Bull. nat. Hist. Mus. Lond. (Geol.) 53(2): 79-115 
Issued 27 November 1997 
Ordovician trilobites from the Tourmakeady_ 
Limestone, western Ireland | 
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q HE NA AAL 
i NISTORY MUSEUM | j 
JONATHAN M. ADRAIN AND RICHARD A. FORTEY | 
Department of Palaeontology, Natural History Museum, Cromwell Road, London SW7 5BD iP PA LAEON TOLOGY 
SYNOPSIS. 
PRESENTED 
Trilobites of the Tourmakeady Limestone, County Mayo, western Ireland, comprise 23 species and genera. This 
fauna is an early example of the Illaenid-Cheirurid biofacies, being dominated by a few species of the families Illaenidae, 
Cheiruridae and Celmidae. It is basal Whiterockian (upper Arenig) in age. Several of the trilobites are early representatives of 
clades which achieved subsequent prominence. All but one of the species are represented by silicified material, and many are 
supplemented by crackout specimens. Four trilobite species have been previously named: Kawina divergens, Niobe ornata, 
Oopsites hibernicus and Illaenus weaveri, and are revised herein. One genus, Mayopyge (type species M. zapata sp. nov.), is new, 
as are the species Phaseolops ceryx, Protostygina coronula, Glaphurus crinitus, Dimeropyge? ericina, Celmus michaelmus, 
Agerina palabunda, Proscharyia platylimbata and Ceratocephalina ramskoeldi. The remaining species are described in open 
nomenclature. 
INTRODUCTION 
The occurrence of trilobites in the upper Arenig Tourmakeady 
Limestone was recognized at the time Gardiner & Reynolds (1909, 
1910) undertook the initial geological description of the Ordovician 
inliers of the Tourmakeady and Glensaul districts. Reed illustrated a 
few sclerites (in Gardiner & Reynolds 1909), and named two 
species. He later (1945) proposed a further two species, based on the 
illustrations of the 1909 work, but the scope and quality of the 
Tourmakeady faunas were not fully appreciated until the discovery 
by Williams in the 1960s that much of the material was silicified. 
Williams & Curry (1985) have subsequently described the rich 
brachiopod faunas of the unit, but the trilobites have only seen 
| limited treatment. The trilobites described here were derived from 
_ the residues sorted by Williams & Curry (1985). Fortey (1975a, 
| 1980) has illustrated some specimens for comparison with conspecific 
or closely related taxa from Spitsbergen. Fortey & Owens (1975) 
_ figured some sclerites to illustrate concepts within their new Proetida, 
_ and Fortey (1975b: 345) gave a preliminary sclerite count in a 
_ discussion of Early Ordovician trilobite communities. 
The aim of the present work is the systematic description of the 
entire Tourmakeady trilobite fauna, based on all available material. 
The fauna is of interest particularly because of its stratigraphic 
| position near the base of the Middle Ordovician. This was a time of 
significant phylogenetic turnover of trilobites, and the Tourmakeady 
assemblage contains several forms important to an understanding of 
this change. 
_ AGE, PRESERVATION, AND COMPOSITION 
_OF THE FAUNA 
The Tourmakeady Limestone occurs as blocks within bedded tuffs 
_ and grits of the ‘Shangort and Tourmakeady Beds’ of Gardiner & 
Reynolds (1909, 1910). This was mapped by Williams & Curry 
| (1985: fig. 1) as part of the Glensaul Group. Evidence of the age of 
the Tourmakeady Limestone is derived from graptolite collections 
reported from beds above and below the limestone blocks, and from 
the trilobites and brachiopods themselves. 
The trilobites from the white crackout limestones are invariably 
| © The Natural History Museum, 1997 
undistorted. Those from the blocks yielding silicified trilobites often 
show a modest degree of distortion (as do most of the brachiopods 
figured by Williams & Curry (1985)). At least one silicified sample 
yielding the type material of Glaphurus crinitus is without distor- 
tion. We do not understand the reasons for these differences. Possibly 
they are related to silicification having occurred near faults. What- 
ever the cause, there appears to be no important difference in faunal 
composition between crackout and silicified collections. Relative 
abundances of particular taxa vary significantly between the crackout 
and silicified samples, but this is certainly due to the ease with which 
smoother, larger, and less convex specimens crack out. 
Graptolites were reported by Dewey et al. (1970) from several 
localities on the western margin of Lough Mask. Three collections 
indicate that the underlying Mount Partry Group belongs to what 
they termed the ‘Didymograptus protobifidus Zone of North 
America’. Williams & Stevens (1988) doubted that the ‘protobifidus 
Zone’ could be reliably distinguished from the North American 
Didymograptus bifidus Zone, which had been claimed to overlie it. 
This interval (bifidus + protobifidus) equates with the Chewtonian 
Stage of the Australian graptolitic standard, and with the mid-part of 
the Arenig Series of the Anglo-Welsh succession. Graptolitic rocks 
of the D. bifidus Zone are associated with the shelly Zone J faunas in 
the western USA (Braithwaite 1976), and hence with the upper part 
of the Ibexian Series of the North American stratigraphic standard in 
Utah (Ross et al., 1993). 
Within the Glensaul Group Dewey et al. (1970) listed a graptolite 
(fauna 4 on their fig. 2) from below the Tourmakeady Limestone 
which they state is ‘intermediate between /sograptus caduceus var 
victoriae Harris and I. caduceus var lunatus Harris’. These varieties 
are usually regarded as subspecies of /. victoriae in current usage 
(e.g. Cooper & Lindholm 1991) and their intergradation occurs 
within the Castlemainian Stage at the junction of Cal and Ca2. A 
graptolite fauna from above the Tourmakeady limestone (Fauna 7 of 
Dewey et al. 1970: 29-30) yielded biserial graptoloids including 
Undulograptus austrodentatus; also listed is a didymograptid de- 
scribed as Didymograptus sp. 2, which seems to belong to the genus 
Xiphograptus Cooper & Fortey, 1982. The combination of 
austrodentatus group biserial graptoloids with Xiphograptus em- 
braces a short interval at the top of the Arenig series (Mitchell & 
Maletz 1995), and at the base of the Darriwillian Stage of the 
Australian scheme. 
