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96 H. A. ARMSTRONG & A. W. OWEN

hence reconstructing biofacies architectures

without recourse to extensive recollecting.

Upper Ordovician euconodonts can be divided

into shelf and oceanic biofacies by this method.

Euconodonts have traditionally been con-

sidered to be markedly provincial during the

Late Ordovician with warm, tropical-water taxa

restricted to the American Midcontinent

Province and cooler-water taxa representative

of the North Atlantic Province. Our analysis

indicates that many genera previously con-

sidered to be representative of the North

Atlantic Province had a pelagic mode of life and

ranged widely across the Iapetus Ocean. This

interpretation challenges the accepted view

of euconodont provinciality and supports

the province biofacies model introduced by

Rasmussen (1998) for early Llanvirn eucon-

odonts of the lapetus Ocean.

Oceanic biofacies are considered to have

reflected water mass structure. The changing

vertical distribution of OB3 biofacies taxa

suggests that they were adapted to cold, nutrient-

rich, oxygen-poor upwelling water. If this is

confirmed then the southern margin of the

lapetus Ocean was characterized by upwelling

currents during the Ashgill. Upwelling was

initiated along the Laurentian margin during the

late Ashgill, coincided with an upward move-

ment of oceanic biofacies and just predates the

glacial maximum. Biofacies architecture patterns

established in the Late Ordovician appear to

have remained into the Early Silurian.

Clade diversities and trajectories in separate

biofacies and latitudes can be attributed to

different causal mechanisms. In Laurentia, the

maximum diversity decline in all biofacies

occurred in the early Ashgill and was coincident

with the onset of ocean cooling. Maximum

diversity decline in Avalonia coincided with its

northward drift into tropical latitudes. The

stability of euconodont biofacies architecture

during Late Ordovician global cooling and plate

reorganization indicates that these events had

a low palaeoecological impact, causing little

disruption of marine habitats despite a substan-

tial decrease in both alpha and beta diversity

(see also Droser et al. 1997, 2000).

Late Ordovician euconodont diversity

reflects the unique environmental conditions

specific to a region and not intrinsic properties

of the clade. The post-extinction recovery of

euconodonts was largely restricted to nekto-

benthic shelf taxa. The failure of many long-

ranging pelagic clades to recover from the

latest-Ordovician mass extinction suggests

intrinsic properties of these clades may have

suppressed their rediversification.

Our observations have implications for the

alpha taxonomy of euconodonts. Genetic evi-

dence for numerous cryptic species in morpho-

logically identical pelagic organisms is consistent

with the idea that they are much more specialized

than would be inferred from their geographical

distributions (Norris 2000). We may now have to

fundamentally revise our morphological species

concepts for pelagic euconodont clades that are

likely to have been much more diverse than

has been generally accepted in traditional

taxonomies.

The authors acknowledge funding from the NERC
(GR3/11834 to A.W.O.rGR9/02834 to H.A.A. and
A.W.O.), The comments provided by the referees
M. P. Smith and J. A. Rasmussen have improved this
paper which is a contribution to IGCP 410. "The Great
Ordovician Biodiversification Event.

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