ORDOVICIAN TRILOBITES FROM THE TOURMAKEADY LIMESTONE 
density to that of first axial ring; doublure forming narrow, convex 
ventral rim, with fine tuberculate sculpture; anterior doublural mar- 
gin ‘v’-shaped, with median posterior embayment. 
DISCUSSION. This species is the second most common in our 
collections and, like other common taxa, displays several 
morphotypes due to tectonic distortion. In Mayopyge zapata the 
distortion seems to be nearly bimodal. Overwhelmingly common 
among the cranidia is the type illustrated on Pl. 8, which displays a 
conical dorsal swelling of the glabella in front of the occipital ring. 
Whether this is a biological structure or a the result of distortion and 
enhancement of an original convexity peak is not known. The 
structure is so pervasive that it seems likely that at least some 
original inflation was present. The second cranidial type is that 
illustrated on Pl. 9, figs 1, 3a, 4, 5a, in which this projection is 
entirely absent and the glabella roundly and evenly inflated. An 
undistorted calcareous crackout cranidium (PI. 7, fig. 6) shows an 
inflated convexity peak at the rear of the glabella. It is conceivable 
that different vectors and amounts of distortion could produce either 
of the silicified morphotypes from such an original morphology. 
Pygidial types do not show as strong a disjunct occurrence as do 
the cranidia, but are even more morphologically discrete. The 
slightly more common form (PI. 10, figs 6-10, 12, 13, 17) includes 
the holotype. It is relatively long versus wide (sagittal length 45— 
50% of anterior width), has long, posteriorly directed spines, a 
second ring furrow that is typically reduced to two lateral pits, anda 
rather sharp doublural embayment (PI. 10, figs 6b, 8b). The second, 
slightly less common, form (PI. 10, figs 11, 14-16) is much shorter 
(sagittal length 27—33% of anterior width), has more laterally splayed 
spines, with the median pairs apparently shorter, a second ring 
furrow that is a medially continuous slot (Pl. 10, figs 11a, 14a, 15a), 
and a relatively shallow doublural embayment (PI. 10, figs 11b, 
15b). 
Given the apparent presence of two cranidial and pygidial 
morphotypes, there are three possibilities. First, the variation may be 
genuine and reflect sexual dimorphism. Second, the variation may 
be genuine and reflect the presence of two closely related species. 
And third, the dimorphism may be artefactual, and a result of the 
tectonic distortion affecting the entire fauna. Sexual dimorphism is 
improbable; it remains unproven in the trilobites as a whole (Adrain 
& Kloc 1997; Hughes & Fortey 1996; Ramsk6ld & Chatterton 1991; 
Ramsk6old & Werdelin 1991). If sexual dimorphism were the case it 
might be expected that other acathoparyphines would also exhibit it, 
which species known from abundant silicified material manifestly 
do not. It cannot be entirely disproved that there are two, closely 
related species: since cheirurid pygidia are distinctive the name is 
attached to the best specimen of the commonest morph. Overall, we 
consider that bimodal tectonic distortion is responsible for the 
Variation, since ‘long’ and ‘short’ forms have also been recognised in 
| Illaenus weaveri and Celmus michaelmus, where the cause is de- 
| monstrably tectonic. However, the pygidial differences remain a 
cause for concern, as the different ring furrows are not readily 
accounted for by distortion alone. 
PLATE 10 
99 
Subfamily CHETRURINAE Hawle & Corda, 1847 
Genus CERAURINELLA Cooper, 1953 
TYPE SPECIES. Ceraurinella typa Cooper, 1953, from the Edinburg 
Formation (Mohawkian), Virginia, U.S.A.; by original designation. 
Ceraurinella sp. Pl. 11, figs 1-9 
MATERIAL. Assigned specimens It. 26072—26080. 
DISCUSSION. The fragmentary nature of the available material 
does not allow a determination of the species, and several important 
morphological differentia (e.g., eye position) are not fully pre- 
served. Nevertheless, the Tourmakeady species is clearly a primitive 
member of the Ceraurinella group (including Sycophantia Fortey, 
1980). The Irish species differs from the Spitsbergen plesiomorph S. 
seminosa Fortey, 1980, in its much shorter anterior cranidial border 
(in which it resembles later and presumably more advanced species) 
and in the complete loss of the small median pygidial spine retained 
in S. seminosa. Both the available librigenae and fixigenal frag- 
ments, however, indicate that the Irish species retained very wide 
posterior fixigenae, similar to S. seminosa. Of other early species, 
the Tourmakeady taxon resembles C. polydorus (Billings, 1865) 
(see Whittington 1965: pl. 60) in the length of the anterior cranidial 
border, but differs in possessing less inflated lateral glabellar lobes 
(compare Pl. 11, fig. 1 with Whittington 1965: pl. 60, fig. 4), a 
broader posterior fixigena, and a narrower pygidium lacking an 
independently defined median spine. 
Family ENCRINURIDAE Angelin, 1854 
Subfamily CYBELINAE Holliday, 1942 
Cybeline indet. (not figured) 
MATERIAL. Assigned specimen It. 26201. 
DISCUSSION. A single very poorly preserved cybeline cranidium 
has been recovered from the silicified residues. It is obscured by 
silicified debris, and identifiable only to subfamily level. It is noted 
here for completeness. 
Family LECANOPYGIDAE Lochman, 1953 
Genus BENTHAMASPIS Poulsen, 1946 
TYPE SPECIES. Benthamaspis problematica Poulsen, 1946, Ibexian, 
Ellesmere Island, Canadian Arctic; by monotypy. 
Benthamaspis aff. B. diminutiva Hintze, 1953 
Pl. 11, figs 17-19, 21-23 
MATERIAL. Assigned specimens It. 26089-26094. 
Figs 1-17 Mayopyge zapata gen. et sp. nov. 1a-b, It. 26055, thoracic segment, dorsal and anterior views, x7.5. 2a-b, It. 26056, thoracic segment, dorsal 
and anterior views, x7.5. 3a-b, It. 26057, thoracic segment, dorsal and anterior views, x7.5. 4a-b, It. 26058, thoracic segment, dorsal and anterior views, 
x7.5. 5a-b, It. 26060, thoracic segment, oblique and dorsal views, x7.5. 6a-c, It. 26059, holotype, pygidium, right lateral, dorsal, and ventral views, x7.5. 
7, It. 26061, pygidium, dorsal view, x10. 8a-b, It. 26064, pygidium, dorsal and ventral views, x7.5. 9, It. 26062, pygidium, dorsal view, x7.5. 10, It. 
26063, pygidium, dorsal view, x10. 11a-b, It. 26066, pygidium, dorsal and ventral views, x7.5. 12, It. 26065, pygidium, dorsal view, x10. 13, It. 26067, 
pygidium, dorsal view, x10. 14a-b, It. 26068, pygidium, dorsal and posterior views, x7.5. 15a-b, It. 26069, pygidium, dorsal and ventral views, x7.5. 16, 
It. 26070, pygidium, dorsal view, x7.5. 17, It. 26071, pygidium, dorsal view, x10. 
