Zoological Journal of the Linnean Society, 66: 31-62. With 15 figures
May 1979
A primitive eubrachythoracid arthrodire from
Gogo, Western Australia
R. S. MILES AND KIM DENNIS
British Museum (Natural History),
Cromwell Road, London SW7 5BD
Acceptedfor publication October 1977
A new eubrachythoracid arthrodire, Hanytoombsia ekganr gen. et sp. nov., is described from the
Upper Devonian Gogo formation of Western Australia. Its structure and relationships are discussed
within the framework provided by a new sketch cladogram for arthrodires. H . eleganr is
morphologically similar to Coccosteus cuspidatus, but its relationships among primitive
eubrachythoracids are unknown.
KEY WORDS: - Arthrodira - Australia - Devonian - morphology
- phylogeny - taxonomy.
CONTENTS
Introduction
. . . . . . . . . . . . .
Description . . . . . . . . . . . . . .
Hartytoombsca gen. nov.
. . . . . . .
Arthrodire characters . . . . . . . .
Phlyctaenioid characters
. . . . . . .
Brachythoracid characters
. . . . . .
Eubrachythoracid characters . . . . . .
Additional points of structure
. . . . .
Discussion
. . . . . . . . . . . . . .
Thesketchcladogram . . . . . . . .
Re-examination of the family Coccosteidae
.
Relationships of Harrytoornbsia
. . . . .
Acknowledgements
. . . . . . . . . . .
References
. . . . . . . . . . . . . .
Abbreviations used in figures . . . . . . . .
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INTRODUCTION
The fish fauna of the Upper Devonian (Frasnian) Gogo formation, Western
Australia, includes at least 20 species. About half of these are the
eubrachythoracid arthrodires which will form the subject of a series of papers.
All of these arthrodires appear to belong to new genera, new species, or both.
The general composition of the Gogo fauna, its occurrence, preservation and
significance have been reviewed by Brunton, Miles 8c Rolfe ( 1969)and Gardiner &
31
0024-4082/79/05003 1-32/$02.00/0
0 1979 The Linnean Society of London
32
R. S. MILES AND K. DENNIS
Miles 1975). This last paper includes a comprehensive bibliography. The
described members of the fauna are as follows :
PLACODERMS
Holonema westolli Miles ( 19 7 1)
Ctenurella gardineri Miles & Young ( 19 7 7
Campbellodus decipiens Miles 8c Young ( 19 7 7 )
DIPN OANS
Griphognathus whitei Miles ( 197 7 )
Holodipterus gogoensis Miles ( 197 7 )
Chirodipterus australis Miles ( 197 7 )
Chirodzpteruspaddyensis Miles ( 19 7 7 )
ACT1N 0PTERYGIANS
Mimia toombsi Gardiner & Bartram (1977)
Moythomasia durgaringa Gardiner & Bartram ( 19 7 7 1
A current problem in placoderm studies is the mass of unordered data which
has been accumulated in attempts to give “complete” descriptions of the
specimens. One aim in describing the Gogo eubrachythoracids will be to avoid
adding unnecessarily to this mass. At the outset we shall recognize the impossibility of giving a “complete” description of any specimen. This is because,
to a large extent, theories determine the observations that are made and the
importance that is attached to them, and “There are no modes of description
which remain invariant under all changes of theory” (Harrk, 1972 : 25). This last
conclusion and the knowledge that our theories are changing all the time have
stimulated the form of this paper.
In organizing the content we have been guided by the belief that a phylogeny is
the best general reference system in biology, because it has an objective existence
independent of the investigator or of the use to which it is put. We have also
accepted that the cladogram -in which all extinct species are treated as terminal
taxa -provides the clearest and most thought-provoking representation of
phylogeny for practical purposes. A cladogram can be converted into a classification by the adoption of various conventions (e.g. Farris, 1976; Patterson &
Rosen, 1977). However, it is important that these conventions are made clear.
We shall avoid a long written description by leaving the labelled figures to
speak for themselves. This approach is based on the proposition that the dermal
bones of eubrachythoracids are now so well known (e.g. Heintz, 1932, 1934;
Miles 8c Westoll, 1968; Miles 197 1) that it is not necessary to describe new species
in detail where only differences in shape and proportion are involved. The
characters will be ordered phylogenetically, i.e. according to the order in which
they arose in arthrodire phylogeny, rather than in anatomical regions. The basis
of this arrangement lies in the concept of “level of universali
of Wiley ( 19751,
and has also been set out in a series of axioms and theories y Lsvtrup (1977).
We prefer to summarize our reasons in the following statements:
( 1 ) The set of properties defining one taxon may be a proper subset of the set
defining another.
(2) N o property defining a taxon can have originated later than any defining a
subset of the taxon.
(3) N o property defining a subset can have originated earlier than any defining
the set of which it is a part.
T’
ARTHRODIRE FROM AUSTRALIA
33
Ph ylogenetic ordering of characters makes it easier to describe new forms in
the context of general statements about arthrodire structure (Miles & Young,
1977 : 124) and clearly ties the description to its conceptual background. The
sketch cladogram in Fig. 15 provides our phylogenetic framework.
Buchanosteus is shown with relationships determined by Young (in press) in
contradistinction to the conclusions reached by White & Toombs ( 19 72).
I t is hoped to elaborate this phylogeny during the description of the Gogo
eubrachythoracids". N o formal classification will be used at this stage. The terms
employed come from the classification of Miles ( 19731, unless alternative
references are given.
DESCRIPTION
Harrytoombsia gen. nov.
Etymology. After Mr H. A. Toombs who collected at Gogo in 1963 and 1967
and thus initiated the systematic study of the Gogo fossil fishes.
Diagnosis. A moderately-sized eubrachythoracid at the coccosteomorph level of
organization (sensu Miles, 1969); the head shield is broad and shallow with
squarish preorbital plates, centrals narrow anteriorly and the submarginal
ending posterior to the suborbital; the median dorsal plate has a short, broad
posterior spine that ends well short of the ventral shield; the ventral shield is
broad and strongly arched, with a large posterior median ventral plate, both the
spinal plate and pectoral fenestra are raised high above the base level, the spinal
plate stands out from the flank at a high angle and the posterior ventrolateral
plate has a rounded posterior margin; the ornamentation comprises fine
tubercles which tend, particularly on the head-shield, to be linked together to
form short, irregular ridge.
Most of these characters will not be repeated in the following description.
Quantitative characters are expressed accurately for the holotype in the list of
measurements.
Type species. Harrytoombsia elegans sp. nov.
Hanytoombsia ekgans sp. nov.
(Figs4-14)
1970: Fig. o n p . 12;Miles.
1975: Typical coccosteid with well-developed spinal plates and a
strongly-tumid venter, Gardiner 8c Miles: 75.
Etymolo~.L, eleguns, elegant; an allusion to the graceful form of the trunkshield.
Diagnosis. As for the genus; this is the only known species and any division
between generic and specific characters would be arbitrary.
Halotype. Western Australian Museum No. 70.4.254.This specimen was provisionally numbered as British Museum (Natural History) P509 14. I t comprises
an almost complete skull and trunk-shield, parasphenoid, an anterior and
posterior superognathal, both inferognathals, fragments of jaw cartilages and the
sub-median-dorsal plate. The specimen has been extracted from a limestone
nodule by means of dilute acetic acid.
* Footnoteadded inproof.Afurtherelaborationisgiveninthesecondpaperoftheseries(Dennis&Miles,1979;
Zoologtcal Journal ofthe Linnean Society: in press).
3
R. S. MILES AND K. DENNIS
34
Figure 1. Head and thoracic armour of an arthrodire in dorsal view to indicate measurements given
in the text.
Occurrence. The holotype is the only specimen. I t comes from locality 47 (Miles,
197 1 : fig. 1) near Gogo Station; from the Lower Frasnian Gogo formation,
Canning Basin, northwestern Australia.
Measurements. Our aim here is to present a standard series of measurements,
which will serve for all the articulated, uncrushed Gogo arthrodires as well as to
record the dimensions of the holotype of Harrytoombsia elegans.
Because of the fragmentary nature of most arthrodire specimens, ad hoc series
of measurements have been employed in the past (Heintz 1929a, 1934; Miles 8c
Westoll, 1963, 1968; Miles, 1964, 1966b, 1971; Obrucheva, 1962).Itwould make
no sense to adopt all of these measurements for the Gogo species as they are too
extensive for routine use, deal in unnecessary detail with individual plates, and
leave out certain important measurements which should be recorded for better
specimens. Therefore, we shall adopt the following dimensions and angles. These
ARTHRODIRE FROM AUSTRALIA
35
have been selected to characterize the form and evolutionary grade of wellpreserved arthrodires (Miles, 1969; Moy-Thomas & Miles, 197 1 ) . Indices are
derived by multiplying the ratio of two linear dimensions by 100. In the
following list, the measurements for Harlytoombsia elegans are given in parentheses
after each definition. These dimensions are not generally comparable with those
given in the literature for less well-preserved species. The terminology of the
bones follows that of Miles ( 19 7 1).
A. 1.H (Fig. l ) , length of skull in the middle line (60 mm).
B. b.H. (Fig. 1), breadth of skull across posterolateral angles (approximately
64.5 mm).
C. b.pm (Fig. 11, breadth of posterior margin of skull across posteromesial
angles (42.3 mm).
D. d.H (Fig. 21, depth of skull perpendicular to plane of central plates (26.9
mm).
I
I
I
I
Figure 2 . Head and thoracic armour of an arthrodire in lateral view to indicate measurements given
in the text.
E.
F.
G.
H.
I.
J.
K.
L.
1.prp (Fig. 11, prepineal length of skull (13.8 mm).
LJo (Fig. 2), length of fenestra orbitalis (17.6 mm).
1.Nu (Fig. 11, length of nuchal plate in middle line (24.8mm).
1.laJ length of lateral articular fossa as shown by Obrucheva (1962: fig. 11C)
(5.6 mm).
d h f , depth of lateral articular fossa as shown by Obrucheva (1962: fig. 11C)
(2.2 mm).
sax, angle between the axis of the lateral articular fossa and the dorsolateral
surface of the skull, as in Miles (197 1 : 106, 108); (cf. trunk angles given
byobrucheva, 1962: fig. 9) (approximately37O).
1.Ch (Fig. 21, length of external surface of cheek parallel to the suborbital
lamina of the suborbital plate (38.2 mm).
1. Ch.po (Fig. 21, length of postorbital division of cheek ( 16.7 mm).
36
R. S. MILES AND K. DENNIS
M. 1.ZG (Fig. 21, length of inferognathal (37.8 mm).
N. 1.bd (Fig. Z), length of biting (“functional”)division of the inferognathal(l9.0
mm).
0. 1.Ts (Fig. 31, length of ventral wall of trunk-shield in the middle line (112.2
mm).
P. b. Ts (Fig. 31, maximum breadth of ventral wall of trunk-shield across spinal
plates (105 mm).
Q. d.Ts (Fig. 21, depth of trunk-shield perpendicular to upper surface of median
dorsal plate (72.1 mm).
R. l f k (Fig. Z), rostrocaudal length of flank from ventral point of the obstantic
process to the inner angle of posterior embayment (27.5 mm).
S . 1.pf (Fig. 21, length of pectoral fenestra (approximately 22.0 mm).
T. 1.MD (Fig. 11, length of median dorsal plate in middle line (55 mm).
U. b.MD (Fig. 11, maximum breadth of median dorsal plate (33 mm).
V. 1.Sp. (Fig. 31, length of spinal plate (28.4 mm).
W. asp., angle between spinal plate and rostrocaudal axis as given by Heintz
(1929a: fig. 17) (45O).
X. 1.AVL (Fig.31, length ofanterior ventrolateralplate (47 mm).
Y. 1.Sp.d (Fig. 3), length of spinal division of anterior ventrolateral plate (18.4
mm).
S
Figure 3. Thoracic armour of an arthrodire in ventral view to indicate measurements given in the
text.
37
ARTHRODIRE F R O M AUSTRALIA
C
.e
38
R. S. MILES AND K. DENNIS
Arthrodire characters
Our basic hypothesis is that Harrytoombsia elegans is a primitive
eubrachythoracid arthrodire. It closely resembles Coccosteus cuspidatus Miller ex
Agassiz (Miles 8c Westoll, 1968) but the question of whether or not it can be
classified as a coccosteid sensu strict0 will be left until later (see Discussion).This
phenetic resemblance will be exploited in the following description.
Like other arthrodires, the dermal bones of the skull-roof (Figs 4, 7, 10)
comprise median rostra1 (R), pineal (P) and nuchal (Nu) plates, and paired
postnasal (PN), preorbital (Pro), postorbital (PtO), central (C), marginal (M),
postmarginal (PM), paranuchal (PNu), and, although not preserved, probably
extrascapular plates. The dermal bones of the cheek (Figs 4, 12) comprise paired
suborbital (SO),postsuborbital (PSO)and submarginal (SM) plates. Those of the
trunk-shield (Figs 4, 5 , 6, 10) comprise median dorsal (MD), anterior and
posterior median ventral (AMV, PMV), paired anterior and posterior
dorsolateral (ADL, PDL), anterior and posterior lateral (AL, PL), anterior and
posterior ventrolateral (AVL, PVL), and paired interolateral (IL) and spinal (Sp)
plates. There is a median parasphenoid (Fig. 13) on the ventral surface of‘ the
mentioned
neurocranium. 0 ther dermal bones -the gnathals -are
below.There is also a stout endochondral sub-median-dorsal plate (smd, Fig. 4)
behind the carinal process of the median dorsal plate. This plate is found in all of
the more complete Gogo eubrachythoracids and has been described in
Rhachiosteus pterygiatus Gross (Miles, 1966a) and Coccosteus cuspidatus (Miles &
Westoll, 1968).The lateral-line grooves and pit-lines answer almost perfectly to
those of C. cuspidatus. However, there is a cutaneous sense organ pit on the
postsuborbital (cuso, Fig. 41, but not on the suborbital as in this last form, and
there is no postsuborbital sensory line groove.
Nearly all of the bones mentioned above may be primitive for placederms (for
head, see Denison, 1975 : 111, if it is accepted that the extrascapulars are
primitively incorporated in the nuchal. However, the posterior lateral, posterior
median ventral and posterior ventrolateral plates may be specializations of
arthrodires and antiarchs (Miles 8, Young, 1977 : 134, table 1). We have not
attempted to sort out arthrodiran synapomorphies. These characters may reside
in the form of individual bones or groups of bones, and this aspect of placoderm
analysis is very poorly developed. The sub-median-dorsal plate is not specifically
mentioned as a primitive placoderm character by Denison or Miles & Young, but
we find this interpretation consistent with the presence of a large basal plate, in a
similar position, in ptyctodontids.
It is now possible to consider other characters of Hanytoombsia elegam, one by
one, following the numbering used in Fig. 15 .
1. Pectoral fenestra (p. fen, Figs 4, 5 ) . This is large, raised high above the base
level of the armour and bounded by anterior and posterior lateral, spinal,
anterior and posterior ventrolateral plates. It is separated from the cavity of the
spinal plate (cav, Fig. 61, as this plate retains the primitive mesial wall and
therefore does not have the “pseudospinal” structure found, inter alia, in
Holonemu and several Gogo eubrachythoracids (Miles, 197 1: 166, 167). The
postpectoral lamina (la. pp, Fig. 6) of the posterior ventrolateral plate is similar
to that of Pholidostew (Stensi6, 1959: fig. 9A) except for the surface which bears
ornament in Pholidostew and is smooth in Harrytoombsia. It is a well developed
process, robust and with a rostrocaudally thickened base.
ARTHRODIRE FROM AUSTRALIA
39
Figure 5 . Hunytoombsiu elegans gen. et sp. nov. Thoracic armour in ventral view. P509 14.
We have followed Miles & Young ( 197 7 : 134) in treating the pectoral fenestra
as an arthrodiran synapomorphy. However, there is some doubt about this
(Young, in press) and the character may eventually prove to be a synapomorphy
for arthrodires plus antiarchs.
2 . Great width of skull-roof acrossposterolaterul angles (plaa, Fig. 7 ) . The angles are
well marked, a feature correlated with the characteristic, triangular shape of the
postmarginal plate. The skull-roof of Hurrytoombsia eleguns is notably broad,
having a breadth/length index of 108 (comparable figures for other primitive
eubrachythoracids are not available, but will be provided for other uncrushed
Gogo species in future works).
3 . Two supe~ognathuls.Separate anterior and posterior elements (ASG, PSG,
Figs 4, 8) are present and are notable for the presence ofwell-formed teeth on the
R. S. MILES AND K. DENNIS
40
Figure 6 . HartytmbsM ekg&
gen. et sp. nov. Thoracic amour in posterolateral view. P50914.
inner surface (m.t, p.t, Fig. 8B, G) in addition to the usual ventral (v. tr, v. ts) and
lateral teeth (Lt, posterior element only). An inner-surface tooth has also been
discovered on the posterior superognathal of Coccosteus cuspidatus (Miles8c Westoll,
1968: pl. 11A).
In non-arthrodiran placoderms, the upper gnathal elements are known only
in the ptyctodontids (upper tooth-plate) and antiarchs (mental plate), although
there is some evidence of an element in palaeacanthaspidoids in the form of the
“vomeral area” of Kosormpis (Gross, 1959; StensiiS, 1963). In no case is there
evidence of two elements at each side. It is not clear whether these single elements
are homologous with the arthrodiran anterior or posterior superognathal, with
both or with neither.
Phlyctaenioid characters
Two such characters can be distinguished, after Miles ( 1973).
4. Ginglymoid dermal neckjoint (Figs 6, 7 , 9). The joint is well developed and the
main dimensions and angles have already been given (Measurements).The lateral
articular fossa and para-articular process of the paranuchal, and the glenoid
condyle (kd) of the anterior dorsolateral are the main features; other details
correspond closely to those summarized by Miles ( 197 1 : 129, 158). The shape of
ARTHRODIRE FROM AUSTRALIA
Figure 7. Hurtytoombsiu eleguns gen. et sp. nov. Head in A, ventral view, P50914, and B, plan of dorsal
surface (left half) and ventral surface (right half).
41
R. S. MILES AND K. DENNIS
42
L
I
10mm
Figure 8. Hanytoombsia &gum gen. et sp. nov. Toothplates. Right posterior superognathal in A,
lateral; B, mesial; C, dorsal and D , ventral views. E, F, and G , right anterior superognathal in
anterior, dorsal and ventral views respectively. P509 14.
ARTHRODIRE FROM AUSTRALIA
43
10 rnrn
Figure 9. H u n y ~ b s i Oelegrmr gen. et sp. nov. Thoracic armour in anterior view. P509 14.
the condyle is shown in Figs 6, 9. It is reciprocated by the fossa, the two structures
fitting closely together and thereby allowing the head to rotate freely through
about 27O. When rotated downwards, the joint brings the head-shield into the
relations with the trunk-armour that were listed for Holonema (Miles, 197 1 : 176).
In particular, the subobstantic area (soa) of the head-shield (on the paranuchal,
postmarginal and marginal) lies inside the obstantic process (pro) formed by the
anterior dorsolateral and lateral plates, and the posterior margin of the cheek lies
in the embayment formed by the lower part of the anterior lateral.
How many of the above details apply to all phlyctaenioids is not clear, as the
more primitive forms are poorly known. They are certainly primitive
brachythoracid characters, with the main features already present in
“phlyctaeniids” such as Phlyctaenius acadicus (Whiteaves)(Miles, 1969).
5. Median dorsalplate with ventral ridge (Figs 4, 6, 9). This ridge is developed in a
stout keel as in Coccosteus cuspidatus and other brachythoracids. It is noted further
below (character number 19).
Brachythoracid characters
The following eight characters are derived mostly from Miles (197 1 : 228,
1973: 116):
6. Suture lines remaining distinct; sinuous in skull-roof, whose plates have well-developed
overlaps (Figs 7 , 10). The overlap relations of the head and trunk plates are not
illustrated as they correspond almost exactly with those of Coccosteus cuspidatus
44
R. S. MILES AND K. DENNIS
10mm
Figure 10. H a r r y t d s i a elegam gen. et sp. nov. Head and thoracic amour in dorsal view. P50914.
ARTHRODIRE FROM AUSTRALIA
45
(Miles 8c Westoll, 1968: figs 2b, 2c). The most notable feature of the skull-roof
bone pattern, in contrast to C. cuspidatus, is the separation of the marginal and
central plates by the meeting of the postorbital and paranuchal plates.
Harrytombsia elegans thus exhibits the Plourdosteus relationships of these bones
(Orvig, 1960: 3051, but conditions are so variable within arthrodires (Stensio,
1963, 1969a) that no significance can be attached to this fact.
7 . Nuchal plate posteriorly expanded (Figs 7 , 10). This type of nuchal is usually said
to be trapezoidal, though the description should not always be taken literally.
8. Paranuchal plate with postnuchal process (Figs 7 , 10, not labelled). This character
is correlated with numbers 4 and 7 .
9. Extrascapular plates separately developed. These are not preserved but they are
found in several other Gogo eubrachythoracids, Holonema, Coccosteus cuspidatus
and Millerosteus minor (Miller) (Desmond, 1974). Evidence that they were present
in Hamytoombsia eleguns, rather than ‘fused’ with the nuchal or completely
reduced, is provided by the posteromesial course of the occipital crosscommissural sensory canal (occ, Fig. 4 ) and the presence of a posterior
descending lamina (pdl, Fig. 7B) which supported them. The upper surface of the
nuchal beaB a transverse depression of unknown function, in the transverse
plane of the endolymphatic openings (Fig. 10, not labelled). In Gemuendenaspis
(Miles, 19621, a similar depression was interpreted as the groove of the occipital
cross-commissural canal, and therefore as possible evidence of the inclusion of
the extrascapular plates in the roof. However, Stensio (1969a) has pointed out
that this interpretation is false.
Although we have listed extrascapulars as a brachythoracid character (Stensio,
1945: 53), it is possible that they appeared earlier in arthrodire phylogeny.
Goujet (1973: 79, figs lB, 3A, 3B) has described a median extrascapular in the
actinolepid Sigaspis lepidophora, and Westoll8c Miles (1963: 146, fig. 6) have shown
that the occipital cross-commissure passed across the head behind the nuchal
plate in the arctolepid Phlyctaenius acadicus. However, we feel that Goujet’s
observation requires corroboration and in the latter fish extrascapulars have yet
to be observed. Therefore, for the moment we let character no. 9 stand as stated
above, with the comment that it is not crucial to the arguments put forward in
this paper.
10. Nuchal thickening on visceral sugace ofskull-roof(th.n, Fig. 7). This character is
correlated with numbers 4, 6, 7 and 8.
1 1 . Paired pits on visceral surface of nuchal plate (pt. u, Fig. 7). These are
particularly well developed, separated by a median septum and open into a
broad depression. The function of these pits in arthrodires is disputed, but as
here they usually open anteroventrally and it seems most likely that they received
a paired dorsal process of the neurocranium (Miles & Westoll, 1968 : 400). If so,
their development may be correlated with other characters concerning the hind
end of the skull-roof and the exoskeletal neck-joint (numbers 4, 7-10).
12. Pre-endolymphatic thickening (th. pre, Fig. 7 ) . This is low but extensively
developed on the visceral surface of the central plate. It ends posteriorly just
before the internal opening of the endolymphatic duct (d. end.i), on the centralparanuchal suture.
13. Pectoral j n with long, horizontal bme articulating against laterally-facing
scapulocoracoid. Neither the fin nor the scapulocoracoid is preserved in Hunytoombsia elegans and the evidence comes solely from the pectoral fenestra (above; p.fen,
46
R. S . MILESAND K.DENNIS
Figure 1 I . Hurrytoombsia elegatu gen. et s p nov. Right inferognathal in B, lateral and C, mesial views
with mentomeckelian and articular ossifications. A, Biting region in dorsal view. P50914. D, E,
Sketches of articular ossification in lateral and mesial views respectively.After P50914.
ARTHRODIRE FROM AUSTRALIA
41
Figs 4 , 5 ) . However, this interpretation is supported by other Gogo forms, with wellpreserved scapulocoracoids, which we shall describe at a later date.
Eubrachythoracid characters
These characters come mostly from Miles (197 1 ) and also from an analysis of
White 8c Toombs ( 1972) on Buchanosteus (“Parabuchanosteus murrumbidgeensis”)).
14. Inferognathal with both blade and biting regions, which are complete and co-ossijed
(IG, Figs 4, 1 1 ) . This element is closely comparable with that of Coccosteus
cuspidatus. Its proportions are given above (Measurements). The biting region
bears dorsal and symphysial teeth separated by a worn, tyothless area. The
mesial surface of the blade bears a slight groove (gr.cth, Fig. 1 1 C) as it does in C.
cuspidatus and other Gogo forms. Its function is unknown although it has been
suggested by Miles 8c Westoll ( 1968 : 4 12) that it may have been the path of the R.
mandibularis internus VII. The relationships of the blade and biting region to
the articular and mentomeckelian ossifications are clearly shown. These cartilage
bones are reasonably well preserved and described in more detail below.
In listing continuous blade and biting regions at this point in the sequence of
characters, we have made the assumption that the inferognathals described for
Phlyctaenius acadicus exclude the blade (Heintz, 1933: 133; cf. Miles, 1969: 146).
15. Skull-roof with lateral consolidated region (lcp, Fig. 7 ) . This is moderately well
developed on the visceral surface and bears an extensive, triangular-shaped
concavity behind the orbital cavity, i.e. the post-ocular division of‘ the
supraorbital area of Stensio ( 1963). There is no inframarginal crista as there is in
Coccosteus cuspidatus (Miles 8c Westoll, 1968 : fig. Za), and the consolidated region
as a whole is more closely reminiscent of Pholidosteus (Stensio, 1963: fig. 54B, C).
16. Submarginalplate small and closely incorporated in the cheek unit (SM, Figs 4, 10).
This plate is wedged between the skull-roof and the postsuborbital plate. As a
result, a gap remains between the skull-roof and the suborbital plate,
immediately behind the orbit. It is tempting to regard this gap as the site of the
spiracle (cf. Holonema, Miles, 197 1 : 176). However, although this condition is
marked in other Gogo forms (Gardiner 8c Miles, 1975: fig. 11, in yet others the
area is completely closed over by dermal bone, with the suborbital in firm
contact with the postorbital. Stensio (1963) has also demonstrated this last
condition in several Wildungen forms.
1 7 . Skull-roof with well developed supraorbital vault (suo.v, Fig. 7 ). This character is
correlated with the presence of large, laterally facing orbits. In Harrytoombsia
elegans there is also a “neurocranial” or ventral postocular process (pt.o.pr)
developed from the edge of the supraorbital vault. It has a bifurcate form, the
two parts being separated by a smooth, deep depression that fades away laterally
into the orbital cavity. The larger anterior division is inclined forwards and is
widest at its mesial end where it is triangular in section. The posterior division
comprises a ridge which ends abruptly just behind the orbital cavity. The
neurocranial” process of Dunkleosteus intermedius (Newberry), as labelled by
Heintz ( 1932 : fig. 131, appears to correspond to the anterior division (described
above) only. We have so far been unable to discover if the process has any
phylogenetic significance but it seems unlikely that it characterizes a
monophyletic group. In some Gogo species (e.g. “Coccosteus” sp., Gardiner 8c
Miles, 1975: fig. 1B) it is both preient and absent within sets of similarly-sized
specimens. Where present it hasa similar structure to H . elegans.
L‘
48
R. S. MILES AND K. DENNIS
Figure 12. Hunytomnbsia elegum gen. et sp. nov. Left suborbital plate with articulated autopalatine
and posterior superognathal in A, lateral; B, mesial; C, dorsal; and D, ventral views. A', B', C' and
D' are their respective key figures.
18. Suborbital plate with slender suborbital lamina (Figs 4, 12). This character is
correlated with the large size and lateral position of the orbits.
19. Median dorsal plate with carinul process (cr.pr, Figs 4, 6 , 9 ) . The median dorsal
keel descends to end posteriorly in a stout process which is very closely
ARTHRODIRE F R O M AUSTRALIA
49
comparable to that of Coccosteus cuspidatus. This process has a broad, concave,
posterior face (art. smd),which is assumed to have formed an articulation surface
for the sub-median-dorsal plate. Anterolateral to the base of the carinal process
is a small, anteriorly facing, paired pit of unknown function but previously
considered to be for the anchorage of muscles and ligaments concerned with the
raising of the head (Mil&, 1966b: 17). This pit is also present in Coccosteus
cuspidatus and Protitanichthys rockportensis Case. The presence of a posterior spine
on the median dorsal plate (Figs 4, 6, 10) ensures that the carinal process does
not project beyond the posterior margin, as, for example, in Pholidosteus sp.
(Stensio, 1959: fig. 9A). This spine is short, does not taper and has a rounded
proximal end. It contrasts with those described in certain other species, such as
Dickosteus threiplandi, Coccosteus cuspidatus, Plourdosteus mironoui (Miles, 1966b: fig.
291, as these are long and tapering with pointed tips. Thus, in Harrytoombsia
elegans, the posterior median dorsal spine is much less pronounced. However, it
must be considered a distinct structure when compared to the median dorsal
plates of spineless forms, e.g. Walsonosteus Jetti, Clarkeosteus halmodeus (Miles,
1966b: figs 29,301.
20. Preorbital plate with preorbital lamina (Figs 4, 7). This lamina bears an overlap
area for the small postnasal plate (PN),as it does in Coccosteus cuspidatus (Miles 8c
Westoll, 1968: fig. 9b). It is termed the antorbital process by Stensio (1963: 226)
and recognized as a “non-dolichothoracid” character.
2 1. Suborbital plate with internal laminae (Figs 4, 12). As in Coccosteus cuspidatus
(Miles 8c Westoll, 1968: fig. 13c-h) there is a postocular crista icr.po) which
descends forwards to become the subocular crista (cr.so).The latter is broad and
extends anteriorly beyond the suborbital lamina so that it is exposed on the
lateral surface of the suborbital. Dorsally it bears an overlap area (pre.ov)for the
preorbital lamina of the preorbital plate. Anteriorly it is notched in the usual
fashion (inc, Fig. 12) for (following Stensio, 1934) the r.maxillaris V and the
n.buccalis lateralis. Ventrally it bears a linguiform process (cr.lg). The
autopalatine ossification (au) wedges between the mesial surface of the
linguiform process and the subocular crista, so that it is firmly supported by the
dermal skeleton, although it extends somewhat further forwards. A final point is
that where the postocular and subocular cristae merge they seem jointly to send
out a broad mesial process. However, this process is incomplete and its true
extent and significance are unknown. I t will be left for description in other Gogo
forms.
22. Palatoquadrate with separate autopalatine and quadrate bones (Fig. 12). These
perichondral structures were probably joined by cartilage in life; they have lost
the bony connexion of more primitive forms (Miles, 197 1; Goujet, 1975). They
are not well preserved in Harytoombsia elegam but are similar to superbly
preserved specimens of other Gogo species. Therefore, it will be sufficient to note
that the autopalatine (au) has a large depression on its ventral surface for the
posterior superognathal, and an extensive unossified area on its mesial and
anteromesial edges forming the orbital articulation with the neurocranium. The
quadrate is firmly attached to the postsuborbital plate and appears to have
possessed a rounded articular condyle which was incompletely ossified.
23. Superognathals with posterior processes (p.pr, Figs 8, 12). Both the anterior and
posterior superognathals bear a stout posterior (or dorsal) process which was in
broad contact with the ethmoid region of the neurocranium and the
4
R. S. MILES AND K. DENNIS
50
autopalatine respectively. These processes are associated with the firm bracing of'
the dentition and appear to be correlated, inter alia, with character 2 1.
Additional points of structure
There are several points which have not been mentioned in the diagnosis of
~ar~toombsiu
or in the above description. They are nevertheless worthy of note.
The dermal snout is advanced, in comparison with Coccosteus cuspidatus, in the
reduction of the descending lamina of the rostra1 plate and loss of the interfenestral process of the postnasal. There is no sign of an internasal bone. It
may have been absent, although it is present in C. cuspidatus, Millerosteus minor and
in Eustmanosteus sp. from Gogo. The parasphenoid (Fig. 13)has a well-developed
ventral groove (gr.a.com) and a rostrocaudal canal (v.m.hyp) reaching from the
hind margin to the hypophysial fossa. The function of both structures is
uncertain (Stensio, 1963; White & Toombs, 19721, but it seems unlikely that they
are eubrachythoracid specializations. Essentially the same type of parasphenoid
is found in several other Gogo eubrachythoracids (including Eastmanosteus sp., cf.
Kulczycki, 19561, Coccosteus cuspidatus and Buchanosteus. We believe that the
cr.mh
i
ash
A
B
f.bhy /
\
p.sh
fO.hYP,
C
A
I
1Omrn
p.sh
D
gr.a..com
v.m.hyp
Figure 13. Hungtoopnbsia clegmrc gen. et sp. nov. Parasphenoid in A, ventral; B, visceral;C, left lateral;
and D, posteriorviews. P50914.
ARTHRODIRE FROM AUSTRALIA
51
primitive arthrodiran parasphenoid lacks these two characters. This primitive
type is exemplified by the actinolepid Kydanowiuspis and the “phlyctaeniid”
Dicksonosteus (Goujet, 19751, as well as the primitive rhenanid Kosoruspis (Gross,
1959). However, these characters are also absent in Pholidosteus, Tapinosteus and
several Wildungen aspinothoracids described by Stensio ( 1963, 1969a),and so we
have been unable to come to a satisfactory account of parasphenoid structure in
arthrodiran phylogeny. Therefore, this subject is left for discussion in a later
paper, following the description of other Gogo species.
The ornamentation of the dermal bones is shown in the photographs in Figs
9, 12A and 14. Tubercles are absent from the postmarginal and posterior lateral
plates and from some of the ventral surface of the posterior median ventral and
right posterior ventrolateral plates. Figure 9 also shows the ascending lamina of
the interolateral plate, with parallel rows of sagittate tubercles bearing fine lateral
denticles, exactly as found in Coccosteus cu~pidutus(Miles& Westoll, 1968 : fig. 38c).
The mentomeckelian bone (Mm, Fig. 11) bears a dorsal groove (gr.IG)for the
inferognathal. Anteromesially the bone is unfinished and presumably in life was
developed as an articulation surface. However, the precise relations of the
mentomeckelian to its antimere are unknown. The articular bone (Art, Fig. 11) is
well preserved in contrast to Coccosteus cuspidatus (Miles & Westoll, 19681, but
there seems to be a real difference in structure between the two forms. In
Harrytoombsia eleguns there is a large unossified area, low on the lateral surface
(a.art), which clearly formed the mandibular articulation surface. The corresponding surface is more dorsally placed in C. cuspidatus and all the Wildungen
Figure 14. Hunytoombsiu elegans gen. et sp. nov. SEM micrograph of the dermal ornament. P50914.
(~50).
52
R. S. MILES AND K. DENNIS
forms described by Stensio (1963). Posterodorsally, in H. elegans, there is an
unfinished process (post.pr) which arises mainly from the mesial surface. I t may
correspond to the supraglenoid area in Tapinosteus (“a special cartilaginous area
against which the detent process of the quadratospiracular may have slid during
the maximal lowering of the lower jaw”, Stensio, 1963: fig. 801, but this cannot
be confirmed from the existing quadrate. Anterodorsally there is a small
unossified area on the lateral surface (d.ua) of unknown function, and on the
mesial surface there is an extensive contact area for the inferognathal (a.IG).
The anterior median ventral plate has, on its visceral surface, a moderately
developed median longitudinal ridge. This structure has been previously
described and figured in Dunkleosteus (Heintz, 1932: 173-4, fig. 60), Watsonosteus
(Miles & Westoll, 1963: 913, fig. 6c), and Coccosteus cuspidatus (Miles 8c Westoll,
1968 : 432-3, fig. 39b). Its form in Harrytoombsia is essentially the same.
DISCUSSION
The sketch cladogram
This section of the paper is concerned with the phyletic relationships of
Harrytoombsiu eleguns, which are to be established within the framework of our
sketch cladogram (Fig. 15). It is necessary to start by providing a further
explanation of this cladogram, including an explanation of the numbered
characters not mentioned so far.
The actinolepidoids are the plesiomorph sister-group of all other arthrodires.
Our conception of this group corresponds with Miles & Young (1977);i.e. it is a
monophyletic group, characterized by a sliding dermal neck-joint, comprising
the subdivisions Actinolepidi and Wu ttagoonaspidi. However, we acknowledge
that the sliding neck-joint might prove to be a plesiomorphic character for all
arthrodires, as earlier suggested (Miles, 1969: 146-7). This would leave the
Actinolepidi as the plesiomorph sister-group of all other arthrodires, and the
Wuttagoonaspidi without a firm hypothesis of relationships. The actinolepidoids
in this restricted sense are a monophyletic group characterized by anterior
ventral plates.
The Phlyctaenii of Miles ( 1973) are not united by known synapomorphies and
must still be regarded as a grade group. Their level of organization is represented
in Fig. 15 by Dicksonosteus arcticus Goujet. We have selected this species because it
is fairly completely known and is succinctly described by Goujet ( 1975).
However, other phlyctaeniids are interchangeable with D . arcticus in this
cladogram, e.g. Phlyctaenius acadicus Heintz ( 1933). This interchangeability, in a
cladogram, of fossil species whose relationships are only known in broad terms
of level of organization, has been discussed by Patterson 8c Rosen (1977: 161).
The lines which branch off between characters 6 and 24 in the cladogram
require no further explanation at this stage. Characters 1-23 have been noted
above in the description of Harrytoombsia elegans. Characters 24-27 are as follows.
24. Loss of spinal plate. We here follow Stensio (1959) in assuming that the
absence of this plate defines a monophyletic group, the “Aspinothoracidi”.
However, we differ from Stensio in regarding this group as advanced rather than
primitive among arthrodires. We nevertheless accept the bounds of the group as
recently laid down (Stensio, 1969b), except in excluding the Dunkleosteidae and,
-53
ARTHRODIRE FROM AUSTRALIA
Dicksonosteus arcticus
6-13\
+Holonerna spp.
14-16\
Homostius spp
17-19\
-\
Buchanosteus spp.
20-23t
c.-\occose
tus
”\
spp
2=5
trem:tosteids
leiosteids
hadrheids
pachyostetds
rhino<teids
L
brachydeirids
Figure 15. Cladograin showing a theory of relationships of certain arthrodires. Numbered
chardcters and the status of certain taxa are explained in the text.
provisionally, the Titanichthyidae. There can be n o doubt that a spinal plate is
present in Dunkleosteus (Heintz, 19681, where it is closely similar to that of
Eastmanosteus sp. from Gogo (Gardiner & Miles, 1975: fig. 1A).
For Stensio ( 1969b) the “Superorder Aspinothoracidi” is a monotypic taxon
containing the “Order Pachyosteomorphi”. The latter is divided into the
suborders Pachyostei and Parapachyostei. However, we cannot accept that these
last groups are phylogenetically valid. They are based on ill-defined and
speculatively restored soft characters of the scapulocoracoid and pectoral fin,
which have not been subjected to close, phylogenetic scrutiny. Further, if we
understand Stensio correctly, the suborder Parapachyostei is empty of named
species. In our view the interrelationships of aspinothoracids are poorly
understood, but certain major monophyletic groups can be defined by characters
25-27 in Fig. 15.
25. “Fusion” of preorbital and postnasal plates. We follow Nelson ( 1969: 10) in
using ‘fusion’ for want of a better word. The important point is that Stensio
(1963, 1969a,b) has noted the replacement of separate preorbital and postnasal
plates by a single element in Trematosteidae (Gross, 1932), Leiosteidae (Stensio,
1963). and Hadrosteidae (Gross, 1932). We regard this as a synapomorphy
uniting a line of primitive aspinothoracids. However, the interrelationships of
these three families are obscure.
26. Marginal plate forming posterior margin of enlarged orbitalfenestra. This character
54
R. S. MILES A N D K. DENNIS
has been well documented by Gross (1932) and Stensio (1963, 1969a,b) but its
phylogenetic significance has not hitherto been noted. We regard it as a
synapomorphy uniting a group of advanced aspinothoracids. These advanced
forms include two monophyletic groups, defined by characters 27 and 28
respectively.
2 7 . Dentition comprising many minute teeth. The teeth are found on the functional
surfaces of the posterior superognathal and the inferognathal in Pachyosteidae
(sensu Stensio, 1963) and Rhinosteidae (Stensio, 1963). These families appear to
form the plesiomorph sister-group of the Brachydeiridae.
28. Trullate body form with reduction ofwentral shield. These characters unite the
Brachydeiridae (sensu Miles, 1969 ; i.e. the combined Brachydiridae, Oxyosteidae
and Synaucheniidae of Gross ( 1932)).All members appear to have lost at least the
posterior ventrolateral and median ventral plates (Stensio, 1969b : 25). However,
it is possible that in Oxyosteus the interolateral has also been lost and in
Brachydeirus this plate may be the only representative of the ventral shield
(Stensio, termed the spinal in Miles, 1969: 153).
Re-examination ofthefamib Coccosteidae
Having completed the explanation of the characters in the sketch cladogram, it
is now possible to go on and consider the relationships of Hurrytoombsia elegans.
From the description, it is clear that this species belongs to a line which branches
off between characters 23 (superognathals with posterior processes) and 24 (loss
of spinal plate) in Fig. 15. This is the position occupied by the coccosteids, the
close phenetic resemblance between H . eleguns and Coccosteus cuspidatus having
already been noted. It is therefore necessary to examine the current status of the
Coccosteidae, to see whether H . eleguns can be referred to this family.
A long definition of the family Coccosteidae was given by Miles & Westoll
(1968: 463-4). This was based on Coccosteus cuspidatus, but in addition to
Coccosteus, the genera Dickosteus, Watsonosteus, Protitanichthys, Plourdosteus,
Clarkeosteus, Liwosteus and Eldenosteus were also included. Millerosteus and
Buchanosteus (Miles, 1969, 197 1 : 19 1 , footnote) were added later, but the second
of these can now be removed on the evidence presented by White & Toombs
(1972: 380; seealso Fig. 15).
A critical examination of the characters in Miles & Westoll’s definition shows
that most of them are characteristic of taxa of higher rank than the
Coccosteidae; i.e. they are more correctly referred to as placoderm, arthrodire,
phlyctaenioid, brachythoracid or eubrachythoracid synapomorphies. These are
the characters that are listed between numbers 1 to 23 in Fig. 15, or are missing
from this Fig. because they are placoderm or arthrodire synapomorphies. If all
of these characters are removed from the definition, we are left with the
following:
“Nuchal plate covering at least 33 per cent of the mid-line length (of the skullroof), not more than 50 per cent . . . Central plates with stepped lateral margin
and well-developed lateral wings . . . Separately ossified internasal bone with
well-developed anteroventral lamina and internasal process . . . Thoracic armour
with . . . median dorsal plate . . . ; posteriorly drawn out into a spine in some
forms . . . Pits for sensory cells typically present on the suborbital and
postsuborbital plates . . . Main lateral line runs posteroventrally on the anterior
ARTHRODIRE FROM AUSTRALIA
55
dorsolateral plate, as a deep groove, down to the anterior margin of the posterior
lateral plate. A shallow dorsal branch diverges on the anterior dorsolateral plate
and runs posterodorsally across the posterior dorsolateral plate up to the
radiation centre of the median dorsal where it meets its antimere.”
These characters require further evaluation below. We have also carefully
considered Stensio’s (1969b: 41-3) definition of the Coccosteidae. His
conception of the family is close to that of Miles & Westoll and he includes in it
the same eight genera (Millerosteus and Buchanosteus being placed in their own
monotypic families). We are satisfied that Stensio’s definition includes no
additional characters that are important for this discussion.
The relative length of the nuchal plate does not stand up as a diagnostic
character of coccosteids if other arthrodires are examined. For example, this
plate covers 33% of the mid-line length in Pholidosteus friedeli Jaekel (Stensio,
1963: fig. 7313, 44% in Buchanosteus (“Parabuchanosteus murrumbidgeensis”, White &
Toombs, 1972, fig. 3; Young, in press), 36% in Rhachiosteus pterygiatus Gross
(Miles, 1966a: 3781, 36%in Brachydeirus ? bicarinatus Stensio (1963: fig. 108A)and
40% in Leptosteus bickensis v. Koenen (Stensio, 1963: fig. 122A). These
brachythdracids are not closely related, and in addition it seems clear that the
relative length of the nuchal in actinolepids (see Denison, 1958 : fig. 1051, and in
Groenlandaspis antarcticus Ritchie ( 1975: fig. 2a) among phlyctaeniids, falls within
the range given for coccosteids. We are able to draw four main conclusions from
these figures. ( 1) The relative length of the nuchal is highly variable at each level
of arthrodire organization described by Miles (19691, and this variation is
independent of changes in the relative length of the snout. (2) The nuchal is
relatively very long in some actinolepids (Aethaspis spp., Denison, 1958, 1960;
Baringaspis dineleyi Miles, 1973) and some eubrachythoracids (Homostius spp.,
Heintz, 1934; ‘euleptaspid B’, Izlrvig, 197 1 : fig. 4) but the significance of this trait
is obscure. (3) The nuchal plate becomes embayed from behind and therefore
shortened in many advanced eubrachythoracids (Miles, 19691, but this change
takes place in several lines and is not, in any case, a clear-cut taxonomic
character. (4)The relative length of the nuchal plate is not generally useful in
unravelling arthrodire phylogeny and fails to discriminate coccosteidsfrom other
forms.
The form of the central plates with a stepped lateral margin and welldeveloped lateral wings is expressed slightly differently by Denison (1975: 17).
He takes into account the posterior process which projects between the nuchal
and paranuchal plates, and thus includes “centrals which tend to be divided into
anterior, lateral and posterior lobes” among the characters of his “Suborder
Coccosteina”. This group includes the Gemuendenaspidae, Buchanosteidae,
Coccosteidae, Pholidosteidae, Homostiidae (including Euleptaspidae) and the
Rhachiosteidae. If Denison is correct, the lobed form of the centrals is not
distinctive of coccosteids, but it might characterize a larger monophyletic group
to which the coccosteids belong. But if his statement is to have any real meaning,
it requires that all members of the ‘Coccosteina’ have or have evolved from an
ancestor with lobed centrals, and that lobed centrals are absent in other
arthrodires. We must now consider whether these conditions are met.
Actinolepids and phlyctaeniids (e.g. Denison, 1958: fig. 105) lack lobed
centrals. This is consistent with Denison’s statement and these groups may
therefore be eliminated from the discussion. Of the ‘Coccosteina’, the
56
R. S. MILES A N D K . DENNIS
Gemuendenaspidae (Miles, 19621, Coccosteidae, Pholidosteidae (StensiB, 1963,
1969a1, Rhachiosteidae (Miles, 1966a) and Homostius (Heintz, 1934) all have
trilobed centrals, which is again consistent with Denison’s statement, although
this character is not always well marked. However, both Buchanosteus
(“Parabuchanosteus murrumbidgeensis”, White & Toombs, 1972: fig 3) and
‘euleptaspid B’ (0rvig, 197 1: fig. 4) clearly lack centrals of the predicted type and
show no sign of having evolved from ancestors with centrals of this type. Indeed,
they show a close resemblance to the undivided centrals of actinolepids and
phlyctaeniids, i.e. to the hypothesized primitive condition. Denison’s statement is
further contradicted by the presence of lobed centrals in species which are not
placed in the “Coccosteina”. Examples are Holonema spp. (Miles, 197 11, Rhinosteus
parvulus Gross, Belosteus acuticeps Gross (Stensio, 1963, 1969a) and Eastmanosteus
spp. (including the Gogo form, Gardiner 8c Miles, 1975: fig. 1A). In fact it seems
that nearly all advanced eubrachythoracids (e.g. dunkleosteids and Wildungen
spp.) tend to have the centrals divided into three lobes. The main problem with
this character is that it is not a clear-cut morphological feature.
We conclude that (1) not all “Coccosteina” tend to have the centrals divided
into anterior, lateral and posterior lobes, (2) this character is found in more
primitive forms, i.e. the holonematids, and (3) it is also found in more advanced
forms, e.g. most aspinothoracids. Thus neither of the two conditions noted
above has been met, and neither the Coccqsteidae (sensuMiles & Westoll) nor the
“Coccosteina” can be considered as a monophyletic group on the evidence of
this character. However, if the non-lobed centrals of actinolepids, phlyctaeniids,
Buchanosteus and Euleptuspis are primitive for arthrodires, as suggested above,
then lobed centrals might be a synapomorphy uniting most of Denison’s
“Coccosteina” with other brachythoracids. But this suggestion raises problems
with respect to holonematids and Homostius, as can be seen from Fig. 15.
Therefore, we postpone further discussion for a later occasion. Several Gogo
species have centrals with a closely similar form to those of Coccosteus cuspidatus
and Hanytoombsia elegans and it is hoped that these will throw further light on the
problem.
Other characters listed by Denison (1975: 17) in his definition of the
‘Coccosteina’ do not modify our conclusion that this is not a monophyletic
group. The most important of these have already been discussed in this paper
and all are probably advanced characters for higher brachythoracids (i.e. for all
forms fitting into Fig. 15 below character number 20).
An internasal bone with the structure summarized by Miles & Westoll has been
found in Coccosteus cuspidatus and Millerosteus minor (Stensio, 1963: pl. 3), but in no
other coccosteid. Perhaps it is reduced or completely absent in species which
have lost the descending lamina of the rostra1 plate (cf. Hunytoombsia elegum).
However, a closely comparable internasal bone is present in Eastmunosbus sp. and
in at least one other non-coccosteid from Gogo, and a reduced internasal, lacking
the anteroventral lamina, is present in Dunkleosteus terelli (Stensih, 1963: fig.
114C). Therefore, it is clear that the internasal is not restricted to coccosteids.
Rather it would appear to be a primitive arthrodiran structure, as the bone is also
found in Kujdanowiaspis (“prerostral”, S tensio, 1963) and Buchanosteus (Miles,
197 1 : figs 104, 105; “Parabuchanosteus murrumbidgeensis”, White & Toombs, 1972).
Unfortunately it is not possible to describe the exact anterior extent of the bone
in these last two forms, as the dermal snout is “fused” into a sutureless capsule.
AK I HKODIRE FKOM AUSTRALIA
57
But there is no reason to suppose that the internasal is radically different from
that of Coccosteus cuspidatus.
A median dorsal with a clearly developed posterior, median dorsal spine has
been described in Coccosteus, Dickosteus, Protitanichthys, Plourdosteus, ~ilierosteusand
Eldenosteus, but in no other brachythoracid arthrodire if for the moment we
discount the short posterior process of Harrytoombsia elegans. I t is possible then,
that the median dorsal spine is a coccosteid specialization. One stumbling-block
to this view is the absence of a spine in Clarkeosteus and Watsonosteus,which Miles
& Westoll ( 1968: 464) referred, with different degrees of confidence, to the
Coccosteidae (Livosteus grundis (Gross) must be left out of this discussion as the
median dorsal is unknown; Obrucheva, 1962). Clarkeosteus halmodeus (Clarke)was
tentatively referred to the Coccosteidae on general phenetic grounds (Miles,
196613: 42), but as it shares no specialization with Coccosteus cuspidatus these
species may not be closely related. Not only does the median dorsal of C.
halmodeus lack a posterior spine, but there is no evidence in its structure that this
spine was present in an immediate ancestor. However, Wutsonosteusjetti(Watson)
is a different matter because the shape of the median dorsal does suggest its
derivation from a spine-bearing ancestor (Miles & Westoll, 1963: fig. 7c).
We are also aware that a short posterior process is present on the median
dorsal plates of certain more primitive arthrodires, e.g. Hugmaspis broggeri
(Heintz, 1929b: fig. 5A), Phlyctaenius acadicus (Heintz, 1933: fig. 3), Neophlyctaenius
sherwoodi (‘the posterior point’; Denison, 1950: pl. 2) and Baringaspis dineleyi
(Miles, 1973: fig. 3). These structures must be regarded as true posterior spines
unless some arbitrary, undefined limits are put on the admissible variety of the
median dorsal spine. This would be inconsistent with our aim of establishing an
objective phylogeny of arthrodires. Thus the presence of a median dorsal spine
in coccosteid and some phlyctaeniid and actinolepidoid arthrodires may be
explained in two ways: ( 1) it is the result of parallel development, having evolved
several times; (2) it is a primitive arthrodire character lost independently in
actinolepidoids, phlyctaeniids and brachythoracids. The wide range of forms in
which this spine is found makes (2) the most acceptable hypothesis and means
that the spine cannot be used to define the Coccosteidae.
Since pits for sensory cells were first described on the suborbital and
postsuborbital of Coccosteus cuspidatus and Plourdosteus canadensis (Woodward) by
0rvig ( 1960: fig. 3B, El, they have been found in a variety of eubrachythoracids.
Thus a pit is also present on the suborbital of Watsonosteus, ‘Coccosteus’ spp. from
Bergisch Gladbach and Gogo (Miles, 1971: fig 110A), and Buchanosteus
(“Purabuchanosteus murrumbidgeend’ White & Toombs, 1972); and on the
postsuborbital of Harrytoombsia, ‘Coccosteus’ spp. from Bergisch Gladbach and
Gogo, Watsonosteus, Dickosteus, Clarkeosteus and Emtmanosteus sp. from Gogo
(Gardiner & Miles, 1975: fig. 1A). However, we discount the supposed suborbital
pit of Gemuendenaspis angusta (Traquair) (Miles, 19621, and suggest that if a real
structure exists in this position in this species, it is likely to be the upper end of
the postsuborbital sensory line groove (as in primitive brachythoracids, cf.
Holonema, Buchanosteus, Homostius).Why the pit should occur more frequently o n
the postsuborbital than on the suborbital is not clear, but we shall not continue
to distinguish between these pits in this discussion. We shall treat the evidence
simply as the presence or absence of sensory pits on the cheek. Thus we suggest
that the distribution of the pits leads to two clear conclusions: (1) they are not
58
K.S . MILESAND K. DENNIS
restricted to, and are therefore not characteristic of, coccosteids; (2) they are
primitive for most if not all eubrachythoracids. The evidence is not good enough
to draw a more precise second conclusion although it is supported by the
presence of a pit on the cheek of Wuttagoonuspis (on the postnasal of Ritchie,
1973; the suborbital of Miles & Young, 1977). One possibility is that groups of
suborbital and postsuborbital sensory cells were primitive for arthrodires, but
only in odd species were they sunk into well-formed pits in the bone surface.
Potentially this suggestion destroys all hope of finding some phylogenetic
significance in the pits of arthrodires.
The last character to be discussed is the pattern of the flank canals, which is
simply expressed in Denison’s ( 19 7 5 : 1 7 ) words : “course of the main lateral lines
parallel to the ventral exposed edges of the anterior dorsolaterals”. This
character is found in all of the coccosteids listed by Miles & Westoll (1968),
including Millerosteus. However, it is also found in Buchanosteus (“Parabuchanosteus
rnurrurnbidgeensis”, White 8c Toombs, 1972), Rhachiosteus (Miles, 1966a: 383, 389),
Homostius (Mark-Kurik, 1963), Holonem (Miles, 197 11, and several Gogo species
in addition to Hanytoombsia elegans. Therefore, it is certain that this character does
not distinguish the Coccosteidae. Whether it distinguishes a larger, monophyletic
group is less certain. However, it does seem possible to establish that it is
a derived condition, because the main lateral-line has a horizontal course across
the anterior dorsolateral plate in all actinolepids, phlyctaeniids, pholidosteids,
dunkleosteids and the Wildungen aspinothoracids. Thus we have a derived
condition that is restricted to Rhuchiosteus plus forms fitting in between characters
13 and 24 in Fig. 15, although the omission of pholidosteids and dunkleosteids
seriously oversifnplifies this picture. Nevertheless, if we exclude Rhachiosteus as
being too poorly known for discussion, then the derived coccosteid pattern
appears to be characteristic of holonematids and primitive eubrachythoracids.
This conclusion has one of two consequences, either the derived pattern has
evolved more than once (as required by Denison’s 1975 phylogeny), or there has
been a reversal in evolution with the primitive pattern reappearing in advanced
eubrachythoracids. We feel unable to discriminate between these hypotheses with
our present knowledge of brachythoracid interrelationships, but hope to return to
the subject in describing further Gogo species.
0rvig (1969) has provided a schema in which either of the above h
becomes plausible. He supposes, inter a h , that the ventrally
coccosteids and other forms noted above, and the horizontal line of remaining
arthrodires, are not the same, but are ventral and main branches of the lateralline respectively. He further supposes that both branches are present in
coccosteids and holonematids, but usually only the ventral branch is represented
by a groove (Miles, 1971). From this supposition it is a short step to the
hypothesis that both branches, plus a dorsal branch and certain connecting
branches (0rvig, 1969), are primitive for brachythoracids or for all arthrodires.
Apparent variations in this basic pattern on the flank can therefore be explained
as relatively trivial (and presumably reversible) changes in the representation of
the branches as grooves in the bone surface. It is never necessary to postulate the
loss, gain or change in position of a line, and the hypothesis can accommodate
any novel condition by the addition of further lines to the primitive type. The
problem with this extreme form of 0rvig’s hypothesis is that in explaining
everything it explains nothing; it tells us nothing about arthrodire phylogeny
ARI'tlRODIRE FROM AUSIRALIA
59
This discussion of the Coccosteidae may be summarized as follows. None of
the characters used by Miles & Westoll ( 1968 : 463-4) to define the family can be
shown to be valid. Therefore, the Coccosteidae cannot be shown to be a
monophyletic group, and we suspect that it is a grade group with a status similar
to that of the phlyctaeniids (Miles, 1973: 116). It follows from this that the
Coccosteidae do not provide an adequate framework in which to discuss the
p hylogenetic relationships of Harrytombsia elegans. Only the type genus Coccosteus
(of the Coccosteidae as conceived by Miles & Westoll) can be shown on the
cladogram (Fig. 15). The type-species, C. cuspidatus, and C . markae can be referred
to this genus with confidence (Miles& Westoll, 1968: 464).
Relationships ofHarrytoombsia
The foregoing description and discussion leave only one possible conclusion
about the relationships of Harytoombsia elegans. It is an arthrodire at the same level
of organization as Coccosteus and interchangeable with this genus in Fig. 15. Its
phyletic relations among eubrachythoracids are unknown.
ACKNOWLEDGEMENTS
We wish to thank Alan Tout for instruction in symbolic logic; Drs Peter Forey
and Gavin Young for reading and commenting on the manuscript; and Mrs S.
Chambers and Miss M. Holloway for help with the illustrations. The
photographs were taken by Mr T. W. Parmenter and Mr U. Samuelson.
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ABBREVIATIONS USED I N FIGURES
AD I.
AL
AMV
Art
ASG
AVL
a.art
a.IG
art.smd
ash
au
av.s
C
cav
ch.pro.pr
ch.pr.sv
cr.lg
cr.mh
cr.po
cr.pr
cr.so
csc
cuso
d.er1d.e
d. end.i
dp.m.cu
d.prp
d.ua
f.bhy
fo.hp
gr.a.com
gr.cth
gr.IG
IG
[L
inc
ioc.ot
ioc.pt
k.sb
kd
la.asc
laf
anterior dorsolateral plate
anterior lateral plate
anterior median ventral plate
articular bone
anterior superognathal
anterior ventrolateral plate
unossified mandibular articulation
surface
contact area for inferognathal
articulation surface for the submedian-dorsal plate
anterior shelf
autopalatine
anterior ventral sulcus
central plate
cavity of spinal plate
channel for dorsal aspect of preorbital
process of neurocraniuin
channel for dorsal aspect of supravagal
process of neurocranium
linguiform process
median longitudinal crest in
hypophysial depression
postocular crista
carinal process
subocular crista
central sensory canal
pit for cutaneous sensory cells
external opening of endolymp hatic
duct
internal opening of endolyrnp hatic
duct
cucullaris depression
dermal preorbital process of skull-roof
formed by preorbital plate
unossified area on lateral surface of
articular bone
paired buccohypophysial foramen
ventral division of fossa hypophyseos
transverse ventral groove
groove for R. rnandibularis internus
VII
dorsal groove of mentomeckelian bone
for reception of inferognathal
inferognathal
interolateral plate
maxillaris-buccalis notch
otic branch of infraorbital canal
postorbital branch of infraorbital canal
suborbital branch of infraorbital canal
glenoid condyle
ascending lamina of interolateral plate
lateral articular fossa
1a.l
Ia.pp
Ic
I T
1d
I . iscc
I. t
M
MD
Mni
mp
m.t
Nu
OCC
orb
oi-b.art
P
PDL
PL
PM
PMV
PN
PNu
Pro
PSG
PSO
PtO
PVL
Pap
pbla
pdl
pfen
plaa
post.pr
PP
P.P'
P.P'O
PP'
pre.ov
Pro
P'%
p.sh
P.1
pt.o.pr
pt.u
lateral lamina of interolateral plate
postpectoral lamina
main lateral line-canal
lateral consolidated part of skull-roof
dorsal branch of main lateral-line
infrascapulocoracoid lamina of
anterior ventrolateral plate
lateral teeth of posterior superognathal
marginal plate
median dorsal plate
mentomeckelian bone
middle pit-line groove
mesial teeth of posterior superognathal
nuchal plate
occipital cross-coinmissural sensory
canal
orbit
orbital articulation of autopalatine
pineal plate
posterior dorsolateral plate
posterolateral plate
postmarginal plate
posterior median ventral plate
postnasal plate
paranuchal plate
preorbital plate
posterior superognathal
postsuborbital plate
postorbital plate
posterior ventrolateral plate
occipital para-articular process
postbranchial lamina
posterior descending lamina of skullroof
pectoral fenestra
posterolateral angle of skull-roof
unfinished process from mesial surface
of articular
posterior pit-line groove
posterior process of nuchal plate
posterior process of anterior and
posterior superognathals
pineal pit
overlap area for the preorbital lamina
of the preorbital plate
obstantic process
supraglenoid process of articular
posterior shelf
posterior tooth on buccal surface of
anterior superognathal
ventral postocular process
paired pits on visceral surface of nuchal
plate
62
R
SM
so
SP
srnd
soa
soc
sorc
su0.v
R. S. MILES AND K . DENNIS
rostra1 plate
submarginal plate
suborbital plate
spinal plate
sub-median-dorsal plate
subobstantic area ofskull-roof
supraorbital sensory canal
supraoral sensory canal
supraorbital vault
th.n
th.pre
v.A.hyp
vsl
v.tr
v.ts
nuchal thickening
pre-endolymphatic thickening
rostrocaudal canal of parasphenoid
transverse ventral sensory line
ventral tooth row of anterior
superognathal
ventral tooth row of posterior
superognathal