86 
odontopleurid subfamilies, resembling Ceratocephalinae Richter & 
Richter, 1925, in pygidial details and Selenopeltinae in cephalic 
morphology. If Ramsk6ld’s (1991: 166-168) character analysis is 
correct, however (and it is followed herein), Ceratocephalina 
ramskoeldi can be assigned with confidence as possibly the most 
primitive representative of Selenopeltinae. The following discussion 
is based primarily on Ramsk6ld’s work. 
The pygidium of Ceratocephalina ramskoeldi differs from those 
of all other assigned selenopeltines in the marginal position of its 
lateral border spines. In typical selenopeltines, these spines are 
supramarginal. The median area of the posterior margin is also 
distinctly triangular and drawn posteriorly, nearly into a median 
spine. In these features, the sclerite agrees with the pattern of three 
marginal spines characteristic of Ceratocephalinae. Ramsk6ld (1991) 
interpreted this pattern as plesiomorphic and probably general to a 
clade encompassing both subfamilies. Hence, the median area of the 
C. ramskoeldi pygidium possibly reflects a transition from a primi- 
tive median-spined condition to the arcuate condition with a fringe 
of small accessory spines seen in Selenopeltinae. The lateral spines 
are similarly in a plesiomorphic marginal position, and have not yet 
begun to migrate dorsally to the position apomorphic for the bulk of 
Selenopeltinae. The cephalic morphology of C. ramskoeldi 1s much 
less ambiguous. The presence of a very short (sag., exsag.) anterior 
border lacking spines or tubercles and of a slender genal spine base 
that overhangs and is not confluent with the posterior and exterior 
borders, but rather which runs onto the interior border on the field, 
are both basal selenopeltine apomorphies. In addition, the species 
bears none of the apomorphies of Ceratocephalinae (e.g., the third 
glabellar spine pair is not set atop an independently inflated swelling 
of the glabella). 
Ceratocephalina ramskoeldi is most similar to C. trispineus 
(Young, 1973) from the Ibexian (Zone H) of Utah. C. ramskoeldi 
differs from the Utah species in the possession of denser, finer 
tuberculate sculpture; a shorter (sag., exsag.) anterior border; more 
prominently inflated L1 and L2; more deeply incised SO; a relatively 
shorter median occipital spine; broader librigenal field; and genal 
spine with a narrower base. As noted by Ramskold (1991), the 
pygidia assigned by Young (1973) to C. trispineus are not those of an 
odontopleurid; they belong in fact to a pilekiid. 
The single fragmentary cranidium described by Fortey & Droser 
(1996: 98, fig. 7.8) as Diacanthaspis sp. is also a primitive 
Ceratocephalina. It is more similar to C. trispineus than to C. 
ramskoeldi in the prominence of its anterior cranidial border, but is 
distinguished from both by its much more robust dorsal tuberculation. 
Family ISOCOLIDAE Angelin, 1854 
DISCUSSION. As presently conceived, Isocolidae is restricted to 
the Middle and Upper Ordovician, and the Tourmakeady species, 
described below, is the oldest known member of the family. How- 
ever, Fortey (1983) and Ingham (in Ingham et al. 1986) have 
PLATE 4 
J.M. ADRAIN AND R.A. FORTEY 
established the Ibexian presence of a group of genera similar to 
Sunwaptan forms currently assigned to the family Catillicephalidae. 
This latter record is herein extended to the basal Whiterockian (see 
below). These catillicephalids have so much in common with the 
isocolids that there is a strong likelihood they are phylogenetically 
related. The familial distinction between them may prove artificial. 
However, Catillicephalidae is itself in need of phylogenetic revision, 
and it remains to be established whether the Sunwaptan/Ibexian 
genera are related to several pre-Sunwaptan genera, including Catilli- 
cephala itself. Pending a comprehensive cladistic review of the 
problems, we follow traditional usage and retain separate families. 
Genus ISOCOLUS Angelin, 1854 
TYPE SPECIES. Jsocolus Sj6greni Angelin, 1854, from the Boda 
Limestone (Ashgill) of Dalarna, Sweden; by monotypy. 
Tsocolus sp. nov. A 
Pl. 3, figs 1-3, 5; Pl. 4, figs 7, 8, 10, 11 
MATERIAL. Assigned specimens It. 25962-25965, 25983-25986. 
DESCRIPTION. Cranidium with sagittal length about 75% of maxi- 
mum width across posterior border and subequal to width across 
palpebral lobes; anterior margin of anterior border with gentle, even 
anterior convexity; anterior border very short (sag., exsag.), anterior 
border furrow sharply incised but extremely short (sag., exsag.); 
glabella widest anteriorly, maximum width about 80% of sagittal 
length excluding LO; preglabellar field very short, expanded later- 
ally into narrow frontal area with considerable dorsal convexity; 
glabella laterally concave, axial furrows bowed inwards; S1 and S2 
deeply incised and slot-like, deeper and longer (exsag.) proximally 
than distally near contact with axial furrow, lengthened into pitlike 
form at proximal end, declined posteriorly at about 30 degrees from 
transverse plane; S3 faint and transversely aligned, contacting axial 
furrow just behind eye ridge; anteromedian lobe of glabella slightly 
swollen and anteriorly expanded; L1 and L2 trapezoidal in outline; 
dorsal glabellar sculpture lacking; SO much shorter (exsag.) than S1 
and 82, evenly incised both medially and laterally, with very shallow 
“W’ shape, anteriorly convex medially; LO longest medially, short- 
ened significantly behind L1, transversely convex, but sagittally 
nearly flat-topped, median node very faint; narrow area of fixigena 
present opposite contact of axial and preglabellar furrow and in front 
of eye ridge; eye ridge faint, slightly declined posteriorly, relatively 
broad (tr.); palpebral lobe tiny; anterior sections of facial sutures 
very short (exsag.), subparallel in front of palpebral lobes, converg- 
ing anteriorly; posterior sections of facial sutures with strong initial 
posterior divergence behind palpebral lobes, then more gentle diver- 
gence posteriorly; posterior fixigena with considerable area, moderate 
dorsal inflation, and lacking dorsal sculpture; posterior border fur- | 
row nearly straight, length (exsag.) similar along most of width, | 
lengthening slightly abaxially; posterior border very short proxi- © 
mally, lengthening greatly distal to fulcrum. Pygidium incompletely 
Figs 1-6 Catillicephalid gen. et sp. noy. la-e, It. 25977, cranidium and right librigena, oblique, right lateral, and left lateral views, x15. 2a-d, It. 25978, 
cranidium, dorsal, posterodorsal, oblique, and ventral views, x15. 3a-b, It. 25979, cranidium, dorsolateral and oblique views, x15. 4a-b, It. 25980, 
cranidium, anterodorsal, left lateral, and dorsal views, x15. 5a-b, It. 25981, cranidium, dorsal and right lateral views, x15. 6a-b, It. 25982, cranidium, 
dorsal and anterior views, x15. 
Figs 7, 8, 10, 11 
Tsocolus sp. nov. A. 7a-e, It. 25983, pygidium, dorsal, ventral, and left lateral views, x15. 8, It. 25984, cranidium, dorsal view, x15. 10, 
It. 25985, cranidium, dorsal view, x15. 11, It. 25986, articulated exoskeleton (destroyed after photography), dorsal view of cephalon, x15. 
Figs 9,12-15 Ceratocephalina ramskoeldi sp. nov. 9a-b, It. 25987, holotype, cranidium, dorsal and oblique views, x15. 12a-c, It. 25988, cranidium, 
dorsal, ventral, and anterior views, x15. 13a-c, It. 25989, cranidium, left lateral, anterior, and dorsal views, x15. 14, It. 25990, left librigena, external 
view, x12. 15, It. 25973, pygidium, dorsal view, x15 (see also Pl. 3, fig. 12). 
