A Devonian brachythoracid arthrodire skull (placoderm fish) from the Broken River area, Queensland

2004·8.4 MB·

by  Gavin C Young

A Devonian Brachythoracid Arthrodire Skull (Placoderm Fish)
from the Broken River Area, Queensland
GAVIN C. YOUNG
Department of Earth and Marine Sciences, Australian National University, Canberra ACT 0200
gyoung @ geology.anu.edu.au
Young, G.C. (2004). A Devonian brachythoracid arthrodire skull (placoderm fish) from the Broken River
area, Queensland. Proceedings of the Linnean Society of New South Wales 125, 43-56.
An incomplete brachythoracid arthrodire skull acid-prepared from the Devonian limestones of the
Broken River area of Queensland is described as Doseyosteus talenti gen. et sp. nov. It supposedly comes
from strata dated by conodonts as late Early Devonian in age (Emsian stage), but shows several derived
features of the skull, typical of Middle-Late Devonian brachythoracids, and not seen in any arthrodire from
the Emsian limestones of the Burrinjuck area of NSW. The alignment with conodont zones of stratigraphic
subdivisions of the Burrinjuck sequence is revised. Published information on the provenance and age of all
previously described placoderm taxa from Broken River is reviewed and amended. The new taxon may be
most closely related to Late Devonian (Frasnian) brachythoracids from Iran and the Gogo Formation of
Western Australia.
Manuscript received 27 May 2003, accepted for publication 22 Oct 2003.
KEYWORDS: Placoderm fishes, Arthrodira, Brachythoraci, Broken River, Devonian, new genus
Doseyosteus, Queensland.
INTRODUCTION gnathostome fishes (e.g. Young 1986; Janvier 1996).
In marine environments of the Late Devonian they
Devonian sedimentary rocks, including many included probably the largest predators of their time.
marine limestones, are well exposed in the Broken The major radiation of brachythoracid subgroups had
River area of Queensland (Fig. 1). Conodonts form apparently already occurred by the Middle Devonian,
the basis for dating the sedimentary sequence (Mawson and primitive representatives were already widespread
and Talent 1989; Sloan et al. 1995). Vertebrate remains in shallow marine environments of the Early Devonian
reported from this sequence include microfossils from (e.g. Young et al. 2001; Mark-Kurik and Young 2003),
many horizons (De Pomeroy 1996; Turner, Basden and are important in considering the origins and
and Burrow 2000), and less well known vertebrate interrelationships of major brachythoracid subgroups
macro-remains. The latter include two genera of (e.g. Leliévre 1995).
antiarch placoderms described by Young (1990), a The stratigraphic occurrence of various
ptyctodont toothplate ascribed to ?Ptyctodus sp. by placoderm remains in the Broken River sequence were
Turner and Cook (1997), a new species of the reviewed by Young (1993, 1995, 1996), De Pomeroy
brachythoracid arthrodire Atlantidosteus Leliévre 1984 (1995, 1996), and Turner et al. (2000), and they have
described by Young (2003a), an isolated suborbital been mentioned in relation to conodont studies by
plate of another arthrodire illustrated by Turner et al. Sloan et al. (1995). There has been conflicting
(2000, fig. 8.7), and jaw remains of an onychodontid information published about the provenance of some
(Turner et al. 2000, fig. 5). Undescribed vertebrate of the described placoderm taxa. These were collected
macro-remains include various placoderm bones, most from the Broken River area many years ago by
of which belong to brachythoracid arthrodires. The Professor John Jell, University of Queensland, and sent
Arthrodira is the most diverse order within the class to Canberra for acid preparation and study. In this paper
Placodermi, and its major subgroup, the Brachythoraci, I describe a new arthrodire skull from this collection,
comprises nearly 60% of about 170 genera within the and review the locality information and age
Arthrodira (Carr 1995). The brachythoracid arthrodires determinations for previously described placoderm
were one of the most successful groups of early taxa.
DEVONIAN ARTHRODIRE SKULL FROM QUEENSLAND
{vv. y CAINOZOIC [incl. basalt]
T-> } L. DEVONIAN-
CARBONIFEROUS
unconformity
wey
MIDDLE DEVONIAN
= [Broken River Group]
wow disconformity
GSo54 EARLY DEVONIAN
~ [4 SJ SILURIAN
a lnconformity
("4] ?CAMBRIAN-ORDOVICIAN
A. FOSSIL FISH LOCALITIES
%,
~ JESSEY SPRINGS
a
eae A
sf
wf
A 8
Nawagiaspis loc . \
OUTSTATIO
Viv
Figure 1. (A) location of the Broken River area in Queensland, Australia; (B) geological
map of the collecting area (modified from Turner, Basden and Burrow 2000, fig. 2),
showing localities for previously described placoderm taxa, and the specimen described
in this paper (ANU V1026).
LOCALITY AND AGE OF DESCRIBED
locality L4399 (not L4339, given in error by Young
PLACODERM TAXA FROM THE BROKEN
RIVER AREA 1990: 45), on the north bank of the Broken River, Grid
Reference 640 460 on the Burges 1:100 000 sheet,
and was assigned a Middle Devonian (?Eifelian) age
Wurungulepis denisoni Young 1990
According to information provided with this within the Broken River Formation (J.S. Jell, letter of
17 April 1980). Judging by the map of the area
specimen, it came from University of Queensland
44 Proc. Linn. Soc. N.S.W., 125, 2004
G.C. YOUNG
published by Sloan et al. (1995: fig. 2), the
locality lies within outcrop referred to as
8undifferentiated Broken River Group9. costatus
A 8Wurungulepis-Atlantidosteus
fauna9, of assumed Eifelian age, was listed in partitus
the macrovertebrate zonation of Young (1993,
1995, 1996). However De Pomeroy (1995: 480)
patulus
assigned Wurungulepis to the late Emsian
serotinus Conodont Zone (CZ), citing a personal
communication of J.A. Talent. This information
serotinus
was repeated by Turner et al. (2000: 498). Later
(pers. comm. 28/8/95) J.A. Talent had advised
A. Basden that this specimen was collected from
inversus-
the grid reference cited above, situated on a bend
laticostatus
of the Broken River in an anticline, in strata
which were pre-Dosey Limestone in the z
sequence, and equivalent to the Bracteata
< perbonus-
Formation and Lomandra Limestone (spanning
z gronbergi
the Emsian-Eifelian boundary; Sloan et al. 1995:
fig. 3).
O
No conodont data were obtained from
>
tu
the specimen, so its precise position relative to
the standard conodont zonation is uncertain. a
dehiscens
Wurungulepis is an early representative of the
a
asterolepidoid antiarchs, with a high short trunk
armour (Young 1990), and was placed within
Oo
the asterolepidoid clade adjacent to
< pireneae
Sherbonaspis, and as sister group to Stegolepis, wi
Asterolepis, Remigolepis and Pambulaspis, by
Zhu (1996: fig. 29). As earlier discussed (Young
kindlei
1990: 48) the initially suggested Eifelian age
was consistent with the oldest asterolepid
(pterichthyodid) occurrence in Europe, cited as
Gerdalepis from the Eifelian of Germany by sulcatus
Denison (1978), although this occurrence is
slightly younger (early Givetian) according to
Otto (1998: 118). However Gardiner (1994) Figure 2. Proposed alignment with conodont zones
cited Young (1974) for an older record (Emsian) of subdivisions of the Early Devonian limestone
of the asterolepid antiarchs, but the 8cf. sequence (Murrumbidgee Group) around
Pterichthyodes9 mentioned by Young (1974) Burrinjuck Dam, N.S.W., revised from Basden et
was based on an erroneous attribution by Hills al. (2000: fig. 2). Abbreviations for stratigraphic
(1958: 88) to the Early Devonian limestone subdivisions are: B - Bloomfield Limestone
sequence of an 8Antiarchan from Taemas9. In Member; CB - Cavan Formation; CR - Crinoidal
fact, the specimen in question came from the Limestone Member; CU - Currajong Limestone
overlying Hatchery Creek Formation, of Member; HC - Hatchery Creek Formation; M -
presumed Eifelian age (Fig. 2). This specimen Majurgong Formation; R - Receptaculites
was assigned to the new genus Sherbonaspis by Limestone Member; SY - Spirifer yassensis
Young and Gorter (1981). Previously, the Limestone Member; W - Warroo Limestone
suggested Emsian age of a pterichthyodid Member; 1-6 - units of Upper Reef Formation.
antiarch from the Georgina Basin (Young V1370 4 horizon for highest known arthrodire in
1984a) was noted as possibly the oldest the sequence.
occurrence of this group anywhere recorded.
New evidence now indicates that two assemblages may in press), with the limestone occurrence yielding the
have been mixed in this region (Burrow and Young, antiarch probably younger than the diverse
Proc. Linn. Soc. N.S.W., 125, 2004 | 45
DEVONIAN ARTHRODIRE SKULL FROM QUEENSLAND
Wuttagoonaspis fauna from underlying sandstones is Middle-Late Devonian (Gardiner 1994, fig. 32.1).
(Young and Goujet 2003). The bothriolepidoid clade had an earlier history in Asia,
The antiarchs are a major subgroup of the and apparently expanded its range to most regions of
class Placodermi, ranging in age from Early Silurian the world in the Givetian (Young 2003b).
to latest Devonian. In recent years there has been a The confusion about the provenance of this
significant expansion in our knowledge of the group. specimen may have resulted from the misconception
A cladistic analysis of their distribution in relation to that it was a recognisable 8skull9 when collected.
phylogeny by Young (1984b) involved 22 taxa and Turner et al. (2000) used this term to refer to the type,
40 characters. In a recent review of antiarch phylogeny, but the specimen as collected was a largely complete
Zhu (1996) noted some 45 genera and 154 species, trunk armour, and the incomplete skull, missing its
and his data matrix used 66 characters for 40 genera. central portion, formed a minor part of the specimen.
The original age assessment of Eifelian for The whole specimen may have appeared to a non-
Wurungulepis from Broken River is most consistent vertebrate worker to represent a 8skull9. Such fish
with our current knowledge of this large and diverse remains, when collected in the field, are generally not
group. determinable until after acid preparation (e.g. the type
specimen of Atlantidosteus pacifica Young 2003a,
Nawagiaspis wadeae Young 1990 before preparation, was assumed to be a ventral plate
This specimen is recorded from locality of the trunk armour, rather than a large suborbital bone
BRJ68D (University of Queensland locality L4428; from the cheek).
8small limestone outcrop on eastern side of gully 1 A summary list of prepared fish remains from
km upstream from Six Mile yard9), Grid Reference the original J.S. Jell collection was provided to J.A.
596 442 on the Burges 1:100 000 sheet, which was Talent in 1995 to check on age and locality data. This
assigned a Middle Devonian (?Givetian) age within list mentioned only one skull, the brachythoracid
the Broken River Formation (J.S. Jell, letter of 17 April specimen described below, of which locality data
1980). Apparently this specimen was found by Dr provided by J.S. Jell are almost the same as stated by
Mary Wade. Sloan et al. (1995) for NV. wadeae. Thus it seems that
Again, De Pomeroy (1995: 480) referred this the specimen described below, previously listed as a
taxon to the significantly older (late Emsian) serotinus 8skull9, has been confused with the type of N. wadeae,
CZ, based on its assigned position within the Bracteata leading to erroneous locality and age information being
Formation in section Br4 of Sloan et al. (1995, fig. 6). given in De Pomeroy (1995), Sloan et al. (1995), and
This information was repeated by Turner et al. (2000: Turner et al. (2000). In the context of the global
498, 506). However Prof. J.A. Talent9s previous advice distribution in time and space of this major placoderm
to the author (pers. comm. 5/8/92), was that this subgroup (see above), it is almost certain that
specimen was considerably younger (ensensis 4 varcus Nawagiaspis is Middle Devonian in age, and a Givetian
Zones; late Eifelian - Givetian). Clearly, there was age, as first suggested by J.S. Jell, is most consistent
some confusion about which fish specimen was being with other information about the stratigraphic
referred to. Subsequent advice given to A. Basden distribution of the more derived antiarchs.
(pers. comm. 28/8/95), was that N. wadeae came from
the bank of Dosey Creek (Grid Reference 616 437), Atlantidosteus pacifica Young 2003a
the location of section Br2 within outcrop of the This specimen came from locality BRJ 67B
Bracteata Formation (Sloan et al. 1995: fig. 2). The (University of Queensland locality L 4472), Grid
different, and presumably correct, locality information Reference 675 485 on the Burges 1:100 000 sheet,
provided with the specimen, as cited above, described as <Top of ridge to three-quarters way down
corresponds to the vicinity of the boundary between western slope, west of road between Six Mile Dam
the Papilio and Mytton Formations on the map of Sloan and Diggers Creek9 (J.S. Jell, letter of 17 April 1980).
et al. (1995: fig. 2). This is consistent with the Givetian This is the locality (with a slightly different grid
age first suggested by J.S. Jell. reference) referred to as 8Fish Hill9 by Turner et al.
Nawagiaspis wadeae is another antiarch, (2000: 507). They assigned it a middle Eifelian age
originally interpreted as possibly a primitive (costatus - australis conodont zone), but noted that
bothriolepidoid (Young 1990), although in Zhu9s Sloan et al. (1995) gave a slightly longer partitus -
(1996) phylogeny it comes out as a basal early kockelianus zonal range for the Fish Hill section.
asterolepidoid. Apart from primitive Chinese antiarchs, This is consistent with the original assignment of a
and the erroneous Emsian pterichthyodid occurrence Middle Devonian (?Eifelian) age within the Broken
discussed above, the stratigraphic record of this group River Formation by Prof. J.S. Jell. This occurrence is
46 Proc. Linn. Soc. N.S.W., 125, 2004
G.C. YOUNG
part of the evidence for proposing an Eifelian advanced than other arthrodires known from the
8Wurungulepis-Atlantidosteus fauna9 in the Burrinjuck sequence in possessing several derived
macrovertebrate zonation of Young (1993, 1995, characters of the skull, the most obvious being the T-
1996). shaped rostral plate, a feature of more derived
eubrachythoracids (character 5 of Carr 1991; character
Doseyosteus talenti gen. et sp. nov. 4 of Leliévre 1995). Eubrachythoracids were the most
This specimen, described below, was the only diverse fish group of the Middle and Late Devonian,
one in the J.S. Jell collection lacking a sample number and the new Broken River brachythoracid described
at the time of preparation. It is highly probable that it below clearly belongs to this group, with a skull which
was a sample collected the year before the other is more advanced in several respects than any of the
material, and was taken to Canberra separately by Dr known Burrinjuck arthrodires (see below). Gardiner
P. Jell (J.S. Jell, letter of 17 April 1980). The following (1994) lists the first occurrence of this grouping (his
locality details, provided by Prof. J.S. Jell (letter of 17 family Coccosteidae) as Coccosteus Miller 1841 from
April 1980), indicate that it is the specimen collected the Middle Devonian (Eifelian) of Scotland, for which
from the alternative erroneous locality for Nawagiaspis a late Eifelian age is indicated by spores of the
just discussed: 8BRJ34 = L 4054. Grid Reference 616 devonicus-naumovae zone (V.T. Young 1995). The
438 Burges 1:100,000 sheet. Western bank of Dosey same species (Coccosteus cuspidatus) is recorded from
Creek, 750 m upstream from its junction with Broken the Kernave Member of the Narva Formation in the
River. Base of thick limestone lens in Broken River Baltic sequence, although a related brachythoracid
Formation, Middle Devonian. ? Eifelian9. 8Protitanichthys9 occurs a little earlier, and in
In a published listing of University of equivalent strata (costatus CZ) in the Rhenish sequence
Queensland locality numbers (Turner et al. 2000: 506), (Mark-Kurik 2000). However Otto (1997: 115)
UQL4054 is assigned to 8basal part of limestone, suggested that remains of early eubrachythoracids
Lomandra/Dosey Limestone, Broken River Group9, (coccosteids) first occur in the early Eifelian of
with a slightly different grid reference (615 438), but Scotland, Germany, and the Baltic sequence.
the same locality description as above. However, it is Dhanguura johnstoni Young (in press a)
assigned to the Emsian serotinus CZ, citing Sloan et comes from a horizon about 420 m stratigraphically
al. (1995). above the boundary equivalent of the Bloomfield and
Again, no conodonts were obtained from the Receptaculites Members of the Taemas Limestone. A
sample, and section Br4 through the Bracteata similar horizon high in the limestone sequence has
Formation at this locality did not produce identifiable produced the large lungfish Dipnorhynchus cathlesae
conodonts (Sloan et al. 1995: caption to fig. 6). Campbell and Barwick 1999. The lungfish locality is
Nevertheless, these authors (p.5) considered the entire close to localities L537 and L538 of Pedder et al.
formation to belong to the serotinus CZ, making it (1970) which yielded tetracorals Vepresiphyllum
equivalent to the upper part of the Burrinjuck (NSW) dumosum, Sulcorphyllum pavimentum,
limestone sequence, which extends from the top of Chalcidophyllum vesper and C. gigas. This represents
the pirenae CZ (latest Pragian) into the serotinus CZ the uppermost 8tetracoral teilzone9 of the
(the second youngest zone of the late Emsian). It is Murrumbidgee Group (Pedder et al. 1970: fig. 4), and
therefore relevant to make comparisons with the is Coral Fauna F in the scheme of Garratt and Wright
stratigraphic distribution of the diverse arthrodire (1989). These authors considered the succeeding G
assemblage described from the Burrinjuck limestone and H Coral Faunas to overlap, and belong to the late
sequence. Emsian, rather than Eifelian as previously assessed.
The described arthrodire fauna from the Garratt and Wright (1989) also aligned Coral Fauna F
Burrinjuck sequence (White 1952, 1978; White and from Wee Jasper (and the Sulcor Limestone of northern
Toombs 1972; Young 1979, 1981, in press a, b; Young NSW) with the mid-Emsian inversus CZ (see column
et al. 2001; Mark-Kurnik and Young 2003) includes 10 13 of Young 1995, 1996). However Basden et al.
genera of brachythoracids, amongst which the most (2000: fig. 2) showed the uppermost beds of the
derived taxa (Cathlesichthys and Dhanguura) come limestone sequence at Wee Jasper (containing Coral
from the upper part of the Wee Jasper limestone Fauna F) extending well into the next youngest
sequence. Basden et al. (2000, fig. 2) showed the serotinus CZ. Evidence supporting this (summarised
youngest arthrodire skull from the Wee Jasper section by Basden 2001, table 2.1) derives from reassignment
(ANU V1370; the holotype of Dhanguura) to come of some of the conodonts from the highest productive
from the uppermost unit 6 of the 8Upper Reef sample (C62) in Pedder et al.9s (1970) section 2,
Formation9 of Young (1969). This specimen is more referred by them to Polygnathus linguiformis
Proc. Linn. Soc. N.S.W., 125, 2004 47
DEVONIAN ARTHRODIRE SKULL FROM QUEENSLAND
linguiformis, but reassigned to Polygnathus inversus anth, anterior nuchal thickening;
by Klapper and Johnson (1975), and to Polygnathus Ce, central plate;
serotinus (delta morphotype) by Mawson (1987). On cf.Ce, area overlapping Ce plate;
the other hand, the age in terms of conodont zone cf.M, area overlapping marginal plate;
alignment of several constituent members of the cf.PM, area overlapping postmarginal plate;
Taemas Limestone, as indicated by Basden et al. (2000, cf.PtO, area overlapping postorbital plate;
fig. 2), seem to be too young, and should be revised cr.im, inframarginal crista;
downwards on the following evidence. Lindley (2002a: csc, central sensory line canal;
275) noted that the occurrence of the index species of d.end, openings of dermal tube for endolymphatic
Coral Fauna D (Chalcidophyllum recessum) in the duct;
Currajong Limestone Member indicates that it should dep, depression;
be aligned with the dehiscens rather than the perbonus gr.M, groove on Ce plate which received the edge of
CZ. The overlying Bloomfield Limestone Member the marginal plate;
may also have lower beds of dehiscens rather than the ifc.ot, otic branch of infraorbital sensory groove;
perbonus CZ age (Basden 2001: table 2.1). The Warroo if.r, infranuchal ridge;
Limestone Member contains perbonus CZ elements if.pt, infranuchal pit;
(Nicoll, in Lindley 2002b), and the uppermost kb, knob-like thickening of inframarginal crista;
Crinoidal Limestone Member in the Taemas sequence Iep, lateral consolidated part of skull roof;
may align with both the inversus and the serotinus CZ llc, main lateral line sensory canal;
(Basden 2001: table 2.1). M, marginal plate;
These revised alignments are summarised in mp, middle pitline;
Fig. 2. Correlation with the upper part of the Wee mppr, posterior median process of nuchal plate;
Jasper sequence is unclear, because the constituent Nu, nuchal plate;
members of the Taemas Limestone are difficult to oa.Ce, area overlapped by Ce plate;
recognise in the thicker upper part of the sequence, oa.M, area overlapped by M plate;
represented by units 1-6 of Young (1969). If the new oa.Nu, area overlapped by Nu plate;
arthrodire skull described below from Broken River orb, orbital notch;
is of serotinus CZ age, as proposed by Sloan et al. Pi, pineal plate;
(1995), it is still considerably more derived (see below) plpr, posterolateral process or lobe on Ce plate;
than any arthrodire from the Burrinjuck sequence. If PM, postmarginal plate;
correctly dated, this would indicate that derived pmc, postmarginal sensory groove;
features characterising the Middle-Late Devonian pnp, postnuchal process of paranuchal plate;
eubrachythoracid arthrodires had originated at least PNu, paranuchal plate;
by late Emsian time. Pp, posterior pitline;
To summarise, it is emphasised that there is PrO, preorbital plate;
no overlap in the arthrodire skull characters just PtO, postorbital plate;
discussed between the Burrinjuck and Broken River R, rostral plate;
limestone sequences, even though the youngest soa, subobstantic area;
occurrences in the former sequence are also the most soc, supraorbital sensory canal;
derived taxa within the better-documented Burrinjuck th.end, endolymphatic thickening;
arthrodire fauna. For the new taxon described below, th.pre, pre-endolymphatic thickening;
this evidence would support either a latest Emsian age tnth, transverse nuchal ridge or thickening;
(but younger than the Burrinjuck sequence), or an vg, vascular grooves.
Eifelian age as originally suggested by Prof. J.S. Jell.
ABBREVIATIONS SYSTEMATIC PALAEONTOLOGY
The specimen described below (prefix ANU Class PLACODERMI McCoy, 1848
V) is housed in the Earth and Marine Sciences Order ARTHRODIRA Woodward, 1891
Department, Australian National University, Canberra Suborder BRACHYTHORACI Gross, 1932
(GCY Vertebrate Collection). Standard abbreviations
for placoderm dermal bones are used in the text and Doseyosteus talenti gen. et sp. nov.
figures, and together with other morphological Name
abbreviations are listed as follows: From Dosey Creek, the type locality, and the
Greek osteus (bone). The species name recognises
48 Proc. Linn. Soc. N.S.W., 125, 2004
G.C. YOUNG
Professor John A. Talent, Macquarie University, who upstream from its junction with Broken River (J.S. Jell,
has had a long and distinguished career in Devonian letter of 17 April 1980; see discussion above). Horizon
research, including extensive work in the Broken River was described as the 8base of thick limestone lens in
area of Queensland. Broken River Formation9, assigned to the Bracteata
Formation (Sloan et al. 1995) or the <Lomandra/Dosey
Diagnosis Limestone, Broken River Group (Turner et al. 2000).
A eubrachythoracid arthrodire in which the Age: ?late Emsian - Eifelian (see discussion above).
skull shows an embayed anterior margin of the nuchal
plate resulting from overlap by the central plates, the Description
central plates have strong posterolateral lobes ANU V1026 represents a large part of the
separating the nuchal and paranuchal, and a mesial posterolateral region of a brachythoracid skull roof,
process of the marginal plate extends to the anterior preserved as two separate portions. The larger portion
angle of the paranuchal. Subobstantic area of skull (Fig. 3A,D) includes parts of the Nu, PNu and Ce plates
extending onto marginal plate. Dermal bones smooth, (Fig. 4A,B), and the right postmarginal corner of the
or ornamented with fine tubercles. skull is preserved as a separate portion (Figs. 3B,C,
4C,D). The specimen was extracted from the rock in
Remarks six pieces, but they are well preserved, suggesting that
Since only the skull is known, and it is it was broken up before incorporation in the sediment.
incomplete, several features characterising the derived The nuchal (Nu) plate is represented by most of its
subgroup <Eubrachythoraci9 are for the present inferred right half, including the midline, so its overall shape
for this new taxon. Definition of the eubrachythoracid can be estimated. Midline length of the Nu is about 70
arthrodires is discussed by Carr (1991: 379-381) and mm. It has an embayed posterior margin, with a
Long (1995: 55). Thus Doseyosteus talenti gen. et sp. prominent posterior median process (mppr, Fig. 4).
nov. is assumed to have had a T-shaped rostral plate, a Except for the posterior lateral corner the right lateral
posteriorly placed pineal plate separating the margin of the Nu plate is fairly well displayed on the
preorbitals, a dermal process of the preorbital plate external surface. The bone is fractured in its middle
forming the anterodorsal margin of the orbit, and region, and shows anteriorly that it was both
trilobate central plates. The holotype shows a strongly overlapped and underlapped by the central (Ce) plate,
developed posterior thickening of the skull roof, which a condition also reported in Holonema (Miles 1971).
in the midline is represented by the anterior nuchal Along the anterior margin of the plate a thin
thickening. This is much more prominent than the overlapping lamina of the Ce plate has broken away
transverse ridge on the posterior margin of the nuchal to reveal an extensive overlap area (oa.Ce, Fig. 4B).
plate, and is a derived feature seen in coccosteomorph In unbroken condition the anterior margin of the Nu
and pachyosteomorph brachythoracids, but generally plate would have been deeply embayed (Fig. 5). On
lacking in Early Devonian taxa, for example the genus its visceral surface extensive contact faces for the
Cathlesichthys from Burrinjuck, NSW (Young, in press central plates are developed in the normal manner
a). The embayed anterior margin and inferred (cf.Ce, Fig. 4A). Other features shown are the
proportions of the nuchal plate, and the strong prominent infranuchal pits (if-pt) and ridge (if.r) and
posterolateral lobe of the Ce plate, are resemblances the transverse nuchal thickening or ridge (tnth).
to the Late Devonian taxa Eastmanosteus and Noteworthy is the strong development of the
Golshanichthys, but the former differs in having the anterior nuchal thickening (anth), which is relevant to
posterior pitline well developed on the posterolateral the question of the age of this specimen (see discussion
lobe of the central plate, and both forms lack the mesial above). This is a derived feature of brachythoracids,
process of the marginal plate inferred for this new and in ANU V1026 is more pronounced than in any
taxon. Emsian brachythoracid from the Burrinjuck fauna.
These have Nu plates which are fairly flat in front of
Material the infranuchal pits. This is the case even in a form
ANU V1026 (holotype), an incomplete skull like Cathlesichthys, which is derived in having a very
preserved as two unconnected portions. strong transverse nuchal ridge (Young in press a). In
posterior view ANU V1026 shows that the anterior
Locality and Horizon nuchal thickening is more pronounced than the
Locality BRJ34 (University of Queensland transverse nuchal ridge, the reverse of the condition
locality L4054), Grid Reference 616 438, Burges 1:100 in Cathlesichthys. This advanced character is also seen
000 sheet; western bank of Dosey Creek, 750 m in most Middle-Late Devonian brachythoracids, such
Proc. Linn. Soc. N.S.W., 125, 2004 49
DEVONIAN ARTHRODIRE SKULL FROM QUEENSLAND
Figure 3. Doseyosteus talenti gen. et sp. nov. Holotype (ANU V1026). Larger (A,D) and smaller (B,C)
skull portions in external (B,D) and internal (A,C) views.
as Golshanichthys, Tafilalichthys, and various Gogo the anterior end of the PNu exposes an overlap area
forms (e.g. Leliévre et al. 1981; Leliévre 1991; Miles (oa.Ce, Fig. 4B), and the edge of a more extensive
and Dennis 1979; Long 1988, 1995; Dennis-Bryan contact face is shown on the visceral surface (cf.Ce,
1987). These taxa all resemble the giant Famennian Fig. 4A). The endolymphatic thickening forms a broad
form Dunkleosteus, where the 8posterior consolidated thickened area mesially (th.end), combining with the
arch9 of the skull roof (8PCA9 of Heintz 1932: fig. 13) thickened portion of the Nu plate (anth). This thickened
is a broad thickening running in front of the infranuchal part of the skull is much more prominent than in
pits, as the main transverse thickening of the skull. In primitive brachythoracids like Buchanosteus or
contrast, in the Early Devonian form Cathlesichthys Taemasosteus (White 1978; Young 1979). Along the
from Burrinjuck, the transverse nuchal ridge located broken edge of the specimen, maximum bone thickness
behind the infranuchal pits forms the main thickening (in the part enclosing the endolymphatic duct) is almost
supporting the posterior skull margin. 15 mm, which is three times the bone thickness at the
The right paranuchal (PNu) plate of anterior preserved extremity of the Nu. The exoskeletal
Doseyosteus gen. nov. is represented externally by an division of the right endolymphatic duct opens on the
elongate portion including the mesial margin forming visceral skull roof surface at the anterior edge of the
sutures with the Nu and Ce plates (PNu, Fig. 4B). There area of thickened bone (th.end), and is also visible on
is also a small broken part of the postnuchal process the broken margin of the specimen (d.end,
(pnp). The PNu and Ce plates were also connected by Fig.4A).This is also an advanced character of the
a complex interlocking suture; a broken part around brachythoracid skull 4 in large Emsian brachythoracids
50 Proc. Linn. Soc. N.S.W., 125, 2004
G.C. YOUNG
Figure 4. Doseyosteus talenti gen. et sp. nov. Holotype (ANU V1026). A,B. Larger portion of skull in
internal (A) and external (B) views. C,D. Smaller skull portion in internal (C) and external (D) views.
from Burrinjuck the endolymphatic duct is not within must be the central sensory canal rather than the
the bone, but anteriorly forms a bony tube attached to supraorbital sensory canal, because of its oblique
or projecting from the inner surface of a much thinner orientation to the midline. Middle and posterior pitlines
PNu plate (Young in press a: figs. 3, 4, 7A, 9B). A are represented by faint markings in the region of the
similar condition occurs in Holonema from Gogo (J.A. ossification centre (mp, pp). Anterolateral and
Long, pers. comm.; Miles 1971: fig. 53). posterolateral margins of the preserved part of the Ce
The preserved part of the right Ce plate is plate are somewhat fractured, but appear to
crossed by a prominent sensory groove (csc), which approximate natural margins. The former is bevelled
Proc. Linn. Soc. N.S.W., 125, 2004 51
DEVONIAN ARTHRODIRE SKULL FROM QUEENSLAND
N
SS9Fp ey
Figure 5. Doseyosteus talenti gen. et sp. nov. Attempted skull roof reconstruction, preserved
portions shaded.
externally, and internally shows a contact face for the position. Since the anterior end of the PNu is well
postorbital plate (cf.PtO), showing that it overlapped shown on the specimen, and is most unlikely to have
the PtO extensively, as in most other brachythoracids extended to this margin of the Ce plate, it seems that
(e.g. Miles and Westoll 1968: fig. 2; Young 1979: fig. the intervening space must have been occupied by a
1; 1981: fig. 5). Holonema is an exception in this mesial projection of the M plate (M, Fig. 5). This
respect (Miles 1971: fig. 12). Subdivisions of the arrangement has not previously been recorded in
posterior part of this contact face suggest that it also brachythoracids. A similar but smaller process of the
overlapped the marginal (M) plate (cf.M, Fig. 4A). M intrudes the Ce plate of Buchanosteus, but this is
The posterolateral margin of the Ce plate is some distance in front of the PNu (Young 1979: fig.
somewhat thicker, and carries a deep groove (gr.M) 1):
for an interlocking suture, the Ce plate providing . There is a long posterolateral projection of
external and internal laminae to enclose the margin of the Ce plate partly separating the Nu and PNu plates
the contiguous bone. The nature of the preserved (plpr, Fig. 4B), a feature seen in various other
margins suggests that they approximate the suture brachythoracids. An early example with this
Proc. Linn. Soc. N.S.W., 125, 2004