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J. Paleont .. 72(4).1998. pp. 698-718
Copyright <C 1998. The Paleontological
Society
0022-336019810072-0698$03.00
SYSTEMATICS OF THE ACANTHOPARYPHINAE (TRILOBITA), WITH
SPECIES FROM THE SILURIAN OF ARCTIC CANADA
JONATHAN M. ADRAIN
Department of Palaeontology, The Natural History Museum,
Cromwell Road, London SW7 5BD, United Kingdom, <j.adrain@nhm.ac.uk>
ABsTRAcr-Cladistic analysis of the trilobite subfamily Acanthoparyphinae Whittington and Evitt, 1954, yields an explicit hypothesis
of relationship for the group. All Silurian species together form a robustly supported monophylum including the genera Hyrokybe Lane,
1972, Parayoungia Chatterton and Perry, 1984, and Youngia Lindström, 1885. Sister to this is the Ordovician type species of Acanthoparypha Whittington and Evitt, 1954. Remaining species that have historically been assigned to either Acanthoparypha or Pandaspinapyga Esker and Levin, 1964, form a rather labile paraphylum. Nevertheless, the entire group thus identified is definitely monophyletic, and supported by several prominent synapomorphic character-states.
The basal structure and basal node of the subfamily are more difficult to assess. The relationships of the genera Hammannopyge
Pribyl, Vanek, and Pek, 1985, Holia Bradley, 1930, and Nieszkowskia Schmidt, 1881, need to be addressed within the wider context
of the family as a whole. The traditional assignment of Holia to the acanthoparyphines is followed.
Wenlock acanthoparyphines from the Cape Phillips Formation of the central Canadian Arctic islands include several species of
Hyrokybe and Parayoungia. They are similar to, and in one case con specific with, coeval forms to the southwest in the southern
Mackenzie Mountains.
Five species are new: Holia glabra, Hyrokybe lightfooti, Hyrokybe youngi, Hyrokybe mitchellae. and Parayoungia mclaughlini. At
least four other potentially new species are reported in open nomenclature.
INTRODUcnON
T
of cheirurid trilobites at all taxonomic
levels has long been problematic. While there is little question that a natural group is represented, the basal node of the
clade remains ambiguous and no robust hypothesis of ingroup
phylogeny has yet been presented. Some subfamilies are
straightforwardly monophyletic (Cheirurinae, Deiphoninae), but
most (Areiinae, Eccoptochilinae, Cyrtometopinae, Sphaerexochinae) are not demonstrably so and their respective content has
been the subject of widely varying opinion. The present work
deals with the subfamily Acanthoparyphinae. A restricted group
of genera, including Acanthoparypha, form a well-supported
clade. Traditionally, additional generic taxa have been assigned.
The basal structure of the subfamily in this broader historical
scope remains difficult to determine, and will likely emerge only
in the context of a major phylogenetic analysis of Cheiruridae.
This work has two goals. First is the description of all acanthoparyphines known from rich, silicified Silurian faunas of the
Cape Phillips embayment, central Canadian Arctic. Second is
the development of a hypothesis of relationship of all adequately
known species of the subfamily, by means of cladistic parsimony
analysis.
HE CLASSIFICATION
NEW MATERIAL
Silicified trilobites from lower Wenlock to lower Ludlow
rocks of the Cape Phillips Formation, central Canadian Arctic
archipelago, have been the subject of several recent systematic
studies (e.g., Adrain, 1994; Adrain and Rarnsköld, 1996, 1997;
Adrain and Edgecombe, 1997a, 1997b). Comprehensive information on provenance has been given by Adrain and Edgecombe
(1997a), and summaries may be found in the above-cited works.
Consequently, only abbreviated (but complete) locality and stratigraphical data are given in the systematics section below.
Cape Phillips cheirurid diversity is similar, and in several
cases conspecific, to that reported from the Wenlock-Ludlow of
the Delorme Group of the southern Mackenzie Mountains (Perry
and Chatterton, 1979; Chatterton and Perry, 1984). Cheirurines
are relatively rare due to preservational factors in the Cape Phillips Formation. Silicification in this unit is biased toward large,
thick-cuticled sclerites. Cheirurines typically have a very thin
cuticle; only the relatively robust librigenae are commonly preserved. Of other Silurian subfamilies, the sphaerexochine
Sphaerexochus is ubiquitous, well preserved, and very common
in Cape Phillips samples. The deiphonine Deiphon is quite rare,
but enough material has accumulated for systematic treatment.
HISTORY OF STUDY
The dorsal exoskeleton of many acanthoparyphines is both
spinose and densely tuberculate, and many species have relatively small pygidia. As a result, these trilobites have historically
proven difficult to collect and prepare by normal mechanical
means. With rare exceptions, species described before the 1950s
are poorly known, often from cranidia or cephala only, and often
from only internal molds of these. It was not until the advent of
widespread acid preparation of silicified faunas that the group
was properly characterized and named. Of the 25 species presently well enough known for meaningful analysis, 24 are known
primarily from silicified material. As a result, much of the taxonomic history of the group is recent.
The first taxon now unequivocally assigned to Acanthoparyphinae to be named was Youngia Lindström, 1885. Lindström
named two Silurian species from the Wenlock of Gotland, Sweden, making comparisons with Cheirurus trispinosus Young,
1868, but he did not specify a type species for his new genus.
Vogdes (1917) subsequently designated C. trispinosus type,
while Ramsköld (1983) eventually transferred Lindström's Gotland species to Hyrokybe. In the latter part of the nineteenth and
first half of the twentieth century, a handful of mainly Silurian
species were proposed. Few are interpretable beyond their obvious acanthoparyphine affinity, and most remain in need of
modem revision.
Better known were a group of Ordovician species assigned to
Niesz/wwskia Schmidt, 1881. Members of this predominantly
Baltic taxon are often large, and share as a prominent apomorphy the development of a sometimes very elaborate dorsal spine
set near the rear of the glabella. The most completely known
species remain those treated by Öpik (1928, 1930, 1937).
Bradley (1930) proposed Holia for a single species from the
Kimmswick Limestone of Illinois and Missouri, noting similarity between his new taxon and Nieszkowskia. Holia magnaspina
Bradley, 1930, is known from only two incomplete cranidia.
698
ADRAIN-SILURIAN
Whittington and Evitt (1954), in a seminal study, named both
the genus Acanthoparypha and the subfamily Acanthoparyphinae, on the basis of superb Middle Ordovician silicified material
from Virginia. Their original concept of the taxon included
Acanthoparypha, Holia, and Nieszkowskia. Youngia was considered too poorly known at that point to classify with confidence.
Männil (1958) described five potential new species of acanthoparyphines, each on the basis of a single fragmentary cranidium. Four he assigned to Nieszkowskia, and a fifth was made
the type species of the monotypic Ainoa Männil, 1958. Männil
(1958, p. 186) made explicit comparisons with Holia magnaspina. Both species are known only from cranidia, but these differ
only in relative proportions, and Ainoa is considered a probable
synonym of Holia below.
Esker and Levin (1964) proposed Pandaspinapyga for a species Esker had earlier (1961) assigned to Acanthoparypha. The
best known species of Pandaspinapyga is P. salsa Esker, 1964
(see Shaw, 1974), but the group as a whole now appears to be
differentiated from Acanthoparypha solely by the presence of
plesiomorphies (see discussion of Acanthoparypha below).
The next major review of cheirurids was by Lane (1971), who
revised the type species of Youngia and assigned the genus unequivocally to the acanthoparyphines.
Lane (1972) erected Hyrokybe as a monotypic genus, with a
North Greenland type species known from three partially exfoliated cranidia and cephala. At the time of its erection, the taxon
was assigned to Sphaerexochinae. Lane and Owens (1982)
named H. meliceris on the basis of two partial cephala and advocated (p. 55-56) the synonymy of Acanthoparyphinae and
Sphaerexochinae.
The morphology of Hyrokybe was fully documented by Chatterton and Perry (1984), who described several new species
based on excellent silicified material from the Mackenzie Mountains of northwest Canada. Chatterton and Perry (1984) also described several species of Youngia, and erected Parayoungia.
With nearly all exoskeletal parts identified for most of their new
species, most significantly the pygidia (which had never previously been illustrated for a Silurian acanthoparyphine), Chatterton and Perry were able to give an account of the subfamily that
forms the basis of the current understanding of the group.
The only subsequent review of the taxon is that of Pribyl et
al. (1985). These workers presented no new systematic data, but
proposed, somewhat haphazardly, eighteen new cheirurid genera. They also outlined a series of evolutionary narratives, including one for Acanthoparyphinae. Few of the genera proposed
in this work have been used by subsequent workers, and none
as yet has a well-supported systematic basis.
PHYLOGENETIC
ANALYSIS
Scope.-All adequately known species were included in a cladistic parsimony analysis. Minimally, information was required
on the cranidium, Iibrigena, thorax, and pygidium (with a single
exception-the
Iibrigena of Pandaspinapyga salsa is not
known). In most cases, the hypostome was also available for
analysis. The data set thus assembled constitutes 24 ingroup species, of which 23 are known from silicified material. Character
analysis yielded 39 characters, of which 31 are binary and 8
multistate. Illustrations of some of the characters are given in
Figure 1.
In three cases, sister species that are separated by subtle differentia not shared with any other taxa were analyzed as a single
terminal taxon. Hyrokybe youngilH. copelandi on the one hand
and H. lightfootilH. mitchellae on the other are distinguished
mainly on the basis of variation in the shape and dimensions of
TRIWBITES
699
7
32,w
i-Drawings
illustrating some of the characters used in parsimony analysis. i, lateral view of cranidium of Hyrokybe julli. 2, external view of librigena of H. julli. 3, dorsal view of pygidium of H.
mitchellae.
FIGURE
the pygidial spines, whereas Holia glabralH. secristi are distinguished by a variety of differing proportions and spine inclinations. None of the differentia are relevant to subfarnilial phylogeny and introducing all of the species to the analysis would
require the addition of several character-states whose only purpose would be to code for specific autapomorphies.
Characters.-Cranidium.
1. Position of genal spine. O--occupying genal angle. I-between fulcrum and genal angle.
2. Size of genal spine. O-large, elongate. I-small, not elongate, thornlike.
3. Size of occipital spine in large holaspids. O-not developed.
I-tiny and thornlike. 2-robust and elongate.
4. Size of anterior border. O-visible in normal dorsal view.
l-obscured
by frontal part of glabella in normal dorsal
view.
5. Style of glabellar sculpture. O-moderately, evenly tuberculate. l~ominated
by fine granules. 2-<:oarsely, densely
tuberculate.
700
JOURNAL OF PALEONTOLOGY, V. 72, NO.4, 1998
6. Glabellar outline (especially in ventral view). O-subrectangular. I-subcircular.
7. Slope of SI in lateral profile. O-sloping posteriorly. 1subparallel to SO, or slightly anteriorly inclined.
8. Convexity of SI in lateral profile. O-evenly anteriorly convex. I-posteriorly
convex "kink" about half way up glabella (Fig. 1.1).
9. Length of S2 (and usually S3). ~efinitely
reaching axial
furrow dorsally. I-terminating
well adaxial of axial furrow.
10. Depth of SI. O-about the same depth as S2. I-Significantly deeper than S2.
11. Width of interocular fixigena. O-wide, palpebral lobe well
separated from glabella. I-very narrow, palpebral lobe almost abutting glabella.
12. Size of posterior fixigena. O-broad, large area. I-significantly reduced, but still protruding laterally in dorsal view.
2~xtremely
reduced, not protruding laterally or only
slightly protruding laterally in dorsal view.
Librigena
13. Thberculation of field. O-little or none, field essentially
smooth. I-field with coarse, crowded tubercles.
14. Extent of lateral border furrow. O-<:ontinuous to posterior
margin. I-shallowed
and terminating well short of posterior margin.
15. Lateral border tuberculation. O-small granules only. 1coarse tubercles.
16. Length of anterior projection. O-as long or longer than
maximum exsagittallength of field. I-shorter than field.
Hypostome
17. Condition of hypostomal suture. O-functional, hypostome
and rostral plate never found fused. I-<:ommonly fused,
hypostome and rostral plate commonly occurring as fused
unit.
18. Middle body sculpture. O-finely granulate. I-tuberculate.
19. Presence. of median, posteriorly imbricate fold on posterior
border. O-absent. I-present
(e.g., Chatterton and Perry,
1984, pI. 19, fig. 1, pI. 20, figs. 14, 19).
20. Shape of posterior margin. O-median part posteriorly convex. I-median part posteriorly concave.
21. Median structure on posterior margin. O-none. I-small
knob. 2-tightly grouped, posteriorly directed denticles.
22. Position of shoulder. O-on anterior half of hypos tome. Ion posterior half of hypos tome.
Thorax
23. Thoracic pleural structure. O-pleural furrow defined as row
of fine pits. I-pleural furrow absent, pleural rib developed.
24. Development of large fulcral accessory spines. O-absent.
I-present.
25. Presence of preannular area behind axial ring. O-absent.
1~eveloped
as broad, smooth area. 2-well expressed,
with separate, deep preannular furrow. 3~xpressed
only
as small lunate area or absent.
Pygidium
26. Relative size of posterior spine pairs. O-second-last pair
much longer than last pair. I-spines subequal in length.
27. Outline of spines in transverse section. O-ftattened. 1subelliptical or subcircular.
28. Attitude of second-last spine pair. O-in same plane as posterior pair. 1~orsally
raised.
29. Size of posterior spine pair. O-transversely spaced, independent spines. I-greatly
reduced, medially crowded
knobs.
30. Ventral spine morphology. O-simple. I-<:omposite, spines
ventrally subdivided.
31. Presence of accessory spines on second-last spine pair. 0absent. I-long dorsal spines developed, set atop distinct
subquadrate bases. 2-10ng dorsal spines developed, with
no distinct ventral base.
32. Presence of small, dorsally upturned, node-like tips on
spines. O-absent. I-present.
33. Retention of pleural furrow/pits on second-last pygidial segment. O-retained. 1~ffaced.
34. Development of first pygidial ring furrow. O-incised, transverse slot. I-reduced to lateral pits.
35. Development of second pygidial ring furrow. O-incised,
transverse slot. I-reduced to single median pit or effaced.
2-reduced to pair of lateral pits.
36. Development of third pygidial ring furrow. O-small transverse slot. I-absent.
37. Shape of last spine pair. O-simple. I-spatulate. 2-strongly subquadrate.
38. Attitude of tips of second-last spines. O-in same plane as
body of spine. I-strongly dorsally upturned. 2-strongly
downturned.
39. Length of last spine pair. O-elongate. I-shortened.
Outgroup.-The most primitive species considered definitely
related to the "core" acanthoparyphine group appears to be Acidaspis unica Thomson, 1857. The species was assigned to Nieszkowskia by Barton (1916), Reed (1931) and Lane (1971), but
lacks any of the obvious autapomorphies of that taxon (Lane,
1971, p. 66, accounted for this with the observation that "Nieszkowskia provides a good example of the range of variation
shown by certain genera of the subfamily ... "). Pribyl et al.
(1985) made the species the type of the monotypic new genus
Hammannopyge. Evaluation of the validity of this taxon must
await a more complete understanding of cheirurid phylogeny in
general. All of the features used to distinguish Hammannopyge
from Niesz/wwskia (e.g., absence of glabellar spine; large size
of posterior fixigena) seem almost certain to be plesiomorphic.
Nevertheless, they do serve to point out the primitive position
of the species in terms of acanthoparyphine phylogeny. Hammannopyge is applied tentatively herein, and H. unica considered the most appropriate candidate for outgroup analysis.
Results.-Parsimony
analysis was performed using PAUP
version 3.1.1 (Swofford, 1993). All characters were unordered,
Hammannopyge unica was designated outgroup, and ACCTRAN optimization was utilized. A variety of heuristic techniques
were employed, but none improved or added to the result obtained using a random addition sequence, tree bisection-reconnection branchswapping, and attempts using various random
seeds. Fourteen equally parsimonious trees were identified, each
with branch length 85, consistency index 0.565, and retention
index 0.833. A strict consensus of the 14 most parsimonious
cladograms is shown in Figure 2. Primary character support is
shown mapped (with the ACCTRAN assumption) on one of the
most parsimonious trees in Figure 3. This topology was selected
by a criterion of maximal stratigraphic congruence via minimalization of ghost lineages (Smith, 1994).
There are two areas of instability on the cladograms. First,
the trees are divided into two groups by the resolution of the
Youngia clade. Seven trees resolve the species Y. johnsoni, Y.
kathyae, and Y. steineri in a polytomy to which Y. kathyae has
a branch length of zero. The other seven trees unite Y. johnsoni
and Y. kathyae (the latter again with a zero branch length), with
ADRAIN-SILURIAN
TABLE
i-Character
Hammannopyge
701
TRILOBITES
matrix for parsimony analysis of Acanthoparyphinae,
Hyrof<Yb6
unica was the designated outgroup in all runs.
Character
Taxon
unica
anacantha
boucoti
brennardi
chiropyra
cimelia
lightfootilmitchellae
copelandilyoungi
echinoderma
eleyae
evitti
folinsbeei
goniopyga
mclaughlini
hadnagyi
johnsoni
julli
kathyae
lenzi
perforata
salsa
secristilglabra
steineri
tuberculata
111111111122222222223333333333
123456789012345678901234567890123456789
OOOOOOOOOOOOOOOO??????OOOOOOOOOOOOOOOOO
0000000000001001?????????00100001001000
102121101111101101000001?10000101011201
102121100111????01110001310000201111000
000101000010000000000000110000011011000
002010000000000000000100001100001021000
111121101112101110011010210000011011101
111121111112101110011010210000011011111
000101000011000001000000110000011001000
102121100110111100110001210010201111001
010101000011000100000000110000011001100
1020111000101111???????1310000201111001
0000000000100001??????00?10000011011000
1021211001101111??????11210010201111001
1110211?1112101110011010211000011011010
110121101111101101000011?10001101011201
111121111112101110011010211000011011010
102121101111101101000011210001101011201
111121111112101110011010211000011011010
000120000010101000010000111000011011000
000101000010?????000?000110000000000000
002000000000000000000100001100001021000
102121101111101101000011210001101011021
0021201001001111??????11110010201111001
Y. steineri sister to this clade. The remaining seven variations
are in the relative positions of species of "Acanthoparypha" and
Pandaspinapyga salsa. Acanthoparypha perforata is sister to the
Hyrokybe/Parayoungia/Youngia clade on all trees, but the disposition of the remaining species is very labile.
The major nodes of the cladogram are discussed, in ascending
order, under generic headings in the systematics section.
SYSTEMATIC
PALEONTOLOGY
Repository.-All
figured material is housed in the collections
of the Department of Palaeobiology, Royal Ontario Museum,
Toronto, with specimen numbers prefixed ROM.
Terminology.-The
term "fulcral spine" is used to refer to a
thoracic or pygidial spine which has its base set directly on the
fulcrum, and which is differentiated from a pleural part distal to
the fulcrum, which mayor may not bear a pleural spine. Such
fulcral spines are features of the early ontogeny of many groups
of trilobites.
Family CHEIRURIDAE
Hawle and Corda, 1847
Subfamily ACANTHOPARYPHINAE
Whittington and Evitt, 1954
Included genera.-Acanthoparypha
Whittington and Evitt,
1954; Holia Bradley, 1930; Hyrokybe Lane, 1972; Pandaspinapyga Esker and Levin, 1964; Parayoungia Chatterton and
Perry, 1984; Youngia Lindström, 1885.
Discussion.-A
core group of acanthoparyphines, including
Acanthoparypha, Hyrokybe, Pandaspinapyga, Parayoungia, and
Youngia, form a well-supported clade, with apomorphies including development of an anterior border obscured medially by the
glabella in dorsal view, a subcircular glabellar outline, much
reduced interocular fixigena, a distinct preannular area on the
thoracic axial rings, and equally-long, coplanar pygidial spines.
Expansion of this group to encompass Holia, which has been
classified as an acanthoparyphine since the establishment of the
subfamily, is more problematic (see generic discussion below).
In search of a basal subfamilial node, another highly autapomorphic taxon, Nieszkowskia, is also relevant and has in the past
been assigned.
2-Strict consensus of 14 most parsimonious c1adograms identified by PAUP analysis.
FIGURE
Whittington and Evitt (1954, p. 70) did not give an explicit
diagnosis of the subfamily, but did list in discussion the aspects
of morphology they considered to be relevant. These were adapted by Lane (1971, p. 66) in a subfamilial diagnosis. Many of
these character-states (e.g., thoracic pleurae with transverse median row of pits) are very widely distributed in other cheirurids
and are surely, in phylogenetic terms, plesiomorphies. The most
relevant state may possibly prove to be a reduction to two pairs
of pygidial spines, but this must await characterization of the
basal node.
In the present state of knowledge, an effective subfamilial
diagnosis is possible if Holia is excluded, as there is abundant
support for the node including the Acanthoparypha/Pandaspinapyga grade plus the Silurian group. If the taxon is to be more
inclusive, as seems desirable, it will be necessary to consider
the explicit phylogenetic position of Hammannopyge, Nieszkowskia, Holia, and the core acanthoparyphines in the context
of the remainder of Cheiruridae. This is beyond the scope of the
present study. Herein, Holia was coded as plesiomorphic for
character 12, the width of the posterior fixigena. This is effective
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3-Apomorphic character-states (black bars) mapped onto one of
the 14 most parsimonious c1adograms, selected for maximal stratigraphic congruence.
FIGURE
702
JOURNAL OF PALEONTOWGY,
V. 72, NO.4, 1998
ADRA1N-SILURIAN
in distinguishing the genus from the core Acanthoparyphinae,
but it is worth noting that the morphology in Holia is actually
quite reduced from that seen in Hammanopyge unica and all
Nieszkowskia. Hence, the position of Holia as sister to the core
acanthoparyphines is probably accurate, if at present weakly
supported.
Chatterton and Ludvigsen (1976) assigned the genera Heliomera Raymond, 1905, and Heliomeroides Evitt, 1951, to Acanthoparyphinae, an action followed by Ludvigsen (1979), Chatterton (1980), and Hammann (1992). Were this proposal to be
accepted, the proper subfami1ial name would be Heliomerinae
Evitt, 1951, which has priority over Acanthoparyphinae. Whittington (1965), however, argued that Heliomera and Heliomeroides (which he regarded as a subgenus of Heliomera) were
sphaerexochines. Chatterton (1980, pI. 12, fig. 29) subsequently
assigned a protaspid to his new Heliomeroides freschaufae that
is of the globular asaphoid type, very similar to those of Sphaerexochus (e.g., S. arenosus Chatt~rton and Ludvigsen, 1976; see
Chatterton, 1980, pI. 12, figs. 1-5), but entirely unlike those of
core acanthoparyphines (e.g., Acanthoparypha evitti Chatterton
and Ludvigsen, 1976; see Chatterton, 1980, pI. 10, figs. 1-4).
The affinities of the autapomorphic heliomerines remain to be
assessed in detail. However, there is no compelling reason to
suspect that they are ingroup acanthoparyphines as understood
herein, and the only convincing point of similarity, the presence
of two pairs of pygidial spines, is likely to have been independently derived.
Biogeography.-The
acanthoparyphines are a predominantly
Laurentian clade, although this may in large part be due to a
monographic effect resulting from the description of rich Laurentian silicified faunas. They have a stratigraphic range from
the Whiterockian through the Ludlow. Primitive species are
known from Baltica, including Nieszkowskia and HoUa maeruensis. The known diversity of Ordovician "core" acanthoparyphines is also mainly Laurentian, although several very poorly
known Baltic species may belong (e.g., Pseudosphaerexochus
tubereulatus Warburg; see Dean, 1971, p. 27), as may some
species from Russia and Kazahkstan known from fragmentary
glabellar material (see species lists below). In addition, a species
possibly belonging to Pandaspinapyga occurs in the Llanvirn
Las Aguaditas Formation of the Argentine Precordillera (G. D.
Edgecombe et aI., personal commun.). In the Silurian, the only
well-known species are northern Laurentian, but enough information is available to be certain that there was a comparable
Llandovery- Wenlock history in south China, despite the fact that
no Chinese species have been adequately described. Similarly,
Youngia is present in the British Llandovery, Hyrokybe occurs
in the Baltic Wenlock, and Parayoungia (= lchiyamella Kobayashi and Hamada; see under Parayoungia below) is known from
the Japanese Ludlow. In Australia, Holloway (1994) has figured
Youngia from the Llandovery of north Queensland, Parayoungia
is present in the Sheinwoodian Boree Creek Limestone of New
South Wales (L. Ramsköld, personal commun.), and there is
material in the Australian Museum of an acanthoparyphine representing either Hyrokybe or Youngia from the Sheinwoodian
703
TRILOBITES
Rosyth Limestone of New South Wales (G. D. Edgecombe, personal commun.). Given the difficulties inherent in mechanical
recovery of acanthoparyphines, strong statements about their
distribution are presently ill-advised. The group appears, however, to have had a reasonably cosmopolitan distribution for
most of its history.
Genus HOUA Bradley, 1930
Ainoa MÄNNIL,1958, p. 186.
Type species.-Holia magnispina Bradley, 1930, Kimmswick
Limestone, Mohawkian, near Batchtown, minois.
Other species.-Holia
anacantha Ludvigsen, 1979, lower
Whittaker Formation, Mohawkian, southern Mackenzie Mountains, N.W.T.; H. cimelia Whittington and Evitt, 1954, Edinburg Formation, Mohawkian, Virginia; H. glabra new species,
Esbattaotine Formation, Whiterockian, southern Mackenzie
Mountains, N.W.T.; Ainoa maeruensis Männil, 1958, Keila
Stage (Mohawkian), Estonia; H. secristi Whittington and Evitt,
1954, Lincolnshire Limestone, Whiterockian, Virginia; Pseudosphaerexochus (?) yakovlevi Weber, 1948.
Diagnosis.-Robust
occipital spine developed in derived species; hypostomal shoulder set on posterior half (sag.) of hypostome; first pygidial spines with strong dorsal elevation; second
pygidial ring furrow reduced to prominent pair of lateral pits.
Discussion.-Holia
is an undoubted clade with several prominent apomorphies. Its relationship to the remainder of the subfamily is difficult to specify with precision. In the present analysis, the HoUa node is separated from the outgroup by only two
character-states using ACCTRAN optimization (Fig. 3), These
are the loss of the first pygidial pleural furrow (character 33[1])
and loss of the third pygidial ring furrow (character 36[1]). Both,
however, are reversed uptree by Pandaspinapyga salsa and using DELTRAN optimization the branch length between the outgroup and Holia is zero.
Männil (1958) established the monotypic Ainoa on the basis
of a single illustrated cranidium. This (Männil, 1958, pI. 4, figs.
3,4) has a large occipital spine, deeply incised glabellar furrows,
a subrectangu1ar glabella, broad interocular fixigena, and a long
anterior border. In these features, A. maeruensis differs only in
slight proportions from Holia magnaspina. Although neither
type species is adequately known, there do not seem to be any
morphological grounds indicating that A. maeruensis belongs to
a clade independent of Holia. Pribyl et al. (1985, p. 151) considered Ainoa to be a subgenus of Holia, but herein it is placed
in subjective junior synonymy.
Discussion of homeomorphic resemblance between Holia and
Parayoungia is given under the latter genus below.
HOUA GLABRAnew species
HoUa cf. secristi Whittington and Evitt, 1954. LUDVIGSEN,1975, pI. 3,
figs. 7, 8.
Holia secristi Whittington and Evitt, 1954. CHA1TERTONANDLUDVIGSEN, 1976, p. 64, pI. 11, figs. 1-42; LUDVIGSEN,1978, pI. 3, fig. 32;
LUDVIGSEN,1979, p. 41; CHA1TERTON,1980, p. 37, pI. 11, figs. 126.
FIGURE4-Hyrokybe youngi new species, from section BH 1 110 m (except where noted otherwise), Cape Phillips Formation, Wenlock (Sheinwoodian; Monograptus instrenuus-Cyrtograptus kolobus Zone), southern Baillie-Hamilton Island, central Canadian Arctic. i, 4, 8, 9, cranidium,
ROM 51307, dorsal, right lateral, anterior, and ventral views, XIO, except 9, X7.5; 2, 6,10, i4, cranidium and left librigena, ROM 51308, dorsal,
left lateral, oblique, and anterior views, x7.5; 3,7, cranidium, ROM 51309, dorsal and left lateral views, XIO; 5, cranidium, ROM 51310, oblique
view, x7.5; 11, /5, cranidium and right librigena, ROM 51311, oblique and dorsal views, x7.5; i2, 13, cranidium, ROM 51312, dorsal and left
lateral views, XIO; i6, left librigena, ROM 51313, external view, 10; 17, right librigena, ROM 51314, external view, X7.5 (BH 1 112 m); i8,
right librigena, ROM 51315, external view, XIO; i9, right libigena, ROM 51316, external view, XIO; 20, left librigena, ROM 51317, external
view, x7.5; 2i, right Iibrigena, ROM 51318, external view, XIO; 22,23, left librigena, ROM 51319, external and internal views, XIO.
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JOURNAL OF PALEONTOLOGY, V. 72, NO.4, 1998
Diagnosis.-Median
occipital spine with blunt tip; interocular
fixigena narrow; posterior fixigena exsagittally short, fixigenal
spines small; hypostomal maculae and shoulder prominent; pygidium wide, first spines slender and only moderately dorsally
protruded.
Discussion.-Holia
glabra new species is so similar to H.
secristi that full description is unnecessary. The following comparison lists all observed differences between the species. Holia
glabra is distinguished from H. secristi in the possession of a
median occipital spine often with a prominently blunt, versus
tapering, tip; narrower interocular fixigena; less elongate posterior fixigena (compare the length of an exsagittal line from the
rear of the palpebral lobe to the posterior border furrow: Chatterton and Ludvigsen, 1976, pI. 11, figs. 1,4, 11 vs. Whittington
and Evitt, 1954, pI. 30, figs. 35-37, pI. 31, fig. l), although this
may be accentuated by ontogenetic variation, as the Esabataottine material is generally larger than the Lincolnshire H. secristi
cranidia; smaller retained anterior and mid-fixigenal spines in
large sclerites; hypostomal shoulders that are much more laterally pointed; very prominent versus subdued hypostomal maculae (although a single hypostome figured by Chatterton and
Ludvigsen, 1976, pI. Il, fig. 27, does have subdued maculae);
wider pygidium, with the first spines more widely spaced, the
first ring and ring furrows with greater transverse extent, and
the second spines set more widely apart; and especially more
elongate, more slender, more evenly arcuate, and much less dorsally produced first spines.
Chatterton and Ludvigsen (1976, p. 64) noted the differences
in interocular fixigena, fixigenal spines, and pygidial spine elevation, but considered that they did "not seem to warrant specific segregation." In light of the five additional, pervasive, differences noted above, it becomes apparent that separate species
are represented, and the Esbataottine taxon is accordingly named
herein.
Material.-Holotype
pygidium, UA 1839 (Chatterton and
Ludvigsen, 1976, pI. 11, figs. 18, 19), from locality All0, Esbataottine Formation, upper Whiterockian (Chazy), central Mackenzie Mountains, N.WT.; paratypes, the material in UA and
GSC collections listed in Chatterton and Perry (1984, p. 100,
explanation of pI. Il).
Etymology.-Latin
glabra, bald.
Genus ACANTHOPARYPHA
Whittington and Evitt, 1954
Type species.-Acanthoparypha
perforata Whittington and
Evitt, 1954, Edinburg Formation, Mohawkian, Virginia.
Other species.-Acanthoparypha
chiropyga Whittington and
Evitt, 1954, Lincolnshire Formation, Whiterockian, Virginia; A.
echinoderma Chatterton and Ludvigsen, 1976, Esbataottine Formation, Whiterockian, southern Mackenzie Mountains, N.W.T;
A. evitti Chatterton and Ludvigsen, 1976, Esbataottine Formation, Whiterockian, southern Mackenzie Mountains, N.WT; A.?
goniopyga Ludvigsen, 1979, lower Whittaker Formation, Mohawkian, southern Mackenzie Mountains, N.W.T.; Pseudosphaerexochus subcircularis Bradley, 1930, Kimmswick Limestone, Mohawkian, Illinois; ?Pseudosphaerexochus asper Weber,
1948.
Discussion.-Acanthoparypha,
as presently conceived, is a
paraphyletic group. As such, a phylogenetically informative diagnosis cannot be given. The type species, A. perforata, occupies a stable and derived position on the cladogram, as sister to
the Silurian group. The remaining species are assigned provisionally, pending the development of additional data and a more
satisfactory estimate of phylogenetic structure. The monophyly
of Pandaspinapyga was not directly tested, since only a single
species is adequately known. Pandaspinapyga salsa possesses
no identified autapomorphies, however, and at present no potential synapomorphies that might support the genus are evident.
The primary feature generally cited (Lane, 1971, p. 68; Shaw,
1974, p. 33) as characteristic is the "knob-like terminal piece"
on the pygidium. This structure results from the retention of the
third ring furrow (character 36(0)) and as such is almost certainly a plesiomorphy. Pandaspinapyga salsa alternates among
the most parsimonious trees with "Acanthoparypha" goniopyga
as the basal sister taxon to most of the remainder of the subfamily. Given the pygidial plesiomorphies of Pandaspinapyga. including retention of a third ring furrow and a pleural furrow on
the first segment, the group is likely little more than the most
primitive element of the "Acanthoparypha" grade. Tripp (1993)
has advocated synonymizing the genera. This may eventually
prove profitable, but given that Acanthoparypha is itself paraphyletic and the phylogenetic structure of its species is poorly
resolved, Pandaspinapyga is recognized, with reservation, herein.
Genus PANDASPINAPYGA
Esker and Levin, 1964
Type species.-Acanthoparypha
projecta Esker, 1961,
Kimmswick Limestone, Mohawkian, Missouri.
Other species.-Pandaspinapyga
dactyla Chatterton and Ludvigsen, 1976, Esbataottine Formation, Whiterockian, central
Mackenzie Mountains, N.WT.; Pandaspinapyga salsa Esker,
1964, Bromide Formation, Mohawkian, Oklahoma (see Shaw,
1974); ?Nieszkowskia stubblefieldi Bancroft, 1949, Smeathen
Wood Beds, Caradoc, England.
Discussion.-See
under Acanthoparypha above.
Lane (1971) assigned Nieszkowskia stubblefieldi Bancroft to
Pandaspinapyga. The species, known from three fragmentary
cranidia and one pygidium, all internal molds, certainly lacks
the apomorphies of core Nieszkowskia. It also lacks many of the
apomorphies defining the "core" acanthoparyphine node (i.e.,
inclusive of "Pandaspinapyga" and "Acanthoparypha"), as it
retains a subrectangular versus round glabella, wide interocular
fixigena, and an anterior border visible in normal dorsal view.
It does, however, show pygidial spines which are apparently of
nearly similar length. Although too poorly known for meaningful analysis, its position on the cladogram arrived at above is
probably sister to the "core" group, i.e., between the "core"
and Holia nodes. As argued above, "Pandaspinapyga" is probably the primitive (paraphyletic) base of the (paraphyletic)
"Acanthoparypha"/"Pandaspinapyga"
grade. In the present
state of knowledge, therefore, it is appropriate to assign P. stubblefieldi to "Pandaspinapyga," pending the development of new
systematic data.
Genus YOUNGIALindström, 1885
Type species.-Cheirurus
trispinosus Young, 1868, Penkill
Beds, Llandovery, Girvan, Ayrshire, Scotland; by subsequent
designation of Vogdes (1917, p. 115) (see Lane, 1971, for modem revision).
Other species.-Y. boucoti Chatterton and Perry, 1984, Whittaker Formation, Llandovery, southern Mackenzie Mountains,
N.W.T.; Ceraurus (Pseudosphaerexochus)
clintoni Foerste,
1894, Brassfield Formation, Llandovery, Ohio; Cheirurus
(Youngia) douglasi Lamont, 1948, Wether Law Linn Formation,
Llandovery, Scotland (see Clarkson and Howells, 1981); Y.
johnsoni Chatterton and Perry, 1984, Whittaker Formation,
Llandovery, southern Mackenzie Mountains, N.WT.; y. kathyae
Chatterton and Perry, 1984, Whittaker Formation, Llandovery,
southern Mackenzie Mountains, N.W.T.; Cheirurus moroides
Marr and Nicholson, 1888, Skelgill Beds, Llandovery, England
(see Lane, 1971); Y. steineri Chatterton and Perry, 1984, Whittaker Formation, Llandovery, southern Mackenzie Mountains,
ADRAlN-SILURIAN
TRIWBITES
705
FIGURE5-i-17, Hyrokybe youngi new species, from sections BH I 110 m (except where noted otherwise), Cape Phillips Formation, Wenlock
(Sheinwoodian; Monograptus instrenuus-Cynograptus kolobus Zone), southern Baillie-Hamilton Island, central Canadian Arctic. i, 2, hypostome
and rostral plate, ROM 51320, ventral and dorsal views, XIO; 3, hypostome and rostral plate, ROM 51321, dorsal view, X7.5 (BH I 109 m); 4,
7, pygidium, ROM 51322, ventral and dorsal views, X 10; 5, 9, 13, pygidium, ROM 51323, dorsal, posterior, and right lateraI views, X 10; 6, JO,
i4, pygidium, ROM 51324, dorsal, posterior, and right lateral views, XIO; 8, pygidium, ROM 51325, dorsal view, XIO; ll, pygidium, ROM
51326, dorsal view, XIO; 12. pygidum and thoracic segment, ROM 51327, dorsal view, X15; i5, 16, thoracic segment, ROM 51328, dorsal and
anterior views, X 10; i7, pygidium, ROM 51329, dorsal view, X 10. i8-23, Hyrokybe spp. I, locality as indicated, Cape Phillips Formation,
Wenlock (Homerian; Cynograptus lundgreni-Monograptus testis Zone), southern Baillie-Hamilton and northwestern Cornwallis Islands, central
Canadian Arctic. i8, right librigena, ROM 51330, external view, XIO (BH 2 6.5 m); i9, pygidium, ROM 51331, dorsal view, XIO (BH 23m);
20, hypostome and attached rostral plate, ROM 51332, ventral view, X7.5 (BH 2 6.5 m); 2i, cranidium, ROM 51333, oblique view, X7.5 (BH 2
3 m); 22, pygidium, ROM 51334, dorsal view, XIO (BH 2 6.5 m); 23, pygidium, ROM 51335, dorsal view, XIO (ABR 19m).
N.W.T.; ?Y. walli Chatterton and Perry, 1984, Whittaker Formation, Llandovery, southern Mackenzie Mountains, N.W.T.
Diagnosis.-Posterior
fixigena with only moderate lateral protrusion; hypostome with medially posteriorly convex posterior
margin and coarsely tuberculate middle body; thorax and pygidium with large dorsal accessory spines on pleura; pygidial spines
with ventral subdivisions.
Discussion.-5everal
of the species listed above are very
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JOURNAL OF PALEONTOWGY, V. 72, NO.4, 1998
ADRAIN-SILUR1AN
poorly known and revision on the basis of new material may
show them to belong to Hyrokybe.
Most species of Youngia have an elongate pair of spines running from the pleurae of the first pygidial segment. The interpretation of this morphology is of key importance to assessing
the phylogenetic structure of the subfamily. Taken at face value,
an elongate first pair of spines is plesiomorphic. In all species
of Youngia, however, what is known of the thoracic morphology
indicates that at least the posterior thoracic segments also bore
long spines originating at the fulcrum, with much smaller spines
directed ventrally beneath them (Chatterton and Perry, 1984, pI.
14, fig. 11, pI. 15, figs. 18, 22, pI. 18, fig. 11). These smaller
spines bear articulatory facets along the anterior edge of their
external surface and posterior edge of their ventral surface and
are as such almost certainly the true pleural spines. The same
facets are present, for example, on the single, ventral, spines of
Acanthoparypha chiropyga (Whittington and Evitt, 1954, pI. 29,
figs. 26, 29, 32). The elongate, dorsolaterally oriented thoracic
spines are therefore best interpreted as an evolutionary novelty
(character 24 above). Importantly, a similar situation can be observed on the first pygidial segment: the elongate posterodorsally
directed spines are without exception set atop swollen, subquadrate bases. These bases, most obvious in ventral view (Chatterton and Perry, 1984, pI. 14, fig. 15, pI. 17, figs. 18, 20) are
nearly identical to the entire second pygidial spines, seem certain
to be the serial homologue of the ventral thoracic spines, and
are hence the true first pygidial spines.
This observation renders the very derived morphology of Parayoungia much easier to intrepret. In advanced species of this
taxon the pygidium is reduced and merged into a more or less
disk-shaped base with two elongate dorsal spines. Parayoungia
also bears the thoracic morphology of small ventral spines on
all segments and elongate fulcral spines on the posterior ones.
Hence, it can safely be assumed that the long pygidial spines of
Parayoungia are the serial homologues of these dorsal thoracic
spines, and that the entire thoracopygidial spine complex is robustly synapomorphic, uniting Youngia and Parayoungia as a
clade.
707
TRILOBITES
Perry, 1984, Wenlock, central Mackenzie Mountains and central
Canadian Arctic; Shiqiania latisulcata Yi, 1989, Xinglong Formation, Wenlock, Szechuan, China; H. lenzi Chatterton and Perry, 1984, Wenlock, central Mackenzie Mountains, N.WT.; Shiqiania luorepingensis Wu, 1977, Llandovery, southwest China;
H. meliceris Lane and Owens, 1982, Lafayette Bugt Formation,
upper Llandovery or lowest Wenlock, North Greenland; Shiqianiapunctata
Chang, 1974, Silurian, southwest China; H.
mitchellae new species, Wenlock, central Canadian Arctic;
?Youngia uralica Tschemyschew, 1893, Silurian, Ural Mountains, Russia.
Diagnosis.-Occipital
spine very short and thornlike, separated from median occipital node; posterior fixigena small, protruding only slightly laterally in dorsal view; hypostomal suture
fused in holaspid; posterior margin of hypos tome posteriorly
concave, with median cluster of small denticles.
Discussion.-Acanthoparyphines
from the Silurian of China
are typically assigned to the genus Shiqiania Chang, 1974. No
species of Shiqiania are adequately known. Most are represented
by cranidia or cephala only, and several are based on single
specimens. Cbatterton and Perry (1984) considered Shiqiania to
be a probable synonym of Hyrokybe, and this opinion is followed herein. Knowledge of the rest of the exoskeleton of species assigned to Shiqiania may reveal features indicating that a
separate clade is represented, but what is presently known of the
morphology (e.g., very small or absent occipital spine, narrow
interocular and posterior fixigena, small or absent genal spine)
indicates most of the species are ingroup Hyrokybe. Only one
species of Hyrokybe, H. meliceris, is known from rocks of potentially Llandovery age. It is equally possible that the very
poorly known Chinese Llandovery species may prove to belong
to Youngia.
HYROKYBEYOUNGInew species
Figures 4,5.1-5.17
Shiqiania CHANG, 1974, p. 178.
Youngia sp. I; PERRY AND CHATIERTON,1977, p. 298, pI. 3, fig. 22
(non figs. 18, 19 = Hyrokybe mitchellae new species; non figs. 16,
17 = Parayoungia new species A; non figs. 20, 21 = Parayoungia
mclaughlini new species),
Youngia sp. 2; PERRYAND CHATIERTüN, 1977, p. 300, pI. 6, figs. 2225,28.
Type species.-Hyrokybe pharanx Lane, 1972, from the upper
Llandovery or lower Wenlock of North Greenland.
Other species.-Hyrokybe
lightfooti new species, Wenlock,
central Canadian Arctic; Youngia copelandi Perry and Chatterton, 1979, Wenlock, Delorme Range, eastern Mackenzie Mountains, N.WT.; H. youngi new species, Wenlock, central Canadian
Arctic; Shiqiania gaotanensis Chang, 1974, Silurian, southwest
China; Youngia globiceps Lindström, 1885, Upper Visby Marl,
Wenlock, Gotland, Sweden (see Ramsköld, 1983); Shiqiania
globosa Yi, 1978, Silurian, Xiadong, China; H. hadnagyi Chatterton and Perry, 1984, Wenlock, central Mackenzie Mountains,
N.WT.; Youngia inermis Lindström, 1885, Slite Beds, Wenlock,
Gotland, Sweden (see Ramsköld, 1983); H. julli Chatterton and
Diagnosis.-Pygidium
very short, spines short and splayed;
pygidial spine tips slightly upturned.
Discussion.-Hyrokbye
youngi new species is very similar to
the contemporaneous H. copelandi Perry and Chatterton, 1979,
from the Delorme Range of the eastern Mackenzie Mountains.
There are, however, several subtle but pervasive cephalic differences, and the pygidia of the two species are quite markedly
distinct. Hyrokybe youngi is distinguished from H. copelandi in
its sagittally shorter occipital ring; more elongate glabella (compare the length of the axial furrow in lateral view, from its contact with SI to contact with the preglabellar/anterior border furrow; Fig. 4.4, 4.6 vs. Chatterton and Perry, 1984, pI. 8, figs. 5,
10); concomitantly longer anterior section of the facial suture
Genus HYROKYBELane, 1972
FIGURE6-Hyrokybe lightfooti new species, talus boulder ABR TID, Cape Phillips Formation, Wenlock (upper Sheinwoodian; Cyrtograptus perneriMonograptus opimus Zone), near Abbott River, northwestern Cornwallis Island, central Canadian Arctic. i, 5, 9, cranidium, ROM 51347, dorsal,
anterior, and right lateral views, X7.5; 2,6, cranidium, ROM 51348, dorsal and left lateral views, XIO; 3,7,11, cranidium, ROM 51349, dorsal,
anterior, and right lateral views, X7.5; 4, 8, cranidium, ROM 51350, dorsal and right lateral views, X7.5; 10, i4, i5, i8, cranidium, ROM 51351,
dorsal, anterior, ventral, and left lateral views, X 10; i2, 13, i6, 17, cranidium, ROM 51352, anterior, dorsal, right lateral, and ventral views, X7.5;
i9, 20, cranidium, ROM 51353, right lateral and dorsal views, XIO; 2i, 25, cranidium, ROM 51354, dorsal and left lateral views, XIO; 22,23,
left librigena, ROM 51355, external and internal views, XIO; 24, left librigena, ROM 51356, external view, XIO; 26, pygidium, ROM 51357,
dorsal view, XIO; 27-29, pygidium, holotype, ROM 51358, dorsal, ventral, and left lateral views, XIO; 30, pygidium, ROM 51359, dorsal view,
XIO; 3i, pygidium, ROM 51360, dorsal view, XIO.
708
JOURNAL OF PALEONTOWGY, V. 72, NO.4, 1998
ADRAIN-SILURIAN
(best seen on librigena); concomitantly slightly taller eye with
an exsagittally shorter base; pygidium with medially transverse
versus medially arcuate first ring furrow (compare Fig. 5.5-5.8
with Perry and Chatterton, 1979, pI. 69, fig. 18; Chatterton and
Perry, 1984, pI. 8, fig. 16); shorter pygidial spines; and much
less dorsally upturned pygidial spine tips. The species share a
propensity for the last thoracic segment to become partially
fused to the pygidium (Fig. 5.12).
Hyrokbye youngi is distinguished from the slightly younger
H. mitchellae new species in the possession of an exsagittally
shorter occipital ring; longer occipital spine in large holaspids;
shorter (exsag.), more inflated LI; fewer librigenal field tubercles; and narrower, less spatulate pygidial spines. Hyrokybe
youngi differs from H. lightfooti new species in its larger eye;
less densely tuberculate librigenal field; narrower librigenallateral border; much shorter pygidal spines, and retention of the
second ring furrow as a median pit, versus usually thoroughly
effaced.
Material.-Holotype
pygidium ROM 51323 (Fig. 5.5, 5.9,
5.13), paratypes ROM 51307-51322, 51324-51329, from section BH 1 109-112 m, Cape Phillips Formation, Wenlock (midSheinwoodian; Monograptus instrenuus-Cyrtograptus kolobus
Zone), southern Baillie-Hamilton Island, central Canadian Arctic.
Etymology.-After
Neil Young.
HYROKYBELIGHTFooTI new species
Figure 6
Diagnosis.-Small,
thorn-like occipital spine retained in large
holaspides; genal spine relatively large; pygidial spines narrow;
second pygidial ring furrow usually completely effaced.
Description.-Cranidium
with sagittal length 77-80 percent
maximum width across posterior fixigenae; anterior border very
short (sag., exsag.), sculpture of single transverse row of small
tubercles along anterior edge, effaced medially; anterior margin
with sagittal point, followed abaxially by straight section slightly
declined from transverse plane, second abrupt break in slope,
and narrower straight section more strongly declined from transverse plane, running to front of anterior section of the facial
suture; anterior border furrow defined abaxially anterior to narrow frontal area, completely merged with preglabellar furrow
adaxially; anterior fixigena forming narrow strip of frontal area,
sculpture of moderate to fine tubercles, maximally two exsagittally aligned tubercle rows; eye ridge best seen ventrally, bounded by shallow furrow anteriorly, with single row of fine tubercles
along dorsal aspect; small sutural ridge developed along anterior
section of facial suture; anterior sections of facial suture forwardly convergent at about 45 degrees from exsagittal plane;
palpebral lobe very narrow, lateral margin gently curved; interocular fixigena completely reduced, palpebral lobe abuts glabella; posterior fixigena subrectangular, tuberculate sculpture similar to that of frontal area; sutural ridge along posterior section
of facial suture slightly more prominent than that along anterior
section; posterior border furrow about same depth as SI, running
transversely straight to just behind base of genal spine, forming
TRILOBITES
709
broad anterior arc in front of genal angle; posterior border dorsally convex and slightly inflated, of constant, relatively short,
length (exsag.) adaxially, lengthening abaxial to base of genal
spine to form slightly lobate genal angle, sculpture of densely
distributed fine tubercles; genal spine with length in mature individuals about equal to that of adaxial part of posterior border,
cylindrical, tapering rapidly distally to sharp point; glabella inflated, with nearly even sagittal convexity, slightly flatter posteriorly; glabellar sculpture of very densely distributed fine,
moderate, and coarse tubercles; SI nearly transverse, running
slightly posteriorly for most of course, deflected back adaxially,
stopping well in front of SO so rear half of LI is confluent with
median glabellar lobe; S2 retained as transverse incision, separated from axial furrow by distance subequal to width (tr.) of
S2; S3 retained as small incision, much shallower than S2 and
about two-thirds width; LI with inflation independent of that of
main part of glabella, projecting slightly ff(;>moutline in plan
view; L2 subrectangular and very slightly inflated; L3 discriminated only by small anterior and posterior incisions of S2 and
S3; SO deep, slightly bowed backwards medially in some specimens, deflected posteriorly and lengthened (exsag.) behind LI;
LO longest medially, shortened behing LI, with tuberculate
sculpture similar to rear of median glabellar lobe, median structure developed as prominent but small, conical spine set at posterior margin; fossula tiny, set at junction of eye ridge and axial
furrow.
Librigena with field reduced in length near eye (exsagittal
length beside eye about 35 percent maximum exsagittal length
beside lateral border), width at midlength of eye about half maximum length; eye small; eye socle composed of narrow (tr.) band
with single row of small, sometimes partially merged tubercles;
field dominated by large, closely spaced tubercles, smaller fine
tubercules interspersed anteriorly and posteriorly; lateral border
furrow broad and deep, separated from anterior margin by small
sutural ridge, running uninterrupted to posterior margin; lateral
border with width 75 percent that of field at midlength of eye,
width constant anteriorly and posteriorly; border with three more
or less ordered exsagittal tubercle rows, middle row composed
of larger tubercles than adaxial or abaxial row, several smaller
tubercles interspersed among middle row; doublure broad and
smooth.
Rostral plate, hypostome, and thorax not identified.
Pygidium with sagittal length almost exactly half anterior
width; anterior width about 85 percent maximum width,
achieved at about half distance posteriorly along first spines;
articulating half-ring short, slightly longer medially, flat; furrow
defining articulating half-ring bowed slightly posteriorly; axial
furrow defined only opposite anterior half of first axial ring; first
axial ring complete, shortest sagittally, lengthening abaxially;
first ring furrow deep and well impressed, slightly shallower
medially; second ring furrow completely effaced, second axial
ring grades without interruption into terminal piece and bases of
second spine pair; first spine flared laterally, with very shallow
furrow running distally from posterior extent of axial furrow,
spine with nearly constant width for most of length, tapering
FIGURE 7-Hyrokybe mitchellae new species, section BHL I 92 m and locality BHH, Cape Phillips Formation, Wenlock (upper Sheinwoodian;
Cyrtograptus perneri-Monograptus opimus Zone), southern Baillie-Hamilton Island, central Canadian Arctic. i, 2, 4, cranidium, ROM 51336,
dorsal, left lateral, and anterior views, X7.5 (BHL I 92 m); 3, 6, 7, cranidium, ROM 51337, dorsal, right lateral, and anterior views, x6 (BHL I
92 m); 5, right librigena, ROM 51338, external view, X JO (BHL I 92 m); 8, 9, right librigena and fixigenal fragment, ROM 51339, external and
internal views, X JO (BHL I 92 m); 10. right librigena, ROM 51340, external view, X JO (BHH-C); 11, 12. i5, i6, pygidium, holotype, ROM
51341, posterior, dorsal, ventral, and left lateral views, X JO (BHH-A); 13, pygidium, ROM 51342, dorsal view, X JO (BHH-C); i4, i8, i9,
pygidium, ROM 51343, dorsal, posterior, and right lateral views, xJO (BHH-A); 17, pygidium, ROM 51344, dorsal view, xJO (BHH-A); 20, 2i,
pygidium, ROM 51345, dorsal and posterior views, X7.5 (BHH-A); 22. pygidium, ROM 51346, dorsal view, xJO (BHH-A).
710
JOURNAL OF PALEONTOWGY,
V. 72, NO.4, 1998
ADRAIN-SILURIAN
rapidly distally to form small, slightly inflated, tip; second spine
similar in morphology to first, shorter, narrower, but with tip
reaching slightly more posteriorly; entire pygidium with dorsal
sculpture of moderate sized tubercles, not as densely spaced as
those of cranidium; doublure forming broad shelf, sculptur.e of
dense granules; underside of spines with slight ventral concavity; exsagittal ridge developed near adaxial edge of first spine;
doublural embayment subrectangular.
Basis of association.-Two
species cooccur in the Cyrtograptus pemeri-Monograptus opimus Zone of both southern BaillieHamilton and northwestern Cornwallis Islands. As certain exoskeletal parts of cooccurring species are very similar to one another, the criteria used to assign particular sclerites to particular
species must be explicit. Two observations allow considerable
confidence in most of the associations. First, whereas two species occur in either of ABR TID and BHL 1 92/BHH, only one
species is shared between the localities. Hence, if two morphotypes can be recognized for a given sclerite at each locality, all
of the morphotypes in common to the two localities belong together as one species. The second species at either locality is
thus identified by exclusion. Secondly, several related species
are known from occurrences either in the absence of a second
species with which there could be confusion (e.g., H. copelandi,
H. youngi) or else with a second species which is not closely
similar (e.g., Mackenzie Mountains occurrences of H. julli). Reference to these taxa allows corroboration of assignments based
on relative occurrence.
For most sclerites, identifying two morphotypes is not problematical. Cheirurid cranidia, however, have a notoriously low
range of variation compared to pygidia, and this is particularly
true of species of Hyrokybe. The first difficulty to be overcome
in the present example is identifying two morphotypes among
the sample from each locality. Examination of the cranidia illustrated in Figures 6-8 will indicate just how little cranidial
variation is present, despite the occurrence of three obviously
distinct pygidial morphotypes. Nevertheless, there is one feature
in which clear disjunct variation can be detected: when viewed
in lateral profile, the main transverse part of SI is in some specimens nearly transversely straight (Figs. 6.6, 6.8, 6.9,6.11,6.16,
7.2, 7.6) whereas in others there is a pronounced posterior deflection about half the distance across LI (Fig. 8.5-8.8, 8.208.22). On this basis, two sets of cranidia were identified at either
locality. Among cranidia with the posterior "kink," no variation
whatever could be detected between the localities. Among cranidia lacking the "kink," however, those from ABR TID (e.g.,
Fig. 6.1, 6.3, 6.10) show retention in large holaspides of a short,
thorn-like occipital spine. This serves to both confirm the distinction based on the course of SI and to differentiate the ABR
TID non-kinked morphotype from that at BHL 1 92/BHH (Fig.
7.1), which lacks a prominent occipital spine. Hence, definite
associations of cranidia with other sclerites is possible. That the
TRILOBITES
711
assignments are correct is confirmed by reference to the occurrence of H. julli in its type area in the Mackenzie Mountains:
the cranidia possess the prominent SI "kink" (e.g., Chatterton
and Perry, 1984, pI. 10, figs. 8,20), exactly as presently assigned
in the Cape Phillips material.
Species pairs occur also in the Mackenzie Mountains material
described by Chatterton and Perry (1984). Associations in those
faunas are not, however, as problematic as those in the Cape
Phillips Formation. There are two reasons for this. First, the
cooccurring species in the Mackenzie Mountains have strongly
disjunct morphologies (e.g., Hyrokybe julli vs. H. hadnagyi).
Secondly, Chatterton and Perry's material is generally smaller
and more juvenile than that available from the Cape Phillips
Formation. The ontogeny of these trilobites involves a transition
from spinose juveniles to tuberculate adults. With this reduction
in spines there is some convergence on a generalized adult tuberculate morphology, and it is considerably easier to recognize
distinct cranidial morphologies in juveniles. Of all Hyrokybe
cranidia illustrated by Chatterton and Perry (1984), only a single
specimen (H. julli, their pI. 10, figs. 3-6) falls in the size range
typical of Cape Phillips material.
Discussion.-Hyrokybe
lightfooti new species is distinguished
from H. julli, with which it occurs, in the possession of a more
prominent median occipital spine; SI that is not bowed posteriorly in lateral view; librigenal field that is narrower beside eye;
librigenal field tubercles that are subrounded versus subpolygonal; anterior section of the facial suture on Iibrigena that is
essentially straight or anteriorly concave versus anteriorly convex; librigenallateral border bearing three versus four exsagittal
tubercle rows; second pygidial ring furrow entirely effaced; and
pygidial spines that are shorter, taper more abruptly to a point,
and are subelliptical versus subcircular in transverse section.
Comparisons with H. youngi and H. mitchellae are given under
discussion of those species.
Material.-Holotype
pygidium ROM 51358 (Fig. 6.27-6.29);
paratypes ROM 51347-51357, 51359, 51360, all from talus
boulder ABR-TID, Cape Phillips Formation, Wenlock (upper
Sheinwoodian; Cyrtograptus perneri-Monograptus
opimus
Zone), near Abbott River, northwestern Cornwallis Island, central Canadian Arctic.
Etymology.-After
Gordon Lightfoot.
HYROKYBEMITCHELLAE
new species
Figure 7
Youngia sp. I; PERRYANDCHATTERTON,1977, p. 298, pI. 3, figs. 18,
19 (non figs. 16, 17 = Parayoungia new species A; non figs. 20, 21
= Parayoungia mclaughlini new species; non fig. 22 = Hyrokybe
youngi new species).
Diagnosis.--Glabellar tuberculate sculpture relatively fine, especially larger median tubercles; occipital spine minute; pygidial
spines broad and spatulate, first pair flaring laterally; first pygidial ring furrow very deep laterally.
FIGURE8-i-24, Hyrokybe julli Chatterton and Perry, 1984, section BHL I 92 m, locality BHH, and talus boulder ABR TID, Cape Phillips
Formation, Wenlock (upper Sheinwoodian; Cyrtograptus perneri-Monograptus opimus Zone), southern Baillie-Hamilton and northwestern Cornwallis Islands, central Canadian Arctic. i, 5, 9, i3, cranidium, ROM 51361, dorsal, left lateral, anterior, and ventral views, x7.5 (ABR TID); 2,
6, JO, i4, cranidium, ROM 51362, dorsal, left lateral, anterior, and ventral views, x7.5 (ABR TID); 3, 7, 11, cranidium, ROM 51363, dorsal, left
lateral, and anterior views, x7.5 (BHL I 92 m); 4,8, i2, cranidium, ROM 51364, dorsal, left lateral, and anterior views, X7.5 (BHH-A); i5, i9,
cranidium, ROM 51365, dorsal and left lateral views, x7.5 (BHL I 92 m); i6, 20, 24, cranidium, ROM 51366, dorsal, right lateral, and anterior
views, x6 (BHL 192m); i7, 2i, cranidium, ROM 51367, dorsal and right lateral views, x6 (ABR TID); i8, 22, 23, cranidium, ROM 51368,
dorsal, left lateral, and anterior views, x 10 (BHH-A). 25-30, specimens, Hyrokybe indet. which may belong to either Hyrokybe mitchellae new
species or H. julli Chatterton and Perry, 1984. 25, hypostome and rostral plate, ROM 51369, ventral view, x 10 (BHH-A); 26, 27, thoracic segment,
ROM 51370, dorsal and anterior views, X 10 (ABR TID); 28, 29, thoracic segment, ROM 51371, dorsal and anterior views, x 10 (ABR TID);
30, thoracic segment, ROM 51372, dorsal view, x 10 (ABR TID).
712
JOURNAL OF PALEONTOWGY,
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1998
ADRAIN-SILURIAN
TRILOBITES
713
Discussion.-Hyrokybe
mitchellae is most similar among de- obviously distinct in its more slender, shorter, and widely
scribed species to H. lightfooti. The species are distinguished on splayed spines. The pygidia of Figure 5.22 and 5.23, with very
blunt, subquadrate spines, also represent a new species. In the
the basis of the occurrence in H. mitchellae of a much smaller
median occipital spine; slightly finer dorsal glabellar tuberculanear-effacement of the second ring furrow and size and distrition; finer and more numerous marginal row of librigenal lateral bution of tuberculate sculpture, they are most similar to H. lightborder tubercles; much broader pygidial spines; genera1.l¥_deeper footi. It is not possible at present to associate the handful of
recovered librigenae, hypostomes, and cranidial fragments with
first pygidial ring furrow; and retention of the second pygidal
ring furrow as a faint to moderate median pit, versus usually
one or the other of the pygidial morphotypes.
wholly effaced.
Material.-Illustrated
specimens ROM 51330-51335 from
Material.-Holotype
pygidium ROM 51341 (Fig. 7.11, 7.12, section BH 2 3-6.5 m, southern Baillie-Hamilton Island, and
7.15,7.16) from locality BHH; paratypes ROM 51336-561340,
section ABR 19m, northwestern Cornwallis Island; both Cape
51342-51346 from locality BHH and section BHL 1 92 m, Cape
Phillips Formation, Wenlock (lower Homerian; Cyrtograptus
Phillips Formation, Wenlock (upper Sheinwoodian; Cyrtograplundgreni-Monograptus testis Zone), central Canadian Arctic.
tus pemeri-Monograptus opimus Zone), southern Baillie-HamHYROKYBEspp. group 2
ilton Island, central Canadian Arctic.
Figure 9.21-9.29
Etymology.-After
Joni Mitchell.
Discussion.-Exactly
how many species of Hyrokybe are repHYROKYBEJULLI Chatterton and Perry, 1984
resented in the interval ABR 2 18-27 m at Abbott River is
Figures 8, 9.1-9.20
uncertain. Most species of other groups are shared with either
Hyrokybe julli CHA1TERTONAND PERRY, 1984, p. 32, pI. 10, figs. 1ABR TID or BHL 1 921BHH. It is therefore somewhat surpris29; pI. 11, figs, 1-31; pI. 12, figs. 1-20; pI. 13, figs. 6-14; text-fig.
ing that what little information is available indicates the poorly
16.
known ABR 2 18-27 species are apparently new. Two pygidia
Discussion.-There
do not appear to be any significant and have been recovered, but they do not seem conspecific. One
pervasive differences between the Cape Phillips Formation sam- (Fig. 9.28) is similar to those of the Baillie-Hamilton H. mitchple and the slightly older type material from the southern Mac- ellae in its broad and spatulate spine morphology, but differs in
kenzie Mountains. Many of the cranidia illustrated by Chatterton
the presence of more robust and much more densely distributed
and Perry (1984, e.g., pI. 10, figs. 2, 7, pI. 11, fig. 3) appear to tubercles. The other pygidium (Fig. 9.29) is similar to those of
differ in the possession of longer median occipital and genal H. lightfooti, but differs in its more arcuate first ring furrow and
spines, as well as several accessory spines on the rear of the narrower spine tips. One of the recovered cranidia (Fig. 9.22,
occipital ring. This is entirely a function of the small size of the 9.27) has a transversely straight SI and, based upon the arguspecimens. The only specimen that is of a size typical of the ments above (see section on associations under H. lightfooti)
Cape Phillips sample (Chatterton and Perry, 1984, pI. 10, figs. could belong with either pygidium. The other cranidium, how3-6) shows these spines reduced to tubercles and is morphologever (Fig. 9.21, 9.26), has a prominently "kinked" SI, and may
ically indistinguishable from the Arctic specimens. One pygid- represent H. julli or a closely related species. Therefore, it is
ium illustrated by Chatterton and Perry (1984, pI. 10, figs. 17, possible that three species of Hyrokybe occur together in this
18) differs from all other assigned pygidia in its very narrow
interval and associated talus boulders.
spines with strongly curved tips; it is doubtfully conspecific.
Material.-Illustrated
specimens ROM 51385-51391 from
Material.-Assigned
specimens ROM 51361-51368, 51373section ABR 2 18-27 m and talus boulder ABR TIC(3), Cape
51384, from locality BHH and section BHL 1 92 m, southern
Phillips Formation, Wenlock (upper Sheinwoodian; CyrtograpBaillie-Hamilton Island, and talus boulder ABR TID, northtus pemeri-Monograptus
opimus Zone), near Abbott River,
western Cornwallis Island; all Cape Phillips Formation, Wenlock
northwestern Cornwallis Island, central Canadian Arctic.
(upper Sheinwoodian; Cyrtograptus pemeri-Monograptus opimus Zone), central Canadian Arctic.
Genus PARAYOUNGIA
Chatterton and Perry, 1984
HYROKYBEspp. group 1
Figure 5.18-5.23
Discussion.-Two
species of Hyrokybe occur in the' Cyrtograptus lundgreni-Monograptus testis Zone of the Cape Phillips
Formation, but few specimens have been recovered. The pygidium of Figure 5.19 is most similar to those of H. julli, although
ichiyamella KOBAYASHIANDHAMADA,1986, p. 456.
Type species.-Parayoungia
eleyae Chatterton and Perry,
1984, from the Wenlock (Sheinwoodian) of the central Mackenzie Mountains, Northwest Territories, Canada.
Other species.-Youngia
brennardi Chatterton and Perry,
1984, Llandovery (Telychian), central Mackenzie Mountains,
FIGURE9-i-20,
Hyrokybe julli Chatterton and Perry, 1984, section BHL I 92 m, locality BHH, and talus boulder ABR TID, Cape Phillips
Formation, Wenlock (upper Sheinwoodian; Cyrtograptus perneri-Monograptus opimus Zone), southern Baillie-Hamilton and northwestern Cornwallis Islands, central Canadian Arctic. i, left librigena, ROM 51373, external view, XIO (BHL I 92 m); 2, right librigena, ROM 51374, external
view, X 10 (BHL I 92 m); 3, 7, right Iibrigena, ROM 51375, internal and external views, X 10 and X7.5 (ABR TID); 4, left librigena, ROM
51376, external view, XIO (ABR TID); 5,9,13, i7, pygidium, ROM 51377, ventral, dorsal, posterior, and left lateral views, X7.5 (BHH-A); 6,
left librigena, ROM 51378, external view, XIO (BHL I 92 m); 8, right librigena, ROM 51379, external view, X7,5 (BHII 92 m); JO, pygidium,
ROM 51380, dorsal view, XIO (BHL I 92 m); 11, right librigena, ROM 51381, external view, XIO (ABR TID); i2, i6, pygidium, ROM 51382,
dorsal and left lateral views, XIO (ABR TID); i4, i8. i9, pygidium, ROM 51383, dorsal, left lateral, and posterior views, X6 (ABR TID); i5,
20, pygidium, ROM 51384, dorsal and left lateral views, XIO (ABR TTD). 2i-29, Hyrokybe spp. 2, section ABR 218-27 m and talus boulder
ABR TTC(3), Cape Phillips Formation, Wenlock (upper Sheinwoodian; Cyrtograptus perneri-Monograptus opimus Zone), southern Baillie-Hamilton and northwestern Cornwallis Islands, central Canadian Arctic. 2i, 26, cranidium, ROM 51385, dorsal and right lateral views, X 10 (ABR 2
18 m); 22.27. cranidium, ROM 51386, dorsal and left lateral views, X7.5 (ABR TIC(3»; 23, right librigena, ROM 51387, external view, XIO
(ABR TIC(3»; 24, right librigena, ROM 51388, external view, x7.5 (ABR 2 27 m); 25. right Iibrigena, ROM 51389, external view, XIO (ABR
227 m); 28, pygidium, ROM 51390, dorsal view, XIO (ABR 2 27 m); 29. pygidium, ROM 51391, dorsal view, XIO (ABR 2 27 m).
714
JOURNAL OF PALEONTOLOGY,
V. 72, NO.4, 1998
ADRAlN-SILURlAN TRIWBrrES
715
ii-Parayoungia
new species A, locality BHH, and talus boulder ABR TID, Cape Phillips Formation, Wenlock (upper Sheinwoodian;
Cyrtograptus pemeli-Monograptus OpimllS Zone), southern Baillie-Hamilton and northwestern Cornwallis Islands, central Canadian Arctic. 1. 5.
cranidium, ROM 51406, dorsal and left lateral views, X10 (BHH-C); 2,6. cranidium, ROM 51407, dorsal and anterior views, X6 (ABR TID);
3. left librigena, ROM 51408, external view, X 10 (ABR TID); 4, right Iibrigena, ROM 51409, external view, X 10 (ABR TID); 7. right Iibrigena,
ROM 51410, external view, X10 (ABR TID); 8, i2, pygidium, ROM 51411, posterior and dorsal views, x7.5 (ABR TID); 9-11, pygidium,
ROM 51412, right lateral, dorsal, and posterior views, x10 (ABR TID).
FlGURE
Canada; Youngia folinsbeei Chatterton and Perry, 1984, Llandovery (Telychian), central Mackenzie Mountains, Canada; Parayoungia mclaughlini new species, Wenlock (Sheinwoodian),
Arctic Canada; P. tuberculata Chatterton and Perry, 1984, Wenlock (Sheinwoodian), central Mackenzie Mountains, Canada;
Parayoungia new species A, herein, Wenlock (Sheinwoo<1ian),
Arctic Canada.
Diagnosis.-Denal
and occipital spines robust and elongate;
librigenal lateral border furrow shallowed abruptly posteriorly,
not reaching posterior margin; posterior border of hypos tome
with transverse, posteriorly imbricate fold across median part;
thorax and pygidium with large dorsal accessory spines; pygidium strongly reduced, first ring furrow reduced to lateral pits, in
some species linked by a chevron-shaped furrow; second ring
furrow entirely effaced.
Discussion.-When
they erected the genus, Chatterton and
Perry (1984) indicated the probable derivation of the taxon from
Llandovery species of Youngia. In the analysis above, a core
group of Youngia forms a potential clade. In agreement with
Chatterton and Perry's suggestion, two species assigned by them
to Youngia appear to be closely related to the Wenlock Parayoungia species, on the basis of a number of shared derived
character-states. To maintain the monophyly of Youngia and in
the interests of a phylogenetic classification, these species are
reassigned to Parayoungia herein.
Discussion of the relationship between Youngia and Parayoungia is given in discussion of the former genus above.
Chatterton and Perry (1984, p. 39) commented upon the similarity between Parayoungia and Holia, concluding that the resemblances were convergent. Their arguments were based on
the considerable stratigraphic gap between the genera, together
with the observation that the Llandovery Youngia "cannot be
regarded as being directly morphologically intermediate between
these two genera." In the context of the character analysis
above, some discussion of the respective morphologies is now
possible.
~
iD-i-20.
22, Parayoungia mclaughlini new species, section BH 1 110 m (except where noted otherwise), Cape Phillips Formation, Wenlock
(Sheinwoodian; Monograptus instrenuus-Cyrtograptus /Wlobus Zone), southern Baillie-Hamilton Island, central Canadian Arctic. i. 5. cranidium,
ROM 51392, dorsal and left lateral views, X5 (BH 1 112 m); 2-4, 6, cranidium and right Iibrigena, ROM 51393, oblique, anterior, ventral, and
right lateral views, X1O; 7. left Iibrigena, ROM 51394, external view, X10 (BHL 10m); 8, right librigena, ROM 51395, external view, X7.5
(BHL 10m); 9, JO, thoracic segment, ROM 51396, dorsal and anterior views, X7.5 (BH 1 109 m); 11, i2, left librigena, ROM 51397, internal
and external views, X1O; 13. 20, pygidium, holotype, ROM 51398, dorsal and posterior views, X1O; i4, i6, 17. pygidium, ROM 51399, dorsal,
left lateral, and posterior views, X5 (BH I 143 m); i5, left librigenal, ROM 51400, external view, X 10; i8. i9. 22, pygidium, ROM 5140 l, right
lateral, ventral, and dorsal views, X 10. 2i, 23-27, Parayoungia sp., section BH 2 42 m, Cape Phillips Formation, Ludlow (Gorstian; Lobograptus
progenitor Zone), southern Baillie-Hamilton Island, central Canadian Arctic. 2i, cranidial fragment, ROM 51402, dorsal view, X5; 23, posterior
border spine or possibly thoracic fuleral spine, ROM 51403, oblique view, X7.5; 24, 25, thoracic segment, ROM 51404, dorsal and anterior views,
X7.5; 26. 27. encrusted pygidium, ROM 51405, dorsal and posterior views, X5.
flOURE
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JOURNAL OF PALEONTOLOGY, V. 72, NO.4, 1998
Holia and Parayoungia resemble each other in the possession
of stout genal and occipital spines, and particularly in the possession of long, dorsally directed pygidial spines and a small tail
with a ring furrow reduced to prominent lateral pits. Few of
these features are potentially synapomorphic. Long genal spines
are symplesiomorphic. The pygidial modifications are superficially similar, but almost certainly not homologous. The dorsally
produced spines in Holia are by every indication the true first
spines, whereas those in Parayoungia are very likely accessory
spines, serial homologues of the similarly produced thoracic accessory spines. The true first spines in advanced species of Parayoungia are likely merged into the disc-shaped pygidial base.
And although pygidia of the two genera share a distinctive reduction of a ring furrow to lateral pits, in Holia the furrow thus
reduced is the second, whereas in Parayoungia it is the first.
Finally, Holia possesses a large suite of plesiomorphic features
while Parayoungia has many prominent apomorphies indicating
the derived position within the subfamily shown on Figures 2
and 3.
Kobayashi and Hamada (1985, p. 213, pl. 28, figs. 3a-3c)
described a single fragmentary cranidium from the lower Ludlow of Japan as Dindymene (?) megacranidia. The specimen is
incorrectly (obliquely) oriented in their figures, but it is a representative of Parayoungia, closely resembling advanced species
like P. eleyae and P. mclaughlini in its fully circumscribed and
considerably inflated LI. The same authors later (1986, p. 456,
pI. 91, figs. 3a-3f) described a second cranidium (reconstructed
as a cephalon in their text-fig. A, although librigenae are not
apparent on the photographs) as a new genus and species, /chiyamella subglobula, again assigned to the Dindymeninae (Dindymene (?) sp. indet. of Kobayashi and Hamada, 1986, pI. 91,
fig. 4, is a lichid cranidium). This specimen is from a locality
nearby to and of the same age as that of Dindymene (?) megacranidia, and based on such evidence as is available they appear
to represent a single species of Parayoungia, of which /chiyamella is placed in subjective junior synonymy.
PARAYOUNGIA
MCLAUGHUNI
new species
Figure 10.1-10.20, 10.22
Youngia sp. 1; PERRY AND CHATI'ERTON, 1977, p. 298, pI. 3, figs. 20,
21 (non figs. 16, 17 = Parayoungia new species A; non 18, 19 =
Hyrokybe mitchellae new species; non fig. 22 = Hyrokybe youngi
new species)
Diagnosis.-Glabella
posteriorly broad; dorsal tuberculate
sculpture moderately coarse; median occipital spine short; pygidial accessory spines with bases widely separate; pits of first
pygidial ring furrow aligned opposite accessory spine bases.
Discussion.-This
species is so similar to the contemporaneous Parayoungia eleyae Chatterton and Perry, 1984, that extended description is unnecessary. The following comparison
notes all differences observed between the species. Parayoungia
mclaughlini new species is distinguished from P. eleyae in the
possession of a posteriorly broader glabella (compare especially
the transverse distance between the isolated LI lobes, Fig. 10.1
vs. Chatterton and Perry, 1984, pI. 21, figs. 1,3,6); an apparently smaller and shorter median occipital spine; more dorsally
incised S2; generally coarser and less dense dorsal tuberculate
sculpture; slightly smaller librigenal field; large pygidial dorsal
accessory spines with bases more widely separated (compare
Fig. 10.17, 10.20 with Chatterton and Perry, 1984, pI. 22, figs.
7, 18); and pair of pits marking first ring furrow aligned opposite
bases of large accessory spines, versus well behind the spines
in P. eleyae (compare Fig. 10.13, 10.14, 10.22 with Chatterton
and Perry, 1984, pI. 21, fig. 21, pI. 22, figs. 5, 25).
Material.-Holotype
pygidium ROM 51398 (Fig. 10.13,
10.20) and paratypes ROM 51392-51397, 51399-51401, from
sections BH 1 109-143 m and BHL 10m, Cape Phillips Formation, Wenlock (mid-Sheinwoodian; Monograptus instrenuusCyrtograptus kolobus Zone), southern Baillie-Hamilton Island,
central Canadian Arctic.
Etymology.-After
Murray McLaughlin.
PARAYOUNGIA
new species A
Figure 11
New genus odontopleurid?; PERRY AND CHATI'ERTON, 1977, p. 315, pI.
I, figs. 24-29.
Youngia sp. I; PERRY AND CHATI'ERTON, 1977, p. 298, pI. 3, figs. 16,
17 (non figs. 18, 19 = Hyrokybe mitchellae new species; non figs.
20, 21 = Parayoungia mclaughlini new species; non fig. 22 = Hyrokybe youngi new species)
Discussion.-A
new upper Sheinwoodian species is represented by material from the large talus boulder ABR TID on
northwestern Cornwallis Island. In the absence of definitive morphological information, a single cranidium from Baillie-Hamilton Island is also assigned based on the many examples of
shared species known from the respective faunas. However, the
cranidium is much smaller than the single large cranidial fragment known from ABR TID, and the possibility exists that it
may prove to represent a separate species. Parayoungia new
species A is most similar among described species to the slightly
older P. tuberculata Chatterton and Perry, 1984. In particular,
the Baillie-Hamilton cranidium (Fig. 11.1, 11.5) is nearly identical to similarly-sized cranidia of P. tuberculata (compare Chatterton and Perry, 1984, pI. 19, fig. 14). The librigena of Parayoungia new species A (Fig. 11.3, 11.4) is also most similar
to that of P. tuberculata (Chatterton and Perry, 1984, pl. 19, fig.
18) in the coarse and crowded field and border tubercles. The
pygidium of Parayoungia new species A is most similar, however, to that of P. eleyae, the main differences being a coarser
and sparser tuberculate sculpture and more prominently retained
second spines in the Arctic species.
Material.-Assigned
specimens ROM 51406-51412 from locality BHH and section BHL 1 92 m, southern Baillie-Hamilton
Island, and talus boulder ABR TID, northwestern Cornwallis
Island; all Cape Phillips Formation, Wenlock (upper Sheinwoodian; Cyrtograptus pemeri-Monograptus
opimus Zone),
central Canadian Arctic.
PARAYOUNGIA
sp.
Figure 10.21, 10.23-10.27, 10.23
Discussion.-Rare
and fragmentary sclerites from the lower
Ludlow of southern Baillie-Hamilton Island represent one of the
youngest acanthoparyphine species yet described (similar in age
to P. megacranidia; see above), although fragmentary material
of a species of Parayoungia has been collected from the upper
Ludlow Douro Formation on Cornwallis Island. The coarsely
encrusted pygidium (Fig. 10.26, 10.27) leaves no doubt that a
species similar to Parayoungia eleyae or P. mclaughlini is represented, but the material is otherwise not sufficient for detailed
comparisons to be made.
Material.-Assigned
specimens ROM 51402-51405 from
section BH 2 42 m, Cape Phillips Formation, Ludlow (Gorstian;
Lobograptus progenitor Zone), southern Baillie-Hamilton Island, central Canadian Arctic.
ACKNOWLEDGMENTS
Fieldwork was made possible through generous logistical support from the Polar Continental Shelf Project, grants under the
Research Agreements Program of the Department of Energy,
Mines, and Resources (Canada) and the Natural Sciences and
Engineering Research Council (Canada), Boreal Alberta Research grants and Northern Science Training grants from the
ADRAIN-SILURIAN
Canadian Circumpolar Institute, and a grant-in-aid from the Paleontological Society. Costs of technical preparation were partly
met by NSERC operating grants to B. D. E. Chatterton (University of Alberta) and A. C. Lenz (University of Western Ontario). L. Ramsköld (University of Uppsala) is thanked for comments on the manuscript, for permission to cite his work in
progress on the Boree Creek fauna, and for pointing me in the
direction of the Japanese Ludlow species. I am especially grateful to G. D. Edgecombe (Australian Museum) for review of the
manuscript, and for information on material in his institution.
Edgecombe, Chatterton, N. E. Vaccari, and B. G. Waisfeld-gave
permission to cite their work in progress on Argentinian Ordovician faunas. Chatterton and H. B. Whittington provided helpful
reviews for the Journal.
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ACCEI'I'ED2 DECEMBER1997
Society
TRILOBITES FROM LOWER MISSISSIPPIAN STARVED BASIN FACIES OF
THE SOUTHERN UNITED STATES
DAVID K. BREZINSKI
Maryland Geological Survey, 2300 St. Paul Street, Baltimore 21218
ABSTRAcr-A distinctive trilobite fauna occurs within condensed statigraphic sections of the Lower Mississippian (Tournaisian) Chappel
Limestone of the Llano region of Texas, the Welden Limestone of Oklahoma, and the Chouteau Limestone of Union County, Illinois.
The seven species comprising this fauna are interpreted to have inhabited sediment-starved basinal environments. The starved-basin
facies existed in the south-central United States throughout the Tournaisian (Kinderhookian to Osagean). Two species from this fauna,
Australosutura llanoensis, and Carbonocoryphe planucauda, are new, The remaining five species, Griffithidella doris (Hall), Griffithidella alternata (Girty), Carbonocoryphe depressa (Girty), Thigiffides roundyi (Girty), and Pudoproetus chappelensis (Hessler), are
restricted to starved-basin facies.
INTRODUCTION
L
trilobites of North America are well
known from shelf facies. Hessler (1963, 1965), Chamberlain (1969, 1977) and Brezinski (1986, I988a, b) are but a few
of the Lower Mississippian trilobite studies that have described
the relatively diverse trilobite faunas from shelf limestone. Brezinski (1990) has shown that a low-diversity Osagean fauna
composed primarily of the genus Australosutura occupied platform-edge Waulsortian reefs in northeast Oklahoma. Unlike the
deep-water European Kulm facies where a distinctive fauna is
well known (see for example Hahn and Hahn, 1988), off-shelf
species from North American Lower Mississippian strata are
very poorly known. The only documented North American occurrences of Carboniferous off-shelf trilobite species is by Hahn,
Paull, and Chamberlain (1980), who described a species of Carbonocoryphe from flysch deposits in Idaho.
Paleogeographic reconstructions of Lane (1978), Lane and De
Keyser (1980) and Gutshick and Sandberg (1983) have shown that
extensive off-shelf, deep-water environments existed during the
Early Carboniferous and stretched from Texas into southern Illinois
and Indiana. Examination of trilobite collections from Texas, Oklahoma, and Illinois indicates that a generically distinct trilobite fauna
OWER MISSISSIPPIAN
inhabited these deep-water, starved-basin milieus during the Lower
Mississippian (Kinderhookian-Osagean). The composition of this
fauna is unlike that known from shelf faunas and appears to have
some generic affinities to the European Kulm facies. The purpose
of this paper is to redescribe the trilobite species known from the
Chappel Limestone of central Texas, the Welden Limestone of the
Lawrence Uplift of south-central Oklahoma, and the Chouteau
Limestone of southern Illinois, and to discuss their paleozoogeographic and paleoenvironmental implications.
This study is based primarily upon existing museum collections of the U.S. National Museum, Chicago Field Museum of
Natural History, and Carnegie Museum of Natural History. Additional collections were made from the Chappel Limestone near
San Saba, Texas and Welden Limestone along Jackfork Creek
near Ada, Oklahoma by the author and A. Kollar utilizing funding from the Carnegie Museum of Natural History.
Terminology for trilobite species conforms to Harrington
(1959) and Richter and Richter (1949). Specimens utilized in
this study are reposited in the Chicago Field Museum of Natural
History (CFM), the Section of Invertebrate Paleontology of Carnegie Museum of Natural History (CM), and the United States
National Museum (USNM).