133845.pdf

EUCONODONT DIVERSITY CHANGES 91

Fig. 5. (A) Constrained sedation matrices and (B) biofacies architecture diagram for celloni Biozone
euconodont faunas of the Welsh Borderland (Avalonia) and North Greenland (northern Laurentia) shelves.
Welsh data from Aldridge & Mabillard (1981); North Greenland data from Armstrong (1990). Pterospathodw
and Pseudolonchodina are only tentatively assigned to OB3. Ozarkodina, Oulodus and Distomodus though
ranging across the Welsh shelf have their acmes in nearshore settings (Aldridge & Mabillard 1981. fig. 1.4).

was indifferent to bottom conditions and occu-

pied a deep-shelf to marginal setting.

Diversity patterns: Laurentia

Generic diversity patterns are illustrated in

Figure 8a, b. Higher diversity is found in SB2

and OB1, in the inner shelf and upper water

column. By the early Ashgill diversity had

declined in all biofacies and low diversity

continued into the Silurian. Inner shelf SB2

biofacies shows a slight increase in diversity in

the upper ordovicicus Biozone with the return of

Oulodus and Plegagnathus but was further