338 
C.J. Burrow 
Gnathal bones 
Ischnacanthid dentigerous jaw bones, and 
fragments thereof, are often readily recognizable 
by their characteristic latero-medially arched, basal 
concavity, which straddled the jaw cartilages. 
Ischnacanthids had the simplest jaw joint of all 
acanthodians, with a single simple articulation 
(Long 1986). In Taemasacanthus the lower jaw 
articulation area is on the end of a "meckelian 
ossification" (Long 1986, figure 3a), rather than the 
dentigerous jaw bone, and the posterior margin of 
the latter is splayed outwards. The posterior 
margins of lower jaw gnathals of Persacanthus (both 
in the holotype material, and a jaw described by 
Reed 1986), Rockycampacanthus (Long 1986) and 
Atopacanthus (0rvig 1957, plate 2, figure 5) have a 
similar shape, and also lack any sign of an 
articulation area. Posterior jaw bone fragments 
comparable to those found in the Trundle beds 
were figured by Gross (1971b, plate 7, figures 
1,13,14), and captioned as lower jaw fragments of 
Nostolepis striata, based on the erroneous 
assumptions that (a) Nostolepis lower jaws had 
dentigerous bones, while the upper jaws had rows 
of tooth whorls, and (b) that the elements were 
from Nostolepis, whereas they are probably from 
Poracanthodes (this misconception is further 
elaborated in the next paragraph). Unfortunately, 
on most other ischnacanthid gnathals that have 
been described, any ossification of the cartilaginous 
jaw articulation areas is either not present or not 
preserved, and only rarely is the posterior end of 
the gnathal preserved intact. Reed (1986:415) 
proposed that some of the jawbones depicted by 
0rvig (1957, plate 2, figures 1,2), which have an 
extra posterior "knob" could be from the upper 
jaw. However, there are no other features to 
distinguish them from 'proven' lower jaw bones. 
Certainly, on all three jaws figured by 0rvig with 
preserved posterior ends, the latter are splayed 
outwards. By comparison with these known cases, 
it seems possible (if only because of the lack of 
features attributable to known lower jaw gnathals) 
that the posterior jaw fragments described in the 
present paper are from the upper jaw. This 
interpretation is supported by a comparison with 
the articulated jaws of Poracanthodes menneri 
figured by Valiukevifius (1992, plate 11, figure 1). 
Although the posterior jaw bone segment is not 
preserved, the palatoquadrate cartilage retains the 
impression of the shape of this bone (see Figure 
7A), and this compares well to the 'negative' of the 
positive' posterior jaw fragment (Figure 3 A). Also, 
the impression left by the posterior end of the 
lower jaw bone in this specimen is consistent with 
that expected for the known lower jaw type. 
Gross (1957,1971b) described two jaw bone types 
from the Late Silurian Beyrichienkalk as Nostolepis- 
and Gomphonchus-type. The Trundle beds jaw 
B 
Figure 7 Articulated jaws of Poracanthodes menneri, 
and climatiid tooth whorls. A, P. menneri 
right upper and lower jaws, after 
Valiukevifius 1991, plate II, fig. 1: preserved 
dentigerous gnathals stippled; B, lateralview 
and C, anterior view, of tooth whorl of 
Nostolepis striata, after Gross 1957, taf. 4, fig. 
7b,c; D, Lalviacanthus, showing four tooth 
whorls, after Schultze and Zidek 1982, fig. 3. 
me = Meckelian cartilage, pq = 
palatoquadrate cartilage. Scale bars = 1.0 
mm; arrow indicates rostral direction. 
fragments (Figure 3C-F) are very similar to 
examples that were attributed by Gross (1971b, 
figure 9A-F) to Nostolepis, but which have since 
been removed to Gomphonchus (Denison 1976). As 
well as having a medial tuberculated ridge, the jaw 
bone described here has a similar arrangement of 
side cusps, with about three cusps both anterior 
and posterior to the main cusp in each tooth group. 
As noted by Denison (1976), the differences 
between the two types of jawbones may indicate 
that the two scale-based species to which he 
tentatively assigned them, G. sandelensis and G. 
hoppei, may not be congeneric. Indeed, following 
the validation of Poracanthodes as a distinct taxon 
by Valiukevi&us' (1992) description of articulated 
specimens of P. menneri, it now seems the 
Nostolepis-type jaw bone belongs to Poracanthodes 
