PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024 Number 3677, 29 pp., 10 figures, 1 table March 4, 2010 The Braincase of Apatosaurus (Dinosauria: Sauropoda) Based on Computed Tomography of a New Specimen with Comments on Variation and Evolution in Sauropod Neuroanatomy AMY M. BALANOFF,1,2 GABE S. BEVER,3* AND TAKEHITO IKEJIRI4 ABSTRACT WedescribeapreviouslyunreportedbraincaseofthesauropoddinosaurApatosaurusfromthe Cactus Park Quarry, Morrison Formation of western Colorado using high-resolution X-ray computedtomography.Thedigitalnatureofthesedataallowedustoprepareanddescribethefirst three-dimensional rendering of the endocranial space in this historically important dinosaur species.Resultsarecomparedwitharangeoftaxadrawnfromacrossthesauropodtreerevealing previously underappreciated variation in the sauropod neurocranium. Examples of variable charactersincludethedegreeofcerebralandpontineflexure,themorphologyoftheparietalbody and superior sagittal sinus and their relationship with the overlying dermal roof, and the conformationofseveralcranialnerveforamina.Weprovidepreliminaryevolutionaryhypotheses anddiscussionformanyofthesefeatures.Therecognitionthatconsiderablevariationispresentin the sauropod neurocranium hopefully will encourage more detailed descriptions of this anatomically complex region, as well as facilitating a synthetic review of the sauropodomorph braincaseas these descriptionsbecome available. 1DivisionofPaleontology,AmericanMuseumofNaturalHistory. 2DepartmentofEarthandEnvironmentalSciences,ColumbiaUniversity. 3DivisionofPaleontology,AmericanMuseumofNaturalHistory. *Current address:Department of Geology and Geophysics, Yale University,210 Whitney Avenue, New Haven,CT 06520. 4MuseumofPaleontologyandDepartmentofGeologicalSciences,UniversityofMichigan,1109GeddesRoad,Ann Arbor,MI48109-1079. CopyrightEAmericanMuseumofNaturalHistory2010 ISSN0003-0082 2 AMERICAN MUSEUMNOVITATES NO. 3677 INTRODUCTION Institutionalabbreviationsusedinthisstudy: AMNH (American Museum of Natural His- Thecomplexnatureof thevertebratebrain- tory, New York, New York); BYU (Brigham case makes this region a rich source of Young University, Earth Science Museum, phylogenetically informative data. The brain- Provo, Utah); CM (Carnegie Museum of caseandneuroanatomyofsauropoddinosaurs, Natural History, Pittsburgh, Pennsylvania); however, historically have played a relatively HMS (Houston Museum of Natural Science, minor role in shapinghypotheses with regards Houston, Texas); YPM (Yale University, tothephylogeneticandevolutionaryhistoryof Peabody Museum of Natural History, New thisuniqueandinterestinggroup.Thisrelative Haven,Connecticut). lack of influence undoubtedly stems from the rarity with which well-preserved cranial mate- MATERIALS AND METHODS rial isrecovered,withthepostcranial elements traditionally considered diagnostic for refined SPECIMEN taxonomiclevelswithinthisgroup(Tidwelland We describe a fairly complete, well-pre- Carpenter, 2003; see Wilson, 2002: table 8). served braincase of Apatosaurus (BYU 17096; This preservational gap translates directly to figs. 1, 2), using HRCT. BYU 17096 was the existing disparity in our understanding of collected from the Jurassic Morrison morphological variation for these respective Formation of the Cactus Park BYU Quarry anatomical systems across Sauropoda and inwesternColorado.Theendocranialspaceis reinforces the need for detailed anatomical completely infilled with a fine-grained sand- descriptionandcomparisonofwhatevercrani- stonematrix.TheallocationofBYU17096to al data are available. One way to increase the Apatosaurus is based in part on the hundreds return of anatomical data from a particular of disarticulated and semiarticulated postcra- specimen is through theuse of high-resolution nial bones diagnosable to Apatosaurus (espe- X-raycomputedtomography(HRCT).HRCT cially cervicals, dorsals, anterior caudals, and isanondestructivetoolforvisualizinginternal relatively robust limb bones) that were found structures and is particularly useful for study- inthesamequarry(CurticeandWilhite,1996; ingthecomplexinternalanatomyofafossilized Foster, 2003). None of the other genera braincase(Carlson etal., 2003).Inaddition to common in the Morrison Formation, such as providing access to internal cranial features DiplodocusandCamarasaurus,wererecovered such as the presence/absence of bony contacts from the Cactus Park Quarry. The diagnosis and processes, HRCT enhances our ability to ofBYU17096toApatosaurusalsoisbasedon visualize and interpret soft-tissue structures of observed braincase apomorphies discussed in thecentralnervous,circulatory,andendocrine the Phylogenetic Diagnosis section below. systemsthatarehousedwithinthebraincase. The purpose of this study is to provide a SCANNING detailed description ofa previouslyunreported braincaseofApatosaurus thatincludesthe first BYU17096wasscannedattheUniversityof observationsontheinternalbraincaseanatomy Texas High-Resolution X-ray Computed of this taxon. Observed morphologies are Tomography Facility on 14 May 2004 using comparedwithselectedsauropodandoutgroup their high-energy system. Scanning was per- taxa allowing preliminary assessments of neu- formed using a brass filter and air wedge, a rocranial variation and evolution within voltageof420 kV,andanamperageof4.8 mA. Sauropoda. We review the published literature The resulting images were then processed for and explicitly establish baseline evolutionary the removal of ring and streak artifacts using hypotheses for a number of selected neurocra- programs written by Richard Ketcham. The nialfeatureswithinSauropoda.Thehopeisthat specimen was scanned along the coronal axis these hypotheses will elicit future testing and foratotalof127slicesatanimageresolutionof provide the groundwork for more synthetic 1024 3 1024 pixels. The interslice spacing is studies examining broad patterns of neurocra- 1.0 mm,andtheslicethicknessis0.8 mm.Each nialevolutionwithinSauropodomorpha. image has a reconstructed field of view of 2010 BALANOFF ETAL.: BRAINCASE OFAPATOSAURUS 3 265 mm.Thereconstructionoftheendocranial briefly described and illustrated in the late endocast was done with the original 16-bit 19th and early 20th centuries (Marsh, 1880, imagery in the volumetric rendering program 1884;Holland,1906;OsbornandMook,1921; VGStudioMax 1.2.1. Contrast of the images Ostrom and McIntosh, 1966). An endocast of E was increased until the infilled endocranial Diplodocus (AMNH 694) was described by spaceandbonewereeasilydistinguishablefrom Marsh (1884, 1896) and reillustrated by eachother.Theendocranialcavitywasselected Hopson (1979). Janensch (1935–36) described using the segmentation tools available in this an endocast of Brachiosaurus as well as two program, separated into its own volume, and diplodocoid taxa, Dicraeosaurus and Tornieria exportedasanisosurface.Allmeasurementsof (Tornieria is referred to as Barosaurus in the braincase and endocast (including volume) Janensch [1935–36]; however, see Remes were taken in VGStudioMax . Endocast vol- [2006] for updated taxonomy). Many of the E ume measurements were taken by calculating morerecentdescriptionsarebasedonsynthetic thevolumeofnegativespaceoftheendocranial endocasts.TheseincludePlateosaurus(Galton, cavity. For ease of description, features of the 1985), Shunosaurus (Chatterjee and Zheng, endocranialcastarereferredtobythenamesof 2002), an early Cretaceous titanosauriform the soft tissues of the brain that they reflect (TMM 40435; Tidwell and Carpenter, 2003), (e.g.,cerebrumratherthancastofcerebrum).It and Camarasaurus (Chatterjee and Zheng, is important to note, however, that what 2005; Sereno et al., 2007). A digital endocast actuallyispreservedisacastoftheendocranial wasextractedfromHMS175anddescribedas space, which also reflects structures other than Diplodocushayi(Franzosa,2004),althoughthe the brain, such as meninges and sinuses. This allocation of this specimen to Diplodocus is cast, however, is useful in determining relative questionable (see Harris, 2006). Comparative sizeand shape of different regionsofthe brain illustrationsofadigitalendocastofDiplodocus as well as recognizing the morphology of the alsoareavailableinSerenoetal.(2007). cranial nerve roots.The originalslice data and movies showing the endocranial cast are DESCRIPTION available at the DigiMorph website (www. digimorph.org/specimens/Apatosaurus_sp). GENERAL FEATURES Figures 1, 2 TAXONOMIC COMPARISONS BYU 17096 is an articulated braincase that Theinclusionofcranial materialintophylo- iscompleteinthatitincludestherightandleft genetic analyses of sauropods (e.g., Wilson, orbitosphenoid, laterosphenoid, exoccipital- 2002; Upchurch et al., 2004; Rauhut et al., opisthotic, prootic, and the midline basisphe- 2005) makes the diagnosis of newly discovered noid, basioccipital, and supraoccipital. The material more confident. Previous descriptions paired frontal, parietal, squamosal, and post- of cranial material allocated to Apatosaurus orbital also are present as components of the include that of Berman and McIntosh (1978), dermal roof fused to the endochondral ele- who described a fairly complete skull (CM ments of the braincase. The braincase is well 11162) of a ‘‘probable’’ Apatosaurus (Berman preserved overall, although the distal extrem- andMcIntosh,1998:21)andapartialbraincase ities of several bones and processes are (YPM1860)fromtheupperJurassicMorrison broken. The dermal roofing components are Formation of Colorado. A partly disarticulat- better preserved on the right side of the skull. ed braincase is known from Como Bluff, The specimen islarge overall(see table 1)and Wyoming (Connely and Hawley, 1998). In thecranialsuturesaresotightlyfusedastobe addition to describing a new braincase of indistinguishable in many cases in external Apatosaurus,wepresenttheonlyendocastthat view(asiscommoninsauropods;e.g.,Tidwell isknownforthistaxon. and Carpenter [2003], Wilson et al. [2005], The number of endocasts that exist for Remes[2006]).Thisadvancedstateoffusionis sauropodomorph taxa is surprisingly large, present in both the dermal roofing and although the majority of these were only endochondral elements, indicating skeletal 4 AMERICAN MUSEUMNOVITATES NO. 3677 Fig. 1. Braincase ofBYU17096in right lateral(A), leftlateral(B), dorsal(C), andventral (D)views. 2010 BALANOFF ETAL.: BRAINCASE OFAPATOSAURUS 5 Fig. 2. Braincase ofBYU 17096inanterior (A)and posterior (B) views. 6 AMERICAN MUSEUMNOVITATES NO. 3677 TABLE1 Hopson (1979) noted this opening may not MeasurementstakenfromBYU 17096 represent the parietal foramen but rather was Allmeasurements are inmm. possibly filled with part of the cartilaginous endocranium or with a portion of the superior Transversewidthofparietals 169.6 sagittal sinus (or a combination of the dorsal Anteroposteriorlengthofparietals 30.2 longitudinalandtransversesinuses;Witmerand Dorsoventralheightofsupraoccipital 52.0 Ridgely, 2009). The opening in BYU 17096 is Transversewidthofsupraoccipital 60.8 confluent with the superior sagittal sinus that Transversewidthofforamenmagnum 24.6 otherwiseisenclosedwithinthepairedparietals Dorsoventralheightofforamenmagnum 27.6 Anteroposteriorwidthofsupratemporalfenestra 21.2 (see description of endocast below). The open- Transverselengthofsupratemporalfenestra 39.7 ingalsoresidesatapositionhomologoustothe Anteroposteriorlengthoffrontoparietalfenestra 33.6 frontoparietal fontanelle of amniote embryos. Transversewidthoffrontoparietalfenestra 25.6 Therefore, it is possible that an anterodorsal Anteroposteriorlengthofskull 94.0 expansionofthesuperiorsagittalsinusintothe Transversewidthofskullacrossmetoticforamina 31.5 space between the developing elements of the Transversewidthofskullacrossparoccipital dermal roof influenced the developmental processes 169.2 dynamics of that region and thus resulted in Anteroposteriorlengthofendocast 72.5 Transversewidthofendocastatwidestpoint 49.4 the paedomorphic retention of the frontopari- etal fontanelle in the adult skull as the frontoparietalfenestra. maturity for these cranial partitions (but not The shape of the frontoparietal fenestra in necessarily indicative of sexual maturation or BYU 17096 is not perfectly circular but rather maturation of other skeletal regions). The exhibits a brief constriction near its posterior HRCT slices did help considerably in locating margin due to a slight medially directed cur- atleastsomeofthesesuturesbyrevealingtheir vature of the parietals (fig. 1C). This constric- location deep to the external surface (fig. 3). tionresultsinanincompletepartitioningofthe Their three-dimensional paths could then be fenestra into a relatively large anterior section tracedsuperficiallytotheexternalsurfaceofthe and a much smaller posterior opening. This specimen. Our ability to do this indicates that partitioning suggests that the frontoparietal the degree of fusion is not uniform along the fenestra may have housed two separate soft- three-dimensionalpathofthesesuturesandthat bodied structures (positioned anteriorly and cranial fusion often occurs first at the external posteriorly, respectively). The anterior compo- surfaceoftheskullandprogressesdeeper. nent may have housed the parietal (pineal) body,withadorsalpeakofthesuperiorsagittal DERMAL ROOF sinusfillingtheposteriorpartition(seeWitmer andRidgely,2009).Thismedialconstrictionof FRONTOPARIETALFENESTRA: Thedorsalsur- the parietals and associated partitioning of the face of BYU 17096 is formed largely by the frontoparietalfenestraiscommoninsauropods dermal roofing elements, which overlie the thatexhibitthisdorsalopening,andacomplete anterior two-thirds of the endocranial cavity. The dorsal surface is slightly concave up in divisionofthefenestrathroughmedialcontact overall shape and is dominated by a large oftheparietalsisknownwithinthisgroup(e.g., central opening that lies at the junction of the Amargasauruscazaui;SalgadoandCalvo,1992; paired frontals and parietals. This opening Salgado, 1999). This posterior opening gener- constitutes a broad communication (table 1) ally is described as the postparietal foramen betweentheexternalspacedorsaltothecranial (Janensch, 1935–36; Salgado and Calvo, 1992; roof and the endocranial cavity. This commu- Salgado, 1999), which was an unambiguous nication often is referred to as the parietal synapomorphy of Dicraeosauridae (Wilson, foramen (e.g., Salgado and Calvo, 1992; 2002)butmaybederivedforthemoreinclusive Chatterjee and Zheng, 2002), presumably be- Flagellicaudata (Harris and Dodson, 2004; causeofaninferredhomologywiththeparietal Harris,2006). foramen that houses the pineal body in other FRONTALS: The frontals are wedge-shaped reptiles (Janensch, 1935–36; Edinger, 1955). bonesindorsalviewwhoseposteriormarginis 2010 BALANOFF ETAL.: BRAINCASE OFAPATOSAURUS 7 Fig. 3. Two-dimensional HRCT slices through the coronal plane of BYU 17096. Arrows denote the frontal-parietalsuture(A),frontal-frontalsuture(B),parietal-supraoccipitalsuture(C),andorbitosphenoid- laterosphenoid suture(D). tapered due to the presence of the frontopa- the frontoparietal fenestra and extends to the rietal fenestra to which they contribute (along supratemporal fenestra along a slightly sinu- with the parietals; fig. 1C). The frontals are ous trajectory. This suture turns anteriorly relatively shorter anteroposteriorly with their and somewhat ventrally at the lateral edge of length being less than twice their transverse thebraincaseresultinginthedorsalsurfaceof breadth (Wilson, 2002). The frontals contact the postorbital process being comprised en- the orbitosphenoid and laterosphenoid ven- tirely by the frontal (the lateral ends of this trolaterally, parietal posteriorly, postorbital suture lie completely rostral to the supratem- posterolaterally, and eachother medially.The poral fenestra unlike Suuwassea; Harris, frontal-parietal suture, which is clearly visible 2006). The frontal-parietal contact lies near in the HRCT slices (fig. 3A), begins lateral to the suture between the laterosphenoid and 8 AMERICAN MUSEUMNOVITATES NO. 3677 Fig. 4. Two-dimensionalHRCT slicethrougha coronalplaneof the braincaseofBYU17096. Dotted line delineates a pairedfossa formedwithinthe frontals. prootic (as in most sauropods; Wilson et al., anteriorly,postorbitalanterolaterally,parietal 2005). The frontal-parietal suture is overlap- and squamosal posterolaterally, prootic ante- pingformuchofitsbreadth(withthefrontals roventrally, exoccipital-opisthotic posteroven- overlying the parietals) before pinching out trally, and supraoccipital posteriorly. Their posterolaterally (fig. 3A). The frontal-frontal median contact is negated by the diameter of suture is present and, as revealed in the CT the frontoparietal fenestra, whose posterior slices, distinctly interdigitating (fig. 3B). This margin is formed by the supraoccipital (see medialcontactissuturedbutnotfullyfusedas below).Themedialmarginsoftheparietalsdo in dicraeosaurids (Salgado and Calvo, 1992; extend a short distance towards the cranial Wilson, 2002) and possibly Tornieria (Harris, midline thereby constricting (slightly) the 2006). The frontals form the dorsal margin of frontoparietal fenestra. The parietal along the large anterior fenestra for the olfactory with the postorbital contributes to the supra- tracts (see Orbitosphenoid below). A pair of temporal fenestra, which is wider than long bilaterally symmetrical fossae, whose origin is (table 1)andpreservedonlyontheleftsideof unclear, penetrates the lateral margin of the the skull. The diameter of the supratemporal frontalsfromwithinthesupratemporalfenestra fenestra(ineitherdirection)isdistinctlylarger (fig. 4). These fossae extend ventromedially than that of the foramen magnum (table 1; within the frontals before terminating at or Wilson, 2002). The distance separating the near the frontal-parietal suture lateral to the right and left supratemporal fenestrae is frontoparietal fenestra. The frontals fail to greater than the largest diameter of the contributetothesupratemporalfossa(Wilson, supratemporal fenestra (Wilson, 2002). The 2002). supratemporal fenestra faces more laterally PARIETALS: The parietals are smaller than than dorsally (as in Apatosaurus and Suu- thefrontals.The parietals contact thefrontals wassea; Harris, 2006). The parietals of BYU 2010 BALANOFF ETAL.: BRAINCASE OFAPATOSAURUS 9 17096containlowarcuateridgesoneitherside BRAINCASE of the supraoccipital that mark the posterior margin of the skull (Wilson et al., 2005). The SUPRAOCCIPITAL: The supraoccipital is a midline ossification that roofs the posterior parietals overlap the supraoccipital and extend portion of the endocranial cavity. The supra- laterally to overlay the exoccipital-opisthotic. occipital contacts the parietals anterodorsally The parietal-exoccipital suture is sinuous as and laterally, and the exoccipital-opisthotic described for Apatosaurus (Berman and ventrolaterally. In dorsal view, the supraoc- McIntosh, 1978) and Tornieria (Remes, 2006) cipital underlies the paired parietals along its and unlike the linear suture of Suuwassea lateral margins and forms the posterior (Harris, 2006). A squamosal-supraoccipital margin of the frontoparietal fenestra contact excludes a parietal participation in the (fig. 2B). The anterodorsal margin of the dorsal margin of the posttemporal fenestra (a supraoccipital that borders the frontoparietal diplodocoid synapomorphy; Calvo and fenestra is slightly concave in posterior view Salgado, 1995; Upchurch, 1998; Remes,2006). (fig. 1C). The anterior surface of the supraoc- The occipital process of the parietal is deep, cipital that forms the caudal wall of the being approximately twice the diameter of the posteriorportionofthefrontoparietalfenestra foramenmagnum(Wilson,2002). is concave, forming a vertical groove that POSTORBITALS: The left postorbital is more would have housed a dorsal extension of the completely preserved than the right (fig. 1B, dural venous sinus system. C). The postorbital contacts the frontal Theocciputoverallisflattoslightlyconcave anteromedially, parietal dorsomedially, and (posteriorly) and therefore lacks the convex squamosal medially. The postorbital contrib- ‘‘supraoccipital wedge’’ described in some utes to the formation of the supratemporal sauropod braincases (fig. 2B; e.g., eusauropod fenestrathroughitssignificantcontributionto from India; Wilson et al., 2005). In posterior the postorbital process where it contacts the view, the occiput is rectangular in shape and frontal along its medial surface. The distal compares closely with the high and vaulted ends of both the jugal and squamosal pro- occiputdescribedforApatosaurus(Bermanand cesses are broken resulting in the postorbital McIntosh,1978).Theheightofthesupraoccip- lacking the triradiate shape present in ital in posterior view is greater than twice the Apatosaurus and other sauropods (Berman heightoftheforamenmagnum,whichappears andMcIntosh,1978).Thepreservedlengthsof to be the plesiomorphic condition for sauro- these processes both indicate that they were pods (Wilson, 2002). A distinct nuchal crest broader transversely than anteroposteriorly extendsalong the midline of the supraoccipital (Wilson, 2002). The postorbital does possess beginning at the posterior margin of the a distinct but short posterior process (Wilson, frontoparietal fenestra and ending at or just 2002). The temporal bar is longer anteropos- above the dorsal margin of the foramen teriorly than transversely, and is shifted magnum. A nuchal fossa lies lateral to the ventrally exposing the supratemporal fossa in nuchal crest on either side in the posterior lateral view (Wilson, 2002). occipitalplate. SQUAMOSALS: The right and left squamo- The supraoccipitalformsacontactwiththe sals are present and contact the parietal exoccipital-opisthotic that begins laterally, medially, postorbital laterally, and exoccipi- just dorsal to the base of the paroccipital tal-opisthoticposteromedially.Thesquamosal process, and curves ventromedially to reach lies in the posterolateral corner of the skull the dorsolateral margin of the foramen mag- directly anterior to the paroccipital process of num.Theangleofthisventromedialcurvature the exocccipital-opisthotic (structure to which is steeper in BYU 17096 than in YPM 1860 thesquamosalisfirmlyfused).Thesquamosal (Berman and McIntosh, 1978). The supraoc- contacts the lateral margin of the parietal cipital forms the dorsal margin of the circular alongarelativelycomplexsuturethatincludes foramen magnum (slight mediolateral com- both overlapping (with the squamosal over- pression). The external occipital fenestra lies lapping the parietal) and interdigitating re- dorsalandlateraltotheforamenmagnumand gions. is clearly visible in posterior view (fig. 2B). 10 AMERICAN MUSEUMNOVITATES NO. 3677 Fig. 5. Stereo rendering of the braincase of BYU 17096 in ventrolateral view. The squamosal and postorbital havebeen digitally removed from the braincase. This fenestra pierces the occipital plate at the which is rare in sauropods (Wilson et al., supraoccipital-parietal suture, where it would 2005). have transmitted the external occipital vein The ventral suture with the basioccipital (caudal middle cerebral vein; Witmer and cannot be discerned so it is unclear to what Ridgely, 2009) out of the endocranial cavity degreetheexoccipitalcontributestothemassive and onto the external surface of the occipital occipitalcondyleand whether the basioccipital plate.Theexternalpathofthisveinismarked iscompletelyexcludedfromtheventralmargin by a distinct groove that runs ventrolaterally of the foramen magnum. The exoccipitals do from the external occipital fenestra, following formalargeproportionoftheoccipitalcondyle the general trajectory of the supraoccipital- in YPM 1860 and completely exclude the parietal suture (this external groove often is basioccipital from the ventral margin of the present in birds; Baumel and Witmer, 1993). foramen magnum (Berman and McIntosh, EXOCCIPITAL-OPISTHOTIC: The exoccipital 1978). In most sauropods where the relative and opisthotic are fused completely and contribution can be discerned (e.g., Shunosau- therefore described as a single complex. The rus; Chatterjee and Zheng, 2002), the basioc- exoccipital-opisthotic contacts the parietal cipital forms the majority of the occipital dorsally, supraoccipital medially, prootic and condyle. The occipital condyle of BYU 17096 squamosal anteriorly, basioccipital postero- as a whole is hemispherical in shape with a ventrally, and basisphenoid anteroventrally. dorsalsurfacethatisconcaveup,differingfrom The exoccipital-opisthotic contributes to the Diplodocus that has a rounded dorsal margin lateral and ventral margins of the foramen (Berman and McIntosh, 1978). The peduncu- magnum. The suture with the overlying late occipital condyle is deflected posteroven- supraoccipital is visible and positioned near trally, projecting from the main body of the the dorsoventral midline of the foramen basicranium at an angle of approximately 80u magnum. A distinct prominence lies lateral fromthehorizontalplaneoftheskullroof. to the foramen magnum and encompasses The exoccipital-opisthotic forms a promi- portions of both the supraoccipital and nent paroccipital process that extends ventro- exoccipital-opisthotic. It is unclear whether laterally to contact the squamosal. Rather this structure represents a proatlantal facet, than extending at a distinct posterolateral