133845.pdf

108 JOSEPH P. BOTTING

(Lockley & Antia 1980; Gabbott 1999; Botting

& Thomas 1999). For example, A. micula

benthie populations would have been less

strongly affected than predicted from the

benthie bloom, because of population mixing

under pseudoplanktic conditions. In contrast,

'articulate' brachiopods are generally facies-

restricted, and appear among predictable faunal

assemblages. It is significant that Sepkoski

(1995) found much greater net diversification

among 'articulates' than 'inarticulates,' since

many linguloids were capable of burrowing,

while 'articulates' were almost exclusively

sessile epibenthos.

Published global diversity curves differenti-

ated into taxonomic groups (Sepkoski 1995)

appear to record patterns that are consistent

with these predictions (Fig. 3). In general,

sessile taxa (brachiopods, bryozoans, corals)

show the greatest diversification, with a highly

consistent pattern between phyla. Bivalves and

gastropods first achieved significant diversity

during the Ordovician, but then show very

rapid diversification, while trilobites fluctuated

irregularly (confirmed by Adrain & Westrop

2000). Graptolites show little overall diversity

increase, although with significant fluctuations

(Cooper 1999); a similar pattern is observed

among acritarchs and chitinozoans (e.g. Kaljo et

al 1995). Among the groups predicted to show

greatest diversification are the ostracodes,

based on their ecological response. Swain

(1996) reported extremely elevated ostracode

turnover, with substantial overall diversifica-

tion, in a bentonite-rich Ordovician sequence of

North America. He tentatively attributed this to

nutrient input but, as discussed above, a

plausible precise mechanism is not obvious.

Welsh Basin beyrichiocope ostracode

diversity

A summary of the Ordovician diversity of pre-

Ashgill beyrichiocope ostracodes in the Welsh

Basin follows; other 'ostracode' groups are not

included, because of uncertainty over mutual

relationships. The area is subdivided into three

subregions (SW Wales, Mid-Wales and Shrop-

shire, North Wales and northern England) and

correlated with major volcanic episodes within

each area. An estimate of bentonite frequency is

included, based on limited descriptions in

stratigraphic literature and personal obser-

vation, although in many cases thin beds are not

obvious in outcrop, and have not been men-

tioned in publications. Where bentonites appear

frequently within an interval lacking a local

volcanic source, they can usually be correlated

with episodes of major volcanism in one of the

other subregions. In addition, a schematic log of

primary lithology is included for each region,

although extreme local variation may occur;

apparent stratification (benthie anoxia) is

denoted by shading (Figs 4, 5, 6).

Ostracode data were obtained primarily from

Jones (1986-1987), with additional Arenig in-

formation from Siveter (in press), and personal

observations. The rarity of pre-Llanvirn taxa is

apparently real, although the simple morphol-

ogy of described examples may disguise greater

taxonomic variation than is practically identifi-

able. However, the same underestimate should

operate in later unornamented taxa, suggesting

that the observed morphological diversification

accurately reflects a real pattern. Data regarding

the stratigraphic position of volcanic episodes,

and dominant lithologies were obtained from

Smith & George (1961), George (1970), Fortey

et al (2000) and Rushton et al. (2000) as primary

sources, supplemented by additional local refer-

ences (e.g. Stamp & Wooldridge 1923; Davies

1933; Jones & Pugh 1941, 1949; Diggens &

Romano 1968; Earp & Haines 1971; Lynas 1983:

Cave & Rushton 1996; Davies et al. 1997).

In SW Wales (Fig. 4), massive diversification

over a brief interval (Llanvirn) is correlated with

black shales and extensive volcanism of the

Fishguard, St David's and Strumble Head

regions. Arenig volcanism in aerobic sand facies

produced little response, although a few

morphologically conservative taxa appeared at

this time. The low diversity in the upper Caradoc

may be partly an artefact; diverse Ashgill faunas

are known to exist, but are yet to be studied

intensively (Jones 1987). However, although

Caradoc ostracodes above the Aurelucian are

almost certainly present (some taxa, such as

Duringia triformosa, reappear in Shropshire

during the Burrellian), they are apparently very

rare; no identifiable specimens from the area

have been described.

In Mid-Wales and Shropshire (Fig. 5), specia-

tion occurred over the Llanvirn and Caradoc,

corresponding to penecontemporaneous vol-

canic centres and restricted oxygenation. A few

species were immigrants from SW Wales, but

most were endemic.

There are limited data for North Wales and

northern England (Fig. 6) but the majority of

species correspond to the Borrowdale volcanic

episode (Caradoc); a high proportion was

immigrant. Additional faunas are predicted to

be discovered from the Llanvirn and lower

Caradoc.

Overall, the patterns appear to show