The role of pyroclastic volcanism in Ordovician diversification
JOSEPH P. BOTTING
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge
CB2 3EQ, UK (e-mail: [email protected])
Abstract: Pyroclastic volcanism has been tentatively correlated with Ordovician faunal
diversification, but it is unclear whether the volcanism itself or associated tectonic effects
were more influential Elevated nutrient flux from uplifted areas has been widely discussed,
but the link between enhanced productivity and diversification is unclear. Ecosystem com-
partmentalization due to irregular topography was probably significant, but it is debatable
whether this was sufficient to explain the entire diversification. This paper introduces a
further volcanic process, which may have been more significant than these established
hypotheses, although it is emphasized that all relevant factors acted in combination. Recent
studies of the local ecological effects of ash-fall have revealed dramatic post-depositional
plankton and benthic blooms associated with overturning of a stratified water mass. The
data are consistent through all studied sections from the Builth Inlier, Central Wales, while
deeper-water sediments showed plankton blooms only. Repeated bloom events over
localized dysaerobic shelf areas may have produced temporary benthic 'islands', enhanced
genetic heterogeneity within affected regions, and increased speciation rates. In order to
maximize bloom intensity (and possible effects on evolution), this process requires wide-
spread pyroclastic volcanism combined with extensive shelf areas with well-developed
stratification. The hypothesis potentially allows explanation of the differential rates of
evolution between different groups, particularly the decoupled planktic-benthic patterns.
A preliminary test using Welsh Basin ostracodes is presented, with predictions for the
global database results.The Ordovician Period contained the most
dramatic, sustained increases in global diversity
during the Phanerozoic, but its cause (or causes)
remains elusive. Unlike most other episodes of
diversification, it did not follow a major extinc-
tion event. There are marked differences
regarding the onset and extent of diversity
changes between palaeocontinents, and a strong
ecological influence. The majority of the diver-
sity increase is accommodated by sessile (and
relatively immobile) benthos, and manifested
partly as alpha and sometimes gamma diversity
(Miller 1997a; Patzkowsky 1995). Beta (inter-
community) diversity may even have decreased
during the Late Ordovician (Miller 1997b).
Gamma (inter-regional) diversity increases are
generally considered to have been limited
(Sepkoski 1988), despite the hypothesized
habitat-fragmentation influence of numerous
separate continents and volcanic islands (but see
Patzkowski 1995). Sepkoski (1988) suggested
that much of the Ordovician diversity increase
was incorporated through the origination of
novel, biogenic subenvironments such as reefs,
bioherms and crinoid thickets. Data presented
by McCormick & Owen (2001), describing
Ordovician trilobite diversity in the Welsh
Basin, show a significant increase in the species:
genus ratio during diversity peaks. Genus-level
data are more usually employed in diversity
analyses, but the species approach should be
encouraged where possible, since the diversifi-
cation operated primarily at low taxonomic
levels. However, the known fossil record of
several classes (particularly sessile epifauna)
begins in the Ordovician, suggesting some high-
level taxonomic origination (Conway Morris
1999). This is unlikely to be due entirely to the
origination of preservable skeletons in previ-
ously soft-bodied lineages. The relationships
between the different diversity measures are
poorly understood (e.g. Adrain et al. 2000), and
the type of diversification observed (alpha, beta
or gamma) may also depend on the taxonomic
level examined. A similar pattern is apparent in
the post-Permian recovery, in which high-level
taxonomy showed few changes, with diversifica-
tion concentrated at family level and below
(Erwin et al 1987).
The primary difficulty in interpreting the
Ordovician Radiation (i.e. extreme complexity)
may also allow its ultimate solution. Differential
diversity patterns occur between different
regions and taxonomic groups, and there are
putative correlations with changes in climate,
sea level, palaeogeography and volcanism. All
these factors probably affected diversity, in
addition to internal, ecological interactions: to
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, 99-113.
0305-8719/02/$15.00 © The Geological Society of London 2002.