92 H. A. ARMSTRONG & A. W. OWENFig. 6. Generic biofacies for each Ordovician time
slice. The assumption was made that a genus was
more likely stay within the same biofacies in the
subsequent time slice. Anomalous occurrences in the
initial coding were corrected by a posteriori revision
of the seriated matrix,? is retained for occurrences
where this could not be achieved.
reduced into the Early Silurian. This increase
may be a sampling artefact. The Ordovician
decline in diversity continued for longer in the
deeper-water SB3 and OB2 biofacies.
Diversity patterns: Avalonia
Generic diversity patterns are illustrated in
Figure 8c, d. Higher diversity is found in SB2 and
OB1. in the inner shelf and upper water column,
as found in the Laurentia data. Diversity
declined in all biofacies during the mid-Ashgill.
coincident with the northward drift of Avalonia
in tropical latitudes (Fig. 9). Low diversity
continued into the early Silurian in oceanic
biofacies whilst diversity increased in shelf
faunas. Shelf faunas in the Early Silurian
contained a mixture of incumbents (Icriodella
and Ozarkodina), recruits (including Oulodus}.
which had an evolutionary origin in the
Laurentian inner shelf, plus Kockelella and
Distomodus, new genera with cryptic ancestry.
Diversity trajectories in shelf and oceanic bio-
facies are parallel and the decline in diversity
continued for longer in the oceanic biofacies.
implying the causal mechanism persisted for
longer in deeper water and affected taxa in all
habitats.Diversity patterns: Baltica
Generic diversity (Fig. 8e, f) is low in the Baltic
succession, comparable to that in Avalonia, and
the paucity of data makes interpretation difficult.
SB2 and SB3 biofacies show slight rises in diver-
sity into the early Ashgill, OB1 remains static
and OB2 shows a slight decline. Llandovery
diversity is higher than in the early Ashgill in
shelf biofacies and is lower in oceanic biofacies.
Diversity patterns in Baltica most closely
compare with those from Avalonia.Diversity in the early Silurian
Our analysis is restricted to the celloni Biozone
of Avalonia (Welsh Borderland data from
Aldridge & Mabillard 1981) and a comparative
section from northern Laurentia (North
Greenland data from Armstrong 1990; though
not part of the lapetus margin at this time these
data are taken as typical for the Laurentian
shelf). A full review of Silurian successions is
beyond the scope of this paper and the Iapetus
Ocean was virtually closed by the Early Silurian
(Armstrong & Owen 2001). Oceanic and shelf
biofacies identical to those in the Ordovician
can be identified in this limited dataset (Fig. 5).
Oceanic biofacies comprise predominantly
coniform taxa. Pterospathodus and Pseudolon-
chodina are tentatively assigned to OB3 as both
range across the Greenland shelf. Pseudolon-
chodina is not present in the Welsh dataset.
Ozarkodina, Oulodus and Distomodus also