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A.M. De Pomeroy 
shallow groove runs parallel to the dorsal margin 
(Figure 6H); on the other lateral surface a narrow 
lengthwise ledge is found in the same position 
(Figure 6G). The posterior part of the bone is 
broken (Figure 6G). 
The scale QMF 31853 (Figure 6J) measures 1mm 
across the greatest diameter, with a gently convex 
base and no discrete neck. The ornament consists 
of a central area of low, broad, closely-packed 
tubercles gently tapering to a rounded point at the 
top, with a subcircular cross-section, and up to ten 
irregularly radiating, sometimes bifurcating, 
ridges, and small, indistinct nodes on the margins 
of the scale. 
Discussion 
Young (1993) mentions the presence in the 
Broken River Formation of large eubrachythoracid 
arthrodires, such as the homostiid Atlantidosteus. 
However, homostiids have a different type of 
infragnathal from that described here (G C Young, 
pers. comm.). 
Infragnathals have been described from other 
Australian arthrodires, all eubrachythoracids: 
Goodrdigbeeon australianum from the Early 
Devonian Taemas Formation at Taemas and Wee 
Jasper, NSW (White 1978), and from the Frasnian 
Gogo Formation in WA Harrytoombsia elegans, 
Camuropiscis concinnus, Rolfosteus canningensis, 
Tubonasus lennardensis, Bruntonichthys multidens, 
Bullerichthys fascidens, kendrickichthys cavernosus, 
Incisoscutum ritchiei and Latocamurus coulthardi 
(Miles and Dennis 1979; Dennis and Miles 1979a, 
1979b, 1980, 1981; Long 1988). While the Broken 
River specimen does not strongly resemble any one 
of these published forms, it does have some 
common characters. 
Arthrodire infragnathals consist of two main 
regions - the anterior biting region, usually 
accupying approximately half the total length of 
the bone, and commonly bearing one or more rows 
of teeth, and a posterior expanded blade (Miles 
and Westoll 1968; White 1978; Miles and Dennis 
1979; Dennis and Miles 1979a, 1979b, 1980, 1981). 
This specimen is interpreted as part of the anterior 
biting region of the infragnathal, with the posterior 
blade missing. It possesses, in common with other 
described specimens, the deep ventral longitudinal 
groove for meckels cartilage, a shallow groove on 
the lateral surface, and denticles or teeth in the 
biting region. The Broken River specimen differs 
from previously described specimens in the 
anterior region, the position of the shallow lateral 
groove, and the shape and position of the denticles. 
The deep ventral groove, found in the Broken 
River specimen, is commonly, but not always, 
present in arthrodire infragnathals. A ventral 
groove for the mentomeckelian bone has been 
described in Goodradigbeeon (White 1978), 
Bruntonichthys, Bullerichthys and Kendrickichthys 
(Dennis and Miles 1980), and in Northern 
Hemisphere arthrodires Coccosteus cuspidatus, 
Dunkleosteus, Titanichthys, Malerosteus and some 
arctolepids, from the Middle Old Red Sandstone of 
Scotland (Miles and Westoll 1968). 
The shallow lateral groove extends to the 
anterodorsal edge of the specimen (Figure 6H). A 
similar shallow groove, on the mesial surface and 
interpreted as the possible path of the ramus 
mandibularis internus VII, is reported in 
infragnathals of Coccosteus, Harrytoombsia, 
Camuropiscis, Rolfosteus and Incisoscutum (Miles and 
Westoll 1968; Miles and Dennis 1979; Dennis and 
Miles 1979a, 1979b, 1981), but occurring only on 
the posterior blade region parallel to the ventral 
margin, not on the anterior biting region as in the 
Broken River specimen. A shallow mesial groove 
has not been reported in Goodradigbeeon, 
Bruntonichthys, Bullerichthys, Kendrickichthys or 
Latocamurus (White 1978; Dennis and Miles 1980; 
Long 1988). 
The nature of teeth on the biting region varies, 
and in some genera is difficult to determine as a 
result of the amount of wear caused by either the 
anterior or posterior supragnathal, or both (e.g. 
Dennis and Miles 1980). Camuropiscis, 
Bruntonichthys and Incisoscutum, for example, have 
two dorsal or mesial rows of teeth and a short 
anterior row of symphysial teeth (Dennis and Miles 
1979a, 1980, 1981). The unworn part of the dorsal 
tooth row in Bullerichthys comprises ten pointed, 
forward-facing teeth, smallest posteriorly (Dennis 
and Miles 1980). Tubonasus, in contrast, has 
shearing edges on the infragnathal rather than a 
biting surface with teeth (Dennis and Miles 1979b), 
while Goodradigbeeon has no apparent teeth or 
cutting edges (White 1978). 
The spatulate denticles on the dorsal edge of the 
Broken River specimen differ from these other 
arthrodires. The biting region is not flattened, but 
the denticles are aligned longitudinally aong a 
narrow dorsal ridge. There are no anterior 
symphysial teeth; the anterior extremity of the 
Broken River infragnathal narrows and extends 
slightly ventrally, in contrast to the dorsally- 
curving anterior end of other described 
infragnathals (e.g. Camuropiscis Dennis and Miles 
1979a, figure 14a, b; Bullerichthys Dennis and Miles 
1980, figure 12A, C; Kendrickichthys Dennis and 
Miles 1980, figure 20A, B). 
Abundant buchanosteid-type scales occur in the 
Emsian Taemas-Buchan fauna (Schultze 1968; 
Giffin 1980; G.C. Young, pers. comm.), in horizons 
of a comparable age to those at Broken River where 
only a single similar specimen was recovered, 
despite extensive sampling (particularly in section 
SD170). These two regions, although separated by 
approximately 2000 km, were both shallow marine 
