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extension instead of a lingual torus, the base must 
have been held in the tooth row in a different way. 
However it is not clear from its orientation whether 
the labial projection functioned as a spacing device. 
The arrangement of foramina and canals for 
vascular supply to the tooth is another feature of 
the base which may be different in closely related 
forms. Hampe (1988b: figure 3a) described the 
system in Orthacanthus as two parallel, labio- 
lingually arranged canals connected to a cavity 
below the crown, with a separate supply to the 
small intermediate cusp. In contrast, in Triodus 
there is a ramifying system to all three cusps 
(Hampe 1989: figure 2). In Phoebodus gothicus there 
is a single canal traversing the base (Gross 1973: 34, 
figure 13b), and a similar foramen is observed on 
the ventral face of the base in Anlarctilamm (Figure 
3A). However a different arrangement is seen in 
Phoebodus australiensis, which has two large 
transverse canals passing through the base (Long 
1990: figure 4E). Although internal structure has 
not been studied, Portalodus (Figure 6C) and 
probably Mcmurdodus (Turner and Young 1987: 
figure 3B) show labial and lingually placed 
foramina on the base, with the intervening canal 
partly or wholly enclosed, or expresse i as a groove 
across the ventral surface - a combination of the 
supposedly distinctive types of vascularisation 
pattern illustrated by Duffin and Ward (1983: 
figure 4A-C). It is not clear at present that these 
different patterns have any phylogenetic 
significance. 
Relationships of the new taxa 
Based on the foregoing discussion, the three new 
taxa described above may be placed in a 
provisional cladistic framework (Figure 11). All the 
new taxa are variants on the diplodont pattern, 
with largest cusps placed at the lateral margins 
rather than centrally, as in cladodont teeth. 
However Aztecodus and Anareodus share features 
not seen in Portalodus (crenulated cutting ridge, 
small accessory cusps at lateral margins of crown), 
which we assume to indicate a close relationship. 
On the other hand, Portalodus resembles the genus 
Omalodus erected by Ginter and Ivanov (1992: 62) 
in the absence of a lingual torus, and development 
of a labial extension to the base, which forms an 
obtuse angle with the crown. By outgroup 
comparison (e.g., Antarctilamna, 'Phoebodus', 
Cladodus' tooth types), the labial extension is 
interpreted as a unique derived feature, whereas 
the absence of a lingual torus must be a secondary 
loss. On available evidence therefore we consider 
Portalodus and Omalodus immediately related, and 
Aztecodus and Anareodus immediately related as 
two sister-group pairs. This implies that the 
diplodont condition evolved independently in 
Portalodus, and as discussed above there may be 
other evidence based on character distribution 
which indicates further homoplasy in this feature. 
However, for the present we suggest that the 
diplodont condition of Antarctilamna, 
Diplodoselache, and crown group xenacanths is a 
synapomorphy by which those taxa are grouped 
together. Lacking information on other features 
(e.g., fin-spine morphology), the Aztecodus- 
Anareodus clade does not have a clear position 
either within or outside the Xenacanthida on 
available evidence. 
Biostratigraphy 
The use of Devonian shark teeth in 
biostratigraphy is becoming increasingly 
important. Many new species have been recently 
identified and their age ranges tied into well-dated 
sections, some intercalated with marine sections 
containing conodonts or spore zonations (Turner 
1982, 1990, 1991, 1992, 1993; Turner and Young 
1987; Long 1990; Ginter 1990; Ginter and Ivanov 
1992). Ginter and Ivanov (1992: figure 9) 
summarise the biostratigraphic distribution of 
Phoebodus teeth through the Late Devonian of 
eastern Europe in relation to the standard 
conodont zonation. They note their absence thus 
far from the early Frasnian, and rarity in the latest 
Frasnian Imguiformis Zone level in sequences in 
Moravia (Hladil et al. in press) which may be due 
to the Kellwasser extinction event. Phoebodont 
maximum diversity apparently corresponds with 
that of palmatolepid conodonts in the Famennian, 
and their widespread distribution is indicated by 
occurrences in Australia (Turner 1982), Thailand 
(Long 1990), and Morocco (Derycke 1992). 
Ginter and Ivanov (1992) give the earliest 
occurrence of Phoebodus teeth as the Givetian of 
North America (Paul Frank Quarry bone beds), 
and they also record Givetian occurrences from 
Poland, Australia, and the Kutsnetz Basin. Stritzke 
(1986) figured a phoebodont tooth from the 
hermanni-cristatus conodont zone of the Rhenish 
Schiefergebirge, Germany. The new Antarctic taxa 
are of similar age (see discussion in Young 1988: 
16-19). The biostratigraphic distribution of the new 
taxa in Antarctic sections is summarised in Figure 
12, and corresponds to zones 6a-e in the scheme of 
Young (1993), which are provisionally equated 
with varcus to hermanni-cristatus Zone conodonts 
(Givetian). 
An older 'Phoebodus' tooth from the jauf 
Formation of Saudi Arabia (Forey et al. 1992) is a 
considerably large tooth that has very small central 
cusps. It has been studied by one of us (JAL) and is 
not regarded here as properly referred to the 
genus. Zidek (in Cappetta et al. 1993) considered 
the earliest Phoebodus to be of Eifelian age (P. 
floweri, a form synonymised with P. fastigatus by 
Ginter and Ivanov 1992), but his evidence of age is 
