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Locomotion in rauisuchid thecodontsJosé F. Bonaparte aa Consejo Nacional de Investigaciones, Museo Argentino de Ciencias Naturales, Av.Angel Gallardo 470, 1405 Buenos Aires, Argentina

Version of record first published: 24 Aug 2010.

To cite this article: José F. Bonaparte (1984): Locomotion in rauisuchid thecodonts, Journal of VertebratePaleontology, 3:4, 210-218

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Journal of Vertebrate Paleontology 3(4):210-218, March 1984

LOCOMOTION IN RAUISUCHID THECODONTS

JOSE F. BONAPARTEConsejo Nacional de Investigaciones, Museo Argentino de Ciencias Naturales,

Av . Angel Gallardo 470, 1405 Buenos Aires, Argentina

ABSTRACT- The systematic approach to the Rauisuchidae suggests that the lowest level oflocomotororganization ofthe Thecodontia is represented by the Proterosuchidae, which can be regarded as formingthe suborder Proterosuchia. The morphological and ecological relationships of the families Erythro­suchidae and Rauisuchidae suggest that they should be referred to a new suborder, the Erythrosuchia,whose locomotor apparatus shows several important characters derived from the primitive conditionpresent in the Proterosuchia (Proterosuchidae).

In the advanced Rauisuchidae the ilia moved from vertical to almost horizontal position, with theacetabulum projecting nearly ventrally. It required significant modifications in the sacral ribs andproximal areas of the pubes and ischia, and resulted in a type ofgraviportal relation with the subverticalfemur. Because of this condition, the femur did not require the anteromedially projecting head andthe prominent fourth and lesser trochanters as in the two orders of dinosaurs.

The Rauisuchidae with vertical hind limbs remained plantigrade, with a crurotarsal tarsus and thetibia shorter than the femur. It is suggested that the evolutionary history of the vertical hind limbs inarchosaurs followed at least two very different models of improvements as shown by the rauisuchidsand the dinosaurs.

INTRODUCTION

Evolution of locomotion in the Archosauria is oneof the exciting topics of vertebrate paleontology be­cause of the varied adaptive types developed duringthe Mesozoic by the different lineages of this subclass.Possibly the most interesting chapters ofthis evolutionare those which occurred during the first stages of theprocess, involving the anatomical improvements tobecome "semi-improved" and "fully improved," fol­lowing the terms and ideas of Charig (1972).

The aim of this paper is to discuss a new model ofvertical hind limbs developed in the rauisuchid thee­odonts, and to suggest that the general picture of theacquisition of verticality of the hind limbs in archo­saurs is only partially known. In this connection theevidence from the Middle and Late Triassic of SouthAmerica is extremely interesting. It includes unstudiedforms such as Lagerpeton Romer (I 972a), which willprobably shed new light on this complex problem.

The rauisuchid thecodonts, first recognized by theGerman paleontologist, F. von Huene (1936, 1938) onthe basis ofsome incomplete specimens from the Mid­dle Triassic of Brazil, have since been documented onevery continent except Australia and Antarctica.Quadrupeds with the femur longer than the tibia, andplantigrade in the hind limbs, they were the biggestcarnivores of the Middle and Late Triassic, spanningabout 20 m.y. Although the rauisuchids did not de­velop the digitigrady ofthe saurischian dinosaurs, theyexhibit significant improvements in the locomotion

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that were probably responsible for their evolutionarysuccess.

DISTRIBUTION AND CLASSIFICATION OFTHE RAUISUCHIDAE

A list of the rauisuchid genera follows, beginningwith the oldest taxon.

Fenhosuchus Young, 1964: late Early Triassic ofChina(at least some ofthe remains assigned to this genusbelong in the Rauisuchidae).

Ticinosuchus Krebs, 1965: early Middle Triassic ofsouthern Switzerland.

Stagonosuchus Huene, 1938: Middle Triassic of EastAfrica.

Luperosuchus Romer, 1971: Middle Triassic of Ar­gentina.

Prestosuchus Huene, 1942 (see also Barberena, 1978):late Middle Triassic of southern Brazil.

Rauisuchus Huene, 1942: late Middle Triassic ofsouthern Brazil.

Saurosuchus Reig, 1959 (see also Reig, 1961; Sill, 1974):early Late Triassic of Argentina.

Heptasuchus Dawley, Zawiskie and Cosgriff, 1979: LateTriassic of North America.

Fasolasuchus Bonaparte, 1978 (also 1981): Late Trias­sic of Argentina.

Poposaurus Mehl, 1915 (see also Galton, 1977): LateTriassic of North America.

Other rauisuchids not formally described are known

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from the East African Manda Formation (e.g. "Man­dasuchus" Charig, 1967); Middle Triassic of Kupfer­zell, southwestern Germany (Wild, 1980); and LateTriassic of Marocco (Dutuit, pers. comm.). The max­illa of Teratosaurus suevicus Meyer, 1861, from theLate Triassic of Germany, usually considered a car­nosaur, ma y be rauisuchid.

The distribution of rauisuchids suggests that theywere the dominant carnivores of the Middle and LateTriassic world. Phylogenetically, they are likely de­scendants of the Early Triassic Erythrosuchidae, fromwhich they inherited their ecological role. The raui­suchids were present since the late Early Triassic (Fen­hosuchus) , which indicates that they possibly replacedtheir ancestors without any substantial break in timeand in the role of large predators.

The versatility of rauisuchids in the exploitation oftheir ecological niche is of utmost significance for theevolutionary history of the terrestrial tetrapod com­munities of the Triassic, as they were able to overcomea radical change in the prey upon which they fed. InSouth America the fossil record documents that duringthe Middle and early Late Triassic the rauisuchidspreyed upon the therapsids (dicynodonts, cynodonts)and rhynchosaurs, while during the latest Triassic theypreyed upon the archosaur fauna (mainly prosauro­pods), a situation that probably reflects a more generaltrend occurring not only in South America, but alsoin the rest ofGondwana and probably also in Laurasia.

The composition and relationships of the Rauisuch­idae have been discussed and substantially clarified byReig (1961). However, up to now the systematic po­sition of this family within the Thecodontia is notclearly understood. Several authors regarded them aspart of the suborder Pseudosuchia (Reig, 1961; Krebs,1965, 1976; Charig, 1967, 1976; Charig and Reig, 1970;Chatterjee, 1982), while others claim that they belongto the suborder Proterosuchia (Romer, 1971a, 1971b,1972a, 1972b; Bonaparte, 1975, 1981; Barberena,1978). I propose that the suborder Proterosuchia berestricted only to the family Proterosuchidae, becausethey represent the most primitive condition among theThecondontia in the organization of the vertebral col­umn, pectoral and pelvic girdles, and the tarsus, andits significance ought to be expressed in the systematicsof the order. I believe it is not proper to include thefamilies Proterosuchidae and Erythrosuchidae in thesame suborder, because the organization of the pec­toral girdle and tarsus is extremely different in the twofamilies . Besides that, profound differences are presentin the shape of the acetabulum and in the orientationof pubis and ischium, all features that suggest quitedifferent specialization of the girdles and limbs for lo­comotion.

The families Erythrosuchidae-Rauisuchidae andCerritosauridae-Proterochampsidae, previously re­garded by Romer and myself as Proterosuchia, shouldbe separated in a suborder of their own. I propose theterm Erythrosuchia Bonaparte (in press) for this newsuborder, which is characterized by rather primitive,

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quadrupedal and plantigrade thecodonts with cruro­tarsal tarsus, without tendency to bipedalism. Thissuborder includes two evolutionary lineages: (1) ratherlarge terrestrial carnivores, the Erythrosuchidae (EarlyTriassic) and the Rauisuchidae (Middle and LateTriassic), which may be regarded as comprising theinfraorder Rauisuchia; and (2) the medium-sized fam­ilies of aquatic? carnivores, the Cerritosauridae andthe Proterochampsidae, which comprise the infraorderProterochampsia.

Suborder ProterosuchiaFamily Proterosuchidae (Late Permian-Early

Triassic)Suborder Erythrosuchia

Infraorder RauisuchiaFamily Erythrosuchidae (Early Triassic)Family Rauisuchidae (Middle-Late Triassic)

Infraorder ProterochampsiaFamily Cerritosauridae (Middle Triassic)Family Proterochampsidae (Late Triassic)

Romer (l972b) and Sill (1974) suggested that theRauisuchidae may be derived from the Erythrosuch­idae. I agree with this view for the following reasons:

a) A significant ecological continuity is recognized be­tween these two families of large terrestrial carni­vores.

b) The anatomy of the skull and of several pelvic andhind-limb bones of the Rauisuchidae suggest thatthe Erythrosuchidae were their ancestors, primarilythrough improvements in the locomotor apparatus.

c) The more elongated pubis and ischium and moreelaborate tarsus of the Rauisuchidae may be rea­sonably interpreted as improvements over theprimitive characters present in the Erythrosuchi­dae .

d) The type of rather large, fenestrated skull with along preorbital region and a keyhole-shaped orbitfavors the idea that the two families are closelyrelated, even though members of the Rauisuchidaelack any vestige ofthe pineal foramen and their oticnotch is better defined by a posterior process of thesquamosal.

e) Some derived characters ofthe rauisuchid skull andpostcranial skeleton do not prevent their inclusionin the same suborder as the Erythrosuchidae, be­cause they probably correspond to a gradual evo­lution from their ancestors, without any significantbreak between the two families.

IMPROVEMENTS IN THE RAUISUCHIDLOCOMOTOR APPARATUS

The knowledge of appendicular skeleton in Triassicthecodonts affords a basic understanding of how thesprawling primitive members of the group were ableto improve their locomotor apparatus and hold thebody off the ground with fully erect hind limbs-acondition that with some variation is present in most

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\..........._._.-

I!e--\ - -,

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10 em

FIGURE 2. Sacra ofthe Rauisuchidae. A, Saurosuchus sp., ventral view; B, cf. Prestosuchus sp., dorsal view; C, Stagonosuchusnyassicus, posterior view.

dinosaurs, the dominant tetrapods of the Jurassic andCretaceous.

Isolated or general aspects of such improvementshave been discussed many years ago (Romer, 1923),but only Charig (1972) developed an organized set ofideas that allowed us to understand several details andstages ofthe history oflocomotor improvements in theArchosauria. Beyond the archaic locomotion of theamphibians and primitive reptiles, the " sprawlers,"Charig recognized the "semi-improved" type of lo­comotion present in pseudosuchians and crocodiles,and finally the "fully improved" type of locomotionpresent in both orders ofdinosaurs, the Saurischia andOrnithischia.

The osteological characters of the "semi-improved"type include imperforate acetabulum, small anteriorprocess of the ilium, weak supra-acetabular crest,somewhat elongated pubis and ischium, obliquely po­sitioned femur, poorly developed and slightly inturnedhead of the femur, untwisted tibia, crurotarsal tarsuswith calcaneal tuber, and a pes with plantigrade pos­ture.

The corresponding characters of the "fully im­proved" type include fenestrated acetabulum, strongly

developed anterior process of the ilium, strong supra­acetabular crest, very elongated pubis and ischium,vertically positioned femur with a distinct, stronglyinturned head, "twisted" tibia, mesotarsal tarsus with­out calcaneal tuber, and a pes with digitigrade posture.

I basically agree with Charig's interpretation to ex­plain the evolution of archosaurian hind limbs. How­ever, the trend recognized by Charig is only one of atleast two different paths followed by the archosaurs toachieve verticality of the hind limbs. It will be shownbelow that the evolutionary picture is not as simpleand that alternative anatomical features were acquiredby the rauisuchid thecodonts to reach verticality ofthehind limbs.

Recent studies of rauisuchid osteology (Bonaparte,1981) have demonstrated the development of impor­tant modifications in the organization ofthe pelvis andhind limbs through a rearrangement of skeletal ele­ments which is quite different from that found in otherarchosaurs. The rauisuchid ilium acquired a ventro­lateral position (as in some aetosaurs; Casamiquela,1961), with the acetabulum nearly ventrally oriented,which resulted in a graviportal relation of the pelviswith the femora. Thus, the anatomical modifications

FIGURE I. Genera of the Rauisuchidae (not to scale). A, Fenhosuchus cristatus, humerus and distal portion of the ischia,after Young (1964) ; B, Ticinosu chus fero x, reconstructed skull and pelvis , after Krebs (1965) ; C, Stagonosuchus nyassicus ,humerus, ilium, and ischium, after Huene (1938) ; D, Luperosuchus fractus, reconstructed skull , after Romer (1971) ; E, Pres­tosuchus sp. , complete skull , after Barberena (1978) ; F, Rauisuchus tiradentes, premaxilla, scute , pubis, and ilium, after Huene(1935-42); G, Saurosuchus galilei, complete skull and pelvis, after Bonaparte (1981); H , Fasolasuchus tenax, premaxilla, nasal ,and maxilla, after Bonaparte (1981) ; I , Heptasuchus clarki, reconstructed skull and tibia, after Dawley, Zawiskie and Cosgriff(1979) ; J, Teratosaurus suevicus (an assumed rauisuchid), medial view of the maxilla, after Huene (1907-8); K, cf. Poposaurusgracilis. ilium and pubis, after Galton (1977) .

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A ~

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FIGURE 3. Ilia and ischia ofarchosaurs, to demonstrate differences with in the subclass. A, the ph ytosaur Machaeroprosopus,after Cam p (1930); B, the rauisuchid Saurosuchus, after Bonaparte (1981); C, the lagosuch id Lagosuchus, after Bonaparte(1975); D, the pro sauropod Riojasaurus, after Bonaparte (1971); E, the camosaur Piatnitzkysaurus, after Bonaparte (1979).

of the rauisuchid ilium and femur are very differen tfrom those found in some advanced thecodonts, saur­ischians, and ornithischians, in which the ilia remainedvertical, with the acetabulum projecting mostly later­ally .

The Rauisuchid Sacrum-A well preserved sacrumis known in Stagonosuchus nyassicus from the MiddleTriassic beds ofTanzania. It was described and figuredby Huene (1938), and is housed at the Tiibingen Uni­versitat, West Germany. In addition, an undescribedcomplete sacrum from the Ischigualasto Formation ofArgentina (Saurosuchus sp.) , and another one from theSanta Maria Formation ofBrazil (cf. Prestosuchus) , aredeposited at the Museum of the Universidad Nacionalde San Juan and the Museum of the Catholic Univer­sity ofPorto Alegre, respectively. They have two sacralvertebrae bearing well developed sacral ribs, and athird sacral with smaller sacral ribs. In the three citedsacra the bigger sacral ribs are robust and decidedlybent downwards, resting on the internal (functionally

dorsal) side of the ilium. Therefore, the distal ends ofthe sacral ribs project ventrally (Fig . 2).

The Rauisuchid Pelvis (Figs . 2, 5)-Practically com­plete pelves are known in Stagonosuchus (Huene, 1938),Ticinosuchus (Krebs, 1965) , cf. Prestosuchus (Fig. 2),Saurosuchus (Bonaparte, 1981) , and partially in cf. Po­posaurus (Galton, 1977) , genera that span the Middleand Late Triassic. The outstanding features of the pel­vis are: (a) ilium inclined ventrolatera lly, morpholog­ically primitive, contrasting with the advanced mor­phology of the pubis and ischium; (b) the proximalregion between the pubes is transversely wide, whilethat between the ischia in transversely narrow; (c) thedorsal side of the pelvis (Fig . 2) attached to the sacrumshows the wide sacral ribs resting on the internal (dor­sal) surface of the ilia , which are dorsally exposed.

The ilium, almost horizontal in advanced generasuch as cf. Prestosuchus or Saurosuchus, has a verylow iliac blade with a long posterior and a very smallanterior projections. The acetabulum is functionally

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F G IB

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FIGURE 4. Pubes and femora of archosaurs, to demonstrate differences within the subclass. A, the phytosaur Machaero­prosopus, after Camp (1930); B, pubis of Saurosuchus sp. , after Bonaparte (1981) ; C, femur of Ticin osuchus (internal view),after Krebs (1965); D, femur of Fasolasuchus. after Bonaparte (1981); E, the lagosuchid Lagosuchus , after Bonaparte (1975) ;F, the prosauropod Riojasaurus, after Bonaparte (1971); G, the carnosaur Piatnitzkysaurus, after Bonaparte (1979).

deep (shallow when the ilium is oriented vertically),large, closed, and bears long contact surfaces for thepubis and ischium. Morphologically it is similar to theilia ofthe phytosaurs, which had rather primitive limbswithin the context of the Middle and Late Triassicarchosaurs. Functionally, the rauisuchid ilium devel­oped notable specializations as it become graviportalin relation to the hind limbs (Fig. 5).

The pubes of the Rauisuchidae are always elongated,

showing different degree of specialization in each ge­nus . In Stagonosuchus and possibly Ticinosuchus theyare elongate, plate-like, and wide throughout theirlength. In cf. Prestosuchus , Saurosuchus, and cf. Po­posaurus, however, the distal portion of the pubis be­came progressively narrower at the same time an axialprocess (a "foot") appeared (Fig. 4), resembling someJurassic camosaurs such as Piatnitzkysaurus (Fig. 4)or Allosaurus in this respect. However, the proximal

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FIG URE 5. Pelvic girdl es and fem ora of Sa urosuchus (upper row) and cf. Lagerpeton (lower row) in lateral and posteri orviews . In the Sa urosuchus (rau isuch id) type the elongated pubis and ischium are associat ed wit h crurota rsal tar sus, and thetibia is sho rte r than the femur. In Lagerpeton the rather primitive pubis and ischium are curiously associated with an adv ancedmesotar sal tarsus, and the tibia is longer than the femur.

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region of the pubes has not progressed as in these car­nosaurs, showing none of the bone reductions presentin them, i.e. the reduction ofbone around the obturatorfenestra. The proximal region ofthe pubes is very widebecause ofthe lateral incl ination ofthe ilia. This featureis characteristic ofthe Rauisuchidae and, together withthe " foot" ofthe pubis, should be considered a derivedcharacter.

The ischia are elongated, each ofthem with a roughlytriangular section along the distal two-thirds, firml yunited through a thick symphysis, resulting in a strongshaft with a thickened end. The ventromedial laminaof the ischium is almost vestigial in the distal two­thirds and modestly developed in the proximal thirdof the bone. The dorsal surface of the two ischia isdepressed towards the axial plane, forming a longitu­dinal groove, whereas the opposite face is ventrallyangled.

The proximal region of the ischium shows the ace­tabular region well developed, with a long contact withthe ilium and a short one with the pubis. The transversewidth between the ischia is clearl y smaller than thatbetween the pubes, as a result of the lateral inclinationof the ilium (Fig. 5).

It is clear that every bone of the sacrum and pelviswas altered morphologically and functionally to achievea graviportal type of pelvis , naturally associated withvertical hind limbs. Consequently, the sacrum and pel­vis of the advanced Rauisuchidae show a group ofderived characters (synapomorphy) which include: (a)ventrally bent sacral ribs; (b) ilium almost horizontal;(c) narrow distal end of the pubis bearing an axialprocess; (d) rather large width between the proximalregions of the pubes; (e) narrow pro ximal region be­tween the ischia; (f) strong reduction of the ventrallamina ofthe ischia; (g)elongate, strong, rod-like ischiawith distal thickening.

The Rauisuchid Hind Limb- If we study the hindlimb of the Rauisuchidae within the concept of theosteology ofa "fully improved" condition (sensu Cha­rig, 1972), we will doubtless regard it as being of the"semi-improved" type. However, as explained below,this is not exactly the case.

The femur is elongated, gently sigmoid, without pro­nounced trochanters, and with a poor anteromedialprojection of the head. The tibia and fibula are mark­edly shorter than the femur, the tibia is not distally"twisted," and the fibula shows no reduction. The tar­sus is of the crurotarsal, crocodilian type , with a pro­nounced tuber calcanei. The pes was probably plan­tigrade, without bilateral symmetry.

FUNCTIONAL ANATOMY

The bending to a nearly horizontal position of therauisuchid ilium results in a ventrally oriented, deepacetabulum provided with a pronounced supra-ace­tabular crest. The transfer of the body weight to thefemur was through the roofofthe acetabulum, a surface

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that anatomically corresponds to its dorsomedial re­gion.

The position of the acetabular section of the iliumis more lateral than the whole ischium and the distaltwo-thirds of the pubis, and only the more proximalsection of the pubis is at the same lateral plane as thesupra-acetabular crest. Functionally, the femur hadroom to mo ve in a vertical plane from the acetabulumforward without touching the pubis, and backward atsome distance from the narrow ischium. The acetab­ulum of the Rauisuchidae did not develop fenestrationbecause the femur rolled against most of it.

The rauisuchid iliac blade is low, quite in contrastwith most ofthe "fully improved" archosaurs in whichit was high. If we consider that even in the primitiveproterosuchids, as well as in euparkeriids and erythro­suchids , the iliac blade is decidedly high , then it isprobable that the low iliac blade of the Rauisuchidaeis a derived character, resulting from its nearly hori­zontal position. Curiously, it resembles the ankylosaurilium, in which the acetabulum is also closed (Romer,1956:326).

The rather sigmoid femur of the Rauisuchidae, withvestigial trochanters and a modest projection of thefemoral head, appears to be primitive. However, it wasfunctionally advanced, and its function was probablyno less effective than that of the femur of primitivedinosaurs, i.e. the Prosauropoda (see Fig. 4) pro videdwith a well defined and medially projecting head, andobvious fourth and lesser trochanters.

My interpretation is that the vertical ilia with lat­erally oriented acetabula required, for the verticalityof the hind limb, a model of musculature and bonearrangement that resulted in the development of theadvanced type of femur and acetabulum present inboth orders ofdinosaurs. However, the improvementsin the rauisuchid hind limbs were achieved by meansof very different adaptations, in a great measure by thelateral inclination of the ilia and projecting the ace­tabulum onto the femur rather than the femur into theacetabulum as in dinosaurs. Such a solution, graduallyachieved, produced related modifications in the pubesand ischia to hold the limbs vertical, but not in thefemora which remained basically primitive.

Without doubt, the vert icality of the hind limbs wasan important trend among the archosaurs. The im­provements oflocomotion seen in the rauisuchids sug­gest that this trend materialized through at least twovery different evolutionary paths in the Thecodontia:

a} Retention of the original vertical position of theilium resulting in a laterally oriented acetabulum(Euparkeriidae, Ornithosuchidae, Lagosuchidae,and later on in Saurischia and Omithischia), withthe development of a progressively specialized fe­mur and the shift from plantigrade to digitigradeposture involving strong modifications in the tar­sus.

b} Development of a laterally inclined ilium with the

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acetabulum ventrally oriented (Rauisuchidae), withthe retention of a rather primitive hind limb withcrurotarsal tarsus and plantigrade posture.

An evaluation ofthe success ofboth types ofverticalhind limbs obviously favors the first type , with theacetabulum oriented laterally, and the femur withprominent trochanters and digitigrady. However, thesuccess of the graviportal type of the rauisuchids isdocumented by their ability to survive the notable fau­nal replacement that occurred during the middle LateTriassic (Bonaparte, 1982). The therapsid fauna wasreplaced by the archosaur fauna, and the prey of therauisuchids thus probably changed from cynodonts anddicynodonts to prosauropods, the latter with " fully im­proved" type of locomotion.

Acknowledgements-I am grateful to Drs . Ana MariaBaez, Alberto Cione, and Osvaldo A. Reig for criticalreading of the manuscript and valuable suggestions;and to Dr. J. Zidek for improving my English .

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