EUCONODONT DIVERSITY CHANGES 93
Fig. 7. Biofacies architecture diagrams for Laurentia and Avalonia at each of the Ordovician time slices. See
Figure 1 for key.
range across the Welsh shelf but Aldridge &
Mabillard (1981) recognized these taxa had
their acmes in nearshore settings and declined
rapidly in abundance offshore. This pattern
suggests that these genera included wide-
ranging, nektobenthic, ecological generalists
that preferred nearshore habitats. Icriodella is
restricted to SB2 in the Silurian and OB3 in the
Ordovician. This suggests either a change from
a pelagic mode of life, or that Silurian and
Ordovician Icriodella are not congeneric.
Silurian nektobenthic genera appear to have
much broader ecological ranges than their
Ordovician counterparts.
Interpretation
Biofacies architecture
In the modern ocean planktonic and pelagic
species inhabit depth-related water masses
defined by oxygen content, salinity, temperature
and nutrient availability, comparable to bio-
facies in continental shelf settings, though often
of much larger geographical extent. Water mass
character is partly set by the shape of the basin,
the direction of surface winds and patterns of
evaporation and precipitation over the oceans
(Emery & Meincke 1986). In general terms
temperature and salinity fall with depth whilst
nutrients increase (see review in Norris 2000).
Specific water masses and their included fauna
can move both vertically and horizontally
depending upon oceanography and climatic
factors, and are divided by physiochemical
surfaces along which species or prey congregate
for growth or reproduction (Norris 2000).
Vertical movements of water masses occur as a
result of changes in temperature and density and
are most marked in thermally stratified oceans
in continental margin upwelling zones. The
structure of the upwelling zone off the SW
