EUCONODONT DIVERSITY CHANGES 95
Fig. 9. Palaeolatitudes for East Avalonia (based on
MacNiocail 2000). These data indicate a closure rate
of c.3 cm a-1. Timescale from Tucker & McKerrow
(1995). Abbreviations: BR, Browgill Redbeds; BV,
Builth Volcanics; MC, Mill Cove Redbeds; MV,
Mendips Volcanics; NB, North Builth; Sv, Stapley
Volcanics; TL, Tortworth Lavas; Trv, Treffgarne
Volcanics; TV, Tramore Volcanics.
Fig. 10. Water mass structure off the SW African
shelf based upon percentage oxygen content of the
water (based on Demaison & Moore 1980).
Upwelling water is generated by onshore to offshore
winds skimming off warm surface water and thus
allowing cold, nutrient-rich water to well up from
intermediate depths. A similar structure can be found
off the Peruvian margin associated with the
Humboldt Current (Demaison & Moore 1980). We
hypothesize that oceanic euconodont oceanic
biofacies reflect water masses and have superimposed
these biofacies onto the water mass structure of the
SW African shelf. The pattern generated is similar to
that found in the biofacies architecture diagram for
Avalonia (Llanvirn-Ashgill) and Laurentia
(ordovicicus Biozone) in Figure 7.
Formation of mid-Wales (Cave 1965; Smith
1999) and thin phosphorite conglomerate at
the base of the Venst0p Formation in the
Oslo-Asker district (Williams & Bruton 1983;
Owen et al 1990) indicates that the water along
the southern margin of the lapetus Ocean was
rich in nutrients and supports the upwelling
hypothesis.
Upwelling persisted along the southern
margin of the lapetus Ocean at least from the
late Caradoc-early Ashgill and perhaps as early
as the uppermost Tremadoc or early Arenig
(Lindstrom & Vortisch 1983) Upwelling appears
not to have been initiated at the low-latitude
Laurentian margin until the Rawtheyan, just
predating the glacial maximum and coincident
with the general upward movement of oceanic
biofacies across the lapetus Ocean.
Regional diversity trajectories
A major reduction in diversity in Laurentian
biofacies correlates with the early Ashgill onset
of ocean cooling along the Laurentian margin
(Armstrong & Coe 1997) and represents the
extinction of taxa adapted to warm, tropical
conditions. A second decline in diversity in SB2
correlates with a return to greenhouse con-
ditions, and extinctions in taxa interpreted as
being adapted to cooler 'glacial' conditions (see
also Brenchley 1988; Brenchley et al 1995a).
The major decline in diversity in Avalonian
biofacies was coincident with the drift of Ava-
lonia into warmer (but cooling) tropical water
(Figs 8, 9). This suggests extinction of taxa
adapted to cold water conditions, introduced
into cooling but none-the-less warmer water of
the tropics.
Baltica moved from intermediate latitudes to
the tropics from the Arenig to the Ashgill and
sutured to Avalonia during the late Ordovican
(see reviews in Cocks & Fortey 1998; Cocks
2001). The Late Ordovician euconodont diver-
sity trajectories (Figs 8e-f) are more closely
similar to those of Avalonia, suggesting similar
underlying controls. The onset of upwelling and
close proximity of Avalonia and Baltica in the
late Caradoc-early Ashgill corresponds with a
rise in diversity largely the consequence of
migrant shelf taxa from Avalonia.
Conclusions
Constrained seriation of presence-absence
matrices provides a method of qualitatively
defining generic associations or biofacies and
