Ptyctodontid fishes (Vertebrata, Placodermi) from the Late Devonian Gogo Formation, Western Australia, with a revision of the European genus Ctenurella Ørvig, 1960

41 Pages·1997·13.7 MB·English

by  John A Long

Ptyctodontid fishes (Vertebrata, Piacodermi)
from the Late Devonian Gogo Formation,
Western Australie, with a révision of the
Européen genus Ctenurella Ory'ig, 1960
John A. LONG
Department of Earth and Planetary Sciences, The Western Australian Muséum,
Francis Street, Perth, Western Australia, 6000 (Australia)
Long J. A. 1997. — Ptyctodontid fishes (Vertebrata, Piacodermi) from the Late Devonian
Gogo Formation, Western Australia, with a révision of the European genus Ctenurella
Orvig, I960- Geodiversitas 19 (3) : 515-555.
ABSTRACT
A new, almost complété specimen of the ptyctodontid placotierm
Campbellodus decipiens Miles Young, 1977 enables description of the skull
roof, trunk shield, viscéral skeleton, pelvic girdie, dermal scale cover, and
parts of the vertébral column. A new reconstruction of the head shield of
Ctenurella gladhachensis 0rvig. I960 from Bergisch-GJadbach permits thk
taxon to be generically defined from the Gogo species previously referred to
that genus. The Gogo foriTi is hea* referred to Austroptyctodus n.g. A new spe¬
KEYWORDS cimen o\ Austroptyctodus gardineti Miles ^^/^Young, 1977» together with new
Ptyctodontida, observations of Chelyopborus vernmtli Agassiz,, 1844 and Ctenurella gbdba-
Devonian, chensis 0rvig, 1960, shows new information for the endocranium, the hyoid
Piacodermi,
Coco, arch and viscéral skeleton, identifying the previously identified ‘^metaptery-
Australia, goid” éléments as paired nasal bones. The large viscéral skeleton bone poste-
Austroptyctodus n.g.,
Ctenurella, rior to the jaw joint in ptyctodontlds is here identified as an elongated
Chelyophortis. interhyal.
RÉSUMÉ
Une nouvelle description du toit crânien, de la cuirasse thoracique, du sque¬
lette viscéral, de la ceinture pelvienne, de Técaillure et de quelques éléments
de la colonne vertébrale est proposée à partir d’un nouveau spécimen sub¬
complet du ptyctodonte Campbellodus decipiens Miles et Young, 1977. Une
nouvelle reconstitution du toit crânien de Ctenurella gLidbachensts 0rvig,
I960 (Frasnicn de Bergish-Gladbach, Allemagne) permet de redéfinir le
genre Ctenurella^ en y incluant les formes de Gogo qui y sont rapportées. Un
MOTS CLÉS nouveau genre Austroptyctodus. est créé. A partir du maicrici qui définit
Ptyctodontida, Austroptyctodus gardineri, espèce-type du genre, et d’une révision de
Devonien, Chelyophonis vemeuilli Agassiz, 1844 et de Ctenurella galdbachemky de nou¬
Piacodermi.
Gogo, velles informations sur l’endocrâne. Tare hyoïde ei le squelette viscéral dc.s
AustraRc, pryctodontes sont fournies. Ainsi le n métapcerygoïde >> est interprété comme
Austroptyctodus n.c.,
Ctemirella, des os nasaux pairs, de même Tes du squelette viscéral situé en arrière de
Chelyophortis. l'articularion de la mandibule serait un interhyal de forme allongée.
GEODIVERSITAS • 1997 • 19(3) 515
Long J. A.
INTRODUCTION although rhe following species hâve been descri-
bed from relacively complété, well-preserved
The ptyctodontid placoderms hâve for many marerial of head and trunk armour: Ctenurella
years been a problematic and poorly-known gladbachensii (0rvig 1960, 1971), Àustro-
group of extincc flshes. Early workers idencified ptyctodm gardineri n.g. (Miles & Young 1977,
ptyctodontids only from their characterisnc cooth here erected as a new genus), Rhampbodùpns
plates and chus allied then^ lo rhe holocephalans tbreiplandi and R. trispmatîi^ (Watson 1934,
(Pander 18S8; Woodward I891)t although when 1938; Vliles 1967), Rhynchodus tetradon
the cxoskeletal trunk armour was described by (Newberry 1 873; Jaekel 1903; Gross 1933;
Jaekel (1907) he compared ir fàvourably with the Stensio 1959 ; Miles 1967). Cbelyophonn ver-
sturgeon Acfpensc7\ an osteichrhyan fish. Roth mieili (Agassiz 1844; EichwaJd 1859; Obrucheva
Dollo (1907) and Goodrich (1909) rejectcd this 1983), and Ptyctodopsis menzeli (Denison 1985).
view and argucd that chey were placoderm fishes. Recenr new discoveries of other relatively com¬
Watson (1934) described more complété macerial plété ptyctodontids hâve been made in
of a new form he called Rhamphodopsis and fur- North America from the Frasnian Mt. Eldetr site
cher characterised chem as placoderm fishes, and in. Arizona (Johnson ât Elliott, in press) and
this view was strengthened by further discoveries from the Pînicon Range Formation, lowa
of well-preserved specimens (Watson 1938). (Hickerson 1993; work in prep.).
Major discoveries of complété, well-preserved The superb three-dimensionally preserved mate-
ptyctodontids from the Bergi.scb-Gladbach région rial of ptyctodontids Irom rhe Frasnian Gogo
of Germany described by 0rvig (1960, 1962, Formation of We.stern Australia has provided the
1971, 19S0) led him to revive the carlicr hypo- most detailed knowledge ot the group to dare.
thesis of a holoccpltalan affinity between ptycto¬ Miles & Young (1977) described several spéci¬
dontids and holocephalans, proposing them to bc mens of a new species they ;tsslgned to the genus
ancestral to the holocephalans, an opinion loUo- Ctenurella 0rvig, I960. Two incomplète speci¬
wed by Stenski (1963, 1969) and Jarvilc (1980). mens were placed in the new genus and species,
However, many researchers working wirh placo¬ Camphellodus decipiens. Further observations on
derm fishes or chondrichthyans opposed this the structure ot the Gogo ^CtenutrlLi' were descri¬
view, regarding ptyctodontids as placoderm fishes bed by Gardiner (1984a) and Forey & Gardiner
{e.g. Patterson 1963; Stahl 1967; Miles 1967; (1986). New finds of Gogo fishes made dunng
Miles Young 1977; Denison 1978; Forey & expéditions in 1986-1989 by rhe auchor include
Gardiner 1986; Young 1986). Today this view is many new arthrodires (Long 1987, 1988a, I9S8b,
widcly held, and the placoderm affinity of ptycto¬ i990K 1994a, 1995b) as well as two nearly com¬
dontids has been borne out by several recent plété, articulated specimens of ptyctodontids,
computer analy.ses of placoderm incerrelation- which belong to the previously described taxa.
ships {e.g. Forey & Gardiner 1986; Carr 1991, The aim of this paper was originally to describe
1995; Goujet & Young 1995). this new material of the Gogo ptyctodontids in
The problematic position ol the group wîthin detail, by comparison with other specimens of
the Placodermi has been raised. Tbey hâve been ptyctodontids, made avajlable for che author to
proposed as a primitive sisrer group to other pla¬ study at the Muséum nanonal dTüstoire natu¬
coderms (Miles & Young 1977: Young 1980), as relle. The wcll-pceserved nature of the Gogo spe¬
a primitive group more derived than acantho- cimens has permitred new further interprétations
thoracids plus rhenanids, but plesiomorphic to of the structure of these other taxa. Compari.sons
ail other placoderms (Forey & Gardiner 1986), with the Bergisch-Gladbach Ctenurella indicared
or as a sister taxon to the petalichthyids (Goujet that its skull roof and viscéral skeleron could be
1984; Goujet & Young 1995; Carr 1995). reinrerpreted. It soon became apparent that this
Most of che kaown ptyctodontid taxa (some species differs in several major features from the
forty-seven species, Carr 1995, amended) are Gogo species, and chus a new genus, AiistrO'
based solely on tooth-plates (Denison 1978), ptyctodus n.g., is here erected for the Gogo species.
516 GEODIVERSITAS • 1997 • 19(3)
Ptyctodontid fishes from the Late Devonian Gogo Formation
DERMAL BONE TERMINOLOGY in some placoderm groups, is always a médian
IN PTYCTODONTIDS bone lacking sensory-llne canals {e.g. as in an-
tiarchs, Denison 1978), or maybe represented by
The central médian bone thac bears the X-sha- a sériés of irregularly-shaped onamestic bones (as
ped confluence of the two main dorsal in the rhenanid AsterosteuSy Stensiô 1969, fig 92).
senson^-linc canals lias bcen previously called the For chc other bones m the head shield and cheek
centro-nuchal (0rvig I960)> the cenno-median of ptyctodoniids, papers published since Miles 6C
(Miles 1967), the nuchaJ {e,g. Miles & Young Young (1977) use a consistant lerminology.
1977; Long 1988a} or the postpineal (Dcnison However, the large paired ventral bones in the
1978, 1985; Johnson & Elliott 1996). rrunk shield that hâve bcen called either the
Confusion over the nume of this bone has ariscn, anterior vcnrrolaterals (Gross 1933; Miles 1967)
largely due to the supposed presence of a small or chc intcrolatcrals (Watson 1938; Denison
posterior médian clement in Rhamphodopsis 1978, 1985) require discussion. 0rvig (1960)
which was termed the nuchal. Examination of regarded them as a combination of both élé¬
specimens of Rhamphodapsh threiplandi in the ments and termed them the interolateral-anterior
collections ol the Natural History Muséum, ventrolarerals The anterior vcnrrolaceral plates in
London, and in the Ficid Muséum, Chicago, primitive placoderms, such as petalichthyids
shows that the posterior nuchal élément is not {Lumspis, Gross 1961), early arthrodires (actino-
présent in this genus, but was misinterpreted by lepids, phlyctaenaspids, Gou)ec 1984b)> phyllole-
Watson due to rhe crushed and fractured nature pids (Long 1984b) and Wuttagoonaspis (Ritchie
of the Edderton specimens. The area of exposed 1973) are named so because there is also a poste¬
bone immediately behind the paired central rior ventrolatcral plate présent posterior to them.
plates is most likely the crushed occipital ossifica¬ Ail of these forms also beat paired interolateral
tions of the braincasc, as this région lacks any plates forming the anterior ventral margin of ihc
trace of dermal ornamentation on the Edderton shield and meeting the spinal plates dorsally, and
specimens. in ail cases having an expanded anteriorlyTacing
In other placodcrm groups that hâve the supraor- postbranchial lamina wiih rows of triangular
bital sensory line cariais as well as posterior pic- rubcrcles. In acanthothoracids (e.g. Romundinay
line canals converging mcsially onto one médian 0rvig 1975) and rhenanids (e.g. Jagorina^
bone, as in ptyctodontids, this bone is generally Sien.sio 1969) there is only one pair of large ven¬
identified as the nuchaL espccially so when it tral plates and ihese also form the anterior mar¬
extends ail the way to the posterior margin of the gin of the ventral lamina of ihe rrunk shield and
skull roof {e.g. as in petalichthyids; Woodward hâve a postbranchial lamina which in life presum-
1941; Gross 1963; Young 1985; Lia 1992). This ably formed the posterior wall of rhe branchial
condition is aiso seen in the primitive arthrodire- chamber. However, it is known that ihe position
like form Wuttagoanaspis (Rîtehie 1973) and in of the scapulocoracoid in arthrodires generally
phyllolcpids. whcrc the nuchal plate is broad sits between the anterior latéral, the spinal and
rather than clongate. bur still bears rhe confluen¬ the anterior ventrolatcial plates (and also be¬
ce of several .scnsory-line canals (Long J 984b). In tween rhe interolateral in Dtcksoîiostezis, Goujet
many primitive ariltrodireît this bone has a .simi- 1984b). Similarly, in antlarchs, it lies within the
lar elongatc shapc and also has similar contact bounds of the anterior vcnirolateruls, even
relationships wich neighbouring paranudial, cen¬ though only remnants of it hâve beeii tound in
tral, and poscorbital or prcorbital bunes, primitive forms (e.g. Procondylolepis^ Young ÔC
although the convergence of the sensory-line Zhang 1992). Thus its condition in ptyctodon-
canals is lacking. tids would impiy that the interolateral has expaii-
l'hus the médian caual-bearing bone in ptycto- ded ventnilly to include the scapulocoracoid, or
doniids is incerpreted here as homologous to the that ic has indeed fused with the anterior ventro-
nuchal of other primitive placoderms, rather lateral lo he a compound bone (as suggested by
then a postpineal as this élément, where présent 0rvig 1960, also Stensiô 1959). The identifi-
GEODIVERSITAS • 1997 • 19(3) 517 1
Lx)ng J. A.
cation of ihe bones in pryctodonrids cornes naturelle, Paris. Llppcr Devonian (Fammenian),
down to i;he overall shape in ihat ürey are narrow Dankov-Lebedyan beds, LISSR.
bones without posieriorly extended ventral lami- — CtcntAteUa gladhachensb 0rvig> I960. Five
nae, bordering, in che irunk shield, die anterior nearly complété well-preservcd specimens held in
ventral margin ol the trunk and contacting ihe rhe Palaeontological collections of the Muséum
spinal plaie dorsally» and ihe faci chat tbey bear national d'Hiscoire naturelle, Paris; other spéci¬
rhe well-developed postbranchial lamina wiih mens in the Natural Histor}' Muséum, London.
rows of triangular tubercles, a featurc never Middlc-Upper Devonian (Upper Giveuin/Lower
found in any placodcrm anterior vcntrolatcral Frasnian), Obérer Plartenkalk, Germany.
plate (bccause interolacerals are aJso présent in — Pt)fctodopsis menzelli Denison, 19S5. Type spe¬
the same armour). Thtis ic is here su^ested that cimen held (on display) in the County Museqrn,
they represent interolateral plates. This hyporhe- lowa City, lowa; additional specimens held in
sis implies that ihe ptyctodontid trunk shield is the Geology Department collection, L^niversity
more specialised than those of other placoderms of lowa, lowa City. Middie Devonian (Upper
in the secondary loss of both anterior and poste- Givetian), Cedar Valley Limestone, USA.
rior ventrolaterals.- In this respect they could par- — Rhamphodopiis threiplandî Warson, 1934.
rallel the évolution of phyllolepids, now regarded Specimens in the Field Muséum, Chicago and in
as derived arthfodirrs in the loss of postenor dor- the Natural History Muséum, l.ondon. Middie
solateral plaies and the fusion of upper jaw élé¬ Devonian (Eifelian), Middie Üld Red Sandstone,
ments (based on materials of a new phyllo-lepid UK.
from New South Wales iindcr study by — Rhynchodus tetrodon. CasL of the holor}'pe held
Dr. A, Ritchic, pers comm. 1995). in the Nntural History Muséum. London. Upper
Devonian (Frasnian). Kellwasserkalk, Germany.
— Specimens of an undescribed new genus of
MATERIALS AND METHODS ptyctodontid from the Spring Grove Meniber
(Givetian), northern Illinois, held by the
The ptyctodontids from Gogo were prepared by Geolog)^ Department, Augustana College, Rock
the standard acetic acid technique using about (sland, Illinois.
5-10% concentration, strengthcned with dilute — Specimens of an undescribed new genus of
B30 Mowital in acétone solution (from ptyctodontid from the Gneudna Formation
Hoescht). The délicate nature of the Austroptyc- (Upper Givetian-Lower Frasnian), Western
todus specimen callcd for epoxy resin transfer Australia, collected by the authc>r and
préparation. Before embedding the specimen in K. Trinajstic in 1995.
resin, a latex peel of rhe exposed surface derail Al! specimens described or cited in this paper are
was made. Ail photographs are ofspecimens whi- reposited in the palaeontology collections of the
tened with ammonium chloride, excepi for the following institutions as denoted by these abbre-
Bergisch Gladbach specinieiis which were phoio- viations:
graphed under alcohol.
MNHN Muséum national d’Histoire naturelle,
The new Gogo speciraens were compared with Paris;
the following ptyctodontid material. These NHM Natural History Muséum, London;
results will form the basis ol a future révision of WAM Western AustraÜan Muséum, Perth,
Western Australia.
the Euramexican ptyctodontids:
— Austroptyctodus gardineri n.g. (Miles et Young,
1977). Ail specimens held in theNatural History
SYSTEMATIC DESCRIPTIONS
Muséum, London. Upper Devonian (Frasnian),
Gogo Formation, Western Australia.
Genus Campbellodus Miles Young, 1977
— Chelyophorus verneuili Agassiz, 1844. Original
Agassiz collection held in the Palacontological Type SPECIES. — Campbellodus decipiens Miles et
collections of the Muséum national d’Histoire Young, 1977.
518 GEODIVERSITAS • 1997 • 19(3)
Ptyctodontid fishes from the Late Devonian Gogo Formation
AaiIiNDED DiAGNOsiS. — A inotleraicly large ptycto- dying the oihcr Gogo ptyctodontid with the compara¬
domid haviiig a PtyctuduS‘\\\<£: crushing dendeion with tive material of ocher European généra thar the com¬
high dorsal process on the upper tooihplate, head plète description, and new reconstruction provided
shield having a breadth/lengrh ratio of 90, che nuch-al here, could be undertaken.
is subrectanguiar, almosi as broad as long and j)artici-
pates in the posterior margm of tht skull-rool-, cxclu-
ding the centrais from mcsial contact; submargin-al Descwition
plate is much deeper postcriorly tban anterlorly.
Head shield
Trunk shield wlth three médian dorsal bones, the
The exceptional préservation of WAM 86.9.672
dorsaimost bcing a broad, fiat splne. Spinal plate very
small; anterior médian ventral plate more tlian twice (Figs lA-C, 2) bas enabled the head shield of
as broad as long, fail covered with large overlapping Campbellodm to be restored in its natural three-
scalcs. dimensional fdrm. This is unique for pryaodon-
tids in which restorarions usually show the head
Campbellodus decipietu Miles er Young, 1977 shield flattcned in dorsal aspect, the one excep¬
tion bcing Ptyctodopsis (Denison 1985) which is
Campbellodus decipiens Miles et Young, 1977:
preserved in latéral view. Overall rhe head shield
145-155, figs 8-14, pis 1, 2A, B, 4A. - Denisoii 1978:
is 90% as broad as long, being broadest across
28. - Long 1987a: 203; 1988a: 443-4, fig. 7; 1988c;
141.143,144, fig. 2B; 1991: 366, 367. fig. 20H; the püscorbital places posterior to the large orbir.
1995a: 108-110. The preorbical plates are cxceptionally large,
Ptyctodiis Gardiner et Miles, 1975. about iwice the size of the other skull roof boites,
“tooth-plates rescmbling those of Rhynchodus" which are approximately the same size as each
(Brunion, Miles &L Rolfe 1969) other. An unusual feature of the skull roof is rhat
“toodi-plates whlch recall those of Ptyctodus** (Miles the prcorbital place.s are not in rnesial contact
1971)
with apparcntly an open pineal notch for rhe
Holotype. — WAM 70.4.252. This number is nor pineal organ. No pineal plate was recovered
published in the Miles &c Younç (1977) as the spéci¬ during the préparation of the specimen, and, as
men had then been alIocatetT by the provisional most of the dclicace gill arch clemencs of both
British Muséum of Naturai History number P50905.
sides werç found, it is assumed rhat a pineal plate
was ci cher very small (as in Austroptyaodus n.g.),
On 1ER MAITtRlAl.. — NHM P50907. compristng the
lefr spinal plate, left upper toothpiate, incomplète Icft or absent. In latéral view, the skull roof shows a
interolatera! plate, partial dorsal spine, a dermal scalc slii-like opening at che junction of the postorbi¬
and parts ol perichondral ossifications of the endocra- tal, marginal and submarginal plates, referred ro
nium (Miles & Young 1977). WAM 86.9.672, an
previously by Miles & Young (1977) as rhe post-
almost complète individual, shows the head shield
orbital fenestra and by Long ( 1988a) as the spira-
mostly complété, missing only pan of ihc posterior
margin on me riglu side and a small part of che left cular slii.
posterior margin. It shows weli che three-dimensional Marginal plate. le was apparently not tightiy
form of the entire arriculated trunk shield. the pelvic connccted with the lacerai margin of the head
girdic and endogirdlc. body .scalc.s and axial skelcral
shield: as its précisé dorsal border does not cor¬
cléments. WAM 95.6.112 shows the left upper and
lower tooth plates, mirror opposites ro those scen in respond CO the well-preserved ventral margin on
the holotypc. the lacerai face of the head shield, it is presumed
to hâve been looscly atcached. The submarginal
Remarks. — The new material is regarded as cogene- was probably aiso free in the skin of the cheek
nc with the holotypc as the dorsal spine is of the same
région, bur anteriorly contained ihe opercular
broad based sbape with sirailar ornamenraiion, and
the isolarcd prcorbital, postorbital and submarginals cartilage which acticulated with the ethmoid
of the Holotypc are ail of similar shape ;md idcncical ossification of the braincase.
proportions in WAM 86.9.672. This genus ha.s bcen Preorbîtal plates. (PrO; Figs lA-C, 2) These are
redefined in the light of nearly complète material of
che largest bones of the head, being just over half
the dcrmal armoun axial skelcton and squamation.
the total length of the skull. Miles & Young
Long (1988a) figured a reconstruction ofthe dermal
armour, and conimentcd briefly on iis ovcrall mor- (1977, fig. 8) dcscribed and figured une of these
phology, but it is only wichin the framework of stu- plates but wrongly identified it as the right ele-
GEODIVERSITAS • 1997 • 19(3) 519
Lx)ng J. A.
Fig. 1. _ Campbellodus decipiens Miles et Young, 1977. WAM 86.9.672. A-C, head shield; A, dorsal view. B. ventral view; C. right
latéral view, 0. E. nght marginal plate: D,, latéral view; E. mesial view F-H. médian dorsal plates in right latéral view; F. médian dor¬
sal spine: G, médian dorsal plate 2; H, médian dorsal plate 1.1, J. prepelvic bone and attached endoskeletaJ pelvic girdie: I, ventral
view; J, dorsal view. K, right quadrate in mesial view. Ail photos whitened with ammonium chloride.
520 GEODIVERSITAS • 1997 • 19(3)
Ptyccodontid fishes from che Late Devonian Gogo Formation
ethm. ri
1 cm
Fig. 2. — Campbellodus decipiens Miles ef Young, 1977, WAM 86.9.672. A-D, sketch of head shield; A, dorsal view; B, ventral view;
C. right latéral view: D, posterior view, art.fl, articular flange ot neck joint on PNu plate; Ce. central plate; dep, dépréssion; ethm.rl,
ethmoid ridge; for. foramen; gr. groove; Mc. main latéral line canal; Nu. nuchal plate (posterior element); orb, orbil or orbital margin;
P, pineal plate or space provided for it in the head shield; pit, pit for insertion of eye muscles: PNu, paranuchal plate; pp, posterior
sensory-line canal on head shield; ppr. posterior process of PNu plate: PrO, preorbital plate: PtO, postorbital plate; soc. supraorbita!
sensorydine canal; spir, spiracuiar sllt; suov, supraorbital vauit
ment when in face it is die lefc preorbital place, as curved, flexing over the mid-point of che orbit.
verified by direct comparison witb rJie arriculatcd The path of the supraorbital sensory-line canal is
head shield of WAM 86.9.672. The preorbital only visible in dorsal view Iti die posterior parr of
plates are subrectangular in form, having a che plate, faindy seen mainly through a single
breadch/length index of berween 56-60, for both row of minute pores opening from within the
specimens. They show a weakly concave latéral spaccs of the reticulatc ornamentation. In viscéral
margin, gently convex anterior margin, strongly viev^v the tubular sen.sory-line canals are clearly
convex posterior margin, and gently concave seea (Fig. 2B, soc, pp, lie), disappearing within
mesial margins witb a well-defined anterior the centre of die boue at the os.sification centre
Dotch for ihe pineal foramen. In latéral view of the plate. The main latéral line sensory-Üne
(Figs IC, 2C), the preorbital plates are weakly canal that cornes oif the marginal plate enters the
GEODIVERSITAS • 1997 • 19(3) 521
Long J. A.
PNu
ppr
Fig. 3. — Campbellodus decipiens Miles et Young,
1977, WAM 86.9.672. Left paranuchal and postorbital
plates in mesiai view, showing spiracuiar sût. PNu,
paranuchal plate: ppr. posterior process of PNu plate;
PtO, postorbital plate; spir, spiracuiar slit; suov,
supraorbital vault.
preorbital at its posteroventral corner and runs no unusual features apart from Ünear thickenings
directly into thc ossification centre of tlie plate. of bone running out from the ossification centre
In viscéral view, the preorbital sJiows areas of CO the margin of ihc spiracuiar .slit (Fig. 3). These
cancellous spongy bonc above the orbits, lor- chickening of bonc aroiind the spiracuiar slit pre-
ming a supraorbital vault (suov, Fig. 2B), and sumably assisted to direct lhe flow of water from
smooth areas of bone surface for contactlng the outside into thc spiracuiar groove.
endocramum, presumed here to be mostly card- Nuchal plate. (Nu; Figs lA. B, 2A. B, D) This
laginous. An anteriorly facing dépréssion central niedian bone of theskull roofis contacted
(Fig. 2B, ethm.ri) is posieriorly bounded by the ainerolaterally by the preorbitals, laterally by rhe
raised supraorbital sensory-linc canal (soc) and a centrais, and forms the posterior indenced margin
mesially direcced ridge, Presumably, this région of thc skull roof, It is a relativcly small but broad
braced the dorsal wall of rhe ethmoid division of bone for ptyctodontids, about half as long as che
the braincasc as it docs in arthrodines. The latéral preorbital, and only a little longer than its
sides of thc supraorbital scnsoiy-linc canal has a bteadeh. It is quite fiat. lis margins arc ail complè¬
pit (Fig. 2B, pit) just posterior to the cthmoidal te and gencly concave to mect che surtounding
ridge, possibly a myodome for eycmuscle attacli- boncs which it ovcrlaps. The confluence of che
ment. Thcrc is a broad triangular dépréssion sensoiy-linc canals on the nuchal ai*e well-defined
(Fig. 2B, dep) defined by the mesiai margin of in dorsal view and, in ventral view, arc clearly
the supraorbital canal and the raised mesiai edge seen by the raised tubes of bone chat carried che
of the supraorbital vauk> and a simiJar postenor- sensory-lines (pp; Fig. 2B), Weak ridges of bone
ly facing dépréssion between the cwo converging radiate outwards from its ossification centre.
sensory-llne canals in rhe posterior half of the Left central plate. (Ce; Figs lA-C, 2A) li is
plate. The ornament of the preorbital plate is largc- well-preserved in this speetmen although only
ly reticulate with patches of very fine tubercles part of the righr élément is présent, but neither
over the orbits and towards the anteriar margin. shows the posteromesial margin completely pre-
Postorbital plate. (PrO; Figs lA-C, 2, .3) It is served, Ir has a strongly concave anterior margin
only parcially visible in dorsal view, showing its for the preorbital plate, straight contact of latéral
largest area in latéral view. 1rs dorsal lamina margin where it meets the macginals, a strongly
contacts the central posteriorly and is notched convex latéral contact margin with the paranu-
into the preorbiral anteriorly. In latéral view, it cha) plate, and a relarively straight mesiai margin
has a smoothly concave anterior margin for the where it lies in contact with the nuchal. The
orbir and an irregularly convex posterior margin short région of thc po.steroinc.sial margin is also
which in part forms thc margin for thc spiracuiar quite straight and forms part of the indented
slit (spir; Figs 2C, 3). In viscéral view, it shows posterior margin of thc skull roof.
522 GEODIVERSITAS • 1997 • 19 (3)
Ptyctodontid fishes from the Late Devonian Gogo Formation
Paranuchal plate. (PNu; Figs lA-Q 2A'D> 3) It which projeers posteriorly from che latéral mar¬
is well prescrvcd on the left side of the skull roof, gin of the paranuchal,. and weakly overlaps the
and partly preserved on the right sidc. k has an anrerior margin of che arcicular process of the
irregular shapc, dominated by concave margins anterior dorsolareral plate. Thcrc is a groove
where it contacts the postorbital anteriorly and along rhe poscerovencral margin of the para¬
the central anteromesially. It has short contact nuchal plate in posterior view (Fig. 2D, gn Fig. 6).
wich the posiorbiial before beiiig indented for Marginal plate. (M; Figs ID, E, 4, 6) It is well-
the spiracular slii (spin Fig. 3). The posterior preserved on the riglir side only. It is a wedge-
margin of the paranuchal has a large, unorna- shaped bone, slighcly longer than the preorbirals
mented posieriorly-facing flange of bone (art.fl; and, with a breadch/length index of about 37* is
Fig. 2D) for contact wiïh che siniilarly fiat pro- narrower ihan for most other pryctodontids. It
cess on the anrerior donsolateral plate (art.conj lacks the strong inflection seen in some oiher
Figs 13, 14). The ventral margin of this articula- forms, such as Rhynchodus and Rhaynphodopshy
tory flange has a smaJl foramen (for; Fig 2B), and is proportionarely much smallcr relative to
corrc5ponding well to che position of rhe endo- the head shield than for chose généra, k has an
lymphatic duct opening on the viscéral surface of almost stralght dorsal margin bordering the
the place in artlirodircs. Yct as the plate lacks an orbic, and relatively straight, short margins for
external (dorsal opening), I conclude ir is possi- concacting che postorbital and paranuchal plates.
bly just a nutritive canal. As rhe rwo arciculatory The viscéral surface shows a pair of mesially
surfaces of rhe paranuchal and rhe anterior dor- dirccted laminae extending our from che centre
solaceral plates do not precisely fir together. a of the plate (Fig. 4A, C, D). These laminae are
chin pad of cartilage probably divided the cwo criangular în dorsal view and hâve a roughened
fiat processes to allow some sUght degree of verti¬ surface betw^een them presumably for contact
cal flexibility in the neck joint. There is also a wirh che orbital ossification of the braincase.
smooth process of dermal bone (ppr; Fig. 2B) This structure in pryctodontids is becter prescr-
Fig. 4. — Campbellodus decipiens Miles et Young, 1977, WAM 86.9.672. A-D, right marginal plate; A, mesial view; B, latéral view;
C, anterior view; D, dorsal view, mes.lam, mesial perichondral lamina extending from marginal plate: ov.PtO, overlap surfaces for
postorbital plate.
GEODIVERSITAS * 1997 • 19(3) 523
Long J. A.
ved in the new Gogo specimen oiAustroptyctodiis Viscéral skeleton
and is descfihtrd in further detail below (see Scveral paired and one unpaired perichondral
Figs 28, 29)- The posterior division of the viscé¬ ossification of the viscéral skeleton werc recox'er-
ral surface of the marginal plate has a smooth ed from chc specimen. Numerous viscéral arch
triangular région emanating Irom behind the pai- bone.s should bc Ibund in the skeleton of any fish
red laminae. The overlâp area for the postorbitàl that had them well-ossified, yet only a few bones
plate is Nvell-developed on the latéral surface have been found in this specimen, and in other
(Fig. 4 B); arciculated specimens of the Gogo form
Submarginal plate. (SM: Figs 5, b) It is preser- Austroptyctodus^ and the German Ctenureila. This
ved only for the left side, but is also well-prescr- suggesrs that only some bones of the anterior gill
ved in the holotype and was figured and arches in ptyctodonrids were ossified. .Schulc7-e
dcscribed by Miles & Young (1977, fig, 10). Tr is ( 1993; 213) points out that in elasmobranchs the
broad, posteriorly, nartowing to a slighrly up- dorsal hyoid arch éléments are the first to chon-
turned, well-roundcd anterior end. It is propor- drify (epihyal, ceratohyal), follnwed by the pha-
tionately deeper in its posterior end than in ail ryngobranchials, epibranchials and cerato-
other ptyciodontids for which it is known, as in branchiaU, then lastly by the hypobranchials and
most other piyctodontid gênera it is almost basibranchiab. It is possible that only rhose élé¬
bar-like. l’he viscéral surface is smooth with the ments of the first gill arch and possibly one pos¬
anterior end having the perichondral she|l of the terior to it were invesred with perichondral hone
opercular cartilage in situ. In ventral view, this in ptyctodonrids. This could be explained by the
perichondral lamina is niesially developed into a need to strengthen the bones in direct contact
strong process (mes.pr; Fig. 5C), which is inter- with the strong jaw mcchanlsm, chc more distal
preted to hâve supported a thick and presumably arch cléments rcmaining as cartiJaginous units.
large opercular cartilage (Fig. 6). The provisional identification of rhese gill arch
éléments is based on comparîsons with articulat-
ed matcrial of the other Gogo ptyctodontid,
AmtroptyctodiLu as weJI as articuJated specimens
of Cteinirella gladbachensis in the MNlfN, Pans.
Therc is no direct évidence, because none of the
smaller cléments arc preserved in life position in
any specimen examined. The position and iden¬
tification of the large!' éléments is based only on
their general shape and articulation surfaces in
relation to other large bones (e,g. the articulât),
and tomparisuns with the general shapes of ihcse
éléments in other primitive gnachostomes (chon-
drichrhyans, other placoderms, primitive osreich-
chyans).
The largesr of the gill arch elementLS in
Campbellodus are a pair of inwardly curved, dis¬
tal ly broad éléments. No comparable shape ro
bones previously idenrified in rhe ptyctodoniid
viscéral skeleton by Watson (1934)» 0rvig
(1960), Gardiner (1984a) or Furey & Gardincr
(1986). These bones (Fig. 7) arc strongly curved
mcsially and have grooves for vascular or nerve
Fig. 5. — Campbellodus decipiens Miles et Young. 1977, WAM
86.9.672. A-C, left submarginal plate; A, mesial view; B. left passages on the latéral surface near the disral
latéral view; C. ventral view, showing opercular cartilage ossifi¬ (smaller) end. The>' are roo curved mesially and
cation. mes.pr. mesial process or\ opercular cartilage; op.car.
opercular cartilage. coo narrow dorsally to be meckeUan cartilages
524 GEODIVERSITAS • 1997 • 19(3)