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.