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Phytogeny of the Reedocalymeninae (Trilobita): implications for


Early Ordovician biogeography of Gondwana


SAMUEL T. TURVEY

Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR,

United Kingdom (e-mail: [email protected])

Abstract: The shallow-water Neseuretus Association has been recognized by many authors
as an important indicator of the former extent of Gondwana during the Early Ordovician.
Phylogenetic relationships of the Reedocalymeninae (Arenig-?Early Silurian) were inves-
tigated using cladistic analysis, incorporating 22 species of Neseuretus as well as represen-
tatives of all other reedocalymenine genera. The area cladogram derived from this analysis
contains four subclades each containing areas that span much of the palaeogeographic
extent of Gondwana, which are interpreted as representing separate biogeographic events
during the evolution of the subfamily. As the majority of taxa included in the analysis are
associated with shallow shelf facies, this suggests that no significant environmental barriers
existed across the continent during the Early Ordovician. Consideration of area relation-
ships both for different subclades within the area cladogram, and within a consensus area
cladogram, also supports the idea of a faunal cline between eastern and western Gondwana
during this time interval. Further cladistic analysis of different trilobite taxa can be used to
test these ideas.

The palaeogeography of the Early Palaeozoic is

poorly understood relative to that of more recent

geological history, and faunal data have been

widely used to determine continental configur-

ations during this time period. Biogeographic

sensitivity varies between different taxa: whereas

some groups (e.g. graptolites) commonly cannot

be used to detect palaeogeographic boundaries,

trilobites have been recognized as being able

to indicate biogeographic patterns which are

congruent with abiotic geological information

such as palaeomagnetism, geodynamic features

and climatically sensitive sediments (Fortey &

Mellish 1992).

Faunal provinciality was very high during the

Early Ordovician (Arenig-Llanvirn), as a result

of both continental and climatic differentiation

(Cocks & Fortey 1988, 1990), and geographic

separation is regarded as a more important

control on faunal composition than biofacies

during this interval (Fortey & Mellish 1992). The

distributions of different trilobite taxa can be

used to differentiate Laurentia, Baltica and

Gondwana, the three main continental regions

which existed during the Ordovician (Zhou &

Dean 1989), although some faunal admixture

between regions at similar latitudes on the

different continents can also be detected, such as

the presence of Baltican genera in carbonates at

Sobova, south central Turkey (Dean 1973,

1975).

Whereas Laurentian and Baltican trilobite

faunas show little intracontinental provinciality

during the Early Ordovician, considerable

faunal differences have long been detected

between different geographically separated

Gondwanan cratonic faunas during this interval

(e.g. Stubblefield 1939). For example, western

and eastern regions of the palaeocontinent are

characterized by different trilobite groups, and

have been recognized respectively by several

authors as the Selenopeltis Province' or 'caly-

menacean-dalmanitacean Province' and the

'Asaphopsis Province' (Whittington & Hughes

1972; Cocks & Fortey 1990). Gondwana, the

largest of the Early Ordovician palaeoconti-

nents, stretched from polar to equatorial lati-

tudes, extending from the South Pole to more

than 30°N (Cocks 2001), so unlike Laurentia or

Baltica it represented a multiclimate region

(Spjeldnaes 1961, 1981; Cocks & Fortey 1988,

1990). As temperature is regarded as a first-

order control on faunal distribution (Fortey

& Mellish 1992), the corresponding well-

developed climatic gradient across the palaeo-

continent can account for much of this faunal

variation, with Palmer (1972) and Whittington

(1973) being among the first to consider that the

distribution of Early Ordovician trilobites

around Gondwana may be related to latitudinal

belts around a North African South Pole. Geo-

graphic separation must also have determined

patterns of trilobite provinciality and endemicity

to some extent, both across the continents

forming the core of Gondwana, and between

these continents and the numerous small

From: CRAME, J. A. & OWEN, A. W. (eds) 2002. Palaeobiogeography and Biodiversity Change: the Ordovician
and Mesozoic-Cenozoic Radiations, Geological Society, London, Special Publications, 194, 53-68.
0305-8719/02/S15.00 © The Geological Society of London 2002.

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