Zoologicheskii Zhurnal (Zoological Journal)

Transcription

1 /9 CRANIAL KINESIS IN LIZARDS; CONTRIBUTION TO THE PROBLEM OF THE ADAPTIVE SIGNIFICANCE OF SKULL KINESIS by N. N. Iordansky Zoologchesk Zhurnal (Zoologcal Journal) volume 45, number 9 pp Translated by Dr. Leon Kelso Edted by Carl Gans SMITHSONIAN HERPETOLOGICAL INFORMATION SERVICES 1968 Addtonal copes avalable from: Dvson of Reptles and Amphbans Unted States Natonal Museum Washngton, D. C

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3 ,, der saurus ncus A, nst 1 The term kness, when used as a reference to the skulls of tetrapods, s defned ether as the movement of the upper jaw accompaned by specfc elements of (or the entre) dermatocran urn, or as movement of the olfactory regon of the neurocranum wth respect to the rear part of the axal skull (.e., of the bran case formed of cartlage-replacement bonesl Ths phenomenon was frst descrbed by Ntzsch n 1822, accordng to Bradley ( 1 9^3)» w h dscovered that the lzard skull was capable of movement n the f ronto-paretal suture and n the artculaton of the paretal bone wth the occptal. Bradley studed the chewng musculature and accompanyng movements n the skull of lzards. He formulated the frst hypothess of the functonal sgnfcance of the movement of the upper jaw n relaton to the axal skull (see below) and made the frst attempt to correlate these movements wth the functons of specfc muscles. The term "cranal kness" was ntroduced by Versluys (1910) for a constructon of the skull n whch such movements would be possble, as well as the movements of the lower jaw. Versluys (l910, 1912, 1922, 1927) made a more thorough analyss of tetrapod cranal kness. He ponted out the prevalence of kness among most of the tetrapod groups, recent and extnct; classfed varous forms of kness, and proposed a theory of cranal kness, accordng to whch the tetrapod skull was prmtvely knetc; kness was nherted by the ancent tetrapods from fsh-lke ancestors and secondarly lost n several lnes of tetrapod evolu t on. ^ From Versluys comes one of the most wdely accepted hypotheses to date on the functonal sgnfcance of cranal kness. Later contrbutons to the study of kness were made by Lakjer (l9 2 7), Marnell (1928, 1936), de Jong and Brongersma (1927), Hofer (960) and varous other authors. Kness n crossopteryg an fshes was shown by Romer ( P er Frazzetta 1962) and n some stegocephal ans ( Pf annenst el, > accordng to Frazzetta, 1962). An exceptonal amount of work on kness was devoted to the more specalzed and unusual forms of brds and snakes. Comparatvely lttle attenton was gven to kness n the more prmtve forms of cranal kness (n lzards). A valuable contrbuton to the study of cranal kness n lzards was the work of Frazzetta (1962). Frazzetta by usng moton pctures, demonstrated the connecton between the movement of the upper jaw and the sezng and consumpton of prey, and made the frst b omechan cal analyss of the work of the jaw muscles, - p responsble for movement of the upper jaw. Frazzetta presented a new hypothess on the functonal sgnfcance of cranal kness (see below). However, Frazzetta, n hs analyss of cranal kness, focused hs attenton on one form ( Varanus ), gvng less attenton to the features of cranal kness of other forms studed by hm. The type of cranal kness exhbted by Varanus, as our work has shown, cannot be consdered as ether the most prmtve or the most wdespread among lzards. For a clear presentaton of the evoluton of skull kness and ts functonal sgnfcance, a comparson of kness n varous skulls begnnng wth the more prmtve forms, s necessary. MATERIALS AND METHODS In add t on, I I We have studed skull structur uncta tus and the fol lowng lzards: Cyclur e n Sphenodon p_ ana sp. ( Iguandae), A_ ama caucas ca *. A. lehmann qu nolenta *. Phrynoceph macleay. I qu 1 ^. sa ret cula tus ( Agamdae), Lacerta med E rem as arquta. E. gramm ca (Lacertdae),.a*, L_. aq 1 s *. Eumeces schne * t Ma b y u a aurata. Trachysaurus ru qosus (Scncdae), aranus qr seus *. V. n Gecko gecko *, f Varan dae) Tera tosc sc ncus * Platydactylus quttat us. Gymnodactylus casp, ( Gekkon dae) Oph apodus * ( ngudae), Tup na Zonurus cordylus * (Zonu mb s sp. ( T e da e ) from the mate ral of the Zoologcal Insttute of th e Akadema Nauk USS R, the Zoologcal Museu of Moscow Un vers t y, the Cha r of Vertebrate Zoolo gy of Moscow Un vers ty and from our own co lectons. We studed the detals s n the alcoholc wet) connectve tssue f cran al k nes preparat ons of skulls of 11 spece s of lzards 2 (w th sk n removed) a d also n the skulls of freshly k le d speces of Varandae the jaw musculature of nne other speces of 1 zards wa s studed. Moton pc tures of the eat ng of prey by Aqama ceucas ca and Lacerta aq 1 s were t aken by an ade [asss tant, or proba t onerj at the Paleont olog cal tute of t ANSSSR, N.N Kalandadze, to whom th e author s very ndebted for hs he lp n the work. We als wsh to expre ss our sncere thanks to B. S. Mateev kyj (char of vertebrat r,d F. Ya. Dzerzhns zoology, Mosc ow Unversty) for ds cusson of our esults and for crt cal remarks, and for g the opportun ty to work wth museum materals, the head of the herpetol ogy dvson of the Zoolog cal In st tute ANSSSR, ead of the evoluton ry morphology dvson Darevsk, and. S. Zoologcal Mu seum, Moscow Un vers t y, D. N. Hofmann *An alternate hypothess of the antquty of the aknetc skull and ndependence of development of kness n varous lnes of vertebrate evoluton was developed by Edgeworth (1935)- Edgeworth's vews do not survve serous crtcsm and have not receved general acceptance. The names of these [llj lzards are desgnated by an astersk n the lst gven above.

4 "1 vertebrates shed: CRANIAL MOVEMENT OF LIZARDS Followng Bradley's work (19O3), on l s ngu the occptal, maxllary, a and s consdered to be mmo d also the bas- and paraspheno ds. The f nterest to us here prmarly s the mov le term "cranal kness". The relatonsh Lready been descrbed by Bradley, Versluys ; have confned ourselves only to ther en e quadrate, squamosal and the supra-tempo -ocesses ( "metak net c axs" of Frazzetta) th the bas pterygo dal processes, sometm ;sdes ths, both segments are joned by t Lements of the bran case. All these conn xllary segment relatve to the occptal <s wth the anteror end upward and forwa ;sdes ths, the maxllary segment durng nssts of a seres of related dvsons - rd skulls, t hree fund mand bular. The occ ble. It s formed of other segm ents are 1 ty of the ax llar s between th maxlla Hofer, Frazz etta, Oel eraton. Th ese relat 1 bones wth the late the paretal wth the of the ep p terygo ds cart lag no us and me tons allow a 1 m ted frst, as a whole, ) ( protract or dow 0- and retra cton cha pa red: ( 1 quadrate -- P t ntal segment s [un tsj were tal segment was re nf orced e occptals and o c bones, able relatv e to t he occ p egment, whc hss gn f ed by and occ p ta ons have 1 sect h (1956) and Webb [1951) and shps nclud e: th e unon of ends of the paroc c p- tal pra-occ p ta 1, the pterygo ds th the proof c bon es. anous orb to -tempo ral gree of move ment the f urns around the me tak ne t c rd and back ard (r e tract or, ) s conf gurat on s nee t nes, (2) ep pteryg ds, (3) palatal dvsons; and unpared: () paretal dvson, and (5) the snout [muzzle], whch always breaks down further nto (5 a ) central and pared (5 D ) lateral parts. The composton of all these dvsons wth the excepton of the quadrate and ep pterygo d. dffers as also dffer the peculartes of kness n the varous forms of lzards (see table). -- f 1 --

5 ngly saurus opposte movement of all the above-mentoned elements of the maxllary segment of the cranun takes place. However, among the varous forms of lzards, there are essental dfferences n the peculartes of cranal kness and n the composton of the varous sectons of the maxllary segment (cf. table). Frst, the kn ds ot lzards studed fall prmarly nto two groups: (l) C yclura. Aqatna. Lacerta, n whch the palatal unt s a sold shaft [corej partcpatng n, and Oph saurus movements as an ndvs ble nteger [whole unt] unquestonably a more prmtve stage; (2) cranal Eumeces, Varanus. Tera tosc ncus, Gecko. Gymnodactylus. and Zonurus. n whch the palatne unt s artculated along the palato-pteryg odal juncture to the anteror and posteror parts, movable relatve to one another. In lzards of the frst group, the palato-pterygo da artculaton s a suture runnng as a whole very oblquely from postero-anter orly medally and soldly untng the two bones (Fg. 2), whle lzards of the se cond group, a very loose syndesmoss runs more drectly laterally [ transversel yj, permttng rotaton aro und the transverse axs (Fg. ). In geckos, the palatne bone and the pterygod hardly adjon drectly, beng separated by loose connectve tssue (mentoned by Lakjer, I927). n lzards of t he second group, durng retracton, the palatne secton s more or less bent upward: the rego n of the palatal-pterygodal artculaton s rased, and the descendng angle between these bon es s decreased. -- Table, p p The nteror movement of the palatne secton n lzards of the second group s correlated wth the greater role of mesokness n the amphknetc skull (Versluys termnology) than n lzards of the frst group and s n general also correlated wth the greater development of kness. Wthn both groups of lzards there are some more pronounced dfferences among the varous forms. Thus, n Cyclura (Fg. l) and Aqama. the -fg. 5- ostorbtal bones are closely joned not 11 the other forms, but wth the latera acerta. the lnk between the quadrate b n the amphknetc skull, movements aro elatvely weak. The most pronounced d vson and the upper temporal arches ygomatc bone s closely connected wth noted by Frazzetta), and has elements o he premaxllary bone n relaton to the s predomnantly around the mesoknetc evertheless, contrary to the opnon of Fgs. J-) skull s the most knetc am ogether along the central part of the s xceed mob le. wth tlhe lateral element of the paretal dvson as n of the snout, and take part n ts movements. In pterygod (mentoned by Frazzetta) s very lax and metaknetc axs predom nate mesok net sm s ess of movements of the central part of the paretal certa and Oph. In Varanus, (Fg. 5) * ne latne dvson and takes part n ts movements hoknetsm (Hofer, I960);.e., the movablty of In the amphknetc skull of Varanus, movement he paretal dvson shfts very lttle (but shfts wth respect to the occptal segment. The qecko skulls of the forms studed: the vomers move her artculaton wth the palatne bones s

6 In favor of the actualty of the moton observed n connectve-tssue preparatons of the skull s the fact that analogous movements on a much larger scale are more notceable n the head of a freshly klled anmal wth fully preserved muscles, lgatures, etc., than n the connectve tssues of preparatons. Thus, the length of the skull of Varanus qrseus from --Fg. 6 the end of the snout to the occptal condyle, vares accordngly: as much as 8.57* f medan skull length (71-3 mm ) durng protracton and retracton n "fresh" heads, n three wet connectvetssue preparatons of skulls of same sze on the average, only 2.45? ( ). For proof of actve movement n the knetc skull, examnaton of lvng lzards and analyss of the lnes of force arsng n the cranum durng the contracton of cranal muscles s necessary. Drect examnaton of skull kness n lzards s dffcult due to ther great rapdty --p ca and Lacerta aq l s gave results that were y these lzards repeatedly rotate the head the knetc movements of the skull are hard the desred for the actual effect. The selecton of lvng forms. Both Lacerta and they do not have movement wthn the palatne whch to whc wth th retract protrac also n lzards of the second group. Thus n Lacerta, y segment are around the metaknetc axs, and Aqama has a very broad and low skull, thanks g a slght lateral nclnaton. Frazzetta, rey by Gerrhonot us coeruleus. He noted that nd compresson of the lzard's jaws, whle uth. Frazzetta's observatons were confrmed lot cus. dd not try to assgn a role to all the muscular parts. Protractors of lzards appear n M. protractor pterygode and M. levator pterygode; the fbers of the large M. adductor mandbulae externus (but never M. p Frazzetta made a graphc analyss of the lnes of force arsng du rng the contracton of the jaw muscles n the cranum of Varanus. Ths analyss seems fundame ntally accurate, but some- what superfcal, snce Frazzetta dd not study the jaw musculature of the lzard n detal, and the jaw mechansm n retractors are prmarly terygodeus, despte Frazzetta's assertons). Varous parts of the M. adductor mandbulae e xternus play completely dfferent roles n the movements of the cranal mechansm: a large par t of the fbers of the muscle have a retractve effect, some are neutral n relatonshp to th e cranal mechansm and some manfest themselves as protractors. The summary effect of contrac on of M. adductor mandbulae externus s retracton. Our studes of the jaw musculature t of lzards showed that parts of M. adductor mandbulae externus, accepted n the works of Lakj er (1926), Lubosch (1933), I. Poglayen-Neuwall (1954), Oelrch (l?56) and others, do not correspon d wth the functonal and topographc unts of the muscles. Furthermore, he (Frazzetta?) doe s not succeed well n dentfyng these parts n varous forms n the order Saura and, moreo ver, wth parts wth the same names n other reptles. The scope of ths artcle, unfortunately does not allow us to add here a detaled analyss of the structure and functon of lzard, ja w muscles; we hope to do ths n future publcatons. In hs analyss of the functons cran e mportant error: accordng to hs opnon, M. pt of the jaws on the prey. Actually, ths mu scle of the cranal mechansm. Its force s app led and downward posterorly of the jaw t artcul at on. pterygodeus does not change n conformty to th wth kness are n no way affected by the length Fg. 11 n Frazzetta's paper). M. pterygo deus m dtons: (l) flexblty of the palatne a rch (a the pterygods); (2) the orgn of M. ptery go deu from the palatne bone or the bones of the snout, The second condton s not fulflled n 1 zards. the maxllary segment les n the transfer of the M. adductor mandbulae externus from the 1 wer ja 1936). mecha m of rds, zetta made one 1 za godeu retra ctor least n the closng s a zards ne ther a re or or protractor he pte ds and to wer jaw, forward t such tach ent tracton of M., y th ranal chan sm. The ments assocated M. pt god eus Fg. 7; t corresponds to trac tor w th fulf llment of two conpalat t be a east he unctu re f th ne bone and j n tero th ge*l ne f the palatne arch e h n f *cf. G og c hesk SI onar Sofano, p. 223). he rol f M pter ygo deus n retract on of tract fore e co the contract on of th palat e ar rds - see Mar nell h (for b -p

7 deal THE PROBLEM OF THE FUNCTIONAL SIGNIFICANCE OF CRANIAL KINESIS The phenomenon of cranal kness has been known for about 1 50 years, but one problem has not been fnally solved--what s the functonal sgnfcance of ths wde-spread complex adaptaton n vertebrates? In general, all are agreed that knetsm s correlated wth feedng and chefly wth the capture of food. But, n regard to the defnte value of cranal kness, there are several d fferent hypotheses. Frazzetta convncngly showed the nadequacy of Versluys' hypothess, accordng to whch cranal kness served as a means of ncreasng the sze of the gape of predatory anmals, through a greater spread between the upper and lower jaws. In the monoknetc metaknetc cranum, protracton does not change the vertcal range of the gape, and durng amphkness or n the monoknetcmesoknetc skull, t decreases the gape. --Fg. 7- Bradley's hypothess s also untenable; t proposed that kness, durng the openng of the mouth for graspng prey, permts adducton of the palatne elements for holdng the prey between them. The evoluton of lzards has gone partcularly n the drecton of ncreasng the wdth of the n ter-pterygo cavty IdepressonJ and the reduct'or. of the palatal teeth, that s, n a drecton dametrcally opposed to that whch would have been expected f such a knetc functon were present. In addton, the protractor muscles are comparatvely weak and cannot produce the consderable compressve force between the left and rght elements of the palatne arch necessary for holdng of prey between them. Romer's hypothess on the amort zat onal role of kness ( 1937 ) on * ne mpact of jaws cannot be dscarded so decsvely. Frazzetta was nclned to dsplay skeptcsm as regards ths hypothess, snce n the capacty of amortzators n the knetc cranum, one must nclude the contracton of the retractor muscles,.e., the above-noted jaw adductors. The flexblty of the cranal bones n the lower forms of lzards mght, accordng to Frazzetta, wth the same success play the amort zat onal role wthout the ad of the complex structure provded by kness. But, skull kness s strongly developed even n small lzards. We can add that from the pont of vew of ths hypothess ncomprehensble s the role of the protractor muscles accordng to whch, durng the openng of the mouth (before the clampng of the jaws) the maxllary segment of the skull s elevated and thus bendng the sprng at the end. Accordng to Frazzetta's hypothess, cranal kness serves for attanng the smultaneous closng of the jaws n sezng the prey. The gape s orented so that the rsng lower jaw and lowerng upper jaw are smultaneously appled to the prey, by whch means the rsk of the prey's escape s lessened, as compared wth the condton n the aknetc skull, where the prey s "caught' by the lower jaw only. It seems to us, however, that Frazzetta lost sght of the fact that the orentaton of the jaws n relatonshp to sezng the object s not attaned by knet c movements, but by movements of the neck and by turnng the head at the occ p to-cerv cal jont,.e., movements whch take place also n the aknetc cranum. The smultanety (or near smultanety) of contact wth the prey by the upper and lower jaws mght be attaned n the aknetc skull by the lowerng of the head smultaneously wth the rasng of the lower jaw. It s unlkely n predators wth an aknetc skull (and of such there are very many, dscountng the mammals), --p that, after contact of the prey by the upper and lower jaws, suffcent tme elapses for the escape of the prey. Even n rushng at concealed prey from ambush a predator dscloses tself sooner than ts jaws (both together or one of them a splt second earler) grasp the prey. It seems to us that kness s not of any essental beneft ether n ganng a "moment of surprse" n sezng prey nor for the catchng of prey n general. In movements of the knetc skull, the and retracton s sgnfcant. The protrac retractors are powerful jaw adductor muscle load should lkely be retracton. Strange most scentsts have sought the functonal hnted at the functonal sgnfcance of th the beak (accordng to hs opnon) was onl upper jaw. Kness, accordng to Marnell the place of prncpal pressure by the jaws corner of the mouth. nequal

8 s vely nes It seems to us that the advantage of the knetc skull n comparson to the aknetc s not any knd of capture, but n holdng the stll lvng, movng prey. The knetc and aknetc <s can be dentcally effectve n catchng prey. But t s easer for knetc jaws to hold t. can note that aknetc straght jaws durng the holdng of prey wll push forward on the object ught (Fg. 8-l); and the resultant forces appled by them to the object held wll also be rected forward. In the aknetc skull ths load les n holdng prey by the teeth. Ths may avoded by the convex form of jaw, but then the jaws would always be "bent down" to some prey of ted sze and could not hold smaller prey. Kness provdes the predomnantly bent jaws wth re effectve jaw pressure on any tem sezed, and elmnates ther _ t he jawsj defcences (Fg. 2). Aknetc jaws may be analogzed to claws; the knetc, to fngers; snce fngers are more table than claws for holdng objects, so knetc jaws are more sutable than the aknetc for e same purpose. The complexty of the movements of cranal kness s a result of the complexty the cranal structure. In graspng prey, the knetc skull has other advantages over the aknetc (analogous to the advantages of fngers over claws): the knetc upper jaw can transfer the prey along the lower jaw, crushng ts resstance and kllng t. Durng squrmng of the prey, the knetc jaws holdnc t can relax slghtly, wthout releasng the vctm, and fnally exhaustng t n the struggle (the "spnnng? prncple"). The amort zat onal hypothess also does not contradct ths role of kness. All these functons of kness can exst together, as kness has a complex sgnfcance, servng as the best means of holdng prey. --p From ths pont of vew, the relatve weakness of the protractor muscles s understandable; ther fundamental role s to return the maxllary unt of the skull from the operatve retractve poston to the normal poston necessary for compact closng of the oral cleft. Also, protracton s accomplshed somewhat farther along than the "normal" central poston of the maxllary segment and always accompanes the openng of the mouth and the closng of the jaws. There of the hypothess of Boltt and Ewer (156*) whch says that protracton serves for free teeth, stuck n the vctm durng the graspng of the prey and for momentum n swallow s the truth ng the upper no t. We ought to g ve here an exp lanat on of t he hst ory of the v ery numerou cases n the h story of vertebrate s of loss of era nal k nes s h ch occurre d ndependen tly n var o us 1 nes of e volurmng t on. The lo ss of k nes s s to be expec ted n (1) n mals wh ch do not need to hold squ prey--for the, k nes s s useles 'all s pec a 1 zed egetaran f rms adect dae and Pare asaurs among the Cot ylosaur a ny Chel on a and ma ny Ornth sch a; even Ver sluys re marked about ths, gvng a dff erent explana t on, h owever, for t he loss of k nes s n v egetara n forms); (2) those forms whch r equre maxmu m press ure on t he f od ben g chewed nes s n th s case would even be harmful (as n some forms of vege tarans, but espec a 1 ly n mollu sk e aters an d other such forms, for example P lacodont a, a lso not ed by Ve rsluy s)j (3) n some st uat ons n hch the pres erva t on of the adapt ve values of k nes s would b e adv antageo us, but when som e other knd of co tng adapta t on s ore mporta nt for a gven form, exclud ng the poss bl ty of r eta n ng k ne ss. Ths stuat o n occurs nfo rms for h ch t he de velopme nt of the cond ary pala te has proven to be espec all portant, appa rentl el m nat ng k nes s; t also occ ITS n the c ase of speca lzed f eed ng on ry small or 1 arge pr ey. Kn es s s adva ntageous ( n ts unspec alzed forms) 1 n specfc form s of predato n, and espec al ly fo r the g eneral zed p reda tors tha t feed manly on prey small (but no t very small) relat ve to t s own s ze; prey of that s : e may be sezed n th predator's ja ws, such as nsects, by 1 za rds, and sma 11 vertebrat es b the la rger [_predato ] When prey s cons derably smalle r than t he pr edator, the advanta ge f k nes s for sezng t s not so ess ent al, s nee for se zng t, long, relat thn, p ncher-1 ke jaws are adv ant ageous. Typcal cases of th s knd are Crocodl a, P hytosau r a, Ichthyo saur a, Me so saura, dolph ns and others ( I orda nsk, 1963). Knes s n th e for of tak nes s ca nnot be ma ta ned n the spec es wth long jaw s due to the lesser f rmness of t he lnk age of the 1 ong jaws to the bran cas e (Versluys, 19 12). The los s of k nes s n Pter osaur a s probably ass oc a ted wth ths. I t should be ponted ou t that n the presen ce of sok n es s an d prok s k nes s that case ca n be preserved; an example of t h wou Id be th Ion g-beake an cht hyop hages. The above per tans only to the m ore pr m t ve forms of k nes eta- an ph k nes It a predator specalzes n feedng on coarser prey, subequal to tself or larger, the load on the cranum durng the capture, kllng and chewng [of food] s so great that kness s dsadvantageous, through lessenng fhe soldty of the skull. Kness may be advantageous n absence of food mastcaton by the jaws but t becomes dsadvantageous for more or less prolonged chewng of the food requrng, as sad above, rather frm pressure on the object processed. The loss of knetsm n the Therapsda lne s assocated wth ths fact. Snakes occupy a specal poston. In snakes, a more hghly specalzed form of kness developed wth an extreme degree of movablty of elements of the maxllary segment n relaton to each other, whch ads n swallowng large prey whole. As a consequence, snakes n many cases kll

9 saurus t a -p ther prey by constrcton or by poson, and the jaws do not exert mechancal pressure n holdng, kllng and chewng the prey. The reducton of kness n chameleons s evdently correlated wth feedng on small prey, captured by the ad of the tongue. The queston of kness or akness n Sphenodon remans open at ths tme. Recently Ostrom (1962) showed that n several types of Hatter a the protractor muscles are well developed and n a mature state. Ostrom suggested that n varous populatons of Sphenodon, kness can be lost or preserved n the adult state dependng on the feedng peculartes of a gven populaton. Our study of the skulls of Hattera leads to the concluson that metakness n these forms and ther ancestors must essentally be dstngushed from the scheme of metakness proposed by Versluys ( 1 91» )- Versluys proposed that n metakness, the maxllary segment of the skull moves as a sngle unt, n whch the palatne arch, the bones of the snout and the skull roof are all rgdly joned to each other. However, the structure of the skull of Ha ter shows that retracton n ths form should take place n the abductng elements of the palatne arch (pterygods, palatnes, ep- and ectopt erygo ds, and vomers). Ths means that n ths metaknetc skull, movement of the lateral parts of the maxllary segment n relaton to ts central part should take place, smlar to those notced n the amphknetc lzard skull. Whether durng retracton the movements of the palatal arch and the lateral elements of the dermatocran um are ndependent, as n lzards, or whether those and others were dsplaced relatve to the mddle elements of the maxllary segment, we cannot determne from avalable museum materals. In any case, retracton of the metaknetc skull type s accompaned by dverson and rotaton around the lnear axs, lateral margns upward, of elements of the palatne arch, by the lateral part of the superposed skull armor. It seems to us permssble to presume that these movements mght be characterstc of the metaknetc skull type n general. But examnaton of the problems of evoluton of cranal kness exceeds the scope of the present paper. CONCLUSIONS (1) Among the varous lzard speces, there exst essental dfferences n the peculartes of cranal kness and n the composton of var ous pa rts of the maxllary segment. Cranal kness of Varanus. used as the bass for analyss of lzard skull kness by Frazzetta (1962), s nether the most prmtve nor the most prevalent form of cranal kness among lzards. (2) The most prmtve type, among the forms studed, s n the Cyclura. Agama. Lacerta, and Opn :. n whch the palatne secton of the maxllary segment s a tough core [stout shaftj, takng part n the movements of the skull as a sngle unt. (3) In the presence of strengthenng of cranal kness n lzards, n the palatne unt there s developed movablty of ts anteror part n relaton to the posteror part. (l) The M. pterygo deus, whch functons as a retractor n the cranal mechansm n Frazzetta's opnon, actually n lzards cannot produce any knd of retractor movements by the latter. (5) It seems probable that cranal kness provdes the best means of holdng prey [whch s tryng to escape] n the predators jaws. Protracton can serve to release the teeth of the upper jaw durng sezure of prey or n swallowng. (6) Kness of the skull n ts unspecal zed forms s especally advantageous for the unversal [generalzed] predator feedng on prey [whch sj small relatve to ts own sze, but not too small prey. (7) Most of all, n the metaknetc type of skull, retracton and protracton of the maxllary segment are accompaned by abducton and adducton of the palatal arches, and the lateral elements of the superposed skull and by ther rotaton around the longtudnal axes, that s, defnte movement takng place wthn the maxllary segment of the skull, despte the pattern of metaklnetsm proposed by Versluys (1910, 1912).

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