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54 SAMUEL T. TURVEY

tectonic plates and terranes distributed along

its borders or adjacent to it (termed 'peri-

Gondwana'). However, the characterization and

geographic location of these smaller tectonic

units during the Early Ordovician is still strongly

debated, even for geographic regions such as

western Europe which have been exposed to

considerable study, and few authors agree on the

number of oceans, tectonic plates and terranes

involved (Scotese & McKerrow 1990; Paris 1998;

Cocks 2001),

Different authors have used trilobite data

to propose differing biogeographic patterns

for Early Ordovician Gondwana and peri-

Gondwana. Area relationships across the

palaeocontinent have largely been investigated

on the basis of overall faunal similarity, using

both quantitative (e.g. Whittington & Hughes

1972; Fortey & Mellish 1992) and qualitative

approaches. Cladistic biogeographic analysis of

Early Ordovician taxa has tended instead to

consider the relationships between Laurentia,

Baltica and Gondwana (e.g. Peers 1997). South

China and Australia have been regarded as

representing a distinct biogeographic region

separate from central Europe during this time

period by some authors (Palmer 1972; Cocks &

Fortey 1988, 1990; Peers 1997). An alternative

division of Gondwana into a Sino-European

Region and a South American-Australian

Region has also been advocated (Li 1994).

Other authors have emphasized the distinction

between SE Asian and European regions.

Zhou & Dean (1989) suggested that trilobite

faunas from all parts of Gondwanan and peri-

Gondwanan east Asia may belong to a single

faunal province, in turn divisible into sub-

provinces, with further research into SE Asian

Early Ordovician trilobites highlighting faunal

similarities between South China, Tarim and

Indo-China (Zhou et al 1998a, b). A distinct

Northern Gondwanan Province, consisting of

North Africa, the Middle East, and western and

central Europe exclusive of Avalonia, has also

been recognized and subdivided into different

domains partly on the basis of trilobite data

(Paris 1998). There seems to be no faunal

evidence for the presence of oceanographic or

other environmental barriers separating

different areas of Ordovician Gondwana (Zhou

& Dean 1989). Spjeldnaes (1961, 1981) and

Cocks & Fortey (1988,1990) have suggested that

the relationships between different Gondwanan

cratonic trilobite faunas are best understood in

terms of a geographic and climatic cline

across the entire palaeocontinent. Gondwanan

faunas can thus be interpreted as being grada-

tional across the large palaeocontinent, with

intermediate faunal regions variously allied by

different authors to both eastern and western

biogeographic provinces representing 'mixed'

faunas.

The Neseuretus Association

Biofacies has been recognized as an important

determinant of the quality of the biogeographic

signal provided by different trilobite faunas. The

generalized palaeogeographic model proposed

by Fortey & Owens (1978) suggests that, for

a series of different trilobite associations occur-

ring in different water depths along a continent-

edge profile, the associations occurring nearest

inshore should display the highest degree of

endemicity; the environmental barrier separ-

ating them from neighbouring geographic

regions is higher than for deeper-water associ-

ations and so is more likely to act as a barrier to

gene flow and induce speciation. According to

this model, shallow-water trilobite associations

can thus potentially provide the highest resolu-

tion biogeographic information. However,

shallow-water trilobite associations can only be

of use in biogeographic analysis if the genera or

subfamilies they contain are geographically

widespread rather than localized.

The trilobite association expected to provide

the best biogeographic information according to

these criteria is the Neseuretus Association, first

identified by the term 'Calymene Tristani-Stufe'

(Born 1918; see also Hammann 1983) and also

referred to variously as 'Neseuretus shales', the

'Neseuretus Community' or the 'Neseuretus

Fauna'. This association, described by Fortey &

Morris (1982), is generally found in coarse- to

fine-grained elastics such as decalcified iron-

stones, mudrocks and the Gres Armoricain

or Armorican Quartzite, which extends from

Brittany to Iberia and also probably occurs in

eastern Newfoundland (the Armoricain Grit' of

Van Ingen 1914). The reedocalymenine genus

Neseuretus is the dominant faunal component. It

occurs either on its own, or associated with a

sparse, low-diversity trilobite fauna, which

includes genera such as Kerfornella, Plaesia-

comia, Eohomalonotus, Iberocoryphe, Cro-

zonaspis, Taihungshania, Ogyginus, Merlinia

and Liomegalaspides in different parts of the

association's geographic range (Fortey & Owens

1978; Fortey & Morris 1982; El-Khayal &

Romano 1985; Rabano 1990; Zhou et al. 1998a).

The Neseuretus Association has the widest

geographic distribution of any Early Ordovician

Gondwanan trilobite association, occurring in

Avalonia, southern and western Europe, North

Africa, the Middle East, Indo-China, South