ORDOVICIAN TRILOBITES FROM THE TOURMAKEADY LIMESTONE 
sentatives of the illaenid-cheirurid biofacies (///aenus, Glaphurus, 
Ischyrotoma, Kawina, etc.) are represented quite widely in contem- 
porary deposits (e.g., Ingham ef al., 1986). The pelagic trilobites 
Opipeuter and Oopsites are globally distributed at low palaeolatitudes 
(Cocks & Fortey 1990: fig. 2). 
The recognition of phylogenetic affinities of some of these taxa 
with much older forms, for example, the glaphurids with the 
Cambrian raymondinids and the isocolids with the catillicephalids, 
serves to remove any notion of novelty from these faunal elements. 
This leaves Phaseolops as the sole example of a precocious genus of 
higher taxa, and we can match such single examples in several other 
faunas at the base of the Middle Ordovician, which was an important 
watershed for Laurentian faunas in general (Droser ef al., 1996). 
Thus, when considered critically, the trilobites of the Tourmakeady 
do not provide support for the notion of a special generative role for 
this particular insular fauna. However, such islands may have pro- 
vided important refugia during times of global regression. 
The closest biogeographic comparisons of the whole fauna are 
with illaenid-cheirurid biofacies faunas on the Laurentian 
palaeocontinent, and there is no reason to doubt that the South Mayo 
trough lay on the equatorial side of the Iapetus Ocean (Williams & 
Curry 1985). Similar conclusions were reached for the contempo- 
rary Dounans Limestone fauna (bathyurid biofacies) on the northern 
edge of the Midland Valley of Scotland (Ingham ef al., 1986). 
Amongst the Tourmakeady fauna there are several genera which are 
not confined to Laurentia, particularly those extending into Baltica: 
Agerina, Celmus, Niobe, Illaenus, Nileus and Geragnostus. Only 
two genera, Protostygina and Geragnostus, have Middle Ordovician 
_ Gondwanan records. The Baltic connections are of interest because 
_ the brachiopods (Williams & Curry 1985: 188) are stated to be quite 
_ different from those of that palaeocontinent. 
SYSTEMATIC DESCRIPTIONS 
_ REPOSITORY. Figured specimens are housed in the Sedgwick Mu- 
| seum, Cambridge (prefix SM) and the Natural History Museum, 
London (prefix It.). 
| Family METAGNOSTIDAE Jaekel, 1909 
Genus GERAGNOSTUS Howell, 1935 
TYPE SPECIES. Agnostus Sidenbladhi Linnarsson, 1869, from the 
| Tremadoc of Sweden; by original designation. 
_ Geragnostus clusus Whittington, 1963 
Pl. 1, figs 10, 13-16; PI. 2, fig. 9 
| 
_ MATERIAL. Assigned specimens It 25944-25948, 25961. 
| 1963 Geragnostus clusus Whittington: 28-30, pl. 1, figs 1-17, text- 
| fig. 3. 
DISCUSSION. Whittington (1963) fully described this species from 
i undistorted material from the Lower Head boulder. Because the 
| terminal piece of the pygidium is longer than the postaxial field this 
) species conforms to Geragnostus as opposed to Arthrorhachis in the 
_ Tevision of Fortey (1980). Cephalic shields of Geragnostus are all 
‘| somewhat similar. Another closely similar form was figured by 
) Ahlberg (1992) as Geragnostus sp. B, from the Lanna Limestone 
: | (Volkhov Stage) of Sweden. The pygidium is the more distinctive 
_, part of the exoskeleton. The best Irish pygidium (PI. 1, fig. 16) 
| 
| 
; 
81 
compares closely with that of the holotype (Whittington 1963: pl. 1, 
figs 1-6) in having the axis two-thirds the total pygidial length, while 
the terminal piece occupies two-thirds of the axial length. On the 
holotype, the terminal piece tapers backwards from the preceding 
part of the axis; however, on another of Whittington’s specimens 
(1963: pl. 1, fig. 14) the terminal piece is slightly wider (tr.) than the 
second axial ring, as it is on the Tourmakeady specimen. Whittington 
(1965) commented on how closely similar G. clusus was to G. 
longicollis (Raymond, 1925) from the middle Table Head Forma- 
tion. Possibly the only convincing difference was a more pronounced 
angulation in the cephalic outline of the former; this is not visible on 
the specimen of PI. 1, fig. 15, which has been tectonically elongated, 
but is apparent on a less distorted specimen (Pl. 1, fig. 13). 
Geragnostus clusus seems to be the best name to apply to the Irish 
specimens. 
Genus DIVIDUAGNOSTUS Koroleva, 1982 
TYPE SPECIES. 
designation. 
Dividuagnostus minus Koroleva, 1982; by original 
Dividuagnostus sp indet. od ta ie ote We 
MATERIAL. Incomplete cephalic shield, It 25951. 
DIscuSSsION. A small headshield shows a transglabellar furrow 
with the form of a shallow inverted ‘v. This is typical of 
Dividuagnostus according to the revision of Zhou Zhi-yi (1987). 
The unusually large, triangular occipital lobes are also typical of this 
genus. Of species of Arenig age, the Irish specimen differs from D. 
whitlandensis (Fortey & Owens, 1987) in its wide cephalic border, 
but appears to be like D. scoltonensis (Whittard, 1966; see also 
Fortey & Owens 1987, fig. 17b-c) inthe same feature. Dividuagnostus 
scoltonensis ranges through the Fennian Stage in South Wales. 
Without an associated pygidium the determination must be tentative. 
Family NILEIDAE Angelin, 1854 
Genus NILEUS Dalman, 1827 
TYPE SPECIES. Asaphus (Nileus) armadillo Dalman, 1827, from 
the Arenig of Husbyfjol, Skarpasen, Sweden; by monotypy. 
Nileus sp. Pl. 1, figs 1-6 
MATERIAL. Assigned specimens It. 25935-25940, SM A10417. 
DISCUSSION. The Tourmakeady species is similar to Nileus affinis 
from western Newfoundland (Whittington 1963, 1965). The speci- 
mens share similar posterior dimensions of the cephalic doublure 
(compare PI. 1, fig. 3c with Whittington 1965: pl. 30, fig. 5); eyes of 
the same relative size and oblique inclination; hypostomes with 
narrow lateral rims (cf. Pl. 1, fig. 4 with Whittington 1965: pl. 31, fig. 
6); pygidia with nearly identical shape and restriction of subdued 
dorsal terrace lines to the lateral margins; and pygidial doublure of 
nearly equal size with similar median embayment (cf. Pl. 1, fig. Sc 
with Whittington 1965: pl. 31, fig. 10). 
The question of conspecificity cannot be evaluated with confi- 
dence given the few Irish specimens. The single cranidium recovered 
(Pl. 1, fig. la) is very narrow, but the specimen is a juvenile. 
Similarly, the Tourmakeady hypostome (Pl. 1, fig. 4) is nearly 
subquadrate, in contrast with the wider form known inN. affinis. The 
Irish specimen is small however, and lateral expansion with maturity 
can be observed in the ontogeny of various nileid hypostomes (e.g., 
