133845.pdf

Euconodont diversity changes in a cooling and closing Iapetus

Ocean

H. A. ARMSTRONG

1

& A. W. OWEN

2

1

Department of Geological Sciences, University of Durham, Durham DH1 3LE, UK

(e-mail: [email protected])

2

Division of Earth Sciences, University of Glasgow, Gregory Building,

Glasgow G12 8QQ, UK

Abstract: Constrained seriation of euconodont generic presence-absence matrices for four

time slices between the late Llanvirn and late Llandovery provides a qualitative method

for defining shelf and oceanic biofacies, reconstructing biofacies architectures and

analysing biodiversity within a regional context.

We propose many North Atlantic Province taxa had a pelagic mode of life and ranged

widely across the Iapetus Ocean. Oceanic biofacies are considered to reflect water mass

structure. Changes in vertical distribution of one such biofacies (including Amorphog-

nathus and Spinodus) suggest adaptation to cold, nutrient-rich, oxygen-poor upwelling

water. Biofacies distributions suggest that upwelling occurred along the Avalonian margin

throughout the Ashgill, but was only initiated along the Laurentian margin immediately

prior to the Hirnantian glacial maximum.

Clade diversities and trajectories differ between biofacies and latitudes, reflecting

different causal mechanisms. In Laurentia, diversity fell in the early Ashgill, coincident with

the onset of ocean cooling. Diversity declined in Avalonia when the microcontinent drifted

into tropical latitudes. The stability of euconodont biofacies architecture during the Late

Ordovician indicates that global cooling and plate reorganization had a low palaeoecolog-

ical impact despite decreases in alpha and beta diversity.

Over the past 20 years the differential success of

clades has been attributed to either intrinsic

properties of the clades themselves (e.g. com-

petitive ability or origination rates) or to

changes in their biogeographical, environmental

and palaeoecological context (Erwin 1998;

Jablonski 1998). If intrinsic properties of the

clade are the primary control on diversity then

clade diversity is considered independent from

geological setting. Attempting to correlate

changes in clade diversity with independently

established changes in environmental con-

ditions would provide a test of these competing

hypotheses.

Two recent studies on Palaeozoic clades

support the hypothesis that diversity reflects

changing palaeoenvironmental conditions at a

regional scale. Miller (1997) compared the early

Ordovician radiation in six palaeocontinental

regions and found regional differences in the

evolutionary history of trilobites, brachiopods

and molluscs. Miller & Mao (1995) showed that

in a generic dataset, corrected for included

species number, there was a correlation

between Ordovician diversity trends and the

extent of siliciclastic sedimentation, considered

by them to be a proxy for mountain-building

activity. However, aggregate map areas of

marine tectonic provinces through the Ordovi-

cian indicate a slight increase in the area!

extent of the rock record during the period of

maximum diversity decline (Miller & Mao

1995). There is therefore no correlation

between decrease in diversity and total

preserved rock record during the Late

Ordovician.

The changing patterns of Phanerozoic biotic

diversity have been recognized almost entirely

from synoptic global datasets that, by their

nature, average very different palaeoenviron-

mental signals (see Miller 2000 and references

therein). Detailed studies of local successions

can be ecologically well constrained but raise

questions about the quality of the record,

particularly with respect to preservation,

restricted sampling and the response of the biota

to rapidly changing substrates. Regional com-

parative studies provide the best compromise of

taxonomic scope and palaeoenvironmental

acuity (Sepkoski 1993; Miller & Mao 1995;

Miller 2000).

The late Ordovician was a time of major

global environmental change with the late

Ordovician glaciation punctuating a period

of prolonged global greenhouse climate

(Hambrey 1985; Berner 1990, 1992; Crowley &

From: CRAME, J. A. & OWEN, A. W. (eds) 2002. Palaeobiogeography and Biodiversity Change: the Ordovician
and Mesozoic-Cenozoic Radiations. Geological Society, London, Special Publications, 194,85-98.
0305-8719/02/$15.00 © The Geological Society of London 2002.