aero get tee dy tore init se AIS neers vite rari inet: eter Rew! HARVARD UNIVERSITY LIBRARY OF THE Museum of Comparative Zoology ; Oy ae le evan ha i : ton rh MUS. COMP, ZOOL LIBRARY. OCT 2 1 1964 HARVARD UNIVERSITY. Postilla PEABODY MUSEUM OF NATURAL HISTORY YALE UNIVERSITY NEW HAVEN, CONNECTICUT, U.S.A. Number 86 September 25, 1964 INTRACRANIAL MOBILITY IN MOSASAURS DALE A. RUSSELL PEABODY MUSEUM OF NATURAL HIisToRY, YALE UNIVERSITY INTRODUCTION Mosasaurs are an extinct family of large marine lizards which have been found abundantly in, and are apparently restricted to, sediments deposited in shallow epicontinental seaways during late Cretaceous time. Among the diverse living groups included in the Lacertilia, mosasaurs resemble the varanids or monitor lizards most closely, a fact that has been generally recognized since the beginning of the nineteenth century. In the course of their adapta- tion to an aquatic existence, however, the heads and bodies of mosasaurs became more streamlined and their limbs were modified into paddles. As in most lacertilians, the mosasaur cranium was constructed of several rigid associations of bones which were sepa- rated by regions of flexibility making it possible for them to be moved with respect to one another. The present discussion is con- cerned with how these associations may have functioned in life. Although the nature of intracranial movement in mosasaurs appears to have been simple, its explanation is burdened by the use of a complex anatomical terminology. So far as can be deter- mined the muscles of the mosasaur head (see figs. 2-4) were arranged essentially as in Varanus. The works of Lakjer (1926) z Postilla Yale Peabody Museum No. 86 and Frazzetta (1962) are recommended for descriptions and figures of the cranial musculature of this genus. The term Kinesis is applied to the general condition in which elements of the dermal skull roof and palatoquadrate (maxillary segment) move more or less as a unit with respect to the brain- case (occipital segment). The principal axis of rotation in a kinetic skull (metakinetic axis) is located in the extreme posterior region of the head and is fixed on either side by the contact of the paroccipital processes or bones sutured thereto (occipital seg- ment) with the overlying dermal roof bones of the maxillary segment. As the maxillary segment rotates on the paroccipital pro- cesses, displacement occurs at the sliding contact (metakinetic joint) between the parietal (maxillary segment) and the supraoc- cipital (occipital segment) above, and on the sliding basal articula- tion formed by the contact of the basipterygoid processes of the basisphenoid (occipital segment) with the pterygoids (maxillary segment) on each side of the ventral midline of the skull. Further, the maxillary segment may be divided into subordinate units by secondary, transversely oriented axes of rotation. Two such axes are the mesokinetic axis, situated between the frontals and parietals on the dorsal surface of the skull, and the hypokinetic axis (new term), situated in the region of the overlapping pterygo-palatine contacts on its ventral surface. The term streptostyly is here used to describe the particular condition in which the quadrate has lost its contact anteriorly with the lower temporal arcade, and is only loosely bound medially to the pterygoid and dorsally to the quadratic suspensorium of the braincase. The quadrate is then firmly sutured to neither the maxillary nor occipital segment, and activation of any muscle attaching to it may alter its position relative to both of these seg- ments. Thus the cranium of a given reptile may be kinetic with- out being streptostylic (Sphenodon, see Ostrom 1962), streptosty- lic without being kinetic (some advanced mosasaurs, see below), or both kinetic and streptostylic (many lacertilians, see Frazzetta 1962). I am very grateful to Charles M. Bogert of the American Museum of Natural History for generously providing me with a head of Varanus niloticus for dissection. I have profited greatly from many instructive conversations with Georg Zappler, my Sept. 25, 1964 Intracranial mobility in mosasaurs 3 former classmate at Columbia University, and Herbert Barghusen of Smith College. The manuscript has been vastly improved by the detailed constructive criticism of John H. Ostrom and James A. Hopson of the Peabody Museum of Yale University, to whom I extend my sincerest thanks. CRANIAL KINESIS IN Varanus Frazzetta (1962) has recently published an excellent analysis of intracranial mobility in Varanus, the modern monitor lizard. A condensation of his work is given here to facilitate understand- ing of the somewhat more complicated situation postulated for generalized mosasaurs. The skull of Varanus is separated by Frazzetta into the two above-mentioned structural segments. The occipital segment is composed of the prootics, opisthotics, supraoccipital, parasphenoid, basisphenoid and basioccipital, which are all firmly sutured to- gether into an inflexible block. The maxillary segment nearly surrounds the occipital segment and meets it at three points, the metakinetic joint above, the metakinetic axis posteriorly and the basal articulation below. Except for the stapes, which is func- tionally unimportant in the kinetic mechanism of Varanus, the rest of the bones of the skull are included in the maxillary seg- ment. This segment is in turn divisible into five structural subunits: 1. The parietal unit, composed of the parietal, supratemporals, postorbitofrontals and squamosals. This unit articulates with the muzzle unit anteriorly through the mesokinetic axis, and with the Occipital segment ventrally through the metakinetic joint and metakinetic axis. 2. The quadrate units, articulating dorsally with the suspen- sorial processes of the occipital segment, medially through liga- ments with the quadratic rami of the pterygoids and ventrally with the glenoid fossae of the mandibles. The ventral ends of the quadrates are free to swing in an anteroposterior plane. 3. The basal units, composed of the pterygoid, ectopterygoid and jugal on each side of the posterior roof of the oral cavity. They are connected posteriorly by muscles and ligaments to the 4 Postilla Yale Peabody Museum No. 86 Occipital segment and quadrate units respectively, and anteriorly through the hypokinetic axis to the muzzle unit. 4. The muzzle unit, including the premaxilla, nasals, septo- maxillae, vomers, maxillae, prefrontals, lacrymals, palatines and superciliares. This unit meets the basal units posteroventrally through the hypokinetic axis and the parietal unit posterodorsally through the mesokinetic axis. 5. The epipterygoid units, each composed of a single strut anchored to the basal unit below, and connected ligamentously to the occipital segment and parietal unit above. According to Frazzetta, depression of the mandibles and protrac- tion of the muzzle unit are brought about by the activation of mechanically unrelated sets of muscles. Both movements, how- ever, occur simultaneously due to coordinated nervous control. The lower jaws are opened by contraction of the M. depressor mandibulae, aided by longitudinal throat musculature. Protrac- tion of the muzzle unit is caused by the contraction of muscles of the constrictor dorsalis group, linking the two major kinetic seg- ments of the skull.* The M. protractor pterygoid arises on the prootic beneath the trigeminal incisure and extends ventroposter- iorly to insert on the quadratic ramus of the pterygoid. It is evident that activation of this muscle elevates and thrusts the basal unit forward. The M. levator pterygoid is a vertical muscle attach- ing dorsally to the parietal and ventrally to the pterygoid. It assists the M. protractor pterygoid in elevating the basal unit. As the basal units are displaced anterodorsally the muzzle unit rotates upward relative to them about the hypokinetic axis, while rotating upward relative to the skull as a whole about the mesokinetic axis. The quadrates are passively pulled anteriorly by ligaments binding them to the advancing basal units. Frazzetta considers elevation of the mandibles and retraction of the muzzle unit to be mechanically interrelated in Varanus. Most jaw adductor muscles arise along the ventral edge of the supratem- poral arcade, lateral face of the parietal and anterior surface of * The M. levator bulbi is also a part of the constrictor dorsalis group. In snakes it is termed the M. retractor pterygoid (Lakjer 1926, p. 22), and serves to draw the basal units posteriorly. There is no evidence that this muscle operated in a similar manner in mosasaurs. Sept. 25, 1964 — Intracranial mobility in mosasaurs 5 the quadrate. They descend anteriorly to insert on the dorsal regions of the coronoid and surangular. The vertical component of force from the contraction of these muscles closes the jaws, while their horizontal component acting through the mandibles pushes the base of the quadrates posteriorly. The basal units are bound to the quadrates by the quadratomaxillary ligaments and to the lower jaws through the M. pterygoideus. Therefore as the lower jaws and quadrate bases are pushed posteriorly the basal units are passively pulled after them. The muzzle unit then rotates downward about the hypokinetic axis relative to the basal units, while rotating downward relative to the skull as a whole about the mesokinetic axis. Fig. 1. Diagram of the functional units of a mosasaur skull. Abbrevia- tions: Am, anterior mandibular unit; Ba, basal unit; Ep, epipterygoid unit; Mu, muzzle unit; Oc, occipital segment; Pa, parietal unit; Pm, posterior mandibular unit; Qu, quadrate unit; St, stapes segment. CRANIAL KINESIS IN MOSASAURS Although the skull of a generalized mosasaur is basically very similar to that of Varanus, there are several differences in the structural subdivision of the maxillary segment (see fig. 1). The upper temporal arcade is firmly attached to the muzzle unit, and 6 Postilla Yale Peabody Museum No. 86 the supratemporal to the quadratic suspensorium of the braincase, leaving only the fused parietals remaining in the parietal unit. The jugal is buttressed against the postorbitofrontal posteriorly and thereby incorporated into the muzzle unit. Because the quadrates could not have been firmly attached to the quadratic rami of the pterygoids (see below) they were probably not as directly involved in the retraction of the basal units as is the case in Varanus. The one feature essential to an understanding of cranial kinesis in mosasaurs is the extensive and solid suturing of the postorbito- frontals to the ventral surface of the frontal. This in effect makes the upper temporal arcades extensions of the muzzle unit that project behind the mesokinetic axis, since the postorbitofrontals and squamosals overlap each other in an immovable tongue-in- groove junction. As the muzzle unit was rotated upward about the mesokinetic axis, the upper temporal arcades were depressed, and vice versa. The squamosal is expanded at its posterior termination and Fig. 2. Temporal region of a generalized mosasaur, Clidastes liodontus (reconstructed after YPM 1335, one-half natural size). Abbreviations: a, angular; ar, articular; c, coronoid; d, dentary; e, epipterygoid; f, frontal; j, jugal; 1, lacrymal; m, maxilla; p, parietal; pof, postorbitofrontal; prf, prefrontal; pt, pterygoid; q, quadrate; sa, surangular; sp, splenial; sq, squa- mosal; st, supratemporal; tym, calcified tympanum. Sept. 25, 1964 Intracranial mobility in mosasaurs 7 Fig. 3. Restored superficial musculature of the temporal region of Cli- dastes liodontus. Abbreviations: AEMS, Mm. adductor mandibulae externus medialis et superficialis;s AEP, M. adductor mandibulae externus profundus, pp, posterior head, pq, quadrate head; AM, M. adductor mandibulae un- divided; AMP, M. adductor mandibulae posterior; CM, M. cervicomandi- bularis; LAO, M. levator angularis oris;s DM, M. depressor mandibulae; Ps, M. pseudotemporalis, pr, profundus, sup, superficialis; B. bodenapo- neurosis. caps the supratemporal, which in mosasaurs is firmly sutured to the paroccipital processes of the occipital segment. Assuming the occipital segment to be solidly attached to the overlying parietal unit, one of three things would happen when the muzzle unit was protracted or retracted and the upper temporal arcades were cor- respondingly depressed or elevated: (a) The posterior ends of the squamosals would swing in ver- tical arcs over the supratemporals. (b) The posterior ends of the squamosals would remain fixed on the supratemporals and the upper temporal arcades would bend in vertical planes. (c) The posterior ends of the squamosals would remain fixed on the supratemporals, the upper temporal arcades would remain rigid and movement of the muzzle unit about the mesokinetic axis would be suppressed. 8 Postilla Yale Peabody Museum No. 86 Alternative (a) is unlikely for in all mosasaurs the plane of con- tact between the squamosal and the supratemporal is undulatory to a greater or lesser extent, the axes of undulation lying at right angles to the hypothetical direction of movement. Alternative (b) may be dismissed for the reason that the upper temporal arcade is deeper than wide and particularly resistant to vertical bending. Alternative (c) would negate any reason for having transverse lines of flexure in the maxillary segment, as the skull would be akinetic. It is therefore concluded that the occipital segment could move beneath the parietal unit. In fossil specimens of generalized mosa- saurs these structural elements are nearly always disassociated, testifying to their loose interconnections. The occipital segment is here postulated to have pivoted in a vertical plane on the Occipital condyle about the atlas vertebra. Any rolling motion would be prevented by the various articulations with the maxillary segment, which limited movement in a fore and aft direction. Thus as the upper temporal arcades were elevated the paroccipital processes were also lifted and the basipterygoid processes lowered and displaced posteriorly. The reverse motions accompanied de- pression of the upper temporal arcades (see fig. 5). Adjustment in the vertical relations between the paroccipital process of the occipital segment and the suspensorial ramus of the parietal took place through slippage on the loosely overlapping parietal- supratemporal contact. The squamosal was capable of pivoting on the lateral face of the supratemporal (metakinetic axis). As will be seen below, the ability of the occipital segment to turn about the atlas-occipital articulation within the maxillary seg- ment could have played an important role in the kinetic mecha- nism of mosasaurs. It should be noted that the atlas is the fixed structure relative to which all other structures in the skull under- went displacement in kinesis. Frazzetta (1962) considers the occipital segment to be the fixed structure relative to which other structures in the skull undergo displacement during kinetic opera- tions in Varanus. Herein lies the fundamental difference between Frazzetta’s interpretation of kinesis in Varanus and this interpreta- tion of kinesis in generalized mosasaurs. If the muzzle unit of mosasaurs was protracted and retracted the same way as it is in Varanus the occipital segment would be Sept. 25, 1964 Intracranial mobility in mosasaurs 9 passively rocked up and down about the atlas with the rising and falling upper temporal arcades. However, important axial muscles must have inserted on the occipital segment ventral and dorsal to the occipital condyle, these being the Mm. rectus capitis ante- rior and posterior. If superficial muscles, like the M. spinalis capit- is above and Mm. sternohyoideus and geniohyoideus below, held the maxillary segment and lower jaws fixed relative to the atlas-occipital articulation, then alternative contraction of the two rectus capitis muscles would rotate the occipital segment up and down about the atlas vertebra. Therefore the occipital segment could at least have aided the kinetic mechanism of mosasaurs by actively pushing the upper temporal arcades up and down with the paroccipital processes. Fig. 4. Restored deep musculature of the temporal region of Clidastes liodontus. Abbreviations: LPt, M. levator pterygoid; PPt, M. protractor pterygoid; Pt, M. pterygoideus undivided; PtP, M. pterygoideus profundus; PtS, M. pterygoideus superficialis; RCA, M. rectus capitis anterior; RCP, M. rectus capitis posterior. When the head of a mosasaur was at rest a line drawn from the metakinetic joint to the basal articulation would descend anteroventrally at an angle of about 45° with respect to the hori- zontal axis of the skull. The line would descend less steeply during protraction, when the occipital segment was rotated upward about the atlas, and more steeply when it was rotated downward. Thus the metakinetic joint and basal articulation were brought more 10 Postilla Yale Peabody Museum No. 86 Fig. 5. Kinesis in mosasaurs. Abbreviations: max, mesokinetic axis; mtj, metakinetic joint; mtx, metakinetic axis; other abbreviations as in figs. 1-4. A. Muzzle unit elevated, anterior mandibular unit depressed. B. Cra- nium at rest. C. Muzzle unit depressed, anterior mandibular unit elevated. closely together vertically in the protracted state of the muzzle unit than in the retracted state. The same geometric relations also obtain for a line drawn from the mesokinetic to the hypokinetic axis. Assuming little or no vertical slipping on the metakinetic Sept. 25, 1964 Intracranial mobility in mosasaurs 11 joint and basal articulation, it will be seen from figure 5 that the vertical separation between them would directly control the verti- cal separation between the mesokinetic and hypokinetic axes, and thereby directly control the degree of protraction of the muzzle unit. Activation of the constrictor dorsalis muscles would merely accentuate the elevation of the muzzle unit in the protracted state by displacing the hypokinetic axis still further anterodorsally. It is evident then that rotation of the occipital segment could have exerted a profound influence over kinetic movements in the head of mosasaurs. Ligaments binding the basipterygoid processes to the pterygoids were probably tensed by the anterodorsal sliding of the basal units during protraction of the muzzle unit. During retraction the basi- pterygoid processes would have moved posteroventrally with the turning anteroventral margin of the occipital segment and exerted through these tensed ligaments the force necessary to pull the basal units back. It is possible that the movement of the occipital seg- ment was entirely responsible for the rotation of the muzzle unit downward about the mesokinetic axis, and the quadrates were freed to move the lower jaw independently of kinesis in the skull. This would represent an advancement over the condition in Vara- nus where the quadrates are a necessary element in the retraction of the muzzle unit. It is noteworthy that the quadrates are movable in all known mosasaurs, while kinesis was completely lost in later forms (e. g. in Mosasaurus, Plotosaurus, Plesiotylosaurus and Prognathodon). In mosasaurs possessing kinetic skulls it is also possible that the quadrates aided in the retraction of the muzzle unit the same way they do in Varanus. STREPTOSTYLY IN MOSASAURS Kauffman and Kesling (1960) have published a carefully exe- cuted study of an ammonite (Placenticeras) conch from the Virgin Creek Member of the Pierre Shale (Upper Cretaceous) which had been bitten repeatedly by a mosasaur. Superimposed rows of tooth impressions on this conch show that the cephalopod was bitten at least sixteen times before the living chamber was crushed and the soft parts were disengaged from the shell, probably to be devoured by the mosasaur. Kauffman and Kesling’s study has 12 Postilla Yale Peabody Museum No. 86 yielded much direct evidence of jaw movement in mosasaurs, some of which will be discussed below. Kauffman and Kesling (/bid., p. 219) note that the series of impressions from the dentary teeth of each mandible always main- tain the same anteroposterior relation to each other, indicating there was no anteroposterior movement between the lower jaws in the symphyseal region. They also observed (Jbid., fig. 4) that the upper and lower jaws did not always align with each other Fig. 6. Streptostyly in mosasaurs. Abbreviations as in figs. 1-4. A. Mandible protracted. B. Mandible retracted. when occluded. This could only occur if the qaudrates were inde- pendently movable (the lower jaws bent simultaneously at the splenioangular joint, /bid., p. 219). Since both basal units are fixed to a single rigid muzzle unit, it follows that in order for the quadrates to have been independently movable they must have been only loosely attached to the quadratic ramus of the ptery- goids. The single solid point remaining upon which the quadrate Sept. 25, 1964 Intracranial mobility in mosasaurs i) could have pivoted is the cotylus on the side of the suspensorial process of the occipital segment, which evidently was therefore not a sliding articulation. Muscles that acted to protract the lower jaw (see fig. 6) were the M. pterygoideus (the horizontal component of force trans- mitted through the mandible would pull the base of the quadrate anteriorly) and the M. depressor mandibulae (rotating the ante- rior portion of the mandible ventrally about the quadrato-man- dibular articulation so that it would not be swung dorsally into the maxillary segment). Could there have been a separate bundle of the M. protractor pterygoid (an M. protractor quadrati) that inserted near the base of the quadrate and acted to pull it forward? Such fibers do insert on the quadrate of Varanus niloticus (Lakjer 1926, p- 14). The horizontal component of force from the contracting jaw adductor muscles acting through the mandible would rotate the quadrate and mandible back about the cotylus on the quadratic suspensorium. The presence of prey between the jaws would have kept them apart and allowed the mandible to be pulled posteriorly. Grooves that parallel the longitudinal cranial axis of the attacking mosasaur cut into the conch of the above-mentioned ammonite bear witness to the force with which the jaws could be retracted (Kauffman and Kesling, 1960, p. 213). This mechanism for swinging the base of the mosasaur quadrate back and forth has already been suggested by Camp (1942, p. 35, 37). MANDIBULAR JOINT IN MOSASAURS As has long been known, the mosasaur jaw is divided into two halves by a joint in the center of each mandible. The articular, angular, surangular and coronoid are incorporated into a posterior structural unit, and the splenial and dentary into an anterior one. Dorsally a thin blade-like process of the prearticular spans the gap separating the two units to penetrate deeply between the splenial and dentary into the mandibular foramen of the anterior unit. Ventrally there is a ginglymoid splenio-angular articulation which is located beneath the lower edges of the dentary and surangular, and makes a pronounced bump in the center of the lower margin of the mandible. Nearly all previous authors have interpreted this Museum No. 86 Postilla Yale Peabody 14 ‘popoenoid sMef oy} pue ‘pojonpqe sjiuN Je[NqipuewW JOlo}uUe pur g]zznu ay} ‘pouedo smeuf oy} SuIMOYsS (9ZIS [BINJBU YJINOJ-9UO ULYY JOdIeL ATIYSIYS) SMI149j91 SNndévIaIv]q JO |[NYS Ps1ojsoy ‘L “BI Sept. 25, 1964 Intracranial mobility in mosasaurs 1S region as a site of lateral flexion in the lower jaw, permitting the ingestment of large objects. Kauffman and Kesling (1960, p. 218), however, from a study of the tooth marks on their ammonite conch, infer that the anterior unit of the lower jaw must have rotated upward about the splenio-angular joint. A vertical keel on the concave articular face of the splenial fits into a groove on the convex articular face of the angular. The joint would be disarticu- lated by only a slight amount of lateral flexion, although vertical movement would not be inhibited. As understood here, the twisting mechanism postulated by Kauffman and Kesling (/bid., p. 222) for the elevation of the anterior mandibular units would operate as follows. Rotation of the posterior units of the mandibles about their long axes would tend to move their upper edges apart. This movement would be transmitted to the upper edges of the anterior units, but the con- tact of the lower edges of the latter units in the symphyseal region would have prevented the ventral margins of the lower jaws from moving medially. The dorsal margins of the lower jaws would, however, move apart, bending between the rigid surangulars and dentaries. Thus, in a vertical plane drawn through the mandibular cotylus to the anterior tip of the dentary, the longitudinal distance between these two points would remain constant along the ventral margin, and be shortened dorsally, the anterior units of necessity being rotated up and back about the splenio-angular joint. A large suprastapedial process curves posteromedially from the dorsal portion of the main body of the quadrate in mosasaurs. The base of the quadrate would be swung laterally as the suspen- sorial cotylus slipped down and back along this suprastapedial process. The lateral movement of the quadrate base then supplied the force to turn the dorsal edge of the posterior mandibular unit laterally and thereby elevate the anterior unit, according to Kauff- man and Kesling. This is an ingeniously devised system and does credit to the creative imagination of its authors. However, it is unlikely that it could have functioned in life for the following reasons: a) The articulation of the quadrate with the suspensorium was not a sliding one. Because the pterygoids were but loosely at- tached to the quadrate there was no point about which the top of the quadrate could have pivoted. The head of the quadrate No. 86 lla Yale Peabody Museum l Post 16 ayy ‘poyovjor smef oy) pue ‘pajonpp ‘pasojo Ajuvou sMmef oy} SUIMOYS (azis [eanjyeu Y}ANOJ-9U0 B SHUN Jv[NQIpurU JOLIo}UB pue 9jZZNUT uy) Jodiey Apyst|s) sI14a}01 sndiv2ajv]q JO [NYS pI1ojsay "g ‘BI Sept. 25, 1964 Intracranial mobility in mosasaurs 17 is covered by a smooth surface which is very finely marked with tiny irregularities. This surface, as in Varanus and Python, probably anchored ligaments binding the quadrate to the sus- pensorium in a contact that permitted pivoting but prevented any significant amount of anteroposterior slippage. As in these two genera, the mandibular condyle of the mosasaur quadrate is surfaced with a more smoothly polished bone and met the underlying mandibular cotylus in a slipping articulation. b) The prearticular bridges the gap between the posterior and anterior units of the mandible dorsally. It is approximately “T-shaped in cross section and would have resisted any tend- ency of the mandible to bend outward at this point. c) The alveolar margins of the dentaries would have spread more widely apart from one another posteriorly when the ante- rior units were elevated, if the above hypothesis were true. Actually the rows of tooth impressions from the dentary teeth were not noticeably more divergent posteriorly when the ante- rior units were elevated (Kauffman and Kesling 1960, p. 218, fig. 4b; e). Another mechanism could conceivably have actively operated the splenio-angular joint. A slip of the M. adductor mandibulae externus superficialis may have inserted on the posterodorsal cor- ner of the dentary through a tendon passing over the coronoid. The lowered position of the splenio-angular joint would have lengthened the lever arm of the muscle and increased its effective- ness in elevating the anterior mandibular unit. In Varanus the M. cervicomandibularis arises beneath the M. constrictor colli from connective tissue on the neck and passes forward around the quadrate to insert on the ventrolateral margin of the angular and Splenial. This muscle may have inserted on a subdued transverse ridge in front of the articular surface of the splenial in mosasaurs, and thus functioned to depress the anterior mandibular unit. The overhanging of the posterodorsal corner of the dentary by the anterior edge of the coronoid, together with the absence of any unusual groove on the superior surface of the coronoid, make it difficult to visualize any portion of the jaw adductor muscles reaching the dentary. It seems more likely that the anterior edges 18 Postilla Yale Peabody Museum No. 86 of the coronoid and surangular were bound to the posterior edge of the dentary by ligaments, as suggested by Barghusen (oral com- munication). As the lower jaws hit the body of a victim the ante- rior units of the mandibles would absorb the shock of impact by rotating down about the splenio-angular articulations, putting the ligaments binding it dorsally to the posterior unit under tension. These tensed ligaments would then act to restore the anterior unit to its former position. CONCLUSIONS In summary, generalized mosasaurs possessed a kinetic skull with an actively rotating occipital segment, although kinesis was entirely lost in later forms. The quadrates were streptostylic and independently movable in all mosasaurs, and acted to protract and retract the lower jaws. The intramandibular joint operated in a vertical plane and, together with elastic ligaments binding the anterior and posterior halves of the mandible together, probably served as a shock absorbing device. Frazzetta (1962, p. 317) concludes, “. . . that kinesis is adap- tively important in that it makes possible a movement downward of the upper jaws . . . and permits the prey to be engaged by both upper and lower jaws simultaneously . . . thereby diminishing. . . the risk of deflecting the prey away from the gaping mouth by the mandibles before a positive grip can be secured.” In larger animals, kinesis may also increase the absolute speed and there- fore the momentum with which the upper jaws strike the body of the prey. This might serve to stun the victim and to impale it more securely on the teeth. Kinesis was evidently not an essential ele- ment in the feeding mechanism of mosasaurs, as is shown by its loss in later forms. Perhaps the viscosity of the aqueous medium in which mosasaurs lived inhibited rapid movement to such an extent that kinetic movement in the head was no longer useful, as it had been in their terrestrial ancestors. It is interesting that kinesis is developed to a varying degree even among the different genera of earlier, more generalized forms. It would seem that these mosasaurs represent an intermediate adaptive level in the evolu- tion of mosasaurs, a level in which kinesis was being lost. Streptostylic quadrates are, however, found in all mosasaurs Sept. 25, 1964 Intracranial mobility in mosasaurs I, and must have been useful adaptations in aquatic feeding. As sug- gested by Camp (1942, p. 37) and Kauffman and Kesling (1960, p. 218) this enabled the mandibles to be retracted, greatly assist- ing a mosasaur in forcing prey into its throat without the aid of gravity, claws or some solid point of leverage. It is doubtful that the inertial feeding method of lizards, described by Gans (1961, p. 218-219), could have been very effective in underwater swal- lowing. If a mosasaur lifted its head above the surface, however, the inertial method together with the aid of gravity, would also greatly facilitate the engorgement of large bodies. In some mosa- saurs (e.g. Clidastes) the marginal dentition is trenchant, and alternative protraction and retraction of the mandibles might have been effective in sawing a large object into pieces of swallowable size. BIBLIOGRAPHY Camp, C. L., 1942. California mosasaurs. Mem. Univ. California, v. 13, p. i-vi, 1-68, 26 figs., 6 pls. Frazzetta, T. H., 1962. A functional consideration of cranial kinesis in lizards. Jour. Morph., v. 111, no. 3, p. 287-319, 12 figs., 1 table. Gans, Carl, 1961. The feeding mechanism of snakes and its possible evolu- tion. Amer. Zool., v. 1, no. 2, p. 217-227, 5 figs. Kauffman, Erle G. and Robert V. Kesling, 1960. An upper Cretaceous am- monite bitten by a mosasaur. Contrib. Mus. Paleont. Univ. Michigan, v. 15, no. 9, p. 193-248, 7 figs., 9 pls., 6 tables. Lakjer, Tage, 1926. Die Trigeminus-Versorgte Kaumuskulatur der Saurop- siden. Copenhagen, p. 1-153, 26 pls. Ostrom, John H., 1962. On the constrictor dorsalis muscles of Sphenodon. Copeia, 1962, no. 4, p. 732-735, 1 fig. ra : se. ne Harvard MCZ Libi Fn a daha el Cap mar sue Sey