300 
J.A. Long, G.C. Young 
accessory cuspule on the lateral edge of the main 
cusp, and in the shape of the base in ventral view. 
However it differs from Aztecodus in having a 
much larger, flatter main cusp, in the more concave 
shape of the base, with sigmoid curvature along its 
length, and the absence of the two nutritive 
foramina. 
Holotype 
WAM 94.2.13, from the Fish Hotel site B, Cook 
Mountains (Figures 8K, 10B). 
Material 
WAM 90.2.38, 90.2.39 ("Gorgon's Head", near Mt 
Hughes, Cook Mountains); also 94.2.9 from Fault 
Bluff, "fish hotel" (site B), Cook Mountains. 
Description 
These teeth are all of small size (less than 1 cm in 
width or height) but nonetheless overlap in size 
range with both Portalodns gen. nov. and Aztecodus 
gen. nov., indicating that they are not likely to be 
juvenile or extreme posterior teeth of one of the 
other Antarctic genera occurring in the same 
horizons. The main cusp of Anareodus gen. nov. is 
relatively flat and broad quite unlike that of the 
previous genera, withr a well-defined thin cutting 
edge, and lacking any external ornamentation on 
the enameloid. In mesial view this cusp shows a 
weak sigmoidal curvature (WAM 94.2.9, Figure 
10D). Incipient crenulations of the type described 
above in Aztecodus are seen in two specimens (e.g., 
WAM 94.2.13, Figures 8K, 10B; WAM 90.2.38, 
Figure 10A). The base is weakly concave to almost 
straight. In the largest specimen, WAM 90.2.38 
(base 8.5 mm width. Figure 10A) the main cusp is 
approximately three times larger than the other 
cusp and diverges from it at a right angle. There is 
a weak development of median crenulations on 
this specimen. The root is coarsely textured with 
three small nutritive foramina present on the 
largest specimen (90.2.38) but not seen on any other 
specimen. None of the specimens shows the basal 
view of the root. 
DISCUSSION 
Heterodonty 
One of the difficulties in dealing with small 
samples of isolated shark teeth is the possibility of 
variation in tooth morphology within the species, a 
problem in Palaeozoic shark systematics in general 
(e.g., Williams 1985: 85). Many modern sharks of 
the Neoselachii have a heterodont dentition, for 
example the largest family (Carcharinidae) in 
which upper and lower dentitions are markedly 
different, and the symphysial teeth have their own 
morphology (e.g., Reif 1985). Heterodont dentition 
has been suggested for various Palaeozoic sharks, 
and demonstrated in some, for example the 
Pennsylvanian Cobelodus, with single cusped teeth 
in the upper jaw, and tricusped teeth in the lower 
(Zangerl and Case 1976: figure 16). However, 
recent work on forms with diplodont teeth (Hampe 
1988a, b, 1989) suggests that undetected 
heterodonty is not a problem for this group. Thus, 
in Xemcanthus (Hampe 1988a: figure 2) the largest 
teeth are located in the middle region of the jaw, 
and small posterior teeth lack the central cusp, but 
otherwise there is little variation in tooth 
morphology, and no known differences between 
teeth of upper and lower jaws. This is also the case 
in the modem Chlamydoselachus (e.g., Pfeil 1983; J. 
Long pers. observ.), and for the present seems a 
reasonable inference for the chondrichthyans under 
consideration here. 
Is Antarctilamna a xenacanth? 
Antarctilamna prisca was placed cladistically by 
Young (1982: figure 9) as the sister-group to the 
late Palaeozoic form Xemcanthus, and subsequently 
referred to (e.g., Maisey 1984; Young 1989a) as the 
most primitive known member of the Order 
Xenacanthida Glikman 1964, a major group of late 
Palaeozoic elasmobranchs characterised by 
'diplodont' teeth, but also by many other derived 
features. The original analysis of Antarctilamm was 
carried out without access to Zangerl's (1981) 
article, nor the description by Dick (1981) of 
another early xenacanth, Diplodoselache, from the 
Early Carboniferous of Scotland. Since then, the 
teeth named Leonodus from the Early Devonian of 
Spain have also been referred to the Family 
Xenacanthidae by Mader (1986). 
The proposed xenacanthid relationship of 
Antarctilamna was followed by Maisey (1984) and 
Lund (1985), but not by other authors (e.g., 
Williams 1985; Mader 1986). Zidek (1990) has 
argued that both tooth types described by Young 
(1982) ( Antarctilamna, and Portalodus gen. nov. as 
described above) belonged to the phoebodontid 
ctenacanth sharks, and any affinity with 
xenacanths was considered to be exceedingly 
doubtful. In the original cladogram (Young 1982, 
figure 9), previous practice was followed (e.g., 
Schaeffer 1981: figure 26) in using one genus, 
Xemcanthus, to represent a higher taxon (the 
Family Xenacanthidae of Zangerl 1981). However, 
there are other genera normally placed in this 
family which clearly have a closer relationship to 
Xemcanthus, and this has caused some confusion. 
The question of whether Antarctilamna is a 
xenacanth must therefore distinguish 'crown- 
group' xenacanths (family Xenacanthidae) from a 
more inclusive higher taxon (e.g., order 
Xenacanthida of Zangerl 1981). Whether 
Antarctilamna is a xenacanth in this latter sense 
