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56 SAMUEL T.TURVEY

such as the Yangtze Platform of the South China

Plate; Neseuretus is now regarded as a eurytopic

genus with exceptional latitudinal tolerance,

adapted to general inshore clastic environments

(Cocks & Fortey 1988).

Neseuretus has traditionally been regarded as

basal within the Reedocalymeninae, represent-

ing the ancestral stock from which other genera

evolved (e.g. Whittington 1966b; Dean 1975).

Dean (1967a, b) and Zhou & Dean (1989) inter-

preted Neseuretus as having originated in

southern and western Europe, continuing to

evolve in its region of origin but also dispersing

eastwards around the Gondwanan margins to

arrive in South China during the Llanvirn.

However, these ideas on faunal migration have

been biased by differing local preservation in

different geographic regions of the appropriate

shallow-water clastic sediments to which

Neseuretus was restricted; for example, appro-

priate transgressive facies are only known from

the lower Arenig in south Wales (Fortey &

Owens 1987). They have also been based on

inaccurate stratigraphic correlation between

different Gondwanan regions; for example,

although the South Chinese Neseuretus species

were originally interpreted as occurring in

Llanvirn deposits (Lu 1975), these are now

considered to have been deposited during the

Arenig (Mu 1974; Chen et al. 1995; Zhou et al.

1998a). Further problems for biogeographic

interpretation have arisen as a result of some

authors regarding Synhomalonotus as a separ-

ate genus from Neseuretus (e.g. Moore 1959;

Ross 1975). Neseuretus stocks in different

Gondwanan regions have alternatively been

interpreted as representing separate lineages,

which may have been environmentally sepa-

rated because of the transience of the Neseure-

tus Association in a marginal setting (Beckly

1989). Fortey & Owens (1987) considered that

individual Neseuretus species probably ranged

widely over Gondwana, as well as observing,

following Hammann (1983), that different

species were associated with particular facies,

suggesting ecological differentiation; however,

so far only N. tristani (Brongniart in Desmarest,

1817), the geologically youngest species of

Neseuretus, has been recorded from several

Ordovician palaeoplates.

Phylogenetic analysis

Although Neseuretus is regarded as an import-

ant genus both biogeographically and for under-

standing the evolution of the Calymenidae, no

quantitative phylogenetic analysis has previ-

ously been conducted to determine the pattern

of relationships between different Neseuretus

species. This is in part because of the large

number of species currently recognized within

the genus. Whereas the seven other genera

assigned to the Reedocalymeninae (Caly-

menella, Calymenesun, Neseuretinus, Pradoella.

Reedocalymene, Sarrabesia and Vietnamia)

contain relatively few species, over 60 different

species or subspecies have at various times been

established for either Neseuretus or Synhoma-

lonotus, many of which are poorly known and

may represent invalid taxa. Representatives of

the Reedocalymeninae are shown in Figure 2.

Cladistic analysis was conducted on the

Reedocalymeninae, incorporating 22 of the

better-known species of Neseuretus and a

representative species from each of the other

reedocalymenine genera. Type material was

studied where possible, although additional

material for many taxa was also considered, and

type species were included for most reedocaly-

menine genera. Reedocalymene expansa Yi.

1957 was used instead of R. unicornis (Reed.

1917), as the type species is poorly known and

the genus was revised comprehensively on the

basis of extensive new material of R. expansa by

Peng et al. (2000). Calymenella preboisseli

Beckly, 1989 was included on stratigraphic

grounds instead of C. boisseli Bergeron. 1890.

as this species represents the only Arenig

representative of an otherwise Late Ordovician

genus. Calymenesun was coded using C. granu-

losa Lu, 1975 instead of C. tingi (Sun, 1931), as

well-preserved material representing this species

was available for analysis. Bavarilla hofensis

(Barrande, 1868), Pharostomina oepiki Sdzuy.

1955 and Protocalymene mcallisteri Ross, 1967

were selected as outgroup taxa to determine

character polarity within the Reedocalymeni-

nae, as they have been interpreted by previous

authors as either closely related to the Caly-

menidae or representing basal calymenids. or in

the case of Protocalymene ( = "aff. Calymenidius

sp. indet.' of Whittington 1965) as a possible

calymenid (Whittington 1966b; Fortey 1983;

Hammann 1983; D.J.Siveter pers. comni. 2001).

Twenty-five exoskeletal characters were

coded for the analysis. As most Neseuretus

species are only known from holaspid cranidia

and pygidia, all of the characters used were

taken from these two exoskeletal regions. This

prevented the cladistic data matrix from con-

taining large amounts of missing values, which

could lower the resolution of the analysis by

producing large numbers of equally parsi-

monious trees, or produce spurious theories of

character evolution (Kitching et al. 1998).

Certain characters which have been used by