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Motion range of the manus ofPlateosaurus engelhardti von Meyer, 1837

Stefan Reiss and Heinrich Mallison

ABSTRACT

The mode of locomotion of the basal sauropodomorph Plateosaurus engelhardti,known from numerous finds from the late Triassic of Central Europe, has been exten-sively debated. Some early and recent research results indicate that the forelimb couldnot play role in quadrupedal locomotion. Other authors suggested facultative or evenpermanent quadrupedality. This would require adaptations of the range of motion andthe stability of the manual digits to the high forces caused by locomotion. An analysisof the hyperextension capabilities of the hand can therefore determine if the manus isadapted for locomotion. This study examines the capabilities of the manus of P. engel-hardti using digital 3D modeling. The motion ranges of the digits were simulated in acomputer-aided engineering (CAE) program, and the hyperextension capability of theentire manus was tested.

We find that the hand of Plateosaurus was not able to support the animal duringquadrupedal locomotion, but may rather have been a specialized grasping organ.Therefore, P. engelhardti must have been an obligate biped.

Stefan Reiss. Steinmann-Institut für Geologie, Mineralogie und Paläontologie, Rheinische Friedrich-Wilhelms-Universität Bonn, Germany. [email protected] Mallison. Museum für Naturkunde - Leibniz-Institut für Evolutions - und Biodiversitätsforschung, Berlin, Germany. [email protected]

Keywords: Plateosaurus; range of motion; virtual palaeontology

INTRODUCTION

The osteology of Plateosaurus engelhardtivon Meyer, 1837, a basal sauropodomorph, wasfirst described in detail by von Huene (1926). Pla-teosaurus is one of the best known dinosaurs with

over 100 individuals found at numerous sites inCentral Europe, including the bone-beds at Frick(Switzerland), Halberstadt, and Trossingen (bothGermany) (Sander, 1992; Moser, 2003; Klein,2004; Mallison, 2010a, 2010b). There is no clearconsensus on the species taxonomy. It seems

PE Article Number: 17.1.12ACopyright: Palaeontological Association March 2014Submission: 19 August 2013. Acceptance: 1 March 2014

Reiss, Stefan and Mallison, Heinrich. 2014. Motion range of the manus of Plateosaurus engelhardti von Meyer, 1837. Palaeontologia Electronica Vol. 17, Issue 1;12A; 19p; palaeo-electronica.org/content/2014/692-plateo-hand

REISS & MALLISON: PLATEO HAND

likely that all individuals with cranial material fromTrossingen, Frick and Halberstadt belong to onespecies, P. erlenbergensis (Galton, 2001; Prieto-Márques and Norell, 2011). A detailed study of thesacra found P. engelhardti to be valid and thesacrum of the type series sufficiently rich in charac-ters that it was designated as lectotype (Moser,2003). Moser also concluded that P. erlenbergen-sis von Huene, 1905 and its junior synonym P. lon-giceps Jaekel, 1913, to which the Halberstadt andTrossingen material was previously referred Galton(2001), are junior synonyms of P. engelhardti. Wehere follow Moser (2003) in regarding all diagnosticmaterial from the upper Löwenstein Formation andthe Trossingen Formation as P. engelhardti,because no study conducted after Moser’s 2003review (e.g., Prieto-Marques and Norell, 2011, whorefer the Trossingen material to P. erlenbergensis)has addressed the issue of sacral characters, andthus made a convincing case for the presence of adifferent species in Trossingen or Halberstadt.According to von Huene (1926) and Yates (2003a),Sellosaurus is a junior synonym of Plateosaurus,but the older species P. (Sellosaurus) gracilis is notthe focus of this study. The great similarity of thetwo species, especially in the forelimb, likelymeans that conclusion on P. engelhardti also applyto P. gracilis.

Based on the dating of the strata in whichspecimens were found, Plateosaurus engelhardtiapparently occurred in the late Norian and disap-peared in the Rhaetian (Galton, 2001; Yates,2003a). Plateosaurus engelhardti reached an esti-mated maximum size between a total length of 7.5m (Klein, 2004) and 10 m, showing significantdevelopmental plasticity and variation in adult bodysize (Weishampel, 1986; Sander and Klein, 2005;Klein and Sander, 2007; see Moser, 2003 andKlein and Sander, 2007 for the influence of tapho-nomic processes on these size estimates). See-bacher (2001) gave the highest mass estimate witha body mass at 1072 kg for an individual of 6.5 mlength, and Sander (1992) estimated an 8 m indi-vidual at nearly 2200 kg. Recent CAD-modelbased estimates vary between 600 and 838 kg,depending on the overall density and amounts ofsoft tissues assumed, for an individual of roughly 6m total length (Mallison, 2010a, 2011a).

The Locomotion Controversy

Different locomotion modes have beendebated in the literature for Plateosaurus engel-hardti. Variations of a lizard-like locomotion wereassumed by Jaekel (1910) and Fraas (1913).

Jaekel soon revised his opinion and proposed akangaroo-style hopping (Jaekel, 1911, 1913), achange of heart for which Tornier (1912) viciouslyattacked him. Several authors concluded or impliedthat P. engelhardti was exclusively (Wellnhofer,1994) or obligatorily quadrupedal (Paul, 1987,1997, 2000), or was facultatively bipedal at highlocomotion speeds (Galton, 1976, 1990; Weisham-pel and Westphal, 1986; Christian et al., 1996; vanHeerden, 1997; Moser, 2003; Galton andUpchurch, 2004). Von Huene’s (1926) conclusionthat P. engelhardti was an obligate biped was reaf-firmed by Bonnan and Senter (2007) and Rauhutet al. (2011). A detailed assessment of the locomo-tion of Plateosaurus conducted using computer-aided engineering (CAE) provided newapproaches for determining body mass, motionrange, posture and, therefore, locomotion mode(Mallison, 2010a, 2010b, 2011a). Quadrupedalreconstructions of Plateosaurus engelhardti in theform of skeletal mounts, models and drawingswere mostly found to have incorrect limb propor-tions and disarticulated joints, usually the wrist andelbow (Mallison, 2010b). Correctly mounted, theforelimb is only about half as long as the hind limb,unsuitable for locomotion (Mallison, 2010a). Fur-thermore, the motion range of the forelimb underload is restricted, again indicating that the forelimbdid not play a significant role in locomotion (Malli-son, 2010b). Digital 3D assessment of the articula-tion of radius and ulna using similar techniques tothose employed here confirmed the conclusions ofBonnan and Senter (2007) that pronation was pos-sible to a very limited degree only, insufficient forlocomotion (Mallison, 2010a, 2010b, 2011a). Fur-ther indications on the locomotion mode of Plateo-saurus engelhardti can be expected from theanatomy of the manus. The pentadactyle handshave robust digits I through III with claw-likeunguals and more delicate and slender digits IVand V, the latter not orientated parallel to the otherdigits, but angled ulnary (Mallison, 2010a, 2010b).This hand was interpreted as a grasping organ byvon Huene (1926).

Study Objectives

Aims of work. It is the aim of this study to investi-gate the capabilities of the hands of Plateosaurusengelhardti with regards to locomotion by deter-mining the motion ranges of the individual joints aswell as of the manus as a whole in a CAE programbased on NASTRAN (NASA Structural AnalysisSystem). For this purpose, the ability to evenlyhyper-extend the digits is of key importance, as is

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PALAEO-ELECTRONICA.ORG

the ability of the metacarpus to support the weightof the animal. Below we formulate two hypotheseson these conditions. We also describe the flexionlimits of the digits, but refrain from a detailed inter-pretation, as it would require a detailed comparisonto other taxa what is beyond the scope of thispaper.Definition: ONP (osteologically neutral pose).The term ONP is used to describe the position inwhich the long axes of two bones articulating witheach other are approximately parallel to each otherin lateral view, as they are in a human hand placedflat on a table with the fingers straight. In dorsalview, ONP means a position in which the articula-tion surfaces in this laterally determined ONP posi-tion show the best fit (in humans this equals aposition with relaxed, not laterally or mediallyflexed fingers).

Hypothesis 1: Plateosaurus engelhardtiwas able to hyper-extend its digits significantly.Quadrupedal locomotion with forelimbs muchshorter than the hind limbs requires a significantamount of hyper-extendibility of the digits to reachan adequate stride length even for slow efficientlocomotion. This hyper-extension ability is tested incase the conclusions of Bonnan and Senter (2007)and Mallison (2010a, 2010b, 2011a) were in error,and pronation of the lower arm was possible to adegree that allowed the palm to face the ground.Hyperextension of all digits that are involved insupport must be even, as the motion limit of thewhole hand is defined by the least mobile digit.

Hypothesis 2: The sum of the shaft crosssection of radius and ulna of Plateosaurusengelhardti were approximately identical tothat of all metacarpals potentially involved inlocomotion. Given the extreme difference inlength between the forelimb and hind limb in Pla-teosaurus it is plausible to assume that the handhad a digitigrade posture if it was used during loco-motion. In that case, metacarpus and ante-brachium carried equal amounts of body weightand were stressed across similar motion angles,and thus were subjected to similar forces. There-fore, the cross section areas of the bones shouldbe nearly identical. A plantigrade (or semiplanti-grade, with a large, weight-carrying heel) posturewould lead to lower cross sections in the metacar-pals, as part of the weight would be carried by thecarpus, which would come into contact with theground. Conversely, forces and thus cross sectionareas may differ significantly if force projectiononto objects during other actions (e.g., grasping)created the main forces on the bones.

These hypotheses can be tested by a classic,non-digital motion range study and bone measure-ments. Using digital files in a CAD program makesthese tasks easier, because extensive support forthe bones is not required (see Mallison, 2010a,2010b, 2011a). However, using the 3D digital mod-els in a motion modeling program allows even eas-ier judgment of the effect of motion in many jointsat the same time, including combinations of inter-mediate positions, not just maximum flexion andextension. Also, results can be visualized withease, by exporting still frames or videos of themain modeling window, which show motionsequences and positions in any desired view.

Abbreviations

PLSST – Paläontologische Lehr- und Schausam-mlung, Fachbereich Geowissenschaften, EberhardKarls Universität Tübingen, Eberhard-Karls-Univer-sity Tübingen (formerly IFGT or GPIT)MfN – Museum für Naturkunde – Leibniz Institutefor Research on Evolution and Biodiversity at theHumboldt University BerlinONP – osteologically neutral postureSMNS – Staatliches Museum für Naturkunde Stutt-gartYPM – Yale Peabody Museum.

MATERIAL

Fossil Material

The left manus of Plateosaurus engelhardti“GPIT Skelett 2” was CT scanned by B. Ludescher(University Hospital, Eberhard-Karls-UniversityTübingen), with a slice thickness of 1 mm and sliceoverlap of 0.5 mm. File extraction was performed inAMIRA 4.0 by HM (see Mallison, 2010a) (note: thespecimen is not officially numbered, but “GPITSkelett 2” or “GPIT skeleton 2” was previouslyused in the literature).

“GPIT Skelett 2” is mounted in the PLSST(Figure 1). The mount consists of two individuals,which were founded in Trossingen, Germany,during the PLSST’s 1921-1923 excavations.Details on which parts belong to which individualcan be found in von Huene (1932). Of the left hand,assigned by von Huene to Plateosaurus quenstedti(also a junior synonym of P. engelhardti, see Gal-ton, 1984; Moser, 2003), are preserved, distal tothe wrist: metacarpals I through V, complete digitsI, II and III, and incomplete digits IV and V (Figure1). The distal phalanges of digit IV are badly pre-served and were not separated during preparation,and were extracted from the CT scans as a single

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object. Therefore, only the two proximal joints ofdigit IV could be used for the simulation. However,this still allows an acceptable approximation of themobility of the digit for the purpose of this study,due to the shortness of the elements and the largeuncertainties of motion range in all its joints. In digitV only the basal phalanx is preserved. Because ofthe assumed functional importance of digit V it wasnecessary to derive a better approximation of theactual conditions than possible by omission of themissing elements. Therefore, a scaled-down digitalcopy of the basal phalanx was used as substitutionfor the second phalanx. The presence and size ofthe missing phalanges is known from other finds,including MFN skeleton C. For the joint betweenmPh-V-1 and mPh-V-2, the preserved distal articu-lar surface of mPh-V-1 offers valid informationabout joint morphology. Information on the moredistal joints is lacking. Therefore, we desisted fromsubstituting the other missing distal phalanges asthis would have introduced highly speculative data.Also, not substituting the missing bones with cop-ies of those of other individuals guaranteed that all

bones come from the same individual, and there-fore definitely from the same species and the sameontogenetic state. By keeping both digits IV and Vintentionally reduced compared to better preservedspecimens our model can account for potentialintraspecific variability, which could be the causefor the much reduced preservation in the GPITskeleton 2 specimen. This means that results hereare valid as minimum scenarios. For hyper-exten-sion the reduction has no bearing, because digitsIV and V are significantly weaker than digits Ithrough III, and thus likely played only a minor rolein locomotion, if at all.

Computer Programs

Computer Aided Design. ‘Rhinoceros 4.0NURBS modeling for Windows®’ (henceforth‘Rhino’; www.rhino3d.com/) by Robert McNeel &Associates (www.en.na.mcneel.com/) is a CADprogram. Rhino was used to test and improve thedigital articulation of the bones files.Kinematic modeling. NX5 by Siemens AG is aComputer-aided Engineering (CAE) program capa-

FIGURE 1. Plateosaurus engelhardti specimen “GPIT Skelett 2” in the Paläontologische Lehr- und Schausammlung,Fachbereich Geowissenschaften of the Eberhard Karls Universität Tübingen (PLSST). Left: Lateral view of the skeletalmount erected under the direction of Friedrich von Huene. Right: Close up view of the lower left forelimb and hand.

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ble of fully kinetic-dynamic modeling. The programis limited to modeling rigid bodies, so that flexiblebodies can only be simulated by connecting seriesof rigid bodies. However, for this study deformingbodies were not required. With respect to accuracyand speed, NASTRAN-based programs such asNX5 can be regarded as intermediate betweensimple biomechanical calculation and highlydetailed musculoskeletal modeling. However,some aspects such as collision conditions are diffi-cult to define appropriately (Mallison, 2011a). Forfurther details on NASTRAN-based modeling seeMallison (2010a, 2011a, 2011b).

METHODS

Digital Skeleton Articulation

The digitized files of the bones were articu-lated in Rhino in order to reconstruct the manus,using the methods employed before by Mallison(2010a, 2010b, 2010c). The configuration wasbased on articulated fossil finds, e.g., SMNS F33,and ‘best fit’ of the bones. A roughly articulatedversion of the manus (see Mallison, 2010a, 2010b)was used as the basis for this study. The articula-tion was checked for errors, which were correctedby altering the 3D association of the individualbone files (see below). This was achieved by rotat-ing and sliding bones individually and in groupsand reviewing the resulting arrangement in sixaxial views and a freely rotating view, all in parallelprojection mode. ‘Best fit’ was determined on thebasis of the preserved proximal and distal facets ofthe bones. The lack of knowledge on the exactshapes and thicknesses of soft tissues involved informing the joints leads to uncertainties in theresults, which are discussed below.

For transfer of the data into the kinetic/dynamic modeling program, each bone wasexported as a separate polygon mesh (STL) file.

Motion Range Modeling

The placement of bone files in Rhino definestheir location in NX5, so that no further correctionson position had to be undertaken. After import ofthe raw bone data, the modeling in NX5 consistedof three steps:

1. Definition of suitable settings for the simula-tion.

2. Modeling the digital joints.3. Modeling grasping scenarios with generic

bodies4. Documentation of the simulation data.

1. Definition of suitable settings for the simula-tion. The general settings for a simulation shouldalways be as simple as possible, avoiding the intro-duction of errors and making simulation resultseasier to compare to other studies. Gravity, forexample, is not relevant in this study and wastherefore switched off. Euler integration with aframe rate of 50 s-1 with two integration steps perframe guaranteed a sufficiently accurate display.Although it is far less accurate than other optionsavailable in NX5 using integration with variabletime steps, for the purpose of this study Euler inte-gration is sufficient, and reduces calculation timesignificantly.

The study did not include a stress analysis, soit was possible to give the bones unrealistically lowdensity values. This further reduced calculationtimes for simulations, because the forces causedby inertia are minimal. 2. Modeling the digital joints. The basic motions,maximum flexion and maximum hyper-extension ofthe digits, were modeled via sinusoid functions. Tosimplify the modeling process and save computa-tion time, flexing and extending motions were mod-eled in separate simulation files.

Using motion modeling instead of simplestatic analysis of maximum flexion and extensionpositions shows the posture of the entire digit ormanus throughout motion in all joints. Joints withcomplicated geometries (i.e., those that differ sig-nificantly from a single-axis rotary joint) could bejudged and adjusted much easier when shown in afluid motion than in static poses. Also, motionallowed better judgment if the determined motionranges for the individual joints added up to a con-sistent whole-digit posture.3. Documentation of the simulation data. Theresults of the simulation were documented as fol-lows:a) The motion of the distal bone in relation to the

proximal bone of each joint was plotted in-program as graphs.

b) Additionally tables were exported giving jointaxis orientation and displacement per time.

CAVEATS

Remarks on the Metacarpus

The metacarpals were regarded as an immo-bile block in the simulations. There are a number ofindications that the metacarpals could move little ornot at all against each other. The presence of pla-nar lateral contact surfaces is inconsistent withmotion. The configuration of the metacarpals in

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articulated fossils supports this interpretation.Specimens SMNS 13200k, SMNS F 33 and MSF23 preserve articulated hands with a compact, par-allel metacarpal placement, which is also seen inother “prosauropod” genera such as Seitaad ruessiSertich and Loewen, 2010 and Sarahsaurus auri-fontanilis Rowe et al., 2010.

Articular Cartilage

A factor significantly influencing the motionrange of a joint is the thickness and shape of thearticular cartilage. Several authors have providedevidence that dinosaurs or archosaurs developedthick cartilage caps at the articular surfaces(Schwarz et al., 2007; Bonnan et al., 2013; Hollidayet al., 2001, 2010; Mallison, 2010b, 2010c). Thebone configuration of the manus of Plateosaurusengelhardti from Mallison (2010b) showed minorcontact or overlap between various neighboringbones. In some cases, moving the bones distally toeliminate overlap caused significant gaps to openbetween articulation surfaces. These gaps mayindicate thick articular cartilage in those caseswhere they are not caused by postmortal dam-ages.

Usually, the corresponding functional articularsurfaces fit each other in living animal without largegaps, whereas this is not the case in Plateosaurus.Also, in mammals, the bony condylar morphologyreflects the morphology of the functional articularend, unlike in alligators and other archosaurs(Haines 1938, 1969; Holliday et al. 2010). In themanus of Plateosaurus engelhardti, there are sev-eral joints in which significant differences exist,e.g., a deep groove between the condyles of thedistal articular surfaces is not countered by a prom-inent keel between the facets of the proximal artic-ular surfaces. This is the case, e.g., in the jointsbetween the ungual phalanges of digits I through IIIand their next proximal phalanges. Smaller differ-ences between corresponding articular surfacesare also found in the joints MC I/mPh-I-1, MC II/mPh-II-1 and mPh-III-1/mPh-III-2. The exact thick-ness of the cartilage in each separate digit jointcannot be determined. In situ finds, however, indi-cate that cartilage thickness was not significantlylarger than in extant reptiles. Additionally, Bonnanet al. (2013) found that “thinner articular cartilagewould appear to be associated with a subchondralbone shape displaying well-developed surfaces,condyles and convexity. In contrast, […] in archo-saurs, thicker articular cartilage is associated withflatter, poorly-developed surfaces and a less con-vex shape.” Where possible, we therefore

assumed thin, mammal- and extant reptile-like car-tilage caps if the articular ends of a bone were well-ossified with distinct, well-developed articular fac-ets. This is in contrast to the ends of longbones,which in dinosaurs were covered by thick cartilagelayers (see Holliday et al., 2010, Mallison, 2010a,2010b, 2011a; Bonnan et al., 2013). Where thearticular ends were smoothly rounded as in thephalanges of digits IV and V, the amount of carti-lage must remain speculative.

This approach guarantees that we do notunderestimate range of motion by virtually “press-ing” bone on bone, but do not induce unnecessaryspeculation.

Postmortem Damages

Some bones of the hand of GPIT skeleton 2show taphonomic deformation. The proximal artic-ular surface of mPh-I-2 is slightly less broad thanthe distal articular surface of mPh-I-1, and the dis-tal end of the phalanx looks pinched in dorsal view.This indicates a possible deformation of theungual. In other specimens the ungual indeed isslightly broader, but this difference may also havebeen caused by variation.

The distal articular surface of mPh-II-1 isremarkably wide compared to the proximal articularsurface of mPh-II-2, indicating a mediolateralcrushing of the latter. Again, other specimens showa differing morphology, with the articular facesroughly equally wide.

A break-like structure in the middle of mPh-III-3 and the slightly oblique medial condyle of the dis-tal articular surface of mPh-III-3 indicate tapho-nomic damage. MPh-IV-2 is poorly preserved andprobably damaged. However, these deformationsare not so large as to make a cautious simulationof the motions impossible, as the results are proba-bly only slightly influenced. Comparison to otherspecimens indicates that our results are correct.

RESULTS

Articulation

Compared to the initial version of the articu-lated manus from Mallison (2010a), minor adjust-ments were required to correct for impossible orunlikely articulations, e.g., if bones overlapped.MPh-II-2 was upside down in the initial version(Mallison, personal commun., 05/2010, howeverthis was corrected before publication of Mallison[2010a]), and was rotated 180° to correct. Figure 2shows the digitally articulated hand in neutral pose(straight digits) in dorsal view.

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FIGURE 2. Articulated, CT-scan based digital scans of the left hand of Plateosaurus engelhardti specimen “GPITSkelett 2” in the PLSST in dorsal view. Compare to Figure 1.


The asymmetric development of the manussuggests partitioning of the digits into two groups,which are easily recognizable by differences inoverall morphology: digits I through III and digits IVand V. This morphological grouping is confirmed bythe motion range analysis.Digits I – III. (Figures 3, 4, 5, 6, 7) In neutral posi-tion (neither flexed nor hyper-extended), the longaxis of digit I is angled 10° medially compared tothe midline of MC III. Digit II and III are straight andorientated nearly parallel, which is also the case inall other examined specimens. In this context it isworth noting that in the feet of individuals assignedto Plateosaurus, there are marked differences, with

some individuals having straight toes, whereas thepedal digits of others are markedly curved laterally.

The distal articular surface of MC I through IIIdiffer somewhat in shape. The medial condyle ofMC I is much smaller than the lateral condyle,whereas in MC II it is only slightly smaller. Themedian sulcus between the condyles of MC I isdeeper than that of MC II. The distal articular sur-face of MC III has a slightly convex shape withoutrecognizable condyles or median sulcus. It isbroad, extends slightly laterally and curves slightlyproximally. All metacarpals have large collateral lig-ament pits.

The preungual phalanges of digits I – III areuniform in morphology and generally show the fol-

FIGURE 3. Digital files of metacarpal I and the phalanges of digit I of the left hand of Plateosaurus engelhardti speci-men “GPIT Skelett 2” in dorsal (left) and lateral (right) views, showing straight (ONP) position, and slight and full exten-sion and flexion. Figures 3 through 8 to same scale.

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lowing features: The proximal articular surface hasmoderately to well-developed lateral and medialcotyles with a median keel. The distal articular sur-face has well-developed lateral and medial con-dyles and a distinct median sulcus, so that theoverall appearance is spool-shaped. There are dis-tinct collateral ligament pits, with varying degreesof excavation. The proximal ends of the phalangesshow dorsal and ventral processus for the attach-ment of the flexor and extensor tendons. The ven-tral processus is usually larger than the dorsalprocessus. More details on the features of the indi-vidual phalanges will be given below. The ungualsof digits I – III have proximal articulation surfacesthat greatly resemble those of the more basal pha-

langes. Overall, the unguals are slightly curved, lat-erally compressed and show a distinct lateralgroove. Digits IV – V. (Figure 8) In digits IV and V the artic-ular surfaces have no prominent keels, sulci, con-dyles or cotyles. The joints are simple sphericaljoints, limited only by prominent rims around thefacet, or have plain articular surfaces. The longaxis of digit III points 35° ulnary compared to themidline of MC III. The long axis of digit V pointsnearly 90° ulnary and 30° palmary compared to themidline of MC III. Collateral ligament pits are smalland shallow or not visible. Proximal dorsal andventral processus are not developed.

FIGURE 4. Digital files of metacarpal II and the phalanges of digit II of the left hand of Plateosaurus engelhardti spec-imen “GPIT Skelett 2” in dorsal view, showing straight (ONP) position, and slight and full extension and flexion. Fig-ures 3 through 8 to same scale.

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FIGURE 5. Digital files of metacarpal II and the phalanges of digit II of the left hand of Plateosaurus engelhardti spec-imen “GPIT Skelett 2” in lateral view, showing straight (ONP) position, and slight and full extension and flexion. Fig-ures 3 through 8 to same scale.


Bone Diameters

The minimal shaft cross sections of metacar-pals I through III are roughly 90% (8.73 cm2) ofthose of radius and ulna (9.48 cm2). The ratioincreases to nearly 1:1 if cross sections are mea-sured at the articular ends (23.56 cm2 vs. 24.12cm2). In contrast, if only metacarpals II and III areconsidered the ratio decreases to just below 50%for the minimal shaft diameters (4.43 cm2 vs 9.48cm2), and ~55% for the diameters of the articularends (13.66 cm2 vs. 24.12 cm2).

Digit Mobility

Flexion and hyper-extension angles in relationto the long axis of the manus for all phalanges aregiven in Table 1. Figures 3, 4, 5, 6, 7 and 8 showviews of the individual digits flexing and extending.

Digits I – III. The well-developed prominent con-dyles, proximal dorsal and ventral processusrestrict the flexibility of the digits. The flexion ofeach joint reaches higher degrees than the hyper-extension.

In maximum flexion, the ungual of digit Ireaches an angle of 151° in relation to the long axisof the manus, while the lateral divergence of digit Ifrom the midline of manus (long axis of MC III) isreduced to less than 2°. In consequence of this ori-entation digit I cannot be opposed. The ungual ofdigit I does not reach the palm. The ungualreaches a higher degree of flexion at nearly 80°than the basal phalanx.

At maximum hyper-extension, an angle ofnearly 50° in relation to the long axis of the manusis reached by the ungual of digit I. Digit I rotates toa slightly medial orientation due to rotation of mPh-

FIGURE 6. Digital files of metacarpal III and the phalanges of digit III of the left hand of Plateosaurus engelhardtispecimen “GPIT Skelett 2” in dorsal view, showing straight (ONP) position, and slight and full extension and flexion.Figures 3 through 8 to same scale.

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FIGURE 7. Digital files of metacarpal III and the phalanges of digit III of the left hand of Plateosaurus engelhardtispecimen “GPIT Skelett 2” in lateral view, showing straight (ONP) position, and slight and full extension and flexion.Figures 3 through 8 to same scale.


I-1 around its long axis, and the medially tilt of digitI reaches 29°.

Digit II and digit III remain nearly parallel inflexion and hyperextension. Maximal flexion anglesthe ungual of digit II against the metacarpus by189°, but the ungual does not or only barely reachthe palm. The highest flexion degree of a distalbone against a proximal bone in digit II is 80°(ungula mPh-II-3 against mPh-II-2). Digit II is notvery flexible in extension. In relation to the longaxis of the manus the ungual is angled 84° at max-imum hyperextension of the digit.

In maximum flexion, the ungual of digit III isangled against the metacarpus by 237° and the tipof the ungual reaches the palm. Maximum flexionof the distal phalanx in relation to the proximalbone reaches between 50° (the two proximal joints)and 70° (the two distal joints) in digit III. MPh-III-1can perform ulnar rotation during flexion. This rota-tion causes an increase of medial deflection of theungual to nearly 7° in relation to the long axis of themanus at maximum flexion. At this angle, its medialorientation places the ungual of digit III nearly intocontact with that digit II when both digits arestrongly flexed.

In hyperextension, digit III is the most flexibleof the first three digits. The basis joint reaches 40°when fully extended, so that the tip of the ungual isangled against the metacarpus by 104°.Digits IV – V. The maximum flexion of the basaljoint of digit IV reaches 80° and the flexion of jointmPh-IV-1/mPh-IV-2 reaches 60°. Flexed, the digitis orientated medially and may have contacted the

palm. Digit IV is less flexible in hyperextension. Itreaches 15° in the basis joint and 20° in the jointbetween mPh-IV-1 and mPh-IV-2.

Similar to digit III, digit IV can rotate slightlyulnary by nearly 7° during flexion and in neutralposition. This rotation causes a more medialdeflection of the distal phalanges. Additionally,mPh-IV-1 can rotate ulnary around the sagittal axisby 20° when hyper-extended.

Digit V is the most flexible digit. In maximumflexion both modeled joints reach 90° and the digitopposes digit III, but was probably not able tooppose to digits I and II because of its short length.Digit V seems to have been able to reach the palm.In hyperextension, the digit is more flexible thandigit IV. The joint between mPh-V-1 and mPh-V-2reaches the largest hyper-extension angle of theentire hand (50°). However, the high values for dig-its IV and V must be regarded with caution,because the lack of well-defined articular surfacesforces speculation into the assessment of mobility.Extension of the unguals of digit IV and V cannotbe assessed for GPIT skeleton 2 due to theirincomplete preservation.

DISCUSSION

Testing the Hypotheses

Hypothesis 1: Plateosaurus engelhardti wasable to hyper-extend its manual digits signifi-cantly. In most joints, large hyper-extension anglesare prevented by the morphology of the articulationsurfaces. A significant and uniform hyper-exten-sion of all digits is prevented by the differing valuesfor maximum extension, indicating that the manusdid not allow for large stride lengths. Not even thetwo longest digits II and III show a uniform patternof extension. - Digit I has limited mobility in the basal joint during

extension and cannot reach significanthyper-extension values. The slightlymedial orientation of the digit during exten-sion and its shortness also indicate thatthe first digit was not able to support bodyweight in a quadrupedal posture, as itwould not touch the ground at a significantlength.

- Digit II has a similarly limited ability to hyper-extend in the basal joint as digit I. Due to ahigher number of distal segments and asignificantly more flexible joint betweenmPh-II-1 and mPh-II-2 than in the corre-sponding joint of digit I, digit II as a wholeis more flexible than digit I during hyper-

TABLE 1. Maximum flexion angles of phalanges in rela-tion to the long axis of the metacarpus.

Bone angle [deg]max. flexionangle [deg]

max. extension

mPh-I-1 59.8 -27.8

mPh-I-2 151 -49.4

mPh-II-1 49.6 -29.6

mPh-II-2 108 -54.7

mPh-II-3 189 -84.4

mPh-III-1 50 -40

mPh-III-2 99.6 -59.7

mPh-III-3 170 -74.7

mPh-III-4 237 -104

mPh-IV-1 77.8 -12.5

mPh-IV-2 128 -32.6

mPh-V-1 90 -29.6

mPh-V-2 172 -73

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extension. Although digit II is also longerthan digit I, it is neither as long nor as flexi-ble as digit III, and thus was not capable offorming a congruent support during loco-motion together with digit III.

- Digit III is more mobile in the basal joint than digitI and II, and was long enough to be usedeffectively in quadrupedal locomotion.However, it is not significantly strongerthan digit II in cross section area, andwould alone not have been strong enoughto support a significant part of the bodyweight. Additionally, the distal joints of digitIII are limited in their mobility during hyper-extension.

- Digit IV is the least extensible digit. It has greaterlateral flexibility than the other digits, but isincapable of forming a useful support dueto its lateral orientation.

- Digit V has a greater flexibility during hyper-exten-sion than digit IV, but it is quite unsuited forquadrupedal locomotion due to its lateral

orientation, which prevents ground con-tact.

In summary, the mainly small hyper-extensionangles possible do not match an adaptation togreater stride length in the forelimb, which seemsnecessary for quadrupedal locomotion. The smallhyper-extension angles of the basal joints (exceptin digit III) also hinder a plantigrade gait. A digiti-grade use of the manus during active locomotion isnot likely due to the low dorsal mobility within thedigits. Only stabilizing of the anterior body duringfeeding or drinking at ground level seems possible,with laterally directed digits and medially facingpalms. Consequently, Hypothesis 1 is not con-firmed.Hypothesis 2: The sum of the shaft cross sec-tion of radius and ulna of Plateosaurus engel-hardti were approximately identical to that ofmetacarpals I through III. The only digits long andmobile enough to bear significant weight in a digiti-grade posture are digits II and III. The cross sec-tion areas of their metacarpals are roughly halfthose of radius and ulna, indication that the manus

FIGURE 8. 1. Digital files of metacarpal IV and the phalanges of digit IV of the left hand of Plateosaurus engelhardtispecimen “GPIT Skelett 2” in dorsal view showing straight (ONP) position, and slight and full extension and flexion. 2.-4. Digital files of metacarpal V and the phalanges of digit V of the left hand of Plateosaurus engelhardti specimen“GPIT Skelett 2” showing straight (ONP) position, and slight and full flexion in 2. lateral (in relation to the entiremanus) and 3. lateral (in relation to digit V) views, and 4. dorsal view showing straight (ONP) position, slight and fullextension. Figures 3 through 8 to same scale.

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was not used to support the body during locomo-tion.

In contrast, the cross sections of all metacar-pals together closely match those of radius andulna. Therefore, it seems likely that significantforces acted in the hand as a whole, during otheractions than locomotion.

Comparison to Other Extinct Taxa

Basal sauropodomoprhs (‘prosauropods’).Many basal sauropodomorphs are incompletelyknown, and those lacking a manus (e.g., Anchisau-rus including Ammosaurus, Ruehleia, Pantydraco[Thecodontosaurus] caducus (Yates, 2003b),Chromogisaurus (Ezcurra, 2010) and Saturnalia(Langer et al., 2007)) or phalanges in the variablydeveloped digits IV and V, as is the case in Theco-dontosaurus antiquus YPM 2195 (Benton et al.,2000), are obviously unsuitable subjects for com-parison here. In the following, we only mention aselection of taxa we studied ourselves, or for whichsufficient data on the manus was either published.

In the basal sauropodomorph Seitaad the dis-tal end of MC I is broader and sturdier than in Pla-teosaurus. The torsion of digit I is similar in the twotaxa. Sertich and Loewen (2010) note that in Seit-aad “digit IV preserves three complete phalanges.The first two are similar in overall morphology tothe proximal phalanges of digits II and III, thoughwith shallow collateral pits and deeply divided distalcondyles.” The well-developed two basal phalan-ges contrast with the strong reduction seen in Pla-teosaurus. Similarly, “the distal phalanx of digit V ismuch longer than the proximodistally short distalphalanges of other basal sauropodomorphs includ-ing Efraasia, Massospondylus, and Plateosaurus”(Sertich and Loewen, 2010).

The metacarpals of Seitaad, based on Sertichand Loewen (2010, figure 7) and own photographs,seem to form an arc reminiscent of that in sauro-pods, in contrast to the nearly flat metacarpus ofPlateosaurus. Measuring the length ratio of meta-carpals IV to III on Sertich and Loewen (2010, fig-ure 7), an admittedly imprecise measurement,indicated that the ratio is roughly 0.84, whereas inPlateosaurus it is around 0.74. Overall, the meta-carpus and digit development of Seitaad thereforeresembles a sauropod manus, and its adaptation toweight bears much more than the manus of Plateo-saurus does. This resemblance adds slight biome-chanical support to the phylogenetic placement ofSeitaad much closer to sauropods than Plateosau-rus (Sertich and Loewen, 2010, figure 12A) basedon an improved version of the matrix of Yates

(2007) rather than a placement close to Plateosau-rus, as Sertich and Loewen recover using thematrix of Upchurch et al. (2007).

Massospondylus carinatus has a metacarpalIV to III ratio of ~0.82 (data from Cooper, 1981),intermediate between Seitaad and Plateosaurus.Otherwise its hand is overall similar to that of bothtaxa (based on figures in Cooper, 1981). Like themanus of Efraasia (figure 9 in Galton, 1973), themanus of Adeopapposaurus apparently is highlysimilar in overall digit proportions and phalangealproportions to that of Plateosaurus (based onMartínez, 2009, figures 18, 19). Martínez (2009)recovered the two as close relatives. Sertich andLoewen’s (2010) analyses, however, do not showan especially close relationship between the twotaxa, independent of the matrix used. The instabil-ity of cladistic analyses of the “prosauropods” mayindicate that a large number of character statesindependently evolved convergently; if so, adapta-tions of the manus for locomotion or, conversely,other functions such as grasping would be likelycandidates. Our results, and even a comprehen-sive study of all available “prosauropod” manus,are unlikely to shed light on interrelationships ofbasal sauropodomorphs. Efraasia, however, isbasal to the difficult to resolve group of “prosauro-pods” in Sertich and Loewen (2010), independentof the matrix they use, which can be seen as anindication that a Plateosaurus-like manus was anancestral characteristic that was subsequently lostonce or repeatedly.

The manus of more derived (according to Ser-tich and Loewen (2010)) taxa like Melanorosaurusis difficult, due to the lack of well-preservedremains. Of Antetonitrus only MC I and II areknown, but these two bones are already muchbroader (sturdier) than in Plateosaurus (Yates andKitching, 2003), and therefore adapted to support-ing the animal during locomotion. The manus ofMelanorosaurus shows reduction of the phalangesof digits II and III akin to later sauropods, a clearadaptation to weight bearing. Ornithischia. Here, only some facultatively orobligatorily bipedal ornithischians will be dis-cussed. All ornithischians are considered to havebeen herbivorous and in this respect similar to Pla-teosaurus engelhardti. Also similar to Plateosau-rus, most bipedal ornithischians have relativelybroad hands, mostly with all five digits developed.Some ornithopods have apparently specializedhands, partly with remarkable superficial similari-ties to Plateosaurus.

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The hand of the basal ornithischian Hetero-dontosaurus is overall the most similar among non-“prosauropod” dinosaurs to that of Plateosaurus.The asymmetric development of the digits, theshapes of the articular surfaces and the robustclaws on the first three digits (Santa-Luca, 1980;Norman et al., 2004) are reminiscent of the respec-tive features of Plateosaurus. This may indicate asimilar use of the hand as a grasping organ in Pla-teosaurus. However, the phalanges of digitis I-III ofHeterodontosaurus are proportionally significantlylonger and more slender than those of Plateosau-rus, and the difference in length between the twogroups of digits thus decidedly more pronounced.The more robust development of the manus of Pla-teosaurus may indicate adaptation to the produc-tion of larger forces. Overall, the similarities appearto support a non-locomotory adaptation of themanus in Plateosaurus. In contrast, the manus ofTenontosaurus Ostrom, 1970, despite showing aslight partitioning of digits into two groups as well,is significantly more massive, the metacarpals arearranged in an arc, and digits IV and V are betterossified than in Plateosaurus. All these are adapta-tions for weight bearing, matching the proportion-ally longer forelimb.

Similarly, the hyper-phalangy of digit V inIguanodontia (Norman, 2004) and the ability togreatly hyper-extend the digits and other special-izations for quadrupedal locomotion in digits IIthrough IV in Iguanodontia (Norman, 2004; Car-penter and Wilson, 2008) are clear discrepanciesto Plateosaurus. These differences are concordantwith the hypothesis that Plateosaurus was obliga-torily bipedal.Summary. The comparison to other (facultatively)bipedal dinosaur taxa reveals that in somerespects the hand of Plateosaurus is more reminis-cent to the hands of bipedal ornithischians thanthose of the more closely related sau-ropodomorphs. It can therefore be reasoned thatPlateosaurus engelhardti had a manus which wasin its basic design characteristic for a large bipedalherbivore – broad and potentially with prehensility.Such a manus may have been typical of basal sau-ropodomorphs.

Ontogenetic Development of Bipedality

Our conclusion that the hand of Plateosauruswas unsuited to play any role in locomotion fitsthose of Bonnan and Senter (2007) and Mallison(2010a, 2010b) that posture and limb proportionsof Plateosaurus made quadrupedal locomotionimpossible in large sub-adults and adults. Hatch-

lings, however, probably had a different locomotionmode, using a crawling quadrupedal gait. Thehatchlings of the proportionally similar and closelyrelated sauropodomorph Massospondylus werequadrupedal (Reisz et al., 2005), and it is likely thatPlateosaurus and other similar basal sau-ropodomorphs also had proportionally large headsand an anterior center of mass position as hatch-lings. Because the adults were bipedal, an ontoge-netic change of the locomotion mode must haveoccurred (Reisz et al., 2005; Bonnan and Senter,2007). Possibly, the hatchlings moved in a mannersimilar to that of lizards, with a subhorizontal orien-tation of the humerus and sideways orientatedhands (Bonnan and Senter, 2007). Alternatively,the hatchlings may have been capable of limitedpronation of the manus, and this capability was lostduring growth (Bonnan and Senter, 2007). How theontogenetic transition occurred alongside thechanges in body, limb and skull proportions isunclear, because of the lack of fossils of youngindividuals of Plateosaurus (Weishampel andWestphal, 1986; Sander, 1992). A gradual devel-opment of, e.g., a pronounced grasping capabilityduring ontogeny is plausible, with the change fromquadrupedal to bipedal locomotion freeing thehand from supporting the body. On the other hand,digits II and III may well have been strong andmobile enough to support the body weight of ahatchling moving slowly, so that there is no reasonto assume an ontogenetic shift in digit proportions.

CONCLUSIONS

The robust digits I through III, the stout meta-carpus and the very robust bones of the forelimbindicate that Plateosaurus was able to exert largeforces with the manus. There is a distinct split intothree function digit groups, with digit I short andpowerful, digits II and III long and powerful, anddigits IV and V much reduced. However, the manuswas not adapted to locomotion, because those dig-its long and strong enough to play a role in support-ing the body (II and III) were together not strongenough to support as much weight as either radiusand ulna or the metacarpus, and did not hyper-extend uniformly and sufficiently. As a sub-adultand adult, Plateosaurus engelhardti was obligato-rily bipedal. How the shift from likely quadrupedalhatchlings to bipedal adults occurred is unclear, asthere are no fossils known of young of Plateosau-rus. The manus of Plateosaurus may have beenwell-adapted to grasping, but further work isrequired.

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ACKNOWLEDGEMENTS

N. Klein (Steinmann-Institut, Bonn, Germany)and D. Schwarz-Wings (MfN Berlin, Germany) sup-ported the author with feedback and access to liter-ature. J, Sertich (Stony Brook University, StonyBrook, New York, USA) provided pictures of Seit-aad. S. Schliwa (Anatomisches Institut, Bonn, Ger-many) granted access to a manus of Homosapiens. Kind reviews of the first draft by M. Belve-dere (MfN Berlin, Germany) and M. Bonnan (Rich-ard Stockton College, New Jersey) greatlyimproved this paper.

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Yates, A.M. and Kitching, J.W. 2003. The earliest knownsauropod dinosaur and the first steps towards sauro-pod locomotion. Proceedings of the Royal Society B:Biological Sciences, 270:1753-1758.

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Motion range of the manus of Plateosaurus engelhardti von Meyer, 1837Stefan Reiss and Heinrich MallisonINTRODUCTIONThe Locomotion ControversyStudy ObjectivesAbbreviations

MATERIALFossil MaterialComputer Programs

METHODSDigital Skeleton ArticulationMotion Range Modeling

CAVEATSRemarks on the MetacarpusArticular CartilagePostmortem Damages

RESULTSArticulationBone DiametersDigit Mobility

DISCUSSIONTesting the HypothesesComparison to Other Extinct TaxaOntogenetic Development of Bipedality

CONCLUSIONSACKNOWLEDGEMENTSREFERENCES

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