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TRILOBITE BIOGEOGRAPHY OF GONDWANA 63

conditions in the Portixeddu and Tuviois for-

mations, Sardinia (Leone et al. 1991, as ICaly-

menesun sp.; Hammann & Leone 1997), and

Sarrabesia teichmuelleri occurs in coarse-

grained sandstones of the Punta Serpeddi

Formation, Sardinia, which are also indicative of

shelf facies (Hammann & Leone 1997). N. long-

inasuta occurs alongside a varied trilobite

assemblage in shales and mudstones of the Sort

Tepe Formation in Turkey, suggestive of similar

environmental conditions (Dean & Zhou 1988).

Both Neseuretinus birmanicus and Vietnamia

douvillei are known from siltstones from the

upper Na Mo Formation in Vietnam; the for-

mation's depositional environment is poorly

understood, but both lithofacies and biofacies

comparisons suggest that it represents shelf

facies (Tong-Dzuy Thanh, pers. comm. 2001).

"Neseuretus" intermedius occurs in outer shelf

facies on the Yangtze Platform, in association

with characteristically shelf-slope taxa such as

raphiophorids and Nileus (Wang et al. 1987, as N.

xiadongensis). Reedocalymene expansa is abun-

dant in shales from the Miaopo Formation of the

Yangtze Platform, which has been interpreted as

representing a deep-water aulacogenic environ-

ment by Chen & Qiu (1986). Calymenesun also

occurs in deep-water facies (e.g. Tripp et al.

1989).

However, all of these other reedocalymenine

taxa still show relatively restricted geographic

ranges, which suggests that their incorporation

into cladistic biogeographic analysis will not

lower the resolution of the pattern of area

relationships (although see the discussion on

the possible geographic range of Neseuretinus

in Hammann & Leone 1997, p. 114). Similarly,

although most of these taxa occurred during the

Caradoc or Ashgill, a time period with different

continental configurations to the Arenig-

Llanvirn interval (Scotese & McKerrow 1990),

the consensus cladogram indicates that most of

the divergences involving these lineages are

stratigraphically constrained to have occurred

during the Early Ordovician. All of the reedoca-

lymenine taxa included in cladistic analysis are

therefore also included in cladistic bio-

geographic analysis. However, the occurrence of

Calymenella in Australia during the Early

Silurian (A. Sandford, pers. comm. 2001) is

excluded from this analysis, as this would appear

to represent a range expansion which occurred

after the time period under consideration here.

The area cladogram in Figure 4 contains four

subclades containing areas that span much of

the palaeogeographic extent of Gondwana

(nodes 1-4 in Fig. 4). This is interpreted as

indicating that several separate, independent

biogeographic events occurred during the

evolution of the Reedocalymeninae, which can

thus be compared with one another to reach an

understanding of Gondwanan area relation-

ships. Node 2 consists of an unresolved poly-

tomy in the area cladogram, and so can provide

no information on the relationships between

the four different areas (France/Spain, South

America, Avalonia and Turkey) included within

this subclade. The other three nodes are either

well-resolved (nodes 1, 4) or adequately

resolved (node 3). Nodes 1 and 4 display the

easternmost Gondwanan areas at the base of

the subclade, with progressively more western

Gondwanan areas tending to be nested progres-

sively further within the subclade. At node 1,

the far eastern Gondwanan region of South

America represents the sister group to Central

Asian and eastern Mediterranean regions

and South China; at node 4, the western

Gondwanan regions of Avalonia, France, Spain

and North Africa are more closely related to

one another than any is to the more easterly

situated SE Asian regions of Indo-China and

South China. Node 3 shows the opposite pattern

of area relationships, with South America and

South China as sister areas, together sister to

Spain, with the westernmost region of Avalonia

as the sister area to the rest of the subclade.

The ordering of successively nested areas

within each of the resolved nodes matches

previous ideas on the relative geographic

positions of Gondwanan and peri-Gondwanan

tectonic units in the Early Ordovician. These

three sets of area relationships can therefore be

interpreted as supporting the idea of a faunal

cline mirroring the climatic cline across

Gondwana during this time interval. However,

the three resolved subclades appear to represent

biogeographic shifts in opposite directions

across Gondwana. The opposing nested patterns

of area relationships thus represent non-

congruent biogeographic events, and contain

paralogous areas which would conflict with

duplications of themselves to generate ambigu-

ous data if combined as a consensus cladogram,

reducing the information available from the area

cladogram in Figure 3a. As it is imperative to

limit paralogy in area cladistics in order to detect

biogeographic congruence (Ebach 1999), com-

bining all three into a single consensus area

cladogram in an attempt to provide further

information on Early Ordovician Gondwanan

area relationships is thus to be avoided.

The subclades contained within nodes 1 and 4

appear to represent faunal shifts in the same

direction across Gondwana (Fig. 5a, b). As these

two subclades share the area of SE Asia in
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