Devonian sharks from Antarctica 
303 
phyllolepid zone 
Alligator Ridge, 
Mt.Ritchie, top units 
[no sharks teeth 
recorded from this 
zone] 
Pambulaspis subzone 
Mt.Ritchie, 
"Gorgons Head" 
karawaka zone 
Mt. Ritchie, 
Mt. Gudmundson-6, 
? 
( —| "Fish Hotel-B" range? |—j 
portalensis zone 
Fault Bluff sites 
Thelodont zone 
B. kohni subzone 
B.askinae subzone 
Q. 
ra 
c 
6 
-5 
1 
2 
c 
Figure 12 Summary of biostratigraphic occurrence of 
shark remains from the Aztec Siltstone, 
using overall faunal list data from new sites 
in Cook Mountains. 
having three main cusps in the crown, with the 
central cusp slightly smaller; and primitively there 
is a bilobed base. Including smaller intermediates 
there may be five cusps, but specimens with four 
cusps are uncommon, and with six or seven 
extremely rare (Ginter and Ivanov 1992). Apart 
from reduction of the central cusp, the only other 
tooth character possibly uniting the group is the 
30-40° inclination of the lateral cusps away from 
the central cusp mentioned by Zangerl (1981), but 
this is seen in other forms (e.g., Mcmurdodus) and is 
not a clear-cut character. Thus on the evidence of 
coronal morphology the phoebodontids would be 
a paraphyletic grouping. 
Regarding the structure of the base, the bilobed 
condition in some phoebodont teeth was 
considered primitive for the family by Ginter and 
Ivanov (1992), presumably by outgroup 
comparison with forms like stethacanthids (e.g., 
Lund 1985). However, in the latter, two tubercles 
are developed on the dorsal surface, whereas in 
phoebodont teeth, including those with a bilobed 
base, and in all xenacanth teeth, a single 'button' is 
developed on the dorsal surface of the lingual 
torus. In Anlarclilamna, re-examination of the tooth 
figured by Young (1982: figure 3C) suggests that it 
may be incomplete, with the base possibly larger 
and bilobed (M. Ginter, pers. comm.). One example 
of a tooth from the holotype was reported to show 
a 'button', but some others which apparently 
lacked it (Young 1982: 827) may be abraded (M. 
Ginter, pers. comm.), so the state of this character 
needs confirmation. 
The lingual torus is regarded as a derived 
character by most authors, and is one of a variety 
of mechanisms evolved within the Chondrichthyes 
to maintain proper spacing between successive 
teeth in a tooth family (Zangerl 1981: 8) until they 
move up into a functional position (e.g., Hampe 
1988a: figure 3). Thus the 'button' on the dorsal 
surface of the lingual torus in Phoebodus fits into a 
depression in the base of the overlying tooth 
(Ginter and Ivanov 1992: figure 2), with teeth 
presumably held together by inter-dental ligaments 
(Lund 1985). In various cladodont teeth the lingual 
torus may have two buttons developed, but the 
single button in phoebodont and diplodont teeth 
seems to be a consistent feature, and may be 
interpreted as a synapomorphy uniting 
'phoebodontids' as a paraphyletic stem-group to 
xenacanths in the broad sense (see Figure 12). A 
differentiated 'basal tubercle', developed from a 
general 'labio-basal thickening' of some 
phoebodonts (Ginter and Ivanov 1992: figure 2), 
which in certain species (e.g., P. australiensis, Long 
1990: figure 4D) is developed as a distinct 'ventro- 
labial boss', may define a less inclusive group 
(some Phoebodus species, and stem and crown- 
group xenacanths; Figure 12). It should be noted 
that Zidek (in Cappetta et al. 1993) has suggested 
that in the Early Devonian form Leonodus 'the 
basolabial boss and basolingual margin show a 
tendency toward splitting', on which evidence he 
suggests a possible ctenacanthoid affinity. But this 
resemblance, if confirmed, may be a 
symplesiomorphy, in which case it would indicate 
only that reduction of the central cusps preceded 
the development of a single basal tubercle in the 
Leonodus lineage. On the other hand, phoebodontid 
teeth, as just discussed, demonstrate the opposite 
situation, so there is clearly some homoplasy 
involved in these detailed tooth characters. 
In Omalodus and Portalodus, which have a labial 
