B. ButT-3m a. a.: Line morphologiscllO und fnnktionelle Analyse796

PAUWELS, F.: Uher die \Terteilnng der spongiosadiehte im coxalen Femurende und ibre Becloo. ; Gegenbaurs morph. Jalirb., Leipzig 125 (1979) 6, 5. 797—817tung für (lie Lehre vom funktionellen Ban des Knochens. Morph. Jb. 95 (1954) 35—54.

— Gesammelte Abbandlungen zur ftmktionellen Anatomic des BewegungsaPParates. Berlin: Sprin-

Aus dern Xoturhitorischen Muse urn Easel, Zoologisclee Abteilungger 1965.

— Atlas zur Biomechanik der gesunden nnd kranken Hüfte. Berlin: Springer 1973.

Roux, W.: Gesammelte AbhandlUngen fiber ieldtmg5me mk cler Organismen. 2 Bde.,

Leipzig: Engelmanfl 1895.SCHMITT, H. P.: Uber die Beziehung zwischen Dicbte und Festigkeit des Knoelrnns am Beiepiel

des mensebbeben Femur. Z. Anat. Entwicklgm 127 (1968) 1—24.

Smuco, K.: Uber die Hüftpfanne. I. Mitt. Z. Morph. Anthrop. 17 (1915) 325—356.

TmbMANN, B.: Die Beansprueltung des mensebliehea Huftgelenks. Hf. Die Form der Faeies

lunata. Z. Anat. Rntwickl.gesCll. 128 (1969) 329—349. 1 A Functional Interpretation of the Varanid Dentition— Zur Lokalisation von clegenerativea eranderungen am Femurkopf bei der coxartlirose. Z.(Reptilia Lacertilia, Varanidae)Orthop. 111 (1973) 23—27.

WALDEYEE, T: Das Beeken. Bonn: Cohen 1899.

WttuEt, H.: Die Dicko der mensebliehen GelenkknOrPel. Mccl. Diss. Univ. Berlin 1897.

Oliver Rieppeli)r. EArNER, Batun, I

Dr. HnNz.JUReEN KURRAT

Anatomiscbes Institut Received for publication 2. February 1979

Dr. WALTER OBERLANDER nod With 14 figures and 2 tables

der Univereität zu Koln, aad in revised form 11. April 1979Lindenburg

D - 5000 Kbln 41A. Introduclion

I Cranial kinesis in Modern lizards is a long standing and much debated problem inthe area of functional anatomy. Classical contributions are those of BuADLEY (1903)and Vnstuxs (1912). More recently, FRAZZETTA (1962) renewed the interest in thefunctin of the lizard skull with the introduction of the quadric crank chain as anI explanatory model. IORDANSKY (1966, 1970) and Gostus and GA5C (1973) fnrtherelaborated the ideas put forward by FRAzzErn (1962). All of these models, as wellI as the one presented in the present contribution, still have to he tested by clectro1 myography. The only electromyographic data on the head muscles of lizards available to date are those presented by TumoelniouroN (1978) on the m. pterygoideusand vi. depressor mandthulae of Uromaslix.The adaptive significance of cranial kinesis has been discussed by FRAzZETTA (1962),IOflDNsuy (1966) and THoIsoN (1967), but analyses of feeding lizards are rare.PRAzZETTA (1962) described feeding in Gerrhoaotus. and TilnocKluonroN (1976) preSeated a einéradiographie analysis of feeding in Iguana and Urornastix, two herbiVorous lizards. The only published data on feeding in T7aranus are the few observations made by BOLTT and Ewun (1964). A detailed cinematographic analysis ofI feeding hi Fan ens has been attempted by hIPEY (1967). His still unpublished acI toat differs in important respects from the observations presented here.‘ The present study reports cinCradiographic analysis of feeding in Papa nus. Themovements in the head of Paranus are related to the function of the dentition. ItWifi be argued that amphikiaesis is correlated with the proper functioning of thebeth in T7aranus, which is critical for successfully seizing and handling large prey.

i

0. Riurrun: A Functional Interpretation of the Varanicl Dentition

Varanus is a lizard genus adapted to relatively large prey. This is reflected in the mechanics of its head. Consequently, no generalisations will be attempted concerningthe amphikinesis among Modern lizards from a mechanical point of view.

The study reported on here is part of a thesis presented to the Faculty of NaturalSciences at the University of Basel in partial fulfillment of the requirements for thedegree of a Dr. phil.

B. Material and Methods

For the study of cranial kinesis on Vurenus, four live specimens of ]7 bengolensis and onespecimen of J7 exenthemeticsis elbiguleris wore available. In addition, observations could be made

on live V. komoclocesis held at the Zoo Basel.

Cinematography was clone with a Canon 814 camera using Kodachrome super 8 movie film,

and with a Baulieu 14-16 E]ectronix camera using Kodak 4-X reversal film 7277 (16 mm). (‘mi.radiography was clone at the Kantonsspital Basel using Kodak film RAE 2498 (35 mm). The

analysis of the X-ray film was clone with a Tagarne-35 analyser.

For their use in stress analysis photoelastic plates were made out of Araldite B. The plates

were molded to the thickness of 4mm at a temperature of 120 °C. The analysis of isochromatic

fringes and of isoclinics was clone by means of a simple polaroid analyser as described by MöNcH

and Forra (1959) at the (‘ii a-Geigy 0 Brnl.

The apprec cli to the ctructure of tI e head of Voranus is based on the following mueuio spe

cimens. Specimens marked by an osterik have been dissected, the others are dried skulls.

Vei-onns ecomithuros (FNNH 98935); V. beegeleeois (three imeatalegued specimens555, MBS

13 162*’; V. dutserilii (1\lBS 3721*); V. duwerilii heteropholis (FMNH 1457]]); V. exoothcosaticus

olbigehois (one uocatalogued specimen5, MBS 5819, FMNH 51683, 17142,17143); V. cranthe

rnotirus microstictvs (MCZ 47369, 47375); J7 gilleni (BMNH 1910.5.28.13); V. geuldi (FMNH

31338,31340, 51706, MCZ 33916); J7• prisms (MBS 5819*, BMNH 1974.2481, 1974.2482; FMNH

3] 308, 51705); V. kosnodeeesis (BMNH 1934.9.2.1., FM1H 22200); V. iodicus (MBS 11099*);

V. vehnlesns (BMNH 1931.1.12.1); T7. vileticus (MBS. 3729*, ]3453*, ]9642, SMF 33251, 33252,

F5]H 17145, 22084, MCZ 1066); T. eec/lotus (FMNH 22354); V. presinus (MBS 10 504*); V. re

dice//is (MBS 18874, FMNH 98947, 131538); J7 so/voter (MBS 3713*, 3736*, 6138*, 10755*,

osteol. 5028, 5878); V. stone (MBS 17891*); V. venus (AMNH 28698, 73361, FMNH 1492).

Abbreviations: AMNH. American Museum of Natural History, New York; BMNH British Mu-

scum of Natural History, London; FMNH, Field Museum of Natural History, Chicago; MBS,

Natural History Museum,Basel, osteol, = osteological collection; MCZ Museum of Comparati’e

Zoology, Cambridge, Mass.

C. The mechanical nnits of the sknll of Varanus

The decomposition of the kinetic lizard skull into various functional components

goes back to VER5LUY5 (1912). He subdivided the lizard skull into the “oeeipfl

segment” and into the “maxillary segment” which represent the braincase and the

dermatocranium respectively. The braincase is movably suspended within the des’

matocranium on its paroccipital processes posterodorsally and on its basipterygotd

processes anteroventrally (Fig. 1). As the maxillary segment moves up and down

around the braincase, the cartilaginous processus ascendens which projects from the

anteromesial tip of the supraoccipital bone slides back and forth in a trough on tIlE

caudal edge of the parietal. This is the metakinetic joint.

The metakinetic joint is regularly fused up through the development of a sutil&

contact between the front edge of the supraoccipital and the hind edge of the par1e

bI in large (adult) speciauens of Varanus exanthematicus, T. niloticus and V. kornodoen sir. The same apparently occurred in the extinct giant varanid Megalanja fromthe Pleistocene of Australia (HEcHr, 1975). The obliteration of the metakinetic jointappears to be size dependent, but the functional reason of its obliteration is notUnderstoodVnsys (1912) recognised a further joint within the derluatocranium of Varanuslocated between the frontal and the parietal bones, which he called the mesokineticoit This system has been more fully explored by FRAzzETTA (1962) who showedmesk5 to eharacterise the great majority of Modern lizards. The lack of mesokinesis seems to be due to secondary fusion or interlocking of the bones of the deratocraniuiu As FuAzzErrA (1962) realised, true iuesokinesis not only involves thePOssibility of flexion between frontal and parietal bones, but also involves a hypokinetic joint within the palate (Fig. 1, hy.j.). The combination of the mesokineticand hypokinetic joints allows the luuzzle unit to rotate dorsally and ventrally relativeto the rest of the dermatocranimu1 The muzzle unit or the snout complex of Varanus consists of the mesial premaxillaand the fused nasals and of the paired maxillae, septomaxillae, jugals, lacrimals,Prefrontals, vomers and palatines (Fig. 2). These bones move as a unit relative to thest of the dermatoeraniusu The dorsal hinge is located at the frontoparietal sutuse.The latter represents a more or less straight transverse line with slight interdigitation°y (Fig. 3). The sutural plane is oblique, in that the lower edge of the hind end of

J

7980. Rirppn: A Functionai Interpretation of the Varanid Dentition 799

Fig. 1. The application of the quadric crank chain model (FRAzzETTA 1962) to theskull of Varonus se/veto,. Abbreviations: a, angular; ar, articular; bs, basisphenoid;c, eoronoid; d, dentary; ep, epipterygoid; f, frontal; hy.j, hypokinetic joint; ju, jugal;I, lacrimal; mes.j, mesokinetie joint; mx maxifla; a, nasal; p, parietal; pmx, premaxihla;p0, postorhitofro. pot, prootie; prf, prefrontal; ps, parasphenoid; pt, pterygoid;q, quadrate; Sm, septomaxilla; sq, squamos; st, supratemporal (From Riuppan. 1978,by permission of the Birkhauser Verlag, Basci)

Fig. 3. The mosokinetic joint of Uoron?8 solcotor (MBS. 5028)

Key to abbreviationS as in Fig. 1. Scale equc Is 10 mm

the frontal lies below the upper edge of the front end of the parietal. Towards the

lateral edges of the suture, the parietal forms distinct anterior lappets which art’

eulate in lateral notches on the hind edge of the frontal. The ventral hinge, the hypo

kinetic joint, consists of a lateral and a mesial component. The mesial cmupoildhlt lieS

between the palatine and the pterygoid. The lateral component lies betu een the

maxifia and jugal of the muzzle unit on the one hand and the ectopterygoid Of the

basal unit (see below) on the other hand.The maxillary segment is broken down into five components by FnazZETTk (1962)

The muzzle unit described above links the parietal unit dorsally to the basal unit

ventrally. The parietal unit is made up from the parietal, supratemPorals 11toit0

frontals and squaiuosals. The basal units are made up from the ptcrygoids and ecto

0. RIErPEL: A Fnnetional Interprotation of the Varanid Dentition

pterygoids. Caudally the quadrates (quadrate units) link the parietal unit to thebasal units. The quadrates receive the caudal tips of the supratemporals and of thesquamosals in two collateral, shallow ai1icular facets on the dorsal surfaces of theircephalic condyles. The epipterygoid bone is the last component of the maxillarysegment of VER5LUY5 (1912). The foot of the epipterygoid rests in the pterygoid orcoluuxellar fossa on the dorsal surface of the pterygoid. The dorsal head of the epipterygoid is bound to the alar process of the prootie and to the deseensus parietalisby fibrous ligaments.

B. The mandibular joint

The mandibular joint of l’aranns is a complex structure (Fig. 4) in order to allowrotational movements both in the vertical and in the transverse plane. The glenoidsurface of the articular bone of the lower jaw is concave from front to back, but itis convex from mesially to laterally. It shows a saddle-shaped surface. The mandibularcondyle of the quadrate bears a mesial and a lateral convex head with a longitudinallyoriented cavity between the two heads to match the convexity of the glenoid surfaceof the artieular. The mesial side of the glenoid surface receiving the ouesial mandibular coudyle of the quadrate lies deeper than the lateral part of the glenoid surfacereceiving the lateral mandibular eomlyle.

I

800 0. thErruL: A functional Interpretation of the Varenid Dentition

801

ju

Fig. 2. The snout complex of T7eraans bciiyeleiisis (MBS. 2410). Key to abbr,viatioflh

as in Fig. 1. Scale equals 10 mm

par.cgI.s

B

I mc

I S.

C

Fig. 4. The mandibular joint of Varoans solcotor (MBS. 3713). A. The hind end of theright lower jaw in dorsal view. B. The land end of the right lower jaw in mesial view.C. The mandibular condyle of the quadrate in ventral view. Abbreviations: elit, cliordatvmparn forainen ; gl.s, glenoid surface on art eular; line, lateral mandibular condylemine, mesial mandibular condyle; pare, prcartieular cendyle ; rap, retrearticular process. Scale equals 5 mm0Jrpli. lb. 125/0

L

0. Riurran: A Functional Interpretation of the Varanid Dentition

The structure of the mandibular joint results in a lateral rotation of the lower jaw

around its long axis as it is depressed and in a mesial rotation around the long axis

as the lower jaw is elevated. Such a rotation of the lower was proposed by BRADLEY

(1903) on the basis of similar arguments.

When the jaws are fully adducted the dentary (as well as the maxillary) teeth pohit

slightly inwards. As the jaw is depressed relative to the long axis of the quadrate it

rotates around the dceper mesial side of the glenoid surface: the teeth are rtated

outward. They thus assume a fully erect position so that they hit the prey with their

tips instead of with the lateral surfaces of their crowns. Convcrsly, as the jaw is ele

vated relative to the iong axis of the quadrate, it rotates aronnd the higher lateral

side of the glenoid surface. Consequently the teeth rotate back inward again and

thus produce a clamping effect on the prey held between the jaws.

The second feature of the mandibular joint which has to he emphasised is the strong

anterior elevation of the glcnoid surface on the articular bone. Anteromesial to the

glenoid surface the surangular is also elevated and forms a prearticular condyle. Thc

anterior elevation of the glenoid surface provides an increase of area on which the

lower jaw can he rotated during depression (TmwcK3ionToN, 1976). But it also

locks the lower jaw firmly against the mandibular condyle of the quadrate whea

retractive forces are transmitted from the lower jaw through the mandibular joint

to the quadrate. As it will be shown below, the depression of the mnzzle unit is func

tionally important during the swallowing process in Varanus. Depression of the

muzzle unit results from retraction of the basal unit which can be achieved by back

w-ard rotation of the quadrate around its dorsal suspensioii. Jaw adductor muscles

which insert obliquely into the lower jaw will exert a force which can be broken down

into an adduetive component working at a right angle to the lower jaw and into a

retractive component which works along the long axis of the lower jaw.

FRAZZETTA (1962) introduced the model of a kinematic chain which was further

elaborated by IORDANSKY (1966, 1970) to describe the movements which occur

within the maxillary segment of an amphikinetie lizard skull during feeding. The

laeertilian dermatoeranium would represent a quadrie crank chain (Fig. 1). A kine

matic chain is an assembly of links jointed together in such a way that if one link

is fixed and one is moved, the whole chain moves in a predictable way (FRAzZErT&

1966; ALEXANDER, 1968). A quadrie crank chain consists of four links jointed together

by hinge joints the axes of which are parallel to each other.

In the skull of Varanus, the four links would be represented by the mozzle un1

(anterior link), by the parietal unit (dorsal link), by the quadrate unit (eaudal link)

and by the basal unit (ventral link). For the function of the quadrie crank chain it

is critical that all the four joints involved be hinge joints, which do not allow tran

lational movements. The critical point here is the quadrate-pterygoid contact. Ex

amination of dissected heads of Varanus where all tendons and ligaments WCTh Ic

i ntaet showed that very little translational movement occurs between the pterygd’

0. Rizp: A Functjonaj interpretation of the Varanid Dent itjon

and the quadrate. The small amount of translational movement possible results fromthe elasticity of the ligaments of the joint capsule.

F. The Analysis of Feeding Movements in Varanus bengaknsjsThe present account deals with Varanus Sengalensis of which four live specimenswere available for study. The animals were regularly fed on mice. Five feeding sequences were filmed with an ordinary super-S movie cassette at a speed of 40 framesper second. Two further feeding sequences were recorded on a 16 mm black andI white film at the speed of 64 frames per second. An eighth feeding sequence wasfilmed with an X-ray camera at a speed of 48 frames per second. Cursorial observations were also made on one specimen of Varanus exantheniatices albigularis andon one adult specimen of Varanus komodoensis.

When Farce us bengaiensi.s locates prey, it immediately starts to pursue it, seizingit wherever possible without any preference for any particular part of the body. Thepi.ey is vigorously shaken and banged against the substrate, rocks, logs, etc. untilit is immobilized or dead. The JTaran mar then usually drops the prey and starts searcliiag for the prey’s head. It has been shown by Loop (1974) that varanids feeding onsmall mammals show a signifieamrt preference to swallow- the prey head first. Oncethe Varaijus has started to swallow- its prey, “kinetic inertial feeding” (GAN5, 1961,1969) is observed. thuS (1961: 218-219) describes “kinetic inertial feeding” as follows: “The jaw-s release the prey quite suddenly and the entire head shifts forwardand laterally, then bites in a new- position. The inertia of the prey reduces preyshifting. The head continues to sluft from side to side in a w-alking sequence thatgraduajly produces ingestion of the prey - .. The sequence results in a pushing ofthe head over the prey rather than drawring the prey into the month.” GANs (1961)i’5 FRAZZEYn who noted that the lateral excursion of the forward stroke of thehead does not occur in all lizards during kinetic inertial feeding. In Varanus bengahens the lateral excursion of the head occurs only occasionally It may also be notedthat J’aranns bee gaiensjs combines the kinetic inertial feeding method witli the posSibihty to draw- or rather to push the prey back into the throat through jaw mobility(see belowr)

Fig. 5 show’s outline tracings of one kinetic inertial feeding cycle from the X-rayifim of Vat-anus bengale,i.sjs and w-ill serve as basis for the description of the headmovements as they occur during one cycle of jaw’ opening and closure (one orbit).1o frame 1, the animal stands on its erect fore-limbs and holds the prey betw-een itsJaws off the ground. Mouth opening lasts until frame 40. The head is rotated dorsallyaround the oeeipitoatlantal joint, and simultaneously the muzzle unit is elevated.can be seen from frames 3 to 9, the muzzle starts to he elevated before any rotaon of the head around the occipital condyle takes place. On frame 9, the maxillarybeth are cleared off the prey. Therefore, muzzle elevation alone w-itliout any move-I %t of the head or of the mandible is sufficient to free the maxillary teeth from thePrey during the initial phases of the orbit. The muzzle is further elevated during the1Pward rotation of the head around the occipital eondyle, and it reaches its most$2’

802803

F. The Quadric Crank Chain Model

N

Fig. 5. Outline tracings of a feeding sequence of Verne us bengatensis recorded by cine

radiography at a speed of 48 frames a second

extremely elevated position on frame 16, that is before the mouth is maximally

opened. Maximal mouth opening is achieved on frame 40. It is essential to note that

opening of the mouth is brought about by upward rotation of the head around the

occipito-atlantal joint. The mandible remains stationary and supports the prey.

As the mouth is maximally opened, the forward stroke of the head characteTiSt1

of the kinetic inertial feeding follows. Initially, the head moves very rapidly in an

anterovesitral direction. The gape is held constant and the head now shifts across

the prey (Fig. 5 B). Following this movement there is a horizontally directed phase

of the forward stroke of the head. The head now rotates downward around the oCCI

pita.l conth’le, and simultaneously the snout is depressed. This closure of the gape

brings the maxillary teeth into contact with the prey again. The final phase of the’

forward stroke of the head is directed anteroventrally again. The tip of the snoUt

now closely approaches the substrate. The head is strollgly rotated downward around

the occipito-atlantal joint and the muzzle unit is being strongly depressed. This strong

and final depression of the muzzle unit pushes the prey further into the mouth and

down the throat.

804 0. Rixrz’xi: A Functional Interpretation of the Vara.nid Dontition 0. RiEpp: A Functional Interpretation of the Varanid Dentition

48

N-—- -

—..-:

16

7/

805

20

A

Fig. 6. The cervical vertebrae (atlas and axis) of A: Betoderma s. 8Uspectwrn (MB$.81594) and B: I’aranus griseus (MBS. 5819). Scale ecjuals mm

50 -- -

N

/7A

A B

-1

D

\\\

99

\

\

Fig. 7. The application of the quadrie crank chain model to illustrate the movements

as they occur in the head of Varanus bengalensis during feeding

J

0. Rtm’rEL: A Functional Interpretation of the Varanid Dentition

Varanus bengalensis does not perform a kinetic inertial feeding cycle with each

orbit. Often a different method of pushing the prey down the throat is shown. Tho

animal presses the prey against the substrate. It opens the mouth, pushes hard against

the prey and closes the jaws tightly again, thereby depressing the muzzle unit.

During the kinetic inertial cycle, it is advantageous for the animal not only to

maximally accelerate the head but also the maximize the excursion of the head

during the forward stroke. The longer the excursion is, the more is the head likely to

shift across the prey. Varanid lizards, which are adapted to relatively large prey,

show an elongated neck which results from an increase in number of cervical verte

brae and from an increase in length of the individual cervical vertebrae (HOFE5TETTER

and GAse. 1969. and Fig. 6).

Fig. 7 shows the applieaioii of FRAzzETTA’s (1962) quadrie crank chain model to

the analysis of another kinetic inertial feeding sequence of Varanus bengalensis as

it is observed on the X-ray film. The model as it is drawn in Fig. 7 does not correspond

to the actual movements of the head which are generated in the neck region. Rather

it is used in order to allow measurement of degrees of rotation. Joint X is considered

to be the fixed reference point within the quadrie crank chain (FAZzETTA, 1962),

and for the sake of easy comparison, the parietal unit is always drawn in a horizontal

position. The analysis shows that starting from the rest position (Fig. 7 A: frame 1)

the muzzle unit is elevated for 9° while the mouth is maximally opened. The qua

drate is protra etc ci for 210. The angle between the basal unit of the skull and the

dentary is 48° at the stage of maximal jaw opening.

Following the forward stroke of the head, the skull is rotated downward around

the oeeipito-atlantal joint while the muzzle unit is depressed. At the stage of maximal

depression of the muzzle unit (Fig. 7 D: frame 99) the latter has rotated around 150

compared to the stage of maximal muzzle elevation: thus the muzzle is 6° lower than

it is at the rest position. Correspondingly, the quadrate has rotated backward around

27° compared with the maximally protracted stage. This is 6° further backward than

it is at rest position. The analysis shows that during the final stages of each orbit,

the snout is depressed below its rest position, and that this final muzzle depression

pushes the prey further down the throat.

The preeeeding description revealed some implications of cranial kinesis with

respect to feeding (jaw movements as related to food intake). The following sections

will be clealmg with the adaptive significance which cranial kinesis may have in rela

tion to the structure anti function of the clentition.

J’cu-anus is adapted to relatively large prey. The functional significance of tooth

shape has been discussed in detail by FEAzzETTA (1966). ‘What is critical in sueeeSs

fully ;eizing and handling prey is not only the absolute size of the tooth but also lts

degree of reeurvature (Fig. 8). The two parameters may be compensatory. A stronglY

reeurved tooth will have the same or even the better holding effect even if it is ab

solutely shorter than a straight tooth. Or, to put it in other words, reeurvature 0f a

tooth allows the reduction of the latter’s length while maintaining the same holdingeffect. At a given maximal gape, the shorter tooth will interfere less with the uptakeof relatively large prey.Reeurvature of a tooth may be subject to some compromise, however. The tooth‘hits the prey most effectively if it hits the prey with its tip. Force is then transmittedthrough a minimal area which results in maximal stress on the prey’s surface. Penetration of the prey’s skin is then most likely to occur. But to erect a strongly recurvedtooth enough so that it hits the prey with its tip wUl require a kinetic skull or a verywide gape.

I II. The Ontogeny of the Varanid Dentition

The tooth development in T’aranus has been described in great detail by BULLET(1942). The replaeemeiit tooth grows in an interdental position and migrates anterohterallv when the preceeding functional tooth is shed. During the ontogenetiedevelopment the pulp cavity of the tooth beeemes subdivided by dentine infolding.The development of such plieidentine is characteristic of T7aranus and of all otherPlatynota lizards. In the adult tooth, plieidentine results in the subdivision of thePulp cavity through a complex system of nnastomosg dentine lamellae (Fig. 9, A,I E). As the replacement tooth moves into its functional site, the surface of the toothI bearing hone forms a system of irregulat anastomosing ridges. To these ridges thebases of the dentine lamellae of the plieidentine become ankylosed through the‘bone of attachment” (PooLE. 1967). When the functional tooth is to be shed, thebone of attachment and the dentine just above it are removed through resorption.In Varann.s resorption has to occur sirnultaneousl3, all along the base of each dentine‘amelIa0 of the plieidentine. Only towards final stages of tooth replacement do largelranuloeytes attack the surface of the dentine mantle low down on the tooth baseT aranus. But as the pulp cavity is subdivided by dentine lamellae. no resorptionPits develop comparable to those of other lizards. In most lizards, the replacementth move into resorption pits antI come up from below the preeceding functional

806 0. Risppn: A Functional Interpietation of the Varanid Dentition 807

/---------

Fig. 8. Prey penetrated by a straight (solid) and a recorved tdotted) tooth. To freeitself, the prey must deform its surfhco or rupture it to the depth h. The straightand the recurved tooth penetrate to the same depth Ii, although the recurvedi tooth isabsolutely shorter by ci than the straight tooth,

G. The Functional Interpretation of Varanid Dentition

I-

tooth. But due to the development of plicidentine, the replacement teeth of platY

notans develop in an interdental position. This results in the discontinuity of func

tion at any given site in the jaws of Varanws or other platynotans, a functiofl 1

disadvantage which must be compensated by a functional advantage of plieidentiTleVaranus is a pleurodont lizard. The tooth becomes ankylosed to an obliquely slop-

1ing dental shelf on the tooth bearing bone. The teeth of Varanus show widely flared

bases which in some cases may be overgrown by bone (adult specimens of Var’° 1niloticus) what gives the teeth further support. Striations on the bases of the teeth Iresult from the formation of plicidentine.

The crorn of the adult tooth of Varanus is a blade — like structure, laterally C01fl1

pressed with a recurved tip and an anterior and posterior serrated cuthng edge. It

is curved in two directions. The transverse section through a dentary tooth of Vartsaleator (Fig. 9) shows the lingual wall of the tooth base to buldge mesially, produdhh1

I/1/

/ I/ 1

/ I/ 4’

/ /

LI4.

Fig. 10. Succossive growth stages of replacement teeth of the 0th tooth family on theright maxilla of Veranus selmtor (MBS. 0138). The functional tooth is to the heft. Thescale equals 1 mm

a convex surface. The tip of the tooth is concave mesially: it is bent in a lingual direetion.In lateral view, the tooth is recurved. Ontogenetic growth series (Fig. 10) showthat the replacement teeth are ahuost symmetrical during the earliest stages of calcification. The recurvature of the tip of the tooth is more pronounced during laterstages of ontogeny than it will be in the functional tooth, however. Apparently, thepostrior cutting edge grows faster than the anterior cutting edge shortly before thetooth moves into its functional site. This developmental pattern leads to the formation of a biconvcx shaft on which the reeurved tip is observed.As it was noted by MERTEN5 (1942) already, the degree of reeurvature changesalong the tooth row. To obtain some objective statements on the degree of toothI reeurvature, the circumferential line was constructed which represents the axis ofthe tooth in lateral projection. Then the tangential line was constructed on this axisthrough the tip of the tooth. Thus the angle could be measured at which this tangential hile intersects the long axis of the tooth bearing bone. The result of a series ofmaxillae and dentaries of T7aranus salvator representing various growth stages aregiven in table 1 and 2. Recurvature increases towards the rear of the tooth row bothin the upper and lower quadrants (Fig. 11). Reeurvature increases relatively morein the maxillary tooth row, and the maxillary teeth are absolutely more stronglyreeru-ved than the dentary teeth.

jI. The Posterior Curvature of the Teeth

Recurvature of the teeth is of adaptive significance in successfully seizing andhandling prey (Fig. 8). Selection will tend to produce a maximum of recurvature.However, the more reeurved a tooth is, the more difficult is it to bring the tooth

808

I; •‘

I I

0. Rrurnn: A Functional Interpretation of the Varanid Dentition. A Functional Interpretation of the Varanid Dentition

4!I

I

809

I I

Fig. 9. Transverse groundx20;B: x00

section through a dentary tooth of Verenus selceber. A:

MBS- Tooth positions

specimen1 2 3 4 6 7 8 9 10 H

numbers

10755 H 74 73.5 58 60 59.8 59 60

10755 L 76 72 62 55 57 48.5 46.5 60 55 66.2

6 138 H 77 78 71.2 66.8 63 73 64 63 73.8

6138L 82 80 77 60 67 71 63 67 72

Table 2

Degrees of reeorvation of the teeth along the tooth row in seven specimens (loft maxillaries) of

Voronus solvotor. The angle is measured between the tangential line indicating tho direction into

which points the tip of the tooth and a horizontal line representing the lower edge of the maxillary

bone (Fig. 23). An increase of the recurvation results in the decrease of the numerical value of the

angle

MBS-specimen Tooth positions

numbers1 2 3 4 6 7 8 9 10 11 12

3713 69.5 78.8 60.8 55.5 56.9 45.8

3834 79 74 53.2 67 59 43

10 755 78.8 79 85.4 69.5 58 59 60.3 54.2

5028 85.5 77 76 74.5 76 65 61 57.2

3 835 74.5 74 67.5 55 66.5 65 62

5878 77.8 69 59.5 55 53.2 51 53.5

5901 68.5 63 67.2 50.5 53 56 62

into a position so that it may hit the prey with its tip. Yet it is critical for the tooth

to hit the prey with its tip. In doing so, maximal stress is exerted on the prey’s

surface and its penetration is more likely to be accomplished. A second critical

factor of tooth function is the axial loading of the tooth. This can only be achieved

if the tooth hits the prey with its tip. Axial loading reduces the bending moments

to which the tooth is subjected during the strike.To bring a revurved tooth into an erect position it is most economical to rotate

the tooth along the circumferential line which represents its longitudinal axis. The

stronger the tooth is recurved, the closer will the center of rotation approach the

posterior cutting edge of the tooth. The lower jaw is hinged on the mandibular joint

which lies far posterior to the tooth row. Yet, the posterior dentary teeth are closer

to the mandibular joint than are the anterior teeth, and hence the posterior tE’eth

show an increase in degree of recurvature which compensates a decrease in absolute

size.

810 0. Rixnxn: A Functional Interpretation of the Varanid Dentition

Table 1

Same values as in Table 2, but for the dentaries of two specimens of Ttcronus solvotorwith complete dentitior.. L: left, R: right

0. Rizrrrr4: A Functional Interpretation of the Varanid Dentition 811

Fig. 11. Recurvation of the teeth along the tooth row in, top: maxilla of Varenns .901-voter (MBS. 5878), bottom: dentary of Forming indicus (MBS. 11099)

meso meta

•occ hy

J

Fig. 12. Loft maxillary dentition of Voronns solvator (MBS. 10755) rotated around the]iypokinetie joint only (by; solid line) or around the occipito.atlantal joint only (0cc;

dasliod

line). The rotation around the hypokinetic joint matchos the roeurvation ofthe teeth. Abbreviations: hy, liypokinetic joint; moso, mesokinetic joint; motn, rnotakinotik joint; oec, occipital condylo. Scab equals 10mm

J [

0. RIErrEL: A Functional Interpretation of the Varanid Dentition

The maxilla is hinged on the occipital condyle far back behind the functionaltooth row, but it is also hinged on the mesokinetic axis. Rotation of the muzzle unitis made possible through the combination of the hypokinetic and the mesokineticjoints (Fig. 12).

As the hypokinetic joint lies immediately behind the maxillary bone, a greaterdegree of reeurvaftre of the maxillary teeth can be expected in comparison with thedentary teeth. This is indeed the case, and again the more posterior teeth approaching the hypokinetie joint more closely show a relative increase in recurvature but adecrease in absolute size in comparison with the anterior teeth. The increase in degreeof recurvature may be critical for the function of the maxillary teeth since it is theupper tooth row which is more active during the feeding process.

The combined rotation of the upper jaw around the hypokinetie and mesokinetie jointsand around the occipito-atlantal joint not only allows an increase in degree of recurvature, but it also alters the line of action of a given tooth (Fig. 13). The line along whicha tooth tip of the upper tooth row will travel was studied with a simple mechanical modelof a TTaranus head, constructed according to the quadric crank chain model of FRAz

ZETTA (1962). If the maxilla rotates around the occipito-atlantal joint only, eerymaxillary tooth tip will describe the upper left quadrant of a circle line (with thesnout pointing to the left, Fig. 13 A). The gape required to bring the tooth into afully erect position is wide, and the tooth points backward at a very early stage of

jaw closure already. Thus the tip of the tooth may point backward as it hits the prey.It may hit the prey with its anterior cutting edge. Moreover, the line of action of thetooth contains a forward component of the strike ST which will affect the directionof the resultant force produced by the tooth as it hits the prey. Thus, if a maxilla

0. Rriri’xe: A Fnnctional Interpretation of the Varanid Dentition

which bears recurved teeth is rotated around the occipito-atlantal joint only, therecurved teeth are more likely to hit the prey with their anterior cutting edges andhence the teeth will not be loaded axially. Also, the resultant force produced by theteeth is likely to point in an anterovenfral direction and the prey is then likely toslip off the anterior cutting edge (I0RDANSKY, 1966).

If the maxilla is rotated sinaultaneously around the occipito-atlantal joint andaround the hypokinetic and mesokinetic joint (Fig. 13 B), the line of action of the teethis changed. The gape required to bring the teeth into a fully erect position is smaller. Theteeth are more likely to hit the prey with their tips which results in axial loading of theteeth. The anterior force component of the strike ST is reduced and combined witha posterior component which results from the simultaneous snout depression duringjaw closure. The resultant force produced by the teeth is now directed posteroventrally, driving the teeth into the prey and the prey further down the throat (IonnxNsuv, 1966).

The dentary teeth of Varanus are not only curved in a posterior direction but alsoin a lhgual direction as it is shown by the transverse section (Fig. 9). Axial loadingis not only critical in the sagittal plane but also in the transverse plane. In order to1 be axially loaded in the transverse plane it is necessary for the teeth to be rotatedlabially before they hit the prey. A labial rotation of the dentary during mouthopening is easily understood on the basis of the structure of the mandibular jointdescribed above.

When Varanus is seizing and handling large prey, a heavy load is imposed on itsteeth and on their attachment to the tooth hearing hones. The flared bases are oneof the characteristics of the varanid teeth. Broad bases reduce the forces workingI per unit area. Yet, tooth attachment is on an obliquely sloping bony surface (pleurodonty) and upon loading of the tooth a couple will build up which produces tensionon the one and eouipression on the other side of the tooth base. Resulting shearstresses would tend to break the tooth off the bone, especially since mineralized hardI tissues have a weak resistance to shear stresses (WAINwRmHT, Bmos, CURRY andGOSLINE, 1976).

The transverse section through a varanid tooth (Fig. 9) shows that the labial wallof the tooth base rests on the obliquely sloping shelf of the tooth bearing bone. Themesial rim of the tooth bearing shelf is horizontal and was termed Basalpicue byLESSMANN (1952). On it rests the lingual wall of the tooth base. The mesial edge ofthe tooth bearing shelf may slant upward and was called Theka by LES5MANN (1952).

The dentine lamellae which subdivide the pulp cavity as a result of the formationof plicideutine appear as dentine trabeeulae in the transverse section (Fig. 9). Thedentine trabeeulae run in an axial direction and meet the tooth bearing bone at rightangles in any part of the tooth base (Fig. 9 B). The dentine trabeeulae ate connectedWith each other by weak transverse anastomoses. Thus the base of the tooth beComes filled by a network of dentine lamellae in a way very similar to spongy bone.

812

813

K. Tooth attachment in J7eranus

B

Fig. 13. A: Maxillary tooth of Vera, us rotated around the oeeipito.atlantal jont only.

B: Maxillary tooth of Vereneis rotated simlIltaneoIsly around tIle oeeipitoatlt

joint and around the Itypokinetie and mesokinetie joint. ST: Forward componen’ °f the

strike

I.

0. Riurrar,: A Functional Interpretation of the Varanid Dentition

This will certainly increase the stability of the tooth base. The formation of pliciden.

tine in platynotan lizards may indeed be correlated with the large. size of their teeth(PEnn. 1968).

To study the stress distribution within the tooth base, plate models were cut from

plates of Araldite B to the size 35 times the size ofthe original tooth. Their shaperepresented transverse sections at various levels through the functional tooth. The

tooth models were loaded and inspected with light passing through crossed polaroids

between which the models were mounted in an iron frame. Isoclinics appear as dark

lines on the model connecting these parts of the model where tbe directions of the

principal stresses are either parallel or perpendicular to the optical axis of the pola

roids. If the polaroids are rotated stepwise around 90°, a series of sets of isoelinics

will be observed wandering across the model. Each set of isoclinics corresponds to

each step of rotation of the optic axis of the polaroids. The observed sets of isoclinics

can be superimposed on an outline drawing of the model. and with the indication

of the correspondiug optical axis of the polaroids the direction of the principal stresses

all across the model can he found. The isoelinics by themselves do not differentiate

between tensile and compressive stresses, but these can be inferred by pressing one’s

finger — nail into the edge of the model. If the isoelinies are attracted toward this

stressing edge, tensile stress is indicated. If the isoelinics withdraw from the stressing

edge, compressive stresses are indicated (Nageiprobe: FöPPL and MöNdH, 1959). Note

that for any point of the model the direction of one principal stress (tension or corn

pression) is by definition perpendicular to the direction of the other principal stress

(centrifugal stress resulting from the deformation of the model).

Models of transverse sections through the functional varanid tooth were loaded

at different angles which resulted in changes of the directions of the resulting prin

cipal stresses (Fig. 14). The results indicate that upon axial loading (Fig. 14 A), ten

sile stresses result on the lingual side of the tooth base while compressive stresses

build up on the labial side of it. Moreover, upon axial loading the one direction of

the principal stresses (tension and compression) exactly parallels the direction of the

dentine lamellae of the plicidentine (Fig. 9 B) and meets the tooth bearing bony 5W’-

face at right angles anywhere within the tooth base. The other principal stress (cen

trifugal stress resulting from deformation) by definition runs parallel to the surface

of the tooth bearing bone. Therefore, no shear stresses will occur along the site of

ankylosis, and the dentine lamellae will be loaded in pure tension and compression

as long as axial loading is maintained.

The same stress distribution is observed in the model if the tooth is loaded verti

cally. The loading force is then no more oriented axially through the tip of the tooth,

but it still passes through the surface of ankylosis. Hence, upon axial or vertical

loading of the teeth of Varanus, shear stresses are absent or minimal on the surface

of ankylosis.If. however, the tooth is loaded abaxially so that the loading force does 110 iiiore

pass through the surface of ankylosis (Fig. 14 C), the directions of the prineivil stres

ses will no more match neither the direction of the dentine lamellae nor the direction

of the surface of ankylosis. As a consequence, shear stresses ivihl build up along thesite of ankylosis, which might cause the tooth to break off the supporting bone.The formation of plieidentine and the degree of the slope of ankylosis may representadaptations of varanid lizards to use their teeth to handle large prey imposing heavyloads; The deee of pleurodonty is modelled such that no shear stresses occur alongthe surface of ankylosis if the teeth are loaded axially or vertically so that the loadingforce passes through the tooth base. The dentine lamellae take up the compressiveor tensile stresses along their long axes.

Acknowledgcmenis

I thank PD Dr. B. 0. San, University of Basel, and Prof. Dr. II. L. McGill Univer.Sity, Montieal, under whose supervision this work was carried out.Prof. Dr. C. GANS, University of Michigan, Ann Arbor, and Prof. Dr. G. Ganuax, Instituteof Dental Medicine, Univcrity of Basel, took a lively interest in tho problems of mandibularmechanics in loran us and engaged in many stimulating discussions. Dr. A. Kaamcuaxoaa proVided the ground sections through the teeth of lumens, and Prof. Dr. M. EaTCE made the X-rayIpossible at the Kantonsspital, Basel. Dr. IV. Ercssaaoan provided the posI Sibility to carrying out tlio experiments with phutoclastic tooth models at the C’iba-Geigy AG,Base1, and Dr. V. Essamean, Eidgenossische Materialprufungsanstalt Dübendorf, provided theof a critical discussion of these experiments.

Many Curators of Museum collections provided generous access to their skeletal and alcoholoicCollections They are Dr. K. Ki4anran, Seckcnberg Museum, Frankfurt a. M., Dr. E. N. An50t0 and A. C. British Muserum (Natural History), London, Dr. B. 0. ZWEIFEt,Amcrican Museum of Natural History, New York, Dr. F. F. WIIAMs, Museum of ComparativeI Zoology, Cambridge Mass.. Dr. B. A. NussBAnr and Dr. A. 0. Ktuea, The Museum of Zoology,toM, Ann Arbor Mi., and Dr. H. M.&ax, Field Museum of Natural History, Chicago.

814 0. Rsarrna: A Functional Interpretation of the Varanid Dentition

1815

in5.

th

bp -

Fig. 14. The direction of tensile and compressive stresses within the tooth base ofVuranus. The tooth is presented in a transverse section under varying loads. Forfurther explanations see text. Abbreviations: th, Theka; bp, Basalplattc; ling, lingually;lab, labially

816 0. Rtnn4: A Functional interpretation of the Varanid Dentition

Summary

A cinéradiographic analysis of the feeding movements iii l7m-enus bengolenais produced the

following results. The mouth is opened by raising the head (upper jaw) rather than by lowering

the lower jaw. Starting from the resting position, the muzzle unit is elevated around 9° reintive

to the rest of the skull during jaw opening; the quadrate swings anteriorly around 21°. During

jaw closure, the snout is depressed around 15° relative to the rest of the skull, hence 6° beyond

the resting position. The quadrate swings backwards around 27°.

Amphikinesis is interpreted as allowing a stronger posterior recurvature of the maxillary teeth

in Verensis. This increases the holding effect of the teeth without increasing their length, an adap

tation of Vorcoius to capture relatively large prey.

The formation of plicidentine (dentino infolding) in the teeth of Vorenus increases the surface

of attachment of the teeth on the supporting bone. Moreover, the dentine lamellne take up ten

sile anti compressive stresses along their long axes upon axial or vertical loading of the teeth.

Tho slope of plourotlonty is modelled so as to minimalize shear stress on the surface of anky

losis upon axial or vertical loading of the teeth.

Zusammenfassuag

\4tiilirentl ties Frehoktes dffnet Voronus benqeien.sis das Maul rioreli Hebung dos Oberkiefers

und nicht dureh Senkong ties Unterkiefers. Wiihrontl ties Offnens tier Kieter wird the Sc loiauze

urn 9° relat iv zum Rest ties Sclikdels angeliohen; tlas Quadratum drelit sick tiahei um 21° nach

verne. Wkhrenti tier ScldieBung tier Kiefer wird die Sehnauze urn 15° reiativ ztim Rest des Sehd

dels gesenkt, tI. h. sic senkt sich urn 6° fiber die Ridielage hinaus. Das Quadrnttim dreht sich dabei

um 27° nach hinten.

Die ampliikinetiscbeii Bewegungen im Sehddel von Coronet eriatthten eine stdrkore Rdekwilrts

krfiminung tier Zkhne. Dabei wird tier Halteeffokt tier Ziilme vergrd6ert oline deren abseiute Lange

zo steigern.

Die Bildung von Pheidentin erhdht the Atifingefidehe cmos Waranzahnes. Zndem aehmen (lie

Dentinfalten bei axialer otler vertikaler Belastung tier Ziihne the Ztig- untl Druekspannungen eat-

lang ihrer Liingsaehse en f.

Der Pleurotientiewinkel ist so eingeriehtet, tiall bci axialer other vertikaler Belastung der Zahnc

Seherkrafte entiang der Atifiagefiiiche minimahsiert werden.

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817

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Dr. OLIvIER RIEPPEL

Paiiiontoiegisehes institutunti Museum tier Universitat ZOrichKfinstiergasse 16CR - 8006 Zurich U

El