64 SAMUEL T.TURVEY
Fig. 5. (a, b) Area cladograms for nodes 1 and 4 of original area cladogram in Figure 4. with revised area
boundaries, (c) Consensus area cladogram of nodes 1 and 4, showing complete pattern of area relationships of
Gondwanan and peri-Gondwanan regions.
common, they can be combined to produce a
consensus cladogram, which represents the
relationships between different Gondwanan
areas indicated by the congruent westward
faunal shifts within nodes 1 and 4 (Fig. 5c). This
consensus cladogram shows that the faunal cline
extended across the entire region of Gondwana
for which Early Ordovician deposits are avail-
able. South America and Avalonia+western
Europe+North Africa respectively represent the
eastern and western limits of the cline, with no
evidence for a faunal connection linking these
two areas directly across the Early Ordovician
southern polar region.
Conclusions and future research
The area cladograms generated from the cladis-
tic analysis of the Reedocalymeninae support
certain previously held ideas on Gondwanan
biogeography. The presence of multiple sub-
clades within the Reedocalymeninae, each
containing a wide range of both eastern and
western Gondwanan areas, suggests that no
significant environmental barriers existed across
the palaeocontinent even for shallow inner-shelf
marine taxa. The pattern of area relationships
both within these subclades and as evidenced by
the consensus cladogram in Figure 5c are
also consistent with the idea of a faunal cline
occurring across the palaeocontinent during the
Early Ordovician. No reedocalymenine taxa
have been recorded from the Tremadoc, but
some Neseuretus species are known from the
base of the Arenig, suggesting that this pattern
of area relationships refers approximately to the
interval between these two Ordovician series.
This cladistic biogeographic analysis offers a
preliminary hypothesis on the relationships
between different Gondwanan and peri-Gond-
wanan geographic areas for the Early Ordo-
vician. However, the inter-relationships between
the various Asian areas included in the analysis
are unresolved and require further study, and
some of the nodes used to generate the area
cladograms in Figures 4 and 5 are only weakly
supported by bootstrap analysis. The hypothesis
thus requires testing against area cladograms
generated with other Early Ordovician taxa.
Cladistic analysis of several other Early
Ordovician trilobite groups could be used to
assess support for this pattern of area relation-
ships. Of the shallow-water taxa which some-
times co-occur with Neseuretus in the Neseuretus
Association, several genera (e.g. Kerfornella,
Plaesiacomia, Eohomalonotus, Iberocoryphe,
Crozonaspis and Taihungshania) are only
known from restricted regions of Gondwana,
and so their phylogenies cannot be used to
generate comparable area cladograms. Other
shallow-water trilobite groups do occur over
much of the same geographic region as
