under load is limited to an anteromedial-posterolateral plane, displacing the elbow laterally during retraction (Fig. 5A, E). The humerus can only cover an angle of approximately 80° in extension and flexion (Fig. 5B, D). This angle is not split evenly by the orthogonal to the scapula long axis through the glenoid, but only 35° extend in front of it in GPIT1 (angle varies for other skeletons due to different deformation of the scapulacoracoids). Thus, the humerus cannot be protracted to a vertical position if the scapula is placed with the long axis steeper than 35°. However, angles lower than 45° for the scapula are unreasonable (Mallison 2010; see also Bonnan and Senter 2007; Remes 2008), so that humerus protraction to vertical was impossible.

Maximal abduction (humeral elevation) and adduction angles are difficult to determine, due to the large amount of cartilage that must have been present in the shoulder. The glenoid is 8.0 cm wide on both sides, while the humeral head measures 5.4 cm on the left and 5.5 cm on the right humerus of GPIT1. Therefore, cartilage made up around 15% of the anteroposterior extent of the humeral head, assuming that cartilage thickness was equal on the humerus and glenoid. More probably, this figure is too low, and the cartilage cover of the glenoid was much thinner than that of the humeral head. It is in any case moot to try to reconstruct the shape of the cartilage cap in detail. Following Senter and Robins (2005), Galton (1971a) and Gishlick (2001) I here assume that the extents of the preserved articular surfaces on each bone represent the limits of possible motion. Large abduction and adduction angles require the glenoid to be free of significant shearing forces, so that the forelimb can not have played a significant role in locomotion, which would require a sprawling position. Fig. 5A, C, E and SOM 1 ( http://app.pan.pl/SOM/app55-Mallison_SOM.pdf) depict the maximum abduction and adduction angles that may have been possible under compacting forces. Larger angles were probably not possible if significant forces acted on the joint. Humeral elevation to the level of the glenoid is impossible expect at large retraction angles (i.e., not possible by abduction, Fig. 5A–D), but the manus can cross the body midline ventrally (Fig. 5A, E). Rotation of the humerus head in the glenoid, as seen in animals with a sprawling posture (e.g., Alligator and Varanus [Goslow and Jenkins 1983; Landsmeer 1983, 1984]), was probably very limited. In apparent contrast to the limited motion range, the wide proximal end of the humerus indicates that the shoulder musculature was voluminous and required large moment arms in Plateosaurus, similar to all other prosauropods (see Remes 2008), indicating a powerful forelimb.

Forelimb: elbow.—The axis through the distal condyli of the humerus is rotated against the transverse axis of the proximal end, which corresponds to the rotation axis in the glenoid, by about 30°, with the ulnar condyle displaced ventrally. This means that flexion and extension of the elbow occurs in a plane that is only slightly rotated against the median plane of the animal. Extension in the elbow is limited to slightly less than a straight position (which is here equivalent to 180°), the zeugopodium forming a nearly straight line with the stylopodium (Fig. 5B). Flexion is possible to at least 70° (Fig. 5B). For SOM 1 ( http://app.pan.pl/SOM/app55-Mallison_SOM.pdf), I chose a spaced articulation between radius and ulna, in order to maximize pronation potential. Because of this, flexion to angles below 120° requires a supinating motion by 15° to sustain articulation between the radius head and the radial condyle.

Manus pronation by longitudinal rotation of the radius around the ulna is not possible to a degree that allows a ventral direction of the palm in Plateosaurus. The proximal articular end of the radius is much wider than the shaft, and strongly elliptical in proximal aspect (Fig. 5F, G) in contrast to, e.g., Homo sapiens and Felis silvestris. These species have a circular proximal radius head with a shallow circular articular pit, allowing rotation of the radius at all flexion angles of the elbow. In contrast, GPIT1 and all other individuals of Plateosaurus examined for this publication consistently exhibit a shallow trough, the curvature of which is the same or very similar to that of the proximal articular facet of the ulna (Fig. 5F). As with the glenoid, much cartilage must be missing from the elbow joint, so the possibility cannot be ruled out that cartilaginous structures allowed a modest amount of radius rotation. However, even assuming a rotation by 90° does not lead to manus pronation sufficient to allow the palm to face fully ventrally. Examination of extant taxa such as Gorilla gorilla, Homo sapiens, several species of Manis and Felis cattus (MFN collection) shows that the degree of rotation of the radius that is possible against the ulna can be reliably estimated by taking a photograph of the radius in proximal view and fitting a circle to the medial outline. The angle corresponding to the arc conforming to the medial outline of the radius head is somewhat greater than the maximum angle the radius can be rotated. In Plateosaurus, the corresponding angle is roughly 80° (Fig. 5G), 40° supinating and 40° pronating from the best fit between radius and ulna. This apparent neutral position is with the palms facing 60° medially from dorsal, thus a 40° rotation leads to medially directed palms at best. For SOM 1 ( http://app.pan.pl/SOM/app55-Mallison_SOM.pdf), pronation capability was maximized by spacing radius and ulna as far apart as possible. Also, the radius rotates 55°, much more than possible. Despite this, the palm faces at best 10° ventrally of medially. These results confirm, together with manual manipulation of specimens and digital manipulation of 3D data from several individuals (e.g., GPIT1, GPIT2, MB.R.4402, MB.R.4404), the results of Bonnan and Senter (2007), who studied AMNH 2409 exclusively. Bonnan and Senter (2007) also show conclusively that alternative methods for manus pronation, e.g., proximo-distal motions of the radius, are not possible in plateosaurid dinosaurs. Additionally, as pointed out by Mallison (2010), the posture of articulated skeletons in the field argues against Plateosaurus being capable of turning the palm ventrally without massive humerus abduction.

Fig. 5.

Range of motion of the fore limb of prosauropod Plateosaurus engelhardti Meyer, 1837 using the digital skeleton mount of GPIT1, from Trossingen, Germany. A–E. Left scapula and fore limb in anterior (A), anterolateral (B), anteromedial (C), lateral (D), and dorsal (E) views. Equal colors are identical positions. B is parallel, C is perpendicular to the main axis (flexion/extension) of the glenoid. Length of humerus 350 mm. Dashed line(s) refer to: body midline (A), orthogonal to scapula blade long axis (B), body midline and main axis of glenoid (C). Red numbers in B refer to elbow, black to humerus flexion/extension. Numbers in C refer to humerus abduction/adduction versus the vertical. F. Left radius and ulna in articulation in (top row) proximolateral, medial view, (bottom row) distal and lateral views. Length of ulna 237 mm. G. Radius and ulna in proximal view. Dotted line indicates main joint axis of elbow. Circle and lines show method for determination of theoretical maximal pronation angle. H–M. Left manus. H, I. Left to right: flexion, neutral position and extension in dorsal (H) and palmar (I) views. Digit IV duplicated in neutral position views to show lateromedial deviation range. J–M. Oblique views of flexion (J, K) and extension (L, M). Length of metacarpal III 97 mm.

Forelimb: carpus, metacarpus and digits.—The blocklike structure of the wrist appears to block rotating motions (see also Bonnan and Senter 2007), but to allow significant extension and flexion. The digits flex strongly, with the claws of digits II and III potentially able to touch the palm. In contrast to the description by Huene (1926), the digits are not widely splayed and the metacarpus does not show a significant transverse curvature. Only the whole of metacarpal V and the distal end of metacarpal 4 are placed clearly palmar in relation to the others (see also Jaekel 1913–14: fig. 20). This is a marked contrast to the tubular manus of sauropods (Bonnan 2003). The distal articular facet of metacarpal IV, however, is slightly tilted against those of metacarpals II and III. This directs the digits slightly medially during flexion. Digit III and especially digit IV also have some lateral mobility between the metacarpal and the basal phalanx, allowing lateral deviation and therefore some countering action of the flexed digit against digits I and II (Fig. 5G–J). The angle is quite limited at ∼25°, comparable to the lateral mobility present in the digit IV of humans. Fully flexed, the digits can grasp around objects, and digit V opposes digits II and III (Fig. 5G–J, Mallison 2010: fig. 6). Digit I, as has also been pointed out for the closely related Anchisaurus by Galton (1976), is not angled strongly against digit II (Fig. 5G–K; Mallison 2010: fig. 6; contra Galton 1971a, b), and flexion reduces the angle to only 13° compared to the long axis of metacarpal III (Galton 1971a, b).

The right manus of SMNS F 33 is preserved in articulation, as is SMNS 13200 k, a right manus assigned to Pachysaurus by Huene (1932), a genus regarded as a junior synonym of Plateosaurus by Galton (1985). These two hands are the sole articulated prosauropod manus from the German Keuper preserved without significant transverse compression and whose elements have not been separated during preparation. In both, metacarpals II through IV are articulated with their shafts nearly parallel, forming an angle of less than 20°, and the entire metacarpus has no transverse arching. The visible parts of the bones do not suggest that this is a taphonomic artifact, confirming the results obtained for GPIT2.

The manus appears to allow significant digit extension (Fig. 5G, H, K, L). The distal condyles of the metacarpals are smoothly rounded trochleae, and there are small and shallow hyperextension pits visible on the distal ends of metacarpals II through IV. The phalanges show only very shallow or no hyperextension pits. Although the manus could certainly by used to support part of the animals weight, it does not show any special adaptations for a significant role in locomotion beyond the strong hyperextension of the digits. For example, the metacarpus is not elongated, and shows a significantly lesser development of inter-element contact surfaces, and thus stability, than the pes.

Hindlimb: hip.—Both hindlimbs of GPIT1 are preserved almost completely. The left pes lacks the distal tarsals and the ungual of digit IV, while the right hindlimb has large plaster replacements on the metatarsus as well as the phalanges. Therefore, a copy of the left tarsals, metatarsus and toes was used instead of the original right side elements. Most bones do not show any indications of significant deformations, except for the left fibula. This bone is damaged slightly below midshaft, and the remaining parts have been glued together in a slightly bent position. Overall, the left fibula is 1.7 cm shorter than the contralateral element. In the virtual skeletal mount it was positioned with the distal end correctly articulated with the tibia and the tarsals, therefore the proximal end is shifted distally compared to the tibia.

In the parasagittal plane, the femur can cover an angle of 65° without abduction (Fig. 6B), if retraction is assumed to end at the level of the ischia, and protraction before collision with the pubes. Further retraction may well have been possible, depending on the exact architecture of the m. caudofemoralis longus. However, resting positions are only feasible if the femora can be protracted at least 15° beyond the pubes (Fig. 6A, C, see below).

These limits on motion of the hindlimb in the parasagittal plane indicate that the long axis of the sacrum must have been held in a roughly horizontal position for speedy locomotion, as already concluded by Christian et al. (1996) and Christian and Preuschoft (1996). In a steep position as suggested for Anchisaurus by Marsh (1893a, b) and for Plateosaurus by Jaekel (1913–1914) and Huene (1907–1908, 1926), hardly any room is left for femur motions in the parasagittal plane, because the ischia approach a near-vertical position.

Adduction and abduction limits are difficult to determine. Bringing the foot under the body midline requires 15° of adduction, measured from the vertical (Fig. 6C), and abduction angles of as much as 45° may have been possible. Clearing the pubes at large protraction angles requires at least 22° of abduction (Fig. 6C). However, the missing cartilage in the hip joint does not allow an accurate assessment.

Hindlimb: knee.—The knee of Plateosaurus appears to be a simple hinge joint at first approximation, as in all extant taxa with parasagittal limbs. The exact degree of inversion and eversion possible cannot be determined due to the lack of preserved articular cartilage. The preserved bone shape of the distal femur end does not form a smooth continuous curvature in lateral aspect. Rather, there is a marked flattened part of the articular end, at a 110° angle to the long axis of the femur shaft. Assuming that this shape corresponds closely to the shape of the actual articular surface leads to the conclusion that there is a preferred articulation angle, in which the joint is positioned more stably than at neighboring flexion values, a sort of low-grade locking mechanism. However, this assumption is unreasonable for a joint that has to undergo rotation at a constant rate under loading, which is the case in the knee joint during locomotion. The distal condyles of the femur must therefore have been covered with a substantial amount of cartilage. Unless one assumes a cap that at its thickest point is 3 cm thick or more on the femur, there is no smooth curve possible over the distal end, and continuous extension and flexion under compression (i.e., during the stance phase of walking) would have been hindered by the “preferred orientation” of the joint. A similar bone and cartilage shape relation can be seen in juvenile and subadult extant birds, e.g., Gallus domesticus and Struthio camelus (own unpublished data), where the lateral aspect of the bony distal femoral condyles is also flattened, while the covering cartilage cap is well rounded distally. The tibiotarsus of Gallus also has a massive cartilage layer on the proximal end, similar in thickness to that of the femur. Here, however, the difference in shape between cartilage and bone is less pronounced, so that the bones give a good approximation of joint mobility.

Fig. 6.

Range of motion of the hind limb of prosauropod Plateosaurus engelhardti Meyer, 1837 using the digital skeleton mount of GPIT1, from Trossingen, Germany. A–H. Left pes in left to right: flexion, probable standing pose, extension, in lateral (A), medial (B), oblique (C–F), plantar (G), and dorsal (H) views. Length of metatarsal III 231 mm. I–K. Pelvis and left hind limb, in lateral (I, J) and anterior (K) views. I, K, probable standing (blue) and minimally possible flexion (resting) pose; J, maximum femur protraction and retraction angles for locomotion, resulting stride length 1.34 m. L. Left hind limb showing knee range of motion. Crus positions left to right: maximal extension, maximum flexion under large loads, maximum flexion for resting. M. Crus in lateral view, showing maximum ankle flexion and extension under load. Length of fibula 463 mm.

A conservative estimate of the motion range is used here, based on the preserved surfaces and my own measurements and manipulations of Gallus knees. Maximum extension includes a 165° angle between the long axes of the shafts of the femur and the tibia, a maximum flexion angle of 100° under loading (e.g., during rapid locomotion), and a flexion to at least 40° without significant loading (e.g., in a resting pose) (Fig. 6D).

Hindlimb: ankle.—Like the knee, the intertarsal articulation allows motion mainly around the transverse axis. The main rotation occurs between the astragalus and calcaneum proximally, and the distal tarsals distally, as has been pointed out by Huene (1926). Only here exists a large convex/concave set of articulation surfaces that allows a continuous rotation over a large angle under load. The distal articular surface of the astragalus covers an angle of 170°. However, it appears unlikely that positions were possible in which the metatarsus is angled posteriorly in relation to the tibia, therefore the motion range can be reconstructed as covering angles between the long axis of the tibia shaft and the long axis of metatarsal 3 from 175° (maximum extension) to 105° (maximum flexion) under large loads (Fig. 6E). If no significant compressing force is active on the joint, flexion between the distal tarsal row and the metatarsus may have been possible, to facilitate the high flexion angles required for resting poses. However, feet preserved in articulation appear to achieve flexion to 130° (MSF 23), 22° (GPIT unnumbered, articulated hindlimb) or even 0° (SMNS F33) between the proximal and distal tarsals, without requiring tarsal/metatarsal motion. Obviously, these angles are affected by postmortem compression of the skeletons, but the fact remains that the limbs could be flexed to a resting pose with ankle flexion at values far below 90° between the astragalus/calcaneum and the distal tarsals alone.

Hindlimb: digits.—The range of flexion and hyperextension in the pedal digits is comparable to that in the manual digits (Fig. 6F–M). The claws of digits III and IV can touch the plantar surface of the metatarsus, while the second toe follows the same curvature path in lateral view, but cannot reach the metatarsus due to the smaller number of elements. The first digit has similar flexion mobility, too. As in the manus, it has a medially angled articulation between the metapodial element and the basal phalanx. Fully flexed, it curves laterally by 25° compared to the long axis of metatarsal III, which is a smaller angle than in the second digit (28°). Digit IV curves medially by 14°.

A telling detail for the interpretation of the manus and pes function is the slight difference in shape of the distal ends of the metatarsals and metacarpals II through IV. In the manus, the distal articular condyles are significantly wider ventrally than dorsally, while in the pes the ventral and dorsal transverse extent is subequal in digits II and III. This indicates that the main forces acting on these articulation occurred during flexion in the hand, while there is no clear preference in the pes. This pattern is not compatible with quadrupedal locomotion. Additionally, in contrast to the knee and the metacarpal/basal phalanx articulations, the metatarsal/basal phalanx joints have a preferred articulation angle. A flattened area of the distal articular end is orientated at roughly 70° to the long axis of the metatarsus on metatarsals II through IV. In contrast to the knee, there is no indication that significant amounts of articular cartilage are missing. Rather, the articulation surfaces are well ossified. Articulating the basal phalanges with the metatarsus to achieve maximum bony contact results in a position conformai to a high digitigrade stance. The basal phalanges create the shift from subvertical to horizontal, and the distal ends of the metatarsals are lifted off the ground (Fig. 6A, D). Under high compressive loads, e.g., when taking up the weight of the animal during rapid locomotion, or on soft substrates, the distal ends of the metatarsals probably contacted the ground. The proximal surfaces of the basal digits are shaped to conform to the flattened area of the metatarsals, creating a weak locking mechanism. If the joint is compressed during the support phase, rotating or sliding motion away from the preferred articulation angle results in intra-joint forces that favor the preferred angle. A similar, though more elaborate structure is present in the wrists of ungulates, where roughly cubic carpals lock the joint in a straight position under pressure, but allow strong flexion (two times to 88°) for folding the manus under the antebrachium when the animal lies down.

In digit IV, the distal end of the metatarsal has a medial condyle that is equally sized dorsally and ventrally, while the lateral condyle is limited to the ventral face (differing from the other two long pedal digits). Dorsally, there is a pronounced gap, leaving the dorsolateral edge of the proximal articulation surface of the basal phalanx (indistinguishable in shape from those of digits II and III) free of an immediate bony contact. The functional significance of this feature is unclear.

Mobility of the complete skeleton

For the creation of a complete skeletal mount, most skeleton parts were easily combined, e.g., the tail and the sacrum. However, taphonomic damage to the first sacral has tilted the anterior surface of the centrum as well as the neural arch backwards by an unknown amount. Therefore, the prezygapophyses have been displaced caudally, and their correct orientation cannot be ascertained, making alignment of the dorsal and sacral series difficult. Most other sacra of Plateosaurus are also damaged (Moser 2003; own observation), but in general the first sacral has roughly parallel caudal and cranial faces of the centrum, and the center of the prezygapophysal articulation surface rests directly dorsal to the cranial face of the centrum. The virtual skeleton was mounted accordingly, causing a wedge shaped gap between the last presacral and the first sacral. The resulting continuous curvature of the vertebral column across the dorsal/sacral transition is further indication that this arrangement is correct.

Fig. 5A–E and SOM 1 ( http://app.pan.pl/SOM/app55-Mallison_SOM.pdf) show the full motion range of the complete forelimb. Fig. 6B, D, E and SOM 2 ( http://app.pan.pl/-SOM/app55-Mallison_SOM.pdf) show the motion range of the hindlimb available for locomotion, without strong femur abduction, while Fig. 6A, C depict maximum limb flexion.

Aside from neutral pose, various flexion/extension combinations were created to assess the overall mobility of the animal. The aim was not to demonstrate the possible extremes of motion, but rather functional poses, such as resting poses, or a pose that brings the head to the ground, e.g., for drinking or feeding. These occasionally highlighted collisions between body parts, limiting overall range of motion. When the dorsal series is ventrally flexed, the distal ends of the posterior ribs quickly reach the level of the pubes, at flexion angles of below 5° in each of the last four intervertebral articulations. While some overlap may have been possible, further ventriflexion compresses the digestive tract to an improbable degree. Similarly, large adduction angles combined with large retraction angles place the distal end of the humeri within the ribcage.

Ground feeding.—One pose (Fig. 7A) was created to show how the “stepping up” of the neck caused by the trapezoidal cervical centra affects the animal's ability to bring the snout to the ground, e.g., for drinking to feeding. Huene (1928) suggests that Plateosaurus had to perform a see-saw motion in the hips, but that is apparently based on the assumption that the back is not sub-horizontal in the habitual standing posture. In fact, from a pose with a horizontal back and nearly straight hindlimbs, a bending of the anterior half of the thoracic vertebral column, combined with a slight increase in limb flexion and maximum ventriflexion of the neck suffices to bring the snout to the ground. The hands are almost able to reach the ground in this position.

Manus at ground level.—Grasping objects at ground level with the manus requires slightly more flexion of the vertebral column, or a slight downward rotation at the hip. In this position the head can reach significantly below ground level (Fig. 7B).

Resting.—The resting pose of Plateosaurus (Fig. 7C, D, SOM 3 at  http://app.pan.pl/SOM/app55-Mallison_SOM.pdff) follows the position of the articulated finds, and that of SMNS F33 especially. Jaekel (1913–1914) assumed that Plateosaurus habitually rested on the distal ends of the ischia. Such a posture is possible (Fig. 6C, D), but requires extreme flexion of the hindlimbs. Resting on the pubis, as is known from theropods (e.g., Gierlinski et al. 2005; Stevens et al. 2008) is not possible, and there is no enlarged pubic foot. The pelvic girdle in the tight articulation and low position chosen here and in Mallison (2010) partly supports the weight of the anterior body, so that the ribcage is not compressed.