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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