80 
Thus the time of Tourmakeady Limestone deposition is bracketed 
by graptolite faunas, which indicate it must be upper Castlemainian 
to Yapeenian, and late Arenig in terms of the Anglo-Welsh standard. 
This is consistent with the trilobite evidence cited below. 
Williams & Curry (1985) stated that the Tourmakeady brachiopod 
fauna was ‘equivalent to Zone K’ of the North American shelly 
standard zonation. Zone K is a thin brachiopod coquina composed 
entirely of Hesperonomiella minor Cooper which is exposed in 
several sections in Utah and Nevada. Ross et al. (1993) no longer 
recognised it biostratigraphically, but incorporated it within the 
terminal Ibexian. Nonetheless, its position above Zone J does seem 
to be consistent with the occurrence of the Tourmakeady Limestone 
above Zone J equivalents in western Ireland. 
Trilobite faunas described below are dominated numerically by 
Illaenidae, Cheiruridae and celmids. As Fortey (1975b) originally 
pointed out, it is an illaenid-cheirurid biofacies fauna, associated 
with a limestone mound. The biogeographic affinities of the fauna 
are overwhelmingly Laurentian. Comparison is made first with two 
faunas previously known from Laurentia: the white limestone boul- 
der at Lower Head, western Newfoundland (Whittington 1963) and 
the Meiklejohn bioherm in the western USA (Ross 1972). The latter 
lies above the Ninemile Formation which yielded a Zone J trilobite 
and graptolite fauna; the Lower Head Boulder (James & Stevens 
1986: fig. 40) lies above a sparse Cal graptolite fauna. Hence, both 
occupy stratigraphically comparable positions to the Tourmakeady 
Limestone. The Lower Head fauna is most similar. A species of 
Geragnostus is in common, while close comparisons are made with 
species of Nileus, Illaenus, Glaphurus, Kawina, Isocolus, Celmus, 
and Phaseolops. The first four genera named are present also in the 
Meiklejohn Peak, known from sparser material. The fact that the 
species are almost all different in points of fine detail allows for 
some difference in age between the Tourmakeady and Cow Head, 
but the difference cannot be great. Both these previously described 
faunas have been regarded as equivalent to Zone L (Psephosthenaspis 
Zone) of the Utah/Nevada stratigraphic standard, lying at the base of 
the Middle Ordovician (1.e., base of the Whiterockian) in North 
American terms. In its type section, this is developed in bathyurid 
biofacies (Fortey & Droser 1996) and hence has a different suite of 
taxa from the Tourmakeady Limestone. Note that younger (Zone M) 
Whiterockian faunas, similar to those of the Table Head Formation, 
western Newfoundland, are known in the Tourmakeady area from 
the base of the younger Glendavock Formation (Pudsey 1984). 
There are a few similarities at species level between the Tourmakeady 
and faunas of generally deeper-water biofacies described from the 
Valhallfonna Formation, Spitsbergen. The pelagic trilobites 
Opipeuter inconnivus Fortey, 1974 and Oopsites hibernicus (Reed 
in Gardiner & Reynolds, 1909) are both known from the Olenidsletta 
Member, where their ranges extend to just below the first occurrence 
of Isograptus victoriae victoriae (Fortey 1980; Cooper & Fortey 
1982). Fortey (1975a: 31) suggested that Niobe occulta was virtually 
indistinguishable from the extremely fragmentary species from the 
Tourmakeady Limestone described by Reed (1945) as Niobe ornata. 
The species has a short range through the upper mid-part of the 
Olenidsletta Member, where it occurs immediately below beds with 
Isograptus victoriae, and immediately above the interval carrying 
Oopsites hibernicus and Opipeuter inconnivus, which was equated 
by Fortey (1980) with Zone J. Hence the Tourmakeady fauna is 
rather tightly constrained by species comparisons as lying within a 
short interval of the late Arenig between latest Chewtonian and early 
Castlemainian in graptolite terms. 
In the platform sections of Utah and Nevada there is a sequence 
boundary just above the “Zone K’ brachiopod coquina and below the 
first trilobite fauna of the Psephosthenaspis (L) Zone. If this regres- 
J.M. ADRAIN AND R.A. FORTEY 
sive event were global, it is possible that this is the time at which 
illaenid-cheirurid ‘mounds’ developed in the volcanic setting of the 
South Mayo trough, since lowstands on the platform equate with 
enhanced deposition of fringing carbonates on offshore island sites. 
The Tourmakeady fauna might ‘fit in’ to the hiatus at the base of the 
North American Middle Ordovician. In the Meiklejohn ‘reef’ there 
is a 2 m interval of birdseye limestone without macrofossils above 
Zone J and below the bioherm itself, which occupies a comparable 
stratigraphical position. 
Numbers of trilobite individuals for each species recovered from 
the entire silicified collection are given in Table 1. } 
Table 1 Number of cranidia (cr), pygidia (pyg.), and individuals (ind.) 
for all species recovered from silicified residues of the Tourmakeady 
Limestone, together with relative proportion of total number (% tot) of 
individuals (894) recovered. The single known pygidium of Niobe 
ornata (Reed, 1945), is a calcareous crackout specimen and is listed for 
completeness; no other species are known exclusively from non- 
silicified material. 
cr Pyg. ind. % tot 
Celmus michaelmus 276 4 276 30.9 
Mayopyge zapata 265 50 265 29.6 
Illaenus weaveri 109 99 109 12.2 
Geragnostus clusus 31 14 31 3.5 
Oopsites hibernicus 28 7 28 3.1 
Dimeropyge? ericina 27 0 27 3.0 
Proscharyia platylimbata 25 5 25 2.8 
Glaphurus crinitus 23 0 28) 2.6 
Agerina palabunda 22 0 22 Des 
Phaseolops ceryx 20 6 20 zp) 
Opipeuter aff. O. inconnivus 12 0 12 1.3 
Nileus sp. i 12 12 3) 
Benthamaspis aff. B. diminutiva 5 10 10 1.1 
Catillicephalid gen. noy. v7 0 a, 0.8 
Tsocolus sp. noy. A 6 2 6 0.7 
Kawina divergens 6 1 6 0.7 
Ceraurinella sp. 5 4 5 0.6 
Ceratocephalina ramskoeldi 4 4 0.4 
Protostygina coronula 2) 2 2; 0.2 
Ampyx cf. toxotis 1 1 1 0.1 
Dividuagnostus sp. indet. | 0 1 0.1 
Cybeline sp. 1 0 1 0.1 
Niobe ornata 0 1 0.1 
ORDOVICIAN INSULAR FAUNAS, 
BIOGEOGRAPHY AND TAXONOMIC 
NOVELTY 
Neuman (1984) has discussed how Ordovician brachiopod faunas | 
from ancient island sites include a mixture of endemic genera, often 
with alleged first occurrences of ‘ancestors’ of forms known from | 
younger rocks in platform successions. The implication is that such | 
islands may have been sources of speciation leading to subsequent ~ 
clades. The Ordovician South Mayo trough included a number of | 
volcanic centres, possibly as part of a marginal basin resembling the | 
Gulf of California (Ryan & Archer 1978). The fact that Williams & 
Curry (1985) reported some twelve new genera from the |~ 
Tourmakeady brachiopods invites the question whether the trilobites 
show a similar ‘insular’ trend. ; ; 
There is one endemic catillicephalid genus which we are unable to | 
name formally. The fauna includes what may be the oldest known 
proetoidean, Phaseolops, a group which became progressively more 
important through the Ordovician and later. The other typical repre- 
