EUCONODONT DIVERSITY CHANGES 89
Fig. 3. Constrained seriation matrix for the Avalonian shelf (Lakesman Terrane) for each of the time slices
described in the text. Locality data from Orchard (1980). Localities have been located onshore (to the left) to
offshore with reference to Bevins et al. (1992). Shaded boxes indicate a presence; biofacies for generic
associations have been defined by blocks of generic associations and occurrences in previous time slices (see
Fig. 6 and text for explanation).
and 25% of genera recorded in Laurentia during Avalonian and Baltic margins (Fig. 7), OB3
the velicuspis Chron belong to this province, consistently lies in an intermediate position
Many of these genera are monospecific or between (or within) OB1 and OB2 from the
contained few species and are found to range velicuspis Chron onwards. OB3 moved into this
from inner to outer shelf locations indicating intermediate position in Laurentia during the
their distribution was independent of benthic Rawtheyan, just prior to the Hirnantian global
facies. eustatic sea-level fall (Brenchley et al. 1995b;
Shelf Biofacies (SB1, SB2 and SB3) contain Crowley & Baum 1991). At the same time the
nektobenthic taxa that have a distribution in the OB1-OB2 boundary across the Iapetus Ocean
matrices that terminate offshore and form dis- moved from a position coincident with SB3 to
crete blocks within each matrix. SB1 occurs only within SB2.
in the velicuspis Chron in Laurentia and includes Constrained seriation enables biofacies to be
?Bryantodina and Rhipidognathus, genera com- defined from presence-absence data and based
monly found in supratidal, or even hypersaline upon the overlapping ecological ranges of taxa.
environments (Armstrong 1990) and this bio- This allows a generalized biofacies architecture
facies is thought to be representative of local to be reconstructed but lacks the detailed litho-
hypersaline conditions. Increasing biofacies logical and palaeoecological data required for
number indicates greater distance offshore and the precise definition of the environmental
hence deeper-water setting. criteria constraining each biofacies. Such data
The relative positions of OB1, OB2 and SB2, are invariably lacking in euconodont mono-
SB3 remain stable across the Iapetus Ocean graphs and historical descriptions of collections,
through the time slices, but differences do occur Our analysis does however include Laurentian
in the relative position of OB3 and the juxta- sections utilized by Sweet & Bergstrom (1984)
position of oceanic and shelf biofacies. On the in their classic study of Late Ordovician
