133845.pdf

(Tuis.) #1
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
Free download pdf