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.
References
AI.DRIDGF. R. J. & MABILLARD, J. E. 1981. Local vari-
ations in the distribution of Silurian conodonts: an
example from the amorphognathoides interval of
the Welsh Basin. In: NFALE, J. W. & BRASIeR. M.
D. (eds) Microfossils from Recent and Fossil Shelf
Seas Ellis Horwood, Chichester. 10-18.
ARMSTRONG. H. A. 1990. Conodonts from the Lower
Silurian of the North Greenland carbonate plat-
form. Bulletin Gronlands Geologiske Under-
s gelse. 159
ARMSTRONG. H. A. 1995. High-resolution bio-
stratigraphy (conodonts and graptolites) of the
Upper Ordovician and Lower Silurian - a
chronometric framework in which to evaluate the
causes of the mass extinction. Modern Geologv,
- 1-28.
ARMSTRONG. H. A. 1997. Conodonts from the Shinnel
Formation. Tweeddale Member (middle Ordo-
vician). Southern Uplands. Scotland. Palaeontol-
ogy 40.763-799.
ARMSTRONG. H. A. 2000. Conodont micropalaeon-
tology of mid-Ordovician aged limestone clasts
from LORS conglomerates. Lanark and Strath-
more basins. Midland Valley. Scotland. Journal of
Micropalaeontologv. 19. 45-59.
ARMSTRONG. H. A. & COF. A. L. 1997. Deep sea sedi-
ments record the geophysiology of the end
Ordovician glaciation. Journal of the Geological
Society London, 154. 929-934.
ARMSTRONG. H. A. & OWEN. A. W. 2001. Terrane
evolution of the paratectonic Caledonides of
northern Britain. Journal of the Geological
Society, London. 158.475-486.
BARNES. C. R.. REXROAD. C. B. & MILLER. J. F. 1973.
Lower Paleozoic provincialism. In: RHODES. F. H.
T. (ed.) Conodont Zoology. Geological Society of
America, Special Papers. 141. 156-190.
BAUER. J. A. 1994. Conodonts from the Bromide
Formation (Middle Ordovician. south-central
Oklahoma. Journal of Paleontology, 68. 358-376.
