A Redescription of Gymnotrachelus (Placodermi: Arthrodira) from the Cleveland Shale (Famennian) of Northern Ohio. U.S.A

19 Pages·1994·12.5 MB·English

by  Robert K Carr

1
KIRTLANDIA
The Cleveland Museum of Natural History
February 1994 Number48:3-2
A Redescription of Gymnotrachelus
(Placodermi: Arthrodira) from the Cleveland Shale
(Famennian) of Northern Ohio, U.S.A.
RobertK. Carr
MuseumofPaleontology
The UniversityofMichigan
AnnArbor, MI48109-1079
Abstract
Therelationshipsamongselenosteidarthrodiresareinadequatelyknown.Therecoveryof
new material for Gymnotrachelus hydei Dunkle and Bungart from the Cleveland Shale
(Famennian)ofnorthernOhio,U.S.A.,offersauniqueinsightintoanimportant,butpoorly
known member ofthis group. Previous analyses ofselenosteids have suggested that
Gymnotrachelusrepresentseitherabasal memberofthe Selenosteidaeorataxonofunre-
solvedaffinities.Thevariousstudieshavedifferedalsointhetaxaincludedwithinthefami-
ly. The currentredescription ofGymnotrachelusprovides the foundation foraparsimony
basedcladistic analysis ofSelenosteidae using multiple outgroups. In contrast toprevious
analyses, Gymnotrachelus hydei is a derived member ofSelenosteidae, sistertaxon to
MelanosteusLelievre,Feist,Goujet,andBlieck.Gymnotrachelushydeiischaracterizedby:
(1)distinctsensorylinegroovesboundedbyapronouncedlip;(2)arostralexpansionofthe
preorbitalplateovertherhinocapsularregion;(3)apinealplateoverlappedbypreorbitaland
centralplates;(4)thepresenceofagapbetweensubmarginal,suborbital,andpostsuborbital
platesandheadshield;(5)anteriorsuperognathalswithtwoposterolaterallongitudinalrows
ofdenticleswitheachrowpossessingonetothreedenticles;(6)aninferognathalpossessing
asinglerowofdenticleswithoneortwoaccessoryrowsposterolaterally;(7)anovateleaf-
shapedparasphenoidwith ashortstem-likeprehypophysial region; (8) an anteriormedian
ventral plate in the form ofan isosceles triangle with equivalent length and width; (9) an
anterior ventrolateral plate similar in shape to the posterior ventrolateral plate of
Heintzichthysgouldii(arounded scalene triangle with thebase facing medially); and (10)
overlapoftheleftposteriorventrolateralplateontotheright.BraunosteusStensioisconsid-
ered here tobe amemberofSelenosteidae although furtheranalysis is neededtoconfirm
thisrelationship.
4 Carr No.48
Introduction Methods
ThecurrentdescriptionofGymnotrachelushydeiispart Phylogenetic hypotheses arebased, inthis discussion,
ofacontinuing study ofpoorly known andundescribed on analyses using PAUP (v. 3.1, Swofford, 1993) and
specimens recovered from the Cleveland Shale MacClade (v. 3, Maddison and Maddison, 1992).
(Famennian) ofnorthern Ohio, U.S.A. Gymnotrachelus Parameters specified in PAUP include the exhaustive
hydeiwas firstdescribedby Dunkle and Bungart (1939) search option with all characters unordered and
basedon asingle incomplete specimen, preservedin part TrematosteidaeandBrachydeiridaesetasoutgroups.The
and counterpart, which hadundergone some weathering mostparsimonious trees (minimal charactertransforma-
withfurtherloss ofinformation. Dunkle andBungartrec- tions)areretainedforthecurrentdiscussion.
ognizedarelationshipbetweenGymnotrachelushydeiand
selenosteid arthrodires; however, they did notprovide a SystematicPaleontology
MclueasrepuhymlogoefnetNiactudriaaglnosHiiss.tIonry196c5o-l1l9e6c6t,eTdheadCdlietvieolnaanld PlacodermiMcCoy, 1848
ArthrodiraWoodward,1891
Gymnotrachelus hydei material during the Interstate 71
BrachythoraciGross, 1932
Paleontological Salvage Project. I here redescribe EubrachythoraceiMiles, 1971
Gymnotrachelushydeibasedonthisnewmaterial,provid-
PachyosteinaDenison,1978
iGnygmnbootthraacnhueplduastehyodfeDiunaknldeaanndewBundgiaargtn'ossidse.scFriinpatliloyn,ofI AspinothoracidiSSteenlseinoo,st1e9i5d9ae{sDenesaun,Mil1e9s01andDennis,1979)
reviewthephylogeneticrelationshipsofGymnotrachelusto
GymnotrachelusDunkleandBungart, 1939
otherSelenosteidae,althoughtherelationshipofselenoste-
idstootheraspinothoracidarthrodiresremainsunclear. Diagnosis
A number ofworkers have evaluated relationships Gymnotrachelus is a typical aspinothoracid
among aspinothoracid arthrodires (Gross, 1932; Stensio, arthrodire characterized by reduction ofthe lateral and
1963, 1969;DunkleandBungart, 1939;Obruchev, 1964; occipital thickenings ofthe head shield and loss ofthe
Denison, 1975, 1978; Lelievre et al., 1987; Miles and spinal plate. The genus is characterizedby: (1) distinct
Dennis, 1979;Gardiner, 1990;GardinerandMiles, 1990; sensory line grooves bounded by apronounced lip; (2)
Carr, 1991).Earlyanalysesoftenlackedphylogenetically rostral expansion of the preorbital plate over the
informativediagnosesfortaxabecauseofpoorpreserva- rhinocapsular region; (3) a pineal plate overlapped by
tion and atthe time alackofseparationbetween primi- preorbital and central plates; (4) the presence ofa gap
tiveandderivedfeatures. Theprimarysourcesforinfor- between submarginal, suborbital, and postsuborbital
mation on aspinothoracid arthrodires are based on platesandheadshield; (5) anteriorsuperognathalswith
materials from two localities: the upper Frasnian two posterolateral longitudinal rows ofdenticles with
KellwasserkalkoftheManticocerasBeds,Germany,and eachrowpossessingonetothreedenticles;(6)aninfer-
the upper Famennian Cleveland Shale, Ohio, U.S.A. ognathal possessing a single row ofdenticles with one
Duringthe Interstate 71 Paleontological SalvageProject or two accessory rows posterolaterally; (7) an ovate
(1965-1966) numerous specimens ofnew or poorly leaf-shaped parasphenoid with a short stem-like prehy-
known taxa were recovered. Carr (1991; see also pophysialregion;(8)ananteriormedianventralplatein
Denison, 1978; Lelievre et al., 1987) noted insufficien- theformofanisoscelestrianglewithequivalentlength
cies in previous analyses of North American and and width; (9) an anteriorventrolateral plate similarin
Europeanaspinothoracidarthrodiresandprovidedalim- shape to the posterior ventrolateral plate of
ited phylogenetic analysis based in part on undescribed Heintzichthys gouldii (arounded scalene triangle with
material from The Cleveland Museum ofNatural the base facing medially); and (10) overlap ofthe left
History.ThecurrentstudyofGymnotrachelushydeipro- posteriorventrolateralplateontotheright.
videsnewdataonapoorlyknownNorthAmericantaxon
thatisimportantformoredetailedanalyses. Typespecies
Anatomical abbreviationsusedinfigures andlistedat GymnotrachelushydeiDunkleandBungart, 1939
theendofthepaperfollowthoseofDennis-Bryan(1987)
andCarr(1991). Specimen numberprefixes denote their Diagnosis
respective institutions: CMNH, Cleveland Museum of Sameasgenus.
NaturalHistory,Cleveland,Ohio;PM,PeabodyMuseum,
YaleUniversity.Thesuffix“id”whenusedtoformtaxo- Holotype
nomic adjectives does notreferto the familial level in CMNH5724.Figure 1 depictsaredrawingofDunkle
Linnean classification, but is used as aconvenience for andBungart’s(1939)cameralucidadrawingoftheholo-
discussinginformaltaxonomicunits. typecorrectedtoreflectcurrentplateinterpretations.
5
1994 RedescriptionofGymnotrachelus 5
left inferognathal (Figure 8C); CMNH 8778, 8779, and
CMNH
8788,isolatedleftposteriorsuperognathals; 8798,
incompleteleftsuborbitalplate;andCMNH8799,incom-
pletedisarticulatedheadandthoracic shields,cheekwith
sclerotics,andfragmentedscapulocoracoids.
Occurrence
All material was found within the Cleveland Shale
Member (Famennian) ofthe Ohio Shale, northern Ohio,
U.S.A.TheholotypewasfoundonTownesCreek,Lorain
County, Ohio. Interstate71 material was recoveredfrom
theHeintzichthyszone(Carr, 1991)whichwasquarriedat
the intersection ofWest 130th Street and Interstate 71,
Cleveland, Ohio. PM 55665 is recorded only as being
foundintheClevelandShale.
Description
HeadShield
Generalfeatures. The head shield is composed of 1
plates(Figure2A&B).Ofthese,sixarepairedwiththree
unpairedmedianplates.Thereisnoevidenceforthepres-
enceofpostnasalorinternasalplates.Groovesforsensory
lines are present and follow the typical arthrodiran pat-
tern; however, they areboundedby awell-developed lip
DFitneuigrngmkuoilrfneeotlha1oe.ngdyhGoyBalmuronentgoyuatpprredtaa’CctsheMed(Nl1b9uHa3s9s,e5hdy7fd2ioe4gin...1tS)hAtecraurccmueteriurrnretaenestrlpuaarcnneidatdlaaytasirdiocsrnh.aawioA-cf oaiGlrnyldmrinivsdoigptdeeru.caalAci,lhmteethlnhouisuss.gcahhmTrahoirindagsgceteacprsroponiimsdniiofnttoheiuonorncnadeciiicdssonvasadrirtisihsatrtbeoildnneitcrlwteyisitvwhweiiittnhfhiaionnnr
numericalpostscriptof1=leftand2=right. cthoensCelqeuveelnacnedoSfhabloenefacuonamparnedssiisoanppdaurreinntglypnroetsearvsaitmipolne.
Allplatesareflattened,makinginterpretationsoforiginal
AdditionalMaterial curvaturedifficult.
PM 55665, articulated anteriorpart ofhead shield in Rostral(R). Adefinitiverostralplateisnotrecognized
internal view, isolated suborbital, and inferognathal inanyofthespecimens.Onthebasisofthesizeandshape
impression (Figure 3); CMNH 8049, disarticulated tho- forthe gap between preorbital plates, therostral plate if
racic shield, in part, with scapulocoracoid, cheek with present appears to have been triangular in shape with a
sclerotics, and gnathals; CMNH 8050, nearly complete broadanteriormargin (Figures 2A, 3). Thepresenceofa
head and thoracic shields in internal view with cheek rostral plate is speculative since there is noevidence for
(includingsclerotics)andgnathalplates(Figures8E, 12B eithertheplateoroverlapareasonadjacentplates.
& C); CMNH 8051, disarticulatedposteriorhead shield, Pineal (P). The pineal plate (Figure 2A) is exposed
cheek with sclerotics, incomplete thoracic shield, and ventrally in the type (CMNH 5724, Figure 1) and PM
inferognathal (Figures 4B&C, 5, 6, 7B, 8A&B, 11); 55665 (Figure3)andexposedexternallyinCMNH8052
CMNH8052,incompletedisarticulatedheadandthoracic (Figure4A)asanisolatedplate.Thelatterspecimenhas
shields, sclerotics, parasphenoid (Figures 4A, 4D&E, bilateraloverlapareasforthecentralplates(oa.C,Figure
9B, 10); CMNH 8053, gnathals, isolated fragments of 4A) and reduced overlap areas for the preorbital plates
head and thoracic shields and cheek (Figures 4F, 9A); (oa.PrO,Figure4A).Incontrast,inHeintzichthysgouldii
CMNH 8054, incomplete, but partially articulated head the pineal overlaps both preorbital and central plates
shield,disarticulatedandincompletecheekwithsclerotics whileinDunkleosteusterrellithepinealoverlapscentral
andthoracicshield,andinferognathals(Figure 12A&D); and rostral plates, but is overlain by the preorbital. A
CMNH8055,incompletecheekwithsclerotics, gnathals, centralanteriorextensionofthepinealplatesuggeststhat
oneplateandfragmentfromthoracic shield(Figures7A, this plate may have contacted the rostral. This central
8D);CMNH8084,incompleterightinferognathal(poste- extension is much narrower than that suggested by
riorocclusalregionandblade);CMNH8776,incomplete Dunkle and Bungart (1939, p. 17). No external overlap
6 Carr No.48
AL
,
1994 RedescriptionofGymnotrachelus 7
area for the rostral plate is discernible and internally
exposedpineal plates do not reveal the anteriorregion,
preventingevaluationforthepresenceofarostralcontact
face.Internallythepinealfossaisboundedposteriorlyby
a well-developed ridge. A pineal foramen is present
externally(Figure4A).
Nuchal(Nu).Thenuchal(Figures2A&B;4B&C)is
embayedposteriorly with a small process in the midline
(p.pr,Figure4C). InCMNH 8050,themedianprocessis
elongate,extendingca. 8mmbeyondtheposteriormargin
oftheheadshield.Longslenderalaeextendposterolater-
ally along the nuchal gap. The anteriorborder ofthe
nuchalistransverse.
The internal nuchal thickening (n.th. Figure 4B) is
reducedandlimitedtothecentralregion.Thethickening
continues asthin ridgesformingthedescendingfacesof
theposterolateralalae. Pairedpits (pt.u, Figure4B)open
posteriorlyandareseparatedbyamedianseptum(m.sept,
Figure4C)whichiscontinuouswiththeposteriormedian
process. Distinct muscle pits are absent (f.lv, Goujet,
1984) with muscle insertion limited to two laterally
extendedshelvesorelongatefossae(fe.lv,Figure4C).
Preorbital(PrO).Thepreorbitalplate(Figures2A&B;3)
possesses a preorbital process, although it is not pro-
nouncedlaterally duetothe large orbitandgentlecurva- Figure 3. Gymnotrachelus hydei. Internal view ofan
ture of the dorsal orbit border (contrast this with incompleteheadshield(PM55665).Scalebarequals 1cm.
Dunkleosteuswheredistinctpre-andpostorbitalprocesses
denote a smallerorbit with apronounced curvature). A
grooveforthesupraorbitalsensorylinetraversestheplate Thisplatformisconfinedtothepreorbitalplateanddoes
andendsneartheanterolateralcorner(soc,Figures2A& notextend medially asdoesthe crista in someEuropean
B).Theratiooflongitudinallengthsofthepreorbitaland selenosteids (e.g., Enseosteus Stensio, 1963, fig. 113A).
centralplatesisca. 1.1 (PrO/C);theratiooflengthsofthe The preorbital plate is expanded anteriorly over the
preorbitalandpostorbitalplatesisca. 1.2(PrO/PtO). rhinocapsularregion(Figure3;alsoseeninHeintzichthys
Internally, a supraorbital vault (suo.v, Figure 3) is gouldii,buttoalesserdegree,Carr, 1991,fig.3A;asimi-
present extending from the preorbital process onto the larregion is seen in tubular snouted coccosteomorphs
postorbitalplate. Medially,thesupraorbitalvaultshowsa RolfosteusandTubonasus, DennisandMiles, 1979). Due
recess for the neurocranial preorbital process (ch.pro.pr. to flattening in preservation, it is not clearwhetherthis
Figure3).Anteriortothisrecess,asupraethmoidcristais expansionisdownturnedinlife. Alongtheplate’s lateral
present,butformsalowplatformversusadistinctridge. margin, there is a shallow notch between the preorbital
dermal process and the anterolateral extension overthe
rhinocapsular region. The supraorbital sensory line
grooveisdetectedinternallyasaraisedridgeonthepre-
Figure 2, Gymnotrachelus hydei. Composite reconstruc- orbitalplate(Figure3).
tionsofheadandthoracicshieldsinA,dorsalviewandB, Postorbital (PtO). The postorbital plate (Figures 2A
lateral view. Only afew head shieldplates arecomplete &B,3,4D)lacksapostorbitalprocess.Groovesforthree
in all dimensions; therefore, plate boundaries are recon- sensory lines are presenton the postorbital plate; central
structedbasedon known platecomponents andpotential sensory line (esc, Figure 4D) and postorbital and otic
boundaries and plate overlaps with adjacent plates. The branchesoftheinfraorbital sensoryline(ioc.ptandioc.ot
posteriormargin ofthe central and anteriordorsolateral respectively,Figure4D).Thecentralsensorylineandthe
platesareincompletelyknown.C,areconstructionofthe otic branch ofthe infraorbital line are generallycontinu-
ventralthoracicplatesindorsalview(noattempthasbeen ous, forming an angle ofca. 95°, although they are dis-
made to interpret original curvature). Estimated bound- juncton therightpostorbitalplate in CMNH 8052 (con-
ariesfortheinterolateralplatesaredrawnindottedlines. joinedonthe left). Thepostorbital branchofthe infraor-
Hiddenplateboundariesaredrawnindashedlines. bital linedoesnotalways unite withtheformertwosen-
8 Carr No.48
oa.M
Figure 4. Gymnotrachelus hydei. A, pineal (lowerleft) and left postmarginal plates (CMNH 8052) in external view. B,
nuchalplateininternalviewandC,close-upofnuchalthickening(CMNH8051).D,rightpostorbitalplateinexternalview
(CMNH8052).E,rightcentralplateinexternalview(CMNH8052).F,leftmarginalplateinexternalview(CMNH8053).
Scalebarsequal 1 cm.
1994 RedescriptionofGymnotrachelus 9
sory lines and is directed posteriorly. The angle formed
between the otic andpostorbital branches is ca. 30-40°.
Dunkle and Bungart (1939) noted a short postorbital
branchofthe infraorbital sensoryline groove. They sug-
gestedthatitcontinuedontothemarginalplatebasedon
thepresenceofamarginalplateoverlapareaatthetermi-
nation ofthe postorbital branch groove (Dunkle and
Bungart, 1939,om.m,fig.2).Theinterpretationofamar-
ginal plate overlap areaand apostorbital process appear
tobeinerror. Theputativepostorbitalprocess islocated
well anteriorto the supraorbital crista. The overlap area
appearstobeadepressiononthepostorbitalplateassoci-
atedwith acontinuation ofthe postorbital branch ofthe
infraorbital sensory line. A distinct overlap areaforthe
marginalplateisseeninCMNH8052(oa.M,Figure4D).
The central-postorbital plate length ratio is ca. 1.2
(C/PtO). Internally, the supraorbital vault is continued
posteriorly.Adistinct,lowridgeformsthemedialbound-
aryofthevaultandcontinuesposterolaterallyasasupra-
orbitalcristatothelateralmarginoftheplate.Thereisno
apparent inframarginal cristaextending posteriorly from
thesupraorbitalcristaonthepostorbitalplate.
Central(C).Thecentralplates(Figures2A&B,3,4E)
areelongate withthe posteriormargin incompletely pre-
servedinavailablematerial.Theyareseparatedanteriorly
bythepinealplate, arejoinedalongasinuous suture for
ca. 33% oftheir longitudinal length, and are separated
posteriorlybythenuchalplate.Twodistinctsensorylines
are present; a supraorbital sensory line (soc, Figure 4E)
and a central sensory line (esc, Figure 4E). A middle
pit-linemaybeoccasionallypresentandisnotedonlyon
CMNH 8054 (seen on the rightcentral plate impression
andexternallyontheleftcentralplate).Itisdirectedpos-
terolaterallyfromtheossificationcenterforminganangle
ofca. 35°withthecentralsensoryline.Thislineappears
tobedistinctfromthe“middleheadline”ofDunkleand
Bungart (1939, i.ac.sc, fig. 1) which, ifcorrectly identi-
fiedasasensoryline,ismorelikelyaposteriorpit-line.A
posteriorpit-lineis not seen on any ofthe new material
CanMdANaHlmoiw8d0de5l4ne.dopliytm-lpihnae,tiacstnhoitcekdenaibnogveis,pirsesfeonutndinotenrlnyalolny FA,igiunrteer5n.alGyamnndotBr,acehxteelrunsalhyvdieie.wsLe(ftCpMaNraHnu8c0h5a1l).plaStcealien
at the ossification center and unlike Dunkleosteus and barsequal 1 cm.
Heintzichthysisnotcontinuouswiththenuchalthicken-
ing. Theraisedridge indicatingthecourseofthe supra-
orbital sensory line groove on the preorbital plate’s becomes the main lateral line on the paranuchal plate.
internal surface is continued onto the central plate Theboundarybetweentheselinesistypicallydemarcat-
(Figure 3). There is no distinct boundary forthe inser- edbythepresenceofapostmarginallineextendingfrom
tionofthecuccularis muscle(compare with thedepres- theossificationcenter;however,thereisnoevidencefor
sionforthecuccularismuscleinDunkleosteus, dp.m.cu, thepresenceofagrooveforthislineinGymnotrachelus.
Stensio, 1963,fig. 112A). DunkleandBungart(1939,p.18,fig.6A&C)interpret-
Marginal (M). The marginal plate (Figures 2A&B, edasingleplatefragmentwithasensorylineasthemar-
4F)possessesasinglesensorygroove;acontinuationof ginal plate intheholotype (CMNH 5724). This pieceis
the postorbital branch ofthe infraorbital line, which interpreted here as a fragment ofthe right preorbital
10 Carr No.48
Figure 6. Gymnotrachelus hydei (CMNH 8051). Postmarginal(PM). Two isolatedpostmarginal plates
Suborbital plate in A, rightexternal and B, left internal are preserved, one in CMNH 8052 and the other in
views. Right postsuborbital plate in C, internal and D, CMNH8799.Thepostmarginalplate(Figures2B,4A)is
external views. E, right submarginal plate in internal triangularin shape with anteroventral andposteroventral
view.Scalebarsequal 1 cm. bordersformingaca. 90° angle. Adistinctivesubobstan-
tic region is absent; however, a thinning ofthe pos-
teroventral margin is notedin CMNH 8799. Thecontact
plate possessing thickenings associated with the dermal margins ofneighboringplates are notpresentinthe two
preorbital process. On the marginal plate, the sensory specimens with postmarginals preserved, hindering the
line groove is situated near and parallel to the plate’s interpretation ofoverlap areas. However, the marginal
medialborder. plate shape and orientation ofan internal inframarginal
Itisdifficulttoevaluatethepresenceofacentral and crista suggest the presence ofoverlap areas for the
marginal plate contact; however, the general spacing of paranuchal (oa.PNu, Figure 4A) and marginal (oa.M,
plates suggests a lackofcontact. Two overlap areas are Figure4A)plateswiththelatterbeinglarger.Internally,a
presentonthemarginalplate,ananteriorpostorbitalplate continuationoftheinframarginalcristaispresent.
(oa.PtO,Figure4F)andaposteriorparanuchalplateover- Paranuchal(PNu).Theparanuchalplate(Figures2A
lap (oa.PNu, Figure4F). Internally, alow inframarginal &B;5)possessesapostnuchalprocessthatextendsonto
cristaparallelstheexternalsensorylinetotheplate’sossi- the descending posterior face ofthe head shield. An
fication center. At this point, a thickening continues overlap area for the nuchal plate is present (oa.Nu,
beneath the main lateral line, while the inframarginal Figure5B)whichispartiallyoverlainbythepostnuchal
cristacontinuesasalowthickening(barelynoticeablein process forming arecess forthe nuchal plate’spostero-
CMNH8051)totheposteroventralcorneroftheplateand lateral ala. Themainlateral linetraversestheplateend-
ontothepostmarginalplate. ingalongtheposteriormarginjustabovethejunctionof
1994 RedescriptionofGymnotrachelus li
the lateral articularfossa (laf, Figure 5B) and occipital Figure 7. Gymnotrachelus hyclei. A, sclerotic plate in
para-articularprocess (pap. Figures 5A&B). An exter- external view (CMNH 8055). Scale barequals 1 cm. B,
nal opening for the endolymphatic duct is present close-upofexternal ornamentation (CMNH 8051). Scale
(d.end.e, Figure 5B). The lateral articular fossa is well barequals0.5cm.
developedwiththeoccipitalpara-articularprocessbeing
shortandnearlyround.
Internally, there is adistinctchannel (ch.pr.sv. Figure posteriorsuperognathalonthesubautopalatinecrista.This
5A) andrecess forthe dorsal aspect ofthe supravagal ventralridgeextendsposteroventrallyasalowridgeonto
processoftheneurocranium.Thechannelextendsposterior- the “blade” portion ofthe suborbital plate (= R3 of
lytotheventrallipofthelateralarticularfossaandisdirect- Heintz, 1932, fig. 22). The subocular and postorbital
edtowardthebaseoftheoccipitalpara-articularprocess. cristaearecontinuous.
Postsuborbital (PSO). The postsuborbital plate
CheekPlates (Figures 2B, 6C&D) is triangular with distinct overlap
Generalfeatures.Thethreeplates(suborbital,postsub- areas forthe suborbital (oa.SO, Figure 6D) and submar-
orbital, submarginal) ofthe cheek are not fused to the ginal (oa.SM,Figure6D)plates.Theshapeofthesubor-
head shield. As in the head shield, sensory line grooves bitaloverlapreflectstheroundedposteriordimensionsof
are bounded by awell-developed lip (ridge). All plates thesuborbitalplate,whereasthesubmarginaloverlaparea
havebeenflattenedsecondarily. formsanopen angle (ca. 100°).Theseoverlapsaresepa-
Suborbital (SO). The shape ofthe suborbital plate rated dorsally resulting in agap between the suborbital
(Figures 2B, 6A&B) is similarto that ofother selen- and submarginal plates and the head shield (Figure 2B).
osteids, with the posteriorregion narrowing gently to Ananteroventralnotchispresentalongthepostsuborbital
form an anterior “handle.” The posterior“blade” region platemargin inCMNH 8054(Figure 6C&D); however,
isround.Theventralborderoftheorbitformsashallow inotherspecimens,theventralmarginformsagentlesig-
concavity. The groove for the supraoral sensory line moidcurvewithoutadistinctivenotch.
(sore,Figure6A)formsaclosedangle(ca. 85°)anddoes On the internal surface, a small thickening denotes
notjoinwiththegrooveforthesuborbitalbranchofthe thepositionofthequadrate(Qu,Figure6C). Abovethe
infraorbital line (ioc.sb. Figure 6A). The suborbital thickening, two low ridges suggest the trajectory ofthe
branch groove ends before the anteriorextension ofthe palatoquadrate. The quadrate and ridges lie beneath the
“handle”(adistanceofapproximatelyonequarterofthe centralregionbetweenexternaloverlapareassuggesting
totallengthofthesuborbitalplate).Thegrooveparallels that the palatoquadrate traverses beneath the gap
theinternalsuborbitalcrista(cr.so,Figure6B)andsepa- between suborbital, postsuborbital, and submarginal
rates the “handle” into two asymmetrical regions, the platesandheadshield.
ventral region being larger (in Dunkleosteus and Submarginal(SM).Thesubmarginalplate(Figures2B,
Heintzichthysthe position ofthe groove is closerto the 6E)isrectangulartosubrectangular(incontrasttotheelon-
ventralmargin). gate submarginal plate ofprimitive aspinothoracid
Internallythereis asubocularshelf(cr.so, Figure 6B) arthrodires)andlooselyabutsanoverlapareaonthepost-
and subautopalatine (cr.sau. Figure 6B) and postorbital suborbital. The ossification center is located postero-
(cr.po, Figure 6B) cristae. There is nocontactface fora dorsally(aposteriorlocationistypicalofeubrachythoracid
12 Carr No.48
Figure8.Gymnotrachelushydei.RightinferognathalinA, andbetweenspecimensrangingfromdenticlesonlyonthe
lateral view andB. close-upofocclusal surface indorsal medianedgetocompletecoverage.
view (CMNH 8051). C, close-up ofleft inferognathal in
medial view (CMNH 8776). D, right and left anterior GnathalPlatesandParasphenoid
superognathalsinventralview(CMNH8055).Notepaired Generalfeatures. Three paired gnathal elements are
lateralrowsofdenticles.E,rightanteriorsuperognathalin present (inferognathal, anterior and posterior supero-
ventralview(CMNH8050).Scalebarsequal 1cm. gnathals). The anterior superognathals do not articulate
with the parasphenoid, which is poorly preserved. All
plateshavebeenflattenedsecondarily.
submarginalplates).Internally,theposteriormarginshows Inferognathal (IG) (Figures 2B, 8A-C). The occlusal
growthridges.Ashallowcontactface(cf.PSO,Figure6E) and adsymphysial region occupy ca. 46% ofthe total
ispresentattheanteroventralcorner.Alowridgeispresent inferognathal length (44-48%). Anteriorly, the adsym-
fromtheposterodorsalcornerthroughthecenterofossifi- physial region possesses 5-6 large denticles in a single
cationandashortdistancebeyond.Thereisnogroovefor row.Typicallythelargestdenticleisfoundattheposteri-
thehyomandibula. or apex ofthe adsymphysis. Posteriorto this cusp, the
Sclerotic(scler).Therearefourscleroticplates(Figures occlusal surfacepossesses asinglerowofevenly spaced
2B, 7) pereye. Typically, each sclerotic has an external denticlesextendingtotheposteriormarginoftheocclusal
overlapareaandan internalcontactfaceatoppositeends region. The anterior denticles are recurved where not
foradjacentscleroticplates,althoughthenatureofcontact worn. Intheposterior40%oftheocclusalregion,oneor
isvariable(e.g.,overlapareasateachendorbothacontact twoadditionallateralrowsofdenticlesarepresent(Figure
faceandoverlapareaatoneend).Eachscleroticpossesses 8B&C).Thedenticleswithinanindividualinferognathal
an ornamentofpunctate denticles (Figure 7). These are arespacedequallyalongtheocclusalsurface(5-10denti-
densestmediallyandextendacrossthesurfaceofthescle- cles percentimeter). The number ofdenticles percen-
rotic.Thelateralextentofdenticlesisvariablebothwithin timeter varies inversely with overall size, andtherefore