a eubrachythoracid arthrodire with a snubnose from gogo, western australia

<oologtcal Journal oJthe Linnean Soczelv (19821, 75: 153 166. With 6 figures

A eubrachythoracid arthrodire with a snubnose from Gogo, Western Australia

KIM DENNIS AND R. S. MILES

British Museum (. laturul History), Cromwell Road, London SW7 5BD

.4creptedfi,r publication September I981

A new eubrachythoracid arthrodire, Simosteus tuberculntus gem et sp. nov., is described from the Upper Devonian Gogo Formation of Western Australia and its structure and relationships are discussed. It is related to the trematosteids. leiosteids and hadrosteids on the evidence of ‘fused’ preorbital and postnasal bones.

KEY U’ORDS:--Arthrodira - .4ustralia ~ Devonian ~ morphology - phylogeny - taxonomy.

CONTENTS

Introduction . . . . Taxonomy. . . . . Description . , , .

General characters . Character phylogeny ,

Other points of structure Discussion , . , , . Acknowledgements. . . References. . . . . Abbreviations used in figures

. . . . . . . . . . . . . . . 153

. . . . . . . . . . . . . . . 153

. . . . . . . . . . . . . . . 155

. . . . . . . . . . . . . . . 155

. . . . . . . . . . . . . . . 157

. . . . . . . . . . . . . . . 163

. . . . . . . . . . . . . . . I63

. . . . . . . . . . . . . . . 164

. . . . . . . . . . . . . . . 165

. . . . . . . . . . . . . . . 165

INTRODUCTION

This unique specimen, described here as a new genus of arthrodire from Gogo, M’estern Australia, was studied by one of us (R. S. Miles) in the Australian Museum, Sydney in April 1980. The account was written at a later date on the basis ofexcellent photographs (Figs 2 -6) provided by Dr A. Ritchie and from notes taken during examination. The preservation, as with the other Gogo specimens, is outstanding-a feature which greatly facilitated further work under somewhat unusual conditions.

1”ZXONOMY

Simosteus gen. nov. , ‘l‘ume: Gr. simos, snub-nosed ; osteon, bone. Diugnom: ,4 eubrachythoracid arthrodire with a relatively long head, moderate

prepineal division and small orbits; a skull-roof that is strongly folded in the 153

OOL+ 4082/82/06oI‘,i+ I4 $O{ oo/o C 1982 The Linnedn Society of London

1.54 K. DENNIS AND R. S. MILES

transverse plane posteriorly. It has an upturned snout; preorbitals that are inseparable from the postnasals and which have well developed descending laminae; trilobed 'coccosteid'-like centrals and a nuchal which lacks a posterior em baymen t .

Type-species: Simosteus tuberculutus sp. nov.

Simosteus tuberculatus sp. nov. (Figs 2 6)

.Vame: L. tuberculum, tubercle. A reference to the prominent tubercular ornament.

Diagnosis: As for the genus; this is the only known species and any division between generic and specific characters would be arbitrary.

Holotype: A specimen belonging to the Australian Museum, Sydney. It comprises a skull-roof with weathered lateral margins; the postorbitals and marginals are incomplete and represented in part by casting material, the postmarginals are missing. Also present is the left cheek unit with autopalatine and part of the ethmoid region of the braincase attached, the right anterior superognathal, the left posterior superognathal, left articular, left and right cribrosal bones, two sclerotic plates and the parasphenoid.

Occurrence: The holotype is the only specimen. It was found at Bob's Bore (Miles, 1971 : fig. 1 ) and comes from the Lower Frasnian Gogo Formation, Canning Basin, northwestern Australia. Its exact locality is unknown.

Measurements: These are only available for parts of the skull-roof and cheek (Miles & Dennis, 1979: figs 1, 2) and are as follows:

Length of skull-roof in the midline 125 mm; Breadth of posterior margin of skull across posteromesial angles c. 73 mm; Prepineal length of skull 29 mm; Length of fenestra orbitalis 21 mm; Length of nuchal plate in the midline 49 mm; Length of lateral articular fossa 5.7 mm ; Angle between the axis of the lateral articular fossa and the dorsolateral surface of the skull-roof c. 40"; Length of external surface of cheek parallel to the suborbital lamina of the suborbital plate 77 mm; Length of postorbital division of cheek 55 mm. Remarks: Statements in the diagnosis involving proportions are illustrated by the

following ratios : ( 1 ) Prepineal length (expressed as a ratio of prepineal length to total length of

skull, multiplied by 100) 23.3. Comparative figures are: Harrytoombsia, 23.0; Camuropiscis concinnus Dennis & Miles, 38.7 ; Rolfosteus, 51 .O; Tubonasus, 40.0 ; Incisoscutum, 29.1.

(2) Size of orbital fenestra (expressed as a ratio of fenestra length to postpineal length of skull, multiplied by 100) 21.9. Comparative figures are: Harrytoombsia, 38.1 ; C. concinnus, 42.1 ; Rolfosteus, 36.6; Tubonasus, 39.2; Incisoscutum, 44.0.

(3) Size of descending lamina of preorbital plate (expressed as a ratio of lamina length to preorbital plate length, multiplied by 100) c. 30.9. Comparative

SNUB-NOSED ARTHRODIRE 155

figures are: Harrytoombsia, 5.9; C. concinnus, 28.3 ; Rolfosteus, 45.1 ; Tubonasus, 36.2; Incisoscutum, 27.1.

All comparative figures are taken from well preserved Gogo material.

DESCRIPTION

General characters

We shall deal first with the general features of the head and then turn to some of the numbered characters in the cladogram (Fig. 1) . The dermal bones of the skull- roof can be seen in Figs 2 & 3, the individual bones being similar to those of other Cbccosteus-like arthrodires. However, the snout is flat and upturned; the rostra1 plate (R) having a thickened and somewhat reduced descending lamina. The postnasals have either been lost or are ‘fused’ with the preorbitals. This ‘fusion’ of preorbital and postnasal to form a preorbito-postnasal plate has also been observed by Gross (1932) in the trematosteids and hadrosteids and by Stensio (1963) in the leiosteids. Neither an internasal nor extrascapulars have been found. Evidence concerning the former is unclear, but the presence of posterior descending laminae (pdl) on the posterior margins of the nuchal (Nu) and paranuchal (PNu) plates suggest that the latter were present in life.

Regarding bone pattern, it should be noted that, like Harytoombsia (Miles & Dennis, 1979: fig. 7B), Simosteus exhibits the Plourdosteus ((arvig, 1960:305) type of

adinolepidolds

LWscmsteus arctiars

phlyCam%bs acadiars -- spp. - spp. HMmstius spp.

f3mhamtws spp.

coccosteus spp.

traIn$osteids IeiOSpidS hadrosteids

pachyosteids rhinofsteids

brachydeirids

Figure I . Character phylogeny of some of the better-known arthrodires

I 1

156 K. DENNIS AND R. S. MILES

skull-roof where the paranuchal sutures with the postorbital (PtO) thereby separating the central (C) and marginal (M) plates.

The dermal bones of the cheek (Fig. 4A) comprise suborbital (SO), postsuborbital (PSO) and submarginal (SM) plates. These have the same arrangement as in Coccosteus cuspidatus (Miles & Westoll, 1968: fig. 13a, b) in that the submarginal maintains a substantial contact with the suborbital plate.

The pattern of lateral line grooves (Figs 2-4) is like that of Coccosteus cuspidatus (Miles & Westoll, 1968: fig. 1). The supraorbital sensory line (SOC) follows the same course despite the modified snout, and on the left side, though discontinuous, it joins the central canal (csc). Middle (mp) and posterior (pp) pitline grooves are also evident on the surface. The groove for the otic branch of the infraorbital canal (ioc.ot) is obscured by sediment but that for the postmarginal canal (pmc) is clearly present on the cast impression of the marginal though its extent is unknown. Posteriorly the grpove for the main lateral-line (lc) continues back to pass off the skull-roof at the level of the neck-joint below the external opening of the ductus endolymphaticus (d.end.e) . The occipital cross-commissure has not been observed on the surface and presumably it occupied a more superficial position. The pineal pit (ppt, Fig. 6) visible on the visceral surface of the pineal (P) plate is a paired structure which has a larger left side than right.

The sensory lines ofthe cheek are difficult to interpret due to residual deposits on the surface. All the usual grooves are present on the suborbital but only the supraoral (sorc) and postorbital branch of the infraorbital (ioc.pt) actually meet. In addition, the course of the suborbital branch of the infraorbital canal (iocsb) is interrupted anteriorly because the bone is damaged. The postsuborbital plate, like Coccosteus cuspidatus (Miles & Westoll, 1968: fig. 13k), bears a postsuborbital sensory line groove (psoc) in front of which is a cutaneous sense organ pit (cuso).

Figure 2. Simostnrs tvbcrcuhhrs gem ct sp. nov. Skull-roof in dorsal view. Holotype.


The ornament (Figs 2-4) is relatively coarse, being made up of large, evenly distributed tubercles with clusters of smaller, more densely packed denticles at the radiation centres of the plates. The tubercles are absent from the preorbital lamina region of the snout and parts of the suborbital plate. In the latter case the lack of ornament may be a result of post-mortem damage. Two sclerotic plates are preserved but their edges are abraded and the ornament unclear. One appears short and flat and the other long and thin.

Character phylogeny

We shall now describe phylogenetically significant characters of Simosteus following the numbering used in Fig. 1. Due to the absence of a trunk-shield, characters 4, 12, 18 and 23 have been omitted; they are explained elsewhere (Miles & Dennis, 1979; Dennis & Miles, 1979a, b, 1980, 1981).

( 1 ) Great width of skull-roof across posterolateral angles. The skull has a typically eubrachythoracid form so, although the postmarginal plates which constitute the posterolateral angles are not preserved, we assume that the character is as described.

(2) Two superognathals. Separate anterior and posterior elements (ASG, PSG, Fig. 5A, B, D, E) are present. The former is from the right side and the latter from the left. The anterior superognathal (Fig. 5 A, B) is incomplete but comprises a dorsal horizontal lamina and an anterior descending lamina. Where preserved, the ‘ventral tooth-row area’ is rounded and without teeth (‘ba’ of Brvig, 1980: figs 1, 5, 6) ; i t also has a curious piece of bone curling up from its lower edge (frag). This bone is thin and capped with rock making i t difficult to understand on either fiinctional or comparative grounds. It may indeed be a post-mortem addition. The dorsal lamina bears a relatively long posterior process (p.pro; ‘dp’ of Brvig, 1980: fig. 4) which curves gently inwards-there are no mesial teeth on its ventral surfiice. The posterior superognathal (Fig. 5D, E) is complete except for the posterior process. The ventral tooth-row has been worn down to a blade-like cutting edge (‘ba’ of Brvig, 1980: figs 3, 10) with only three posteriorly placed nonfunctional teeth remaining (v.t; ‘dfp. Psg’ of Brvig, 1980: figs 2, 3) . The lateral tooth-row (1.tr; ‘dfl. Psg’ of Brvig, 1980: figs 2, 11) is visible as a ridge running down the anterior surface and forming the upright part of the stepped biting edge. The lower part of the biting edge occludes with the inferognathal just behind the cusp (‘cu’ of Brvig, 1980 : figs 18-2 1 ) .

(3 ) Ginglymoid dermal neckjoint. Only the lateral articular fossa (laf, Figs 3B, 4B) and para-articular process (pap) of the paranuchal plate (PNu) are preserved; and that from the left side is damaged. The fossa appears small for the size of skull- roof-a feature emphasized by comparing the various fossa length/posterolateral width ratios. The figure for Simosteus is 7.8 which is substantially less than that obtained for Harryloombsia, 13.2; CamuropisciJ concinnus, 13.3; Tubonasus, 11.9; and Incisoscutum, 12.9. Conversely the para-articular process seems unusually large. There is no sign of the subobstantic area of the head-shield and it must therefore have been lower down on the unpreserved part of the marginal and the postmarginal plates.

(5) Occipital cross-commissure passing off the hind margin of the paranuchal. There is no direct evidence of this sensory line but i t is assumed to follow the same course as in the previously described eubrachythoracid arthrodires from Gogo.

I 1 *


a


Figure 4. Sirnostats tubcrcrclahrs gm. et sp. nov. A, Left cheek unit in lateral view with autopalatine and fragment of neurocranium. B, Skull-roof in posterior view. Holotype.

( 6 ) Suture lines remaining distinct, sinuous in skull-roof, whose plates have well developed overlaps. This character is well illustrated in Figs 2, 3 & 6. The only feature which requires further comment is the absence of a postnasal overlap area on the front of the preorbital plate. The fit of the cheek confirms that the postnasal was not present as a separate bone and we propose that the preorbital and postnasal have become ‘fused’. This is discussed further under character 24.

Nuchal plate posteriorly expanded. This feature is seen in Fig. 2, as is the comparatively broad anterior margin. The nuchal appears somewhat narrow and this is reflected by the breadth/length ratio-1 17.1. Figures for other Gogo arthrodires are: Harrytoombsia, 138.3; C. concinnus, 140.5; Roljosteus, 136.8; Tubonasus, 15 1.9; Kendrickichthys, 135.2; Bullerichthys, 142.9; Incisoscutum, 168.3.

( 8 ) Paranuchal plate with postnuchal process. This process is distinct and forms part of the posterior descending lamina (pdl, Fig. 2) of the skull-roof. As is usual, i t extends dorsomesially from the lateral articular fossa.

(9) Nuchal thickenkg on visceral surface of skull-roof (th.n, Fig. 6 ) . This dermal bone thickening is very well developed, especially posteriorly. It carries the channel for the dorsal part of the supravagal process of the neurocranium (ch.pr.sv; cranio-

(7)

160 K . DENNIS AND R. S. MILES

C .-

d

# e Y C

U

m

SNUBNOSED ARTHRODIRE 161

Figure 6. Simostcrrs tubercuktw gem et sp. nov. Skull-roof in ventral view. Holotype. Hatching on the lateral margins indicates the extent of the casting material.

spinal of Young, 1979: fig. 2) and the internal opening of the endolymphatic duct (d.end.i). Lateral to these structures there is a well developed cucullaris depression (dp. m. cu).

( 10) Pairedpits on visceral surface of nuchalplate (p.tu, Fig. 6). These pits are clearly present, face anteroventrally, and are separated from one another by a fairly broad median septum. Behind them is a transverse thickening which has a U-shaped, somewhat undercut front edge and beyond that is the posterior process (p.pr). This is a small projection which has a small posteriorly facing depression (dp.mc, Fig. 4B) at either side; its form not unlike that of C. cuspidatus (Miles & Westoll, 1968:

( 1 1 ) Pre-endoQmphatic thickening (th. pre, Fig. 6). This is present on the visceral fig. 2a).

surface of the central plates but is only incipiently developed.


( 13) Inferognathal with both blade and biting regions which are complete and co-ossified. The lower jaw is very poorly known but as the upper toothplates follow the usual eubrachythoracid pattern it seems reasonable to assume that the inferognathal did likewise.

The left articular is present and has a form similar to that of Zncisoscutum (Dennis & Miles, 1981 : fig. 11). The lateral surface, though partly obscured by a dark deposit, can be seen to have a distinct mandibular articulation area and a pit lined with endochondral bone which presumably housed the cartilage base of a small posterior process, The mesial surface, which fitted against the inferognathal, is partially broken away.

(14) Skull-roof with lateral consolidated region. The lateral consolidated part is well developed, but whether or not it continued down onto the postmarginal plate as the inframarginal crista is unknown, as the lateral margins of the skull-roof are not preserved.

(15) Submarginal plate*small and closely incorporated in the cheek unit (SM, Fig. 4A). The submarginal plate is very like that of C. cuspidatus (Miles & Westoll, 1968: fig. 13a, b, 1, m) in that it sutures with the suborbital as well as with the postsuborbital, marginal and postorbital plates. It is elongate, slightly abraded anteriorly, with a narrow overlap area for the marginal and small contact face for the postsuborbital on the dorsal and ventral surfaces respectively.

(16) Skull-roof with well developed supraorbital vault (suo. v). This feature is clearly seen in Fig. 6. Not so evident is the rod-like ventral postocular process (pt. 0. pr) extending from the posteromesial edge. Anteriorly the vault is bounded by the dermal preorbital process (d.prp) which has a well developed channel posteromesially for the dorsal aspect of the preorbital process of the neurocranium (ch. pro. pr).

(17) Suborbital plate with slender suborbital lamina. Although the suborbital (Fig. 4A) is typically eubrachythoracid in form, the small eyes of this species result in a fairly short, stout suborbital lamina.

(19) Preorbital plate with preorbital lamina (Fig. 3). The area of the preorbital lamina is extensive and cannot be separated from the postnasal plate (see also character 24).

(20) Suborbital plate with internal laminae. The laminae are not figured but examination of the specimen reveals that they are similar to those of Harrytoombsia (Miles & Dennis, 1979: 49).

(2 1 ) Palatoquadrate with separate autopalatine and quadrate bones. Both are preserved in Simosteus but they exhibit no unusual features and are not figured. The autopalatine (au) is visible in part in Fig. 4A. I t has a fragment of the ethmoid region of the neurocranium (neu) attached to its anterodorsal face. The quadrate is incomplete but the articular condyle is clearly present. Both structures appear to be essentially the same as in Harrytoombsia (Miles & Dennis, 1979: 49).

Two cribrosal bones are also preserved but their description will be left to the more complete examples found in certain other of the Gogo genera.

(22) Superognathals with posterior processes (p.pro, Fig. 5 ) . The toothplates have already been described.

(24) ‘Fusion’ of preorbital and postnasal plates. As in the trematosteids (Gross, 1932), leiosteids (Stensio, 1963) and hadrosteids (Gross, 1932), the preorbital and postnasal plates of Simosteus are replaced by a single element, the preorbito- postnasal plate, at each side. Like the thickened lower part of the rostral, the


descending lamina of this ‘fused’ plate is devoid of denticles except around the orbital margin and has instead an open, pitted texture. This would seem to suggest a soft-snouted arthrodire with plates deeply sited in the tissues.

Other points o f structure

The parasphenoid is preserved and can be seen in dorsal aspect in Fig. 5C. The main body of the bone is thickened but the prehypophysial region which extends anteriorly from it is thin and tapering. There is a distinct fossa hypophyseos (fo.hyp,,) that houses a pair of buccohypophysial openings (f.bhy). These are separated from one another by a median septum (cr.mh), here perforated by a transverse canal of unknown function. There is another shallow depression (p.dep) behind that containing the hypophysial openings which is similar to that seen in Inciososcutum (Dennis & Miles, 1981 : fig. 14A). However, i t is a single, rather than a paired, structure and is devoid of other features. Slight thickenings are evident, especially on the left side of the fossa hypophyseos, which correspond to the posterolateral thickenings (pl.pr) of Bullerichhy (Dennis & Miles, 1980: fig. 9A). Also present is a small posterior shelf (psh) .

The ventral surface of the parasphenoid is not well preserved but can be seen to have a tuberculated median crest and a transverse groove which ends blindly on the left side under a thin cover of bone. Centrally, the groove houses a single buccohypophysial opening; a median septum is not therefore present on this surface. Directly below this groove and opening is the rostrocaudal canal which presumably opened on the posterior surface but the foramen is unclear.

DISCUSSION

Simosteus fits into our cladogram between characters 23 and 25 along the line occupied by the trematosteids, leiosteids and hadrosteids (see Dension, 1978 for review). These arthrodires are characterized by the presence of a single preorbito- postnasal plate (character 24). What then are the relationships of Simosteus within this group?

The trematosteids are identified by the lack of a spinal plate and the presence of a postpineal fenestra. They are narrow-snouted forms with large eyes, a posteriorly embayed skull-roof, a suture between the central and marginal plates and a small anterior superognathal. A close link between this group and Simosteus is unlikely because Simosteus does not possess a postpineal fenestra. I n addition, it has a broad snout, small eyes, a suture between the paranuchal and postorbital plates (which separates the central and marginal) and a normal-sized anterior superognathal. The head is not posteriorly embayed.

The leiosteids are also aspinal and can be recognized by their pointed snout, posteriorly embayed skull-roof, crushing toothplates and smooth bone surface. A postpineal fenestra is generally present. There is a lack here of well defined specialized features, but it would appear that Simosteus is not closely related to this group. It has a broad snout and the hind margin of the skull-roof is straight if not slightly convex. The superognathals are more blade-like, for cutting rather than crushing prey, and the ornament is distinct, comprising large tubercles with smaller ones at the bone radiation centres.


Table 1. Distribution of certain characters among the trematosteids, leiosteids, hadrosteids and Simosteus gen. nov.

Trematosteids Leiosteids Hadrosteids Tronat- Bet- Brachy- Parabet- Cyrt- Sim-

Characten osteus osleus oslew oslew ostew osleu5

Posteriorly -

Postpineal - -

X X X X X X X

embayed skull-roof

fenestra Suture between

central and marginal plates

X X X X X X

X X X X X X - -

Narrow snout - Dermal - - X

X X X X X X X

X X X X X

ornament

of cheek and sku 11-roof

X - - - Interlocking X ? X ? - X ?

x , character present; -, character absent.

The hadrosteids, like the trematosteids and leiosteids, have a posteriorly embayed skull-roof and in common with the former have a suture between the marginal and central plates. The superognathals are characterized by the presence of a single large cusp and the dermal bones are thick and smooth. Again, a close relationship must be ruled out on present evidence because in Simostew the skull- roof is not embayed and it is the paranuchal and postorbital, not the marginal and central plates, which are in contact. Initially the anterior and posterior superognathals of Simosteus bore teeth but these are worn down to blade-like cutting edges: there are no conspicuous cusps. As previouly stated the dermal bones of Simosteus are ornamented.

Interrelationships of the trematosteids, leiosteids and hadrosteids are obscure. Some characters are shared by two groups and others have a spotty distribution among the genera of all three (Table 1). Simosteus is more primitive than these and we have already commented on the skull-roof which is similar to that of coccosteids in general proportions, and on the nuchal which is not posteriorly embayed. In addition, the central plates have well developed lateral wings and stepped lateral margins. Where we have separated Simosteus from the three groups above we have had to rely on primitive characters, and here-in default of any clear synapomorphies-matters must rest. Simosteus is therefore more primitive than the trematosteids, leiosteids and hadrosteids, and interchangeable with them in our cladogram, but its exact relationships are unknown.

ACKNOWLEDGEMENTS

We wish to thank Dr Alex Ritchie of the Australian Museum, Sydney for kindly allowing us to study the specimen and for providing the photographs; and Dr Peter Forey for reading and commenting on the manuscript.


REFERENCES

DENISON. R. H.. 1978. I n H.-P. Schultze. Handbook ~~Prr lmich ihvo lo~F. 2. Placodrnni. Stuttgart: Gustav Fischer

DENNIS, K. I>. & MILES, R. S., 1979a. .4 second eubrachythorarid arthrodire from Gogo, Western Australia.

DENNIS, K. D. & MILES, R. S., 1979b. Eubrachythoracid arthrodires with tubular rostral plates from Gogo,

DENNIS, K. D. & MILES, R. S., 1980. New durophagous arthrodires from Gogo, Western Australia. <oological

DENNIS, K . D. & MILES, R. S., 1981. A pachyosteomorph arthrodire from Gogo, Western Australia. zoological

GROSS, W., 1932. Die .4rthrodira Wildungens. Geologische und Palaontologische .4bhandlungen, 19: 5 61. MILES, R. S., 1971. The Holonematidae (placoderm fishes), a review based on new specimens ofHolonema from

the Upper De\.onian of Western Australia. Philosophical Tramactions OJ the Royal Society of London. B 263: 101 234.

MILES, R. S. & DENNIS, K. D., 1979. A primitive eubrachythoracid arthrodire from Gogo, Western Australia. zoological Journal of the 1.innean Socie!v, 66: 31 62.

MILES, R. S. & WESTOLL, T . S., 1968. The placoderm fish Coccosteus cuspidatus Miller ex Agassiz from the Middle Old Red Sandstone of Scotland. Part I. Descriptive morphology. Tranractiom of the R y a l Sociep of Edinbwgh, 67: 3 7 3 476.

ORVIC;, T.. 1960. New finds of acmthodians, arthrodires, crossopterygians, ganoids and dipnoaus i n the L’pprr Middle Devonian ciilcareous flags (Oberer Plattenkalk) of the Rergisch Gladbach Paffrath Trough. I. Palaontologische <eitschrfi, 34: 295 -335.

ORVIG, ‘I . , 1980. Histologir studies of ostracoderms, plxcoderms and fossil elasmobranchs. 3 . Structure and growth of the gnathalia o f certain arthrodires. <oolo,giral Scripla. 9: I41 159.

STENSIO, E. A,, 1963. Anatomical studies on the arthrodiran head. Part I. PreBce, geological and geographical distribution, the organisation of the head in the Dolichothoraci, Coccosteomorphi and Pachyosteomorphi. Taxonomic appendix. Xungliga Sirmmka I’etenskapsakad~mimns Handlingar, 9: 1 49.

YOUNG, G . C., 1979. New information on the structure and relationships of Buchanosteus (Placodermi, Euarthrodira) from the early Devonian of New South Wales. <oologicalJournal of thr I h e a n Socie[v, 66: :%09 352.

Verlag.

zoological Journal of the Linnearr Sot iep, 67: 1-29.

Western Australia. <oologicol Journal of the [.inneon Socie(p, 67: 297-328.

Journal of the I.innean Sociep, 69: 43-85.

Journal of the Linnean Sociep, 73: 2 I 3 258.

‘4BBREVIATIONS USED IN FIGURES

a11 c ch.pro.pr

cl1.pr.w

cr.mh

csc ruso d .end .e d.end.i d p m c

dp.m.ru d V P f.bhy fo. hypv frag

ioc.ot ioc.pt iocsb laf Ic I.tr

M

autopalatine central plate channel for dorsal aspect of preorbital process of neurocranium channel for dorsal aspect of supravagal process of neurocranium median longitudinal crest in hypophysial depression central sensory canal pit for cutaneous sensory rells external opening of endolymphatic duct internal upening of endolymphatic duct depression in posteromedian rusp of nurhal cucul1;iris depression dermal preorbital process paired huccohypophysial foramen ventral division of fossa hypophyseos ( I n id en t i fied fragm m t on ii n I erior s ti perogniit ha1 otic branch of infraorbital canal postorbital branch of infraorbital canal suborbital branch of infraorbital canal lateral articular fossa main lateral-line canal lateral tooth-ridge of posterior superogn;ithal marginal plate

m p NU neu

P PNu PrO.PN PSO P I 0 pa 1’ p.dep

pdl

PI. P‘ Pmc PP P.P‘ P.P‘O PP‘

psoc pt.o.pr pt.u

R SM so

psh

middle pitline groove nuchal plate fragment of ethmoid region of neurocranium pineal plate paranuchal plate preorbito-postnasal plate postsu borbi tal plate postorbital plate occipital para-articular process depression on dorsal surface of parasphenoid posterior descending lamina of skull- roof * posterolateral process postmarginal sensory canal posterior pit-line groove posterior process of nuchal plate posterior process of superognathals pineal pit posterior shelf postsuborbital sensory canal ventral postocular process paired pits on visceral surface of nuchal plate rostral plate submarginal plate suborbital plate

166

SOC

sorc su0.v

K. DENNIS AND R. S. MILES

supraorbital sensory canal supraoral sensory canal supraorbital vault

th.n nuchal thickening th.pre pre-endolymphatic thickening V . t ventral teeth of posterior superognathal

a eubrachythoracid arthrodire with a snubnose from gogo, western australia

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