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Life Sciences Contributions : 20
Royal Ontario Museum

The Ordovician Trilobite
Pseudogygites Kobayashi

in Eastern and
Arctic North America

Rolf Ludvigsen

ROM

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LIFE SCIENCES CONTRIBUTIONS
ROYAL ONTARIO MUSEUM
NUMBER 120

ROLF LUDVIGSEN The Ordovician Trilobite
Pseudogygites Kobayashi
in Eastern and
Arctic North America

ROYAL ONTARIO MUSEUM
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Research Associate, Department of Invertebrate Palaeontology, Royal Ontario Museum.

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Ludvigsen, Rolf, 1944-

The Ordovician trilobite Pseudogygites Kobayashi in
eastern and arctic North America
(Life sciences contributions; no. 120 ISSN 0384-8159)
Bibliography: p.
ISBN 0-88854-247-X pa.
1. Trilobites. 2. Paleontology—Ordovician.
2. Paleontology — North America. I. Royal Ontario
Museum. II. Title. II. Series.
QE821.L83 DOs) 393 C79-0948 14-1

Publication date: 2 November 1979
ISBN 0-88854-247-X
ISSN 0384-8159

© The Royal Ontario Museum, 1979
100 Queen’s Park, Toronto, Canada MSS 2C6
PRINTED AND BOUND IN CANADA AT THE ALGER PRESS

The Ordovician Trilobite
Pseudogygites Kobayashi
in Eastern and

Arctic North America

Abstract

Pseudogygites Kobayashi is an endemic North American isoteline
trilobite which occurs widely in bituminous shale and limestone units
of Late Ordovician age (Maysvillian and Richmondian) in southern
Ontario and neighbouring areas and the Canadian arctic. The
originally designated type species, Asaphus canadensis Chapman
from Whitby, Ontario, is a junior subjective synonym of Asaphus?
latimarginata Hall from the Watertown area of New York.
Pseudogygites latimarginatus is rare in the Cobourg Formation of
southern Ontario, but is very abundant in the overlying lower Whitby
Formation. Three new species are described: Pseudogygites hudsoni
from Southampton Island, Hudson Bay; P. akpatokensis from
Akpatok Island, Ungava Bay; and P. arcticus from Devon, Bathurst,
and Cornwallis islands.

Preservation of both carcasses and undisturbed exuviae of P.
latimarginatus permits reconstruction of the moulting behaviour of
this species.

In ecologic terms, the Cobourg-Whitby contact identifies the level
at which the environment changed from shallow, well oxygenated,
and warm to shallow, oxygen-poor, and cold. P. latimarginatus was
probably derived from /sotelus gigas by paedomorphosis (neoteny)
and its great abundance in the lower Whitby is attributed to fortuitous
preadaptation to a new ecologic setting. The four known species of
Pseudogygites could be iterative paedomorphs of different species of
Tsotelus.

Introduction

The endemic North American trilobite Pseudogygites occurs in vast numbers at a few
localities in southern Ontario and widely in arctic Canada. Despite the wealth of
material, the genus has remained poorly known since it was established by Kobayashi
in 1934. Its definition was perfunctory and, furthermore, hidden in a descriptive work
on Korean trilobites. Kobayashi chose Asaphus canadensis Chapman, 1856 from

]

Whitby, Ontario, as the type species, but presented no illustrations and, apparently,
made no attempt to trace the type specimens. His diagnosis of Pseudogygites was
brief, but concise: ‘‘Basilicus-like asaphids; hypostoma forked; isoteliform suture;
glabella urceolate, well defined by the dorsal furrow; glabellar furrows rather
indistinct.”’ (Kobayashi, 1934:460, 461). As Fritz (1959:1120) suggested,
Kobayashi probably based his concept of Pseudogygites on the specimen from
Ottawa that was illustrated by Raymond (1913: pl. 6, fig. 1) and not on that illustrated
by Chapman (1858) from the type area because the Ottawa specimen was the only
specimen published at that time showing the attributed diagnostic features. This
specimen has also served as a model for the reconstruction of P. canadensis in the
Treatise (Jaanusson, 1959: fig. 253-5a).

The purpose of this paper is to assess critically the material of Pseudogygites that
occurs in southern Ontario, New York, Southampton Island in Hudson Bay, Akpatok
Island in Ungava Bay, and at various localities in the Canadian arctic (Cornwallis,
Bathurst, and Devon islands), and to clarify the age and origin of this genus.

Pseudogygites has been recorded from eastern and arctic North America numerous
times, but a full description of any species has not been presented since Chapman
described Asaphus canadensis more than a century ago. Material from Craigleith on
Georgian Bay has been illustrated repeatedly, but generally by poorly or indifferently
preserved specimens that do not show critical features.

In the absence of concise morphological information, the classification and origin
of Pseudogygites remain speculative. Raymond (1912:115) suggested that
Pseudogygites was derived from Asaphus and Basilicus; Jaanusson (1959) assigned
it, with query, to the Isotelinae; and Whittington (1966:712) thought it was possibly
related to Ogygiocaris or **Pseudobasilicus’’.

The ventral morphology of Pseudogygites, in particular, needs clarification. The
hypostome has been described as being forked (Raymond, 1912; Kobayashi, 1934), a
statement that must be based on Chapman’s (1859a) original illustration.
Documentation of the morphology of the hypostome is critical because it provides the
best evidence for distinguishing Pseudogygites from its near-homeomorph,
Ogyginus.

The assignment of Pseudogygites to the Asaphidae naturally implies the presence
of a median connective suture crossing the doublure. The available holaspid
specimens from southern Ontario, without exception, lack median sutures whereas
those from arctic Canada apparently possess such sutures. Pseudogygites 1s
unquestionably an asaphid and not a nileid and, therefore, the presence of yoked free
cheeks in at least one species is highly pertinent in evaluating the family assignment
of other asaphidlike genera that lack median sutures.

Systematic Palaeontology

Repositories

The illustrated specimens are in the Royal Ontario Museum, Toronto (ROM); the
Geological Survey of Canada, Ottawa (GSC); the American Museum of Natural

Bathurst
Sian
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NORTHWEST
TERRITORIES

Southampton 2
Island ave Bs
g

Akpatok
Island

\ QUEBEC

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MICHIGAN

Fig. | Locality map of eastern and arctic North America showing occurrences of four species of
Pseudogygites: circles—P. latimarginatus (Hall), squares—P. hudsoni n. sp., diamond—P.

akpatokensis n. sp., triangles—P. arcticus n. sp.

History, New York (AMNH); and the Museum of Comparative Zoology, Harvard
University, Cambridge, Massachusetts (MCZ).

Suborder Asaphinae Salter
Superfamily Asaphacea Burmeister
Family Asaphidae Burmeister

Discussion

Prominent in the diagnosis of the family Asaphidae is the presence of a median
connective suture separating the free cheeks. The absence of such a suture has been
deemed sufficient evidence to reject otherwise asaphidlike genera from this family
(for example, Brachyaspis Salter; Jaanusson, 1959:355) and the presence of a
median suture has been used to assign otherwise nileidlike genera to this family (for
example, Varvia Tjernvik; Jaanusson, 1959:354). Therefore, it is of considerable
interest to encounter an undoubted asaphid, Pseudogygites, which includes species
both with and without median connective sutures. All examined specimens of P.
latimarginatus from southern Ontario possess yoked free cheeks and P. hudsoni n.
sp. and P. arcticus n. sp. from the Canadian arctic possess functional median
connective sutures. A parallel situation is found among species of /sotelus. Most
species of /sotelus possess median connective sutures, but a number of recent reports
of individuals of [sotelus gigas DeKay with yoked free cheeks demonstrate that this is
not an uncommon condition (Henningsmoen, 1975; Jaanusson, 1975; this paper, Fig.
6H).

Whereas the presence of a median connective suture remains an important
diagnostic criterion for recognizing Asaphidae, the available evidence from
Pseudogygites and Isotelus suggests that the mere absence of such a suture is not
sufficient, in itself, to justify exclusion of a taxon from this family.

In this regard, reference should be made to the family assignment of Brachyaspis
Salter. The type species, Brachyaspis rectifrons (Portlock) from Ireland, lacks a
median connective suture. This conclusion led Whittington (1954) to suggest that B.
rectifrons may not be congeneric with similar trilobites with median sutures from
North America which have been assigned to Brachyaspis. Jaanusson (1975) and
Chatterton and Ludvigsen (1976) suggested that B. rectifrons is not an asaphid but a
nileid, and Chatterton and Ludvigsen assigned the North American species of
‘‘Brachyaspis’’ to a new asaphid genus, Nahannia. The absence of a median suture
cannot now be taken as good evidence that B. rectifrons is a nileid. This taxon may
well be an asaphid, but confirmation (that is, an associated hypostome) should be
sought. B. rectifrons remains an incompletely known taxon and it would be unwise,
at the present time, to synonymize Nahannia with Brachyaspis. Setting aside the
question of a median connective suture, the large palpebral lobes and the absence of
distinct segmentation on the pygidial axis would still characterize Nahannia.

By similar reasoning, the genus Varvia which bears ‘‘a remarkable similarity to
species of Nileus’’ (Fortey, 1975:35) should be removed from the Asaphidae and
placed in the Nileidae.

Subfamily Isotelinae Angelin

Genus Pseudogygites Kobayashi, 1934

Type Species

The type species is Asaphus? latimarginata Hall, 1847. The original type species
designated by Kobayashi (1934), Asaphus canadensis Chapman, 1856 is herein
considered a junior subjective synonym of Hall’s species.

Diagnosis

A genus of Isotelinae with well-defined flattened borders on cephalon and pygidium,
a distinctly outlined glabella which expands in front of the eyes, three pairs of faint
lateral glabellar furrows, and genal angles produced into slim spines. Pygidium has 7
to 14 faintly furrowed pleurae and a well-defined axis. Cephalic doublure is broad
and flat medially, narrower and convex laterally; it lacks a vincular socket. Median
connective suture may be present or fused. Hypostome is deeply notched posteriorly.

Discussion

In establishing Pseudogygites, Kobayashi (1934) did no more than present a brief
diagnosis and select Asaphus canadensis as type species. This species was defined in
a series of brief papers in the late 1850s by E.J. Chapman, then Professor of
Mineralogy and Geology at the University of Toronto. Chapman’s first note in 1856
merely announced the discovery of a new species, Asaphus canadensis, in the ‘*Utica
Schist’’ at Whitby, Canada West [Ontario]. A brief description of the new species
was presented in 1857, a more complete description and an illustration followed in
1858, and the hypostome was illustrated and described in 1859a. Chapman did not
select a type, and the complete specimen on which his only illustration was based
(Chapman, 1858:232) cannot now be located in the collections of the University of
Toronto or in the Royal Ontario Museum. Chapman (1858) noted that A. canadensis
occurs in the township of Whitby on Lake Ontario and on Georgian Bay, but the
former must be considered the type locality—this being the only locality mentioned in
the initial paper. Even in the absence of types, there is little problem in determining
the critical features of A. canadensis. Chapman’s (1858) description and illustration
are adequate and a pygidium from the type locality which Chapman identified as A.
canadensis and presented to the Museum of Palaeontology at the University of
Toronto is still extant (Fig. SH). This specimen, now in the Royal Ontario Museum, is
clearly conspecific with the more abundant material from the Craigleith locality of
Georgian Bay. An uncrushed specimen from the Eastview Formation at Ottawa was
assigned to Ogygites canadensis by Raymond (1913) and this specimen appears to
have formed the basis for Kobayashi’s concept of Pseudogygites canadensis and the
model for Jaanusson’s (1959) reconstruction of the cephalon of P. canadensis. The
available material suggests that only a single species of Pseudogygites occurs at the
aforementioned localities on Lake Ontario and Georgian Bay, and in the Ottawa area.
The species Asaphus? latimarginata Hall, 1847 presents fewer taxonomic
problems, but unfortunately it was based on float material consisting of two

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incomplete pygidia on loose pieces of shale from Watertown, New York. Both of
these specimens are extant and the more complete is selected as lectotype and
illustrated photographically for the first time (Fig. 4A).

Most authors since the late 1950s have assumed that Pseudogygites canadensis and
P. latimarginatus are synonymous. Chapman did not mention Hall’s species in his
1856 paper, an omission that James Hall was quick to point out. In a letter to
Chapman in 1856 or 1857 (Chapman, 1857), Hall suggested that Asaphus canadensis
was identical with Asaphus? latimarginata. Chapman (1858), however, was reluctant
to apply Hall’s name to the material from Canada, and, without citing differences,
merely stated that the Canadian material was more complete than that from New
York. Chapman further marshalled the weight of the opinion of Joachim Barrande
who had previously stated (1852:647) that Hall’s specimens of Asaphus were too
incomplete to be determined with any certainty. Hall’s figures of A.? latimarginata
are less than satisfactory and somewhat diagrammatic, but the lectotype pygidium is
sufficiently well preserved to show that it is conspecific with pygidia from the type
locality of A. canadensis. Therefore, Asaphus canadensis becomes a junior
subjective synonym of A.? latimarginata which now becomes the type species of
Pseudogygites.

Pseudogygites shares a number of features with Jsotelus, especially with immature
specimens of /sotelus. From mature specimens of /sotelus, Pseudogygites may be
distinguished by its flat cephalic borders, long genal spines, well-defined axial
furrows on the cephalon and pygidium, longer hypostome, and by the absence of a
vincular socket in front of the genal angle (compare Fig. 12D and Fig. 12H).

In dorsal view, Pseudogygites is surprisingly similar to the ogygiocaridiniid
Ogyginus Raymond from the Llanvirnian and Llandeilian of Britain. Whittard (1964)
has shown that Jaanusson (1959) was in error in depicting Ogyginus with a dorsal
intramarginal facial suture. This suture is marginal in front of the glabella and
provides one of the few unequivocal differences with Pseudogygites. The other
obvious difference lies in the hypostome which is forked in Pseudogygites and entire
in Ogyginus. Pseudogygites is younger than Ogyginus and there is no evidence to
show that they are closely related. Raymond (1912) pointed out that the similarity of
the two taxa appears to be the result of parallel evolution.

Pseudogygites latimarginatus (Hall, 1847)
Figs. 2, 3, 4, 5, 6A—E, 7A-C, 12A—D

Asaphus? latimarginata Hall, 1847:253, pl. 66, figs. 4a, 4b.
Asaphus canadensis Chapman, 1856:482.

Asaphus canadensis—Chapman, 1857:47.

Asaphus canadensis—Chapman, 1858:231, unnumbered figure.
Asaphus canadensis—Chapman, 1859a:1, unnumbered figure.
Asaphus canadensis—Logan, 1863: fig. 201.

Basilicus canadensis—Raymond, 1910:62.

Ogygites canadensis—Raymond, 1912:pl. 1, fig. 2.

Ogygites canadensis—Raymond, 1913:43, pl. 6, fig. 1.
Ogygites canadensis—Parks, 1928:48, 53, 55.

Pseudogygites canadensis—Kobayashi, 1934:461.

WS
NS

ows

Ogygites latimarginatus—Kay, 1937:pl. 10, unnumbered figure.

Ogygites latimarginatus—Hussey, 1952:pl. 9, fig. 22.

Ogygites latimarginatus—Wilson, 1957:pl. 5, fig. 2.

(non) Pseudogygites latimarginatus—A.E. Wilson, in Thorsteinsson, 1958 :87.

Pseudogygites canadensis—Jaanusson, 1959:343, fig. 253-Sa.

Pseudogygites latimarginatus—Fritz, 1959:1118, pls. 1, 2.

Pseudogygites latimarginatus—Liberty, 1964:pl. 5, fig. 8.

(non) cf. Pseudogygites latimarginata—D.E. Jackson, in Nelson and Johnson,
1966 : 567.

Pseudogygites canadensis—Liberty, 1969:71.

Pseudogygites latimarginatus—Norford et al., 1970:pl. 4, fig. 21.

(non) Pseudogygites latimarginatus—Kerr, 1974: 132.

Ogygites canadensis—Levi-Setti, 1975:pls. 3, 58, 59.

Pseudogygites latimarginatus—Ludvigsen: 1978, pl. 6, fig. 53.

Pseudogygites latimarginatus—Ludvigsen: 1979a, figs. 14, 36, 37.

Diagnosis

A species of Pseudogygites with a long preglabellar field (= 18— 23% of sagittal
cephalic length); a subtriangular pygidium with 12 to 14 distinct pleurae, short and
faint pleural furrows evident only on anterior portion of pygidium, well-defined
border furrow, and relatively narrow border. Median connective suture is fused.

Lectotype

I select as lectotype of Pseudogygites latimarginatus the incomplete internal mould of
a pygidium (AMNH 30115, Fig. 4A) originally illustrated by Hall (1847: pl. 66, fig.
4a). The specimen occurs on a piece of brown bituminous shale which was collected
loose near Watertown, New York. Curiously, the label on the lectotype cites the
collecting locality as ‘‘Watertown, N.Y. (Collingwood, Canada)’’ suggesting that
this shale chip was glacial drift from Canada. A bedrock source for the shale does not
crop out at Watertown, but some 13 km to the south, on Gulf Stream near Rodman, a
thin (S—7 cm) layer of brown bituminous shale occurs as a veneer on the Hillier
limestone. A fossil collection from this layer shown to me by J. Riva contains P.
latimarginatus pygidia (Fig. 6A) and this horizon is, in all likelihood, the source of
the lectotype. In Fig. 11, this layer is tentatively referred to the Whitby Formation to
differentiate it from the overlying grey, micaceous, and noncalcareous shales of the
lower Frankfort Formation (Fisher, 1977).

Occurrences

CRAIGLEITH AND COLLINGWOOD AREA

Old collections of P. latimarginatus at the University of Toronto and the Royal

Fig. 3. Pseudogygites latimarginatus (Hall)
A Dorsal view of latex impression of external mould of entire exoskeleton, ROM 35029, x 3.5,
lower Whitby Formation, Craigleith (carcass).
B Dorsal view of dismembered exoskeleton, ROM 30015, X 2.5, lower Whitby Formation,
Craigleith (exuvia; note displaced and overturned hypostome).

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Ontario Museum are labelled ‘‘Utica Shale, Collingwood, Ontario’’. Collingwood
undoubtedly refers to the township of Collingwood and not to the town which is
located in the adjacent township of Nottawasaga. Presumably, these specimens were
collected from the lower Whitby Formation exposed along the shore of Georgian Bay
between Craigleith and Camperdown (Parks, 1928: fig. 2). Recent collections have
been made by members of the Department of Invertebrate Palaeontology, Royal
Ontario Museum, from the lower Whitby Formation, 2 km west of Craigleith Station.
Both of these occurrences are herein identified as ‘‘lower Whitby Formation,
Craigleith’’. P. latimarginatus also occurs sparingly in the upper Cobourg
Formation, about 3 km east of the town of Collingwood, on the south side of
Highway 26.

MANITOULIN ISLAND

P. latimarginatus has been recorded from the lower Whitby Formation in and near the
town of Little Current (Caley, 1936; Liberty, 1968). Hussey (1952) has illustrated a
specimen from ‘‘Collingwood, Manitoulin Island’’. According to Liberty (1968),
Pseudogygites sp. occurs in the highest Lindsey Formation (= Cobourg Formation)
in Little Current. These localities are no longer accessible (T.E. Bolton, pers. comm.
to J. Riva, 1975). P. latimarginatus does not occur in the Whitby and Sheguiandah
Formations exposed farther south near the village of Sheguiandah.

NORTHERN MICHIGAN

P. latimarginatus has been recorded from drift near Newberry, northern Michigan
(Ruedemann and Ehlers, 1924). The genus was recorded from the Groos Quarry
Member at the Bichler Quarry, north of Escanaba (Hussey, 1952).

OSHAWA-WHITBY AREA

Old collections of P. latimarginatus in the Royal Ontario Museum are labelled
‘*Collingwood Shale, Whitby, Ontario’’. Whitby possibly refers to the township of
Whitby. These collections probably came from the lower Whitby Formation exposed
in creek beds in Whitby or nearby Oshawa (Parks, 1928:55; Liberty, 1969:67). In
the Canada Cement Quarry at Bowmanville P. latimarginatus is very abundant in the
lower Whitby Formation and rare in the upper part of the Cobourg Formation.

Fig. 4 Pseudogygites latimarginatus (Hall)

A Dorsal view of exfoliated pygidium, lectotype, AMNH 30115, x 2.3, Whitby Formation (?),
Watertown, New York (original of Hall, 1847:pl. 66, fig. 4a).

B_ Dorsal view of latex impression of external mould of dismembered individual lacking free
cheeks, ROM 158, X 1.1, lower Whitby Formation, Oshawa (exuvia; lower cephalic unit was
apparently carried away by the moulting trilobite).

Cc Ventral view of yoked cheeks, ROM 35031, X 1.8, lower Whitby Formation, Craigleith.

D Dorsal view of weathered internal mould of entire exoskeleton; glabella has been removed to
expose hypostome, ROM 35033, x 1.2, lower Whitby Formation, Craigleith (carcass).

E,F Dorsal view of internal mould of thorax and pygidium and external mould of yoked cheeks and
hypostome, X 2.0, and ventral view of latex impression of external mould of hypostome and
doublure, x 4.0, ROM 35027, lower Whitby Formation, Craigleith (exuvia).

G Bedding surface (immersed in water) showing two cranidia and one yoked cheek, ROM 35023,
x 2.0, lower Whitby Formation, Craigleith.

NEW YORK STATE

See discussion under Lectotype.

OTTAWA AREA

P. latimarginatus has been reported from both the Eastview and Billings Formations
(Wilson, 1957; Baird, 1972). Old collections in the Royal Ontario Museum labelled
‘*The Butts, Rideau River, Ottawa’’ probably came from the Billings Formation.

QUEBEC AREA

P. latimarginatus was also reported from the lower Utica Shale at Beauport and
Montmorency Falls near Quebec by Ells (1888) and Low (1892). J. Riva (pers.
comm., 1977) has questioned these occurrences and notes that he has never seen P.
latimarginatus in either Utica or Trenton collections from this area.

Age

Pseudogygites latimarginatus first appears in the grey and tan argillaceous limestones
of the Cobourg Formation of southern Ontario (Sproule, 1936; Liberty, 1969), but it
is never a common fossil in this unit. Its numerical acme is reached at and near the
base of the overlying black and dark brown bituminous shales and black limestones
regionally assigned to the lower Whitby, Eastview, or Billings Formations (Fig. 11).
The species has a short vertical range in these units, but a considerable geographic
range, apparently occurring along a 1700 km long belt between northern Michigan
and Quebec (Fig. 1). Liberty (1969) noted that the species is confined to the entire
10 m thick lower member of the Whitby Formation at Craigleith and similar vertical
ranges are implied for other areas.

Sweet and Bergstrom (1976) assigned the Cobourg and Hillier Formations to
Midcontinent conodont Fauna 11 and noted that the Amorphognathus superbus — A.
ordovicicus transition of the North Atlantic zonation occurs within the uppermost

Fig. 5 Pseudogygites latimarginatus (Hall)

A Dorsal view of pygidium, ROM 35026, x 2.0, lower Whitby Formation, Craigleith.

B Dorsal view of latex impression of external mould of yoked cheeks, ROM 35032, x 2.0, lower
Whitby Formation, Craigleith (note presence of short fused median suture on front part of
doublure).

c Ventral view of external mould of thorax and pygidium and internal mould of portion of yoked
cheeks and hypostome, ROM 35024, x 2.0, lower Whitby Formation, Craigleith (exuvia; note
telescoped thoracic segments).

D Dorsal view of latex impression of external mould of pygidium, ROM 807 U, x 1.2, Billings
Formation (?), Rideau River, Ottawa.

E Ventral view of latex impression of internal mould of pygidium, ROM 35025, x 2.0, lower
Whitby Formation, Craigleith.

F Ventral view of hypostome, ROM 35028, X 2.6, Cobourg Formation, Collingwood.

Dorsal view of internal mould of entire exoskeleton; front portion of cephalon has been
removed to expose broad doublure, ROM 35030, x 2.0, lower Whitby Formation, Craigleith
(carcass).

H Dorsal view of latex impression of external mould of pygidium, ROM 154, x 1.0, lower
Whitby Formation, Oshawa (specimen identified as Asaphus canadensis Chapman by E.J.
Chapman).

Les isp
My i
pitt,
iy

Hillier Formation. They concluded that the Cobourg and Hillier are mid-Edenian to
mid-Maysvillian in age or, in terms of the British sequence, latest Caradocian to early
Ashgillian.

In the Craigleith area, the lower Whitby Formation is assignable to the upper part
of the Climacograptus pygmaeus Zone (Ruedemann and Ehlers, 1924; Riva, 1974; J.
Riva, pers. comm., 1977). In western New York, the C. pygmaeus Zone occurs in
the ‘‘Atwater Creek’’ and **‘Deer River’? Members (= lower Frankfort; Fisher, 1977;
Riva, 1974) which overlie the P. latimarginatus horizon on top of the Hillier
Formation. In northern Michigan near Newberry P. latimarginatus occurs associated
with C. pygmaeus Zone graptolites on loose pieces of shale (Ruedemann and Ehlers,
1924). These may have been derived from the nearby outcrops of the ‘‘Haymeadow
Creek’’ Member (= lower Bill’s Creek Formation, Liberty, 1968) which also
contains graptolites referable to the C. pygmaeus Zone (Berry, 1970). In Ottawa, the
Billings Formation contains graptolites of the C. pygmaeus Zone (J. Riva, pers.
comm., 1977).

In eastcentral North America Pseudogygites latimarginatus occurs within the
Climacograptus pygmaeus Zone and possibly in slightly older strata, but it does not
occur in the succeeding Climacograptus manitoulinensis Zone. Riva (1974)
considered the C. pygmaeus Zone correlative with the lower half of the Pleurograptus
linearis Zone of Britain which Williams et al. (1972) have interpreted as straddling
the Caradocian-Ashgillian boundary.

Description

Entire exoskeleton is oval in outline; length (sag.) is 1.5 times maximum width at
midthorax. Cephalon and pygidium are of equal length and each is slightly longer
than one-third total length. Convexity is difficult to determine because of the
compressed state of nearly all specimens, but it appears to be moderate to slight.
Cephalon is parabolic to semicircular in outline; twice as wide across base of genal
spines as long (sag.) and, apparently, moderately vaulted. Glabella is well defined by
furrows; it occupies less than one-third cephalic width at level of eyes and it is
outlined laterally by moderately deep and wide (tr.) axial furrows. These converge
slightly from posterior margin towards a line joining palpebral lobes then diverge and
curve around the broadest part of glabella to become preglabellar furrows.
Preglabellar field is flat and broad, about one-fifth the length (sag.) of cephalon. An
occipital furrow does not occur on holaspid specimens, but on meraspid specimens
(Fig. 12B) it is a straight, medially impressed furrow located immediately behind the

Fig. 6 A-E Pseudogygites latimarginatus (Hall)
A Dorsal view of pygidium, AMNH 42302, xX 1.2, Whitby Formation, Gulf Stream near

Rodman, New York.

B-E Dorsal and oblique lateral views of incomplete and uncompressed cephalon, x 2.0, detail of
anteromedial portion of cranidium, x 9.0, and detail of front portion of glabella, x 27.0, ROM
37775, lower Whitby Formation, Craigleith.

F-H Isotelus gigas DeKay
Detail of anteromedial portion of cephalon, x 9.0, dorsal view of cephalon, x 3.0, and
ventral view of doublure showing fused median connective suture, X 4.8, ROM 35367,
Verulam Formation, Lakefield Quarry, Lakefield, Ontario.

asa

median glabellar tubercle. A ‘‘pseudo-occipital furrow’’ does cross the posterior
portion of the glabella (Fig. 3A), but this furrow is merely the impression of the front
edge of the articulating half ring of the first segment. Three pairs of lightly impressed
lateral glabellar furrows are evident on well-preserved specimens. First furrow is a
composite furrow; it comprises a posterior furrow which is lenticular in outline and
transversely directed and surrounded by a yoke-shaped ridge which opens adaxially;
its anterior portion consists of a pair of obliquely disposed shallow slitlike furrows.
Second furrow is an oval to triangular depression located in line with palpebral lobes.
Third furrow is a very faint concavity located just in front of a line joining anterior
edges of palpebral lobes. None of these furrows extends to the axial furrow. A tiny
median glabellar tubercle is situated in line with the anterior part of the first glabellar
furrow. Palpebral lobe consists of elevated, laterally-convex flap which is located
near the axial furrow just behind midlength of cranidium. Palpebral furrow is
crescent-shaped and very faint. Holochroal eye surface is strongly curved in a
horizontal plane, moderately so in a vertical plane. Free cheek is almost as wide (tr.)
as glabella and slopes gently from shallow furrow at base of eye to lateral border
furrow which defines a 90 degree arc from near genal corner to juncture with axial
furrow. Lateral border is narrow near genal angle; becomes wider and flatter towards
the front to merge with preglabellar field. Posterior border furrow is firmly impressed
on fixed cheek. It is slightly offset anteriorly at facial suture and continues on free
cheek as a shallow depression that does not reach the lateral border furrow. Posterior
branch of facial suture swings outward and backward in an arcuate curve from eye.
Anterior branch of facial suture curves outward and forward along a path parallel to
axial furrow, then curves adaxially to cross anterior margin on sagittal line. Genal
spine is long, slim, and backwardly directed. It attenuates to a fine point opposite
fifth thoracic segment. Cephalon is covered by minute, shallow, circular pits which
are approximately 100 um in diameter (Fig. 6D). Fine wrinkles parallel the cephalic
margin on the anterior‘and lateral borders.

Cephalic doublure, is one-third the length (sag.) of cephalon and flat to gently
concave in front of hypostome. A vestige of a median connective suture is evident as
a short (sag.) fused suture on anterior part of doublure. Laterally, doublure becomes
much narrower and sharply incurved. A small panderian opening is located inside the
genal corner. A vincular socket is not present. Doublure carries fine terrace lines that
run subparallel with cephalic margin.

Hypostome is rectangular in outline, five-sixths as wide (tr.) as long (exsag.). Its
posterior margin is located below the first glabellar furrows. A deep rounded
posteromedial notch extends forward for one-third the length of the hypostome and
separates a pair of subtrapezoidal posterior projections. The ventral surface of each
projection is flat and faces obliquely outward. Anterior margin is convex forwardly
with a faint posterior curvature medially. Hypostome is widest across triangular
anterior wings, behind these it narrows into a pair of U-shaped antennal notches and
then expands into rounded shoulders which carry low carinae along their lateral
margins. Central body is quadrate, slightly inflated, and not defined by furrows; its
posterior margin is defined by a shallow square depression located in front of
posterior notch. Maculae are relatively deep, anterolaterally directed slots located
near the posterolateral margins of the central body and slightly anterior of midlength
of hypostome. Median portion of hypostome is smooth; its flanks carry terrace lines
that run subparallel with lateral margins.

Thorax consists of eight segments. Moderately vaulted axis is slightly narrower
(tr.) than one-third cephalic width; widens a little from first to fourth segment, then
narrows slightly towards pygidium. Axis is transversely divided by a furrow that
shallows towards sagittal line and which separates an anterior portion from a shorter
posterior portion. The latter portion mirrors the articulating half ring of the next
segment. Axial furrow on each segment is scalloped, convex side facing inward.
Inner portion of pleura is subhorizontal; outer portion declines gently. Firmly
impressed pleural furrow proceeds diagonally from anterior side of pleura, near axial
furrow, to midlength of pleura on upper part of declined portion, and terminates just
inside broad anterolateral facet. Anastomosing terrace lines on facets; remaining
thorax is finely pitted. Declined portion of pleura is encased by flat doublure which
contains a small panderian opening.

Pygidium is subtriangular in outline with bowed lateral margins and is only slightly
inflated; width equals 1.3 to 1.5 sagittal length. Prominent facets occur at
anterolateral corners. Axis is moderately convex and well defined by firmly
impressed axial furrows which are convergent at about 15 degrees and are straight or
slightly convex adaxially. Anterior portion of axis is crossed by five to seven shallow
axial ring furrows which become faint to imperceptible close to sagittal line. Posterior
portion of axis is unfurrowed and its termination is bluntly rounded to square-tipped.
Posterior end of axis is well in front of border furrow. Pleural field is somewhat
inflated and bounded by distinct border furrows. Concave border is narrow and it
maintains the same width around the lateral and posterior edges of pygidium.
Interpleural furrows are continuous from axial furrow to border furrow and nearly
straight. There are 10 or 11 firmly impressed interpleural furrows lateral to axis and
an additional three or four faint interpleural furrows behind axial termination. The
direction of the interpleural furrows changes gradually from slightly posterior of
transverse near front of pygidium to sagittal behind axis. Length (exsag.) of pleurae
decreases from front to back. Pleural furrows are narrow (tr.) slitlike depressions on
the posterolateral flanks of each of the first five or six pleurae. Only on the two most
anterior pleurae do the pleural furrows continue to the axial furrow as extremely faint
furrows. Fine terrace lines on border run at a low angle to margin; remainder of
pygidium is finely pitted. On interior, axial ring furrows are more prominent than on
exterior, and continue posteriorly to axial termination. Doublure is broad (about twice
as wide as border) and reaches as far forward as tip of axis. Doublure carries terrace
lines that run subparallel to margin.

Discussion

The description of each of the following species of Pseudogygites will take the form
of a comparison with P. latimarginatus.

Raymond (1913:43) synonymized Asaphus halli Chapman, 1858 and Asaphus
hincksii Chapman, 1859b with Ogygites canadensis (Chapman). The illustration of
A. halli presented by Chapman (1858 : 236) shows an asaphid trilobite with a ribbed
pygidium and a cephalon with rounded genal corners and an indistinct glabella. The
cranidium is transversely divided by a W-shaped furrow which joins the facial suture
slightly in front of the eyes. Another furrow is forwardly convex and terminates at the
axial furrows of the first thoracic segment. The peculiar transverse cephalic furrows
are probably fractures. In any case, the remaining cephalic features of A. halli clearly

exclude it from Pseudogygites latimarginatus and, because no type specimen exists,
this species remains unrecognizable. Likewise, Asaphus hincksii cannot be
considered synonymous with P. latimarginatus because this species was described as
having a smooth pygidium. However, A. hincksii does deserve notice because in the
description of that species Chapman (1859b) noted and illustrated the morphological
feature to which the name of panderian opening was later applied—apparently
independently of its discovery two years earlier by Pander in Asaphus expansus.

Pseudogygites hudsoni n. sp.
Figs. 7D, 8B—G, 10A

cf. Pseudogygites latimarginata (Hall), D.E. Jackson, in Nelson and Johnson,
1966 :567.
?Pseudogygites sp., Jackson, 1971.

Diagnosis

A species of Pseudogygites with a short preglabellar field (= 12% of sagittal cephalic
length); a semicircular pygidium with 9 to 10 distinct pleurae with faint but complete
pleural furrows, poorly defined border furrow, and relatively broad border. Median
connective suture is functional.

Holotype

A complete, but compressed and exfoliated cranidium (GSC 47517) collected from
rubble of the ‘‘Oil Shale interval’’ (= Boas River Shale) near East Bay, Southampton
Island by S.J. Nelson (GSC loc. C-26369). According to Nelson and Johnson
(1976: fig 3), the *‘Oil Shale interval’’ at East Bay is about 17 m thick.

Occurrences

SOUTHAMPTON ISLAND

GSC loc. 84651, Boas River Shale, 0 to 0.7 m above base of exposure, central
Southampton Island, 64°22'50''N, 84°31'10’’W, collected by B.V. Sanford. GSC
loc. 84653, same locality and collector as above, 0.7 to 2.3 m above base of
exposure. GSC loc. C-26369, ‘‘Oil Shale interval’? (= Boas River Shale), near East
Bay, 64°01'N, 81°28’W, collected by S.J. Nelson.

Fig. 7 A-C_ Pseudogygites latimarginatus (Hall)

A Dorsal view of latex impression of external mould of entire exoskeleton, ROM 35034, x 1.5,
lower Whitby Formation, Bowmanville (carcass; note deformed thoracic segments on the left
side, probably indicating an unsuccessful attempt at moulting).

B,C Dorsal and oblique lateral views of nearly complete uncompressed specimen, Gsc 7817,
x 1.5, Eastview Formation, Ottawa.

D Pseudogygites hudsoni n. sp.

Bedding plane with three cranidia and a pygidium, Gsc 47510, x 2.8, Boas River Shale,
Southampton Island (Gsc loc. 84651).

jee

me ig

Age

The type material of Pseudogygites hudsoni n. sp. was selected from three lots of
fossils. One was collected by S.J. Nelson from rubble of the ‘‘Oil Shale interval’’
near East Bay, Southampton Island (Nelson and Johnson, 1966, 1976) and the other
two were collected by B.V. Sanford from the Boas River Shale in central
Southampton Island (Sanford, 1970; Heywood and Sanford, 1977). According to
Nelson and Johnson (1976), the ‘‘Oil Shale interval’’ is Richmondian in age and
occurs at the top of the Ordovician section on Southampton Island, between the
Churchill River Group and the Silurian, and is a separate and younger unit than the
Boas River Shale which, according to Heywood and Sanford (1977), is Edenian in
age and occurs between the Bad Cache Rapids Group and the Churchill River Group.

An evaluation of published evidence suggests that the fossiliferous ‘‘Oil Shale
interval’’ rubble and the Boas River Shale is the same unit and, further, that this unit
is probably of late Maysvillian age and occurs between the Bad Cache Rapids Group
and the Churchill River Group (Fig. 11). The ‘‘Oil Shale interval’’ rubble and the
Boas River Shale contain the same species of Pseudogygites, occur along the same
belt, and the available hand samples are indistinguishable lithologically. According to
Riva (1974:5), the graptolites of the Boas River Shale and of the ‘‘Oil Shale
interval’’ probably belong to the Climacograptus manitoulinensis Zone and, as such,
are probably somewhat younger than those of the lower Whitby Formation of
southern Ontario, which belong to the C. pygmaeus Zone. The Churchill River
Group and the Red Head Rapids Group which overlie the Boas River Shale on
Southampton Island have yielded conodonts of ‘‘late Maysvillian to Richmondian
age, probably Richmondian’’ (Barnes, 1974:235).

Discussion

The following features distinguish Pseudogygites hudsoni n. sp. from P.
latimarginatus (Hall).

CEPHALON

The preglabellar field is shorter and the glabella expands more in front of the eyes. A
faint sagittal depression is present on the front part of the glabella. The lateral
glabellar furrows are deeper. A median connective suture crosses the doublure.

HYPOSTOME

The single available hypostome of P. hudsoni is very similar to that of P.
latimarginatus, but the maculae are deeper and the posterior notch narrower (tr.).

PYGIDIUM

The pygidium is semicircular in outline. The axial ring furrows are greater in number
(up to 12), more distinct, and continue to near the axial termination. The pleurae are
fewer in number and the pleural furrows are faint, but continuous from the lateral
border furrow to the axial furrow.

Pseudogygites arcticus n. sp.
Figs. 8A, 9, 10B_D

Pseudogygites latimarginatus (Hall), A.E. Wilson, in Thorsteinsson, 1958 :87.
Pseudogygites latimarginatus—Kerr, 1974: 132.

Diagnosis

A species of Pseudogygites with a short preglabellar field (= 8-10% of sagittal
cephalic length); a semicircular pygidium with seven to nine faint pleurae and very
faint but complete pleural furrows, well-defined border furrow, and relatively narrow
border. Median connective suture is functional.

Holotype

An incomplete external mould of a cranidium (ROM 35388) from the base of a7 m
thick tongue of the Cape Phillips Formation within the lowest part of the Allen Bay
Formation on Grinnell Peninsula, Devon Island (74°41'N, 95°35'W); collected by
G.S. Nowlan and C.R. Barnes.

Occurrences

DEVON ISLAND

Same locality and collectors as holotype.

CORNWALLIS ISLAND

Cape Phillips tongue within the lower Allen Bay Formation (75°10’N, 95°10'W);
collected by J. Arengi. See Thorsteinsson and Kerr (1968 :7).

BATHURST ISLAND

GSC loc. 67001, 18 m above base of Cape Phillips Formation, Driftwood Bay
(75°57'N, 97°50'W); collected by J.W. Kerr.

Age

In commenting on the age significance of Pseudogygites arcticus (as P.
latimarginatus) in Member A of the Cape Phillips Formation on Cornwallis Island,
Thorsteinsson (1958:90) stated: ‘‘The graptolites with which P. latimarginatus is
associated in Cape Phillips strata appear to bear no relation to Collingwood or Utica
forms. Moreover, the association of P. latimarginatus with Climacograptus latus
indicates that either the former ranges higher than previously recorded or the latter
ranges lower.’’ Thorsteinsson’s assessment of P. arcticus as being younger than P.
latimarginatus in the Collingwood Shale (= lower Whitby) is supported herein. John
Riva has examined graptolites associated with P. arcticus and identified two species:
one is a probable new species of Glyptograptus with extremely long virgella,
antisicular spines, and mesial spine on th 1’ and the other is a species of
Glyptograptus or *‘Pseudoclimacograptus’’. He stated (pers. comm., 1977) that
these graptolites are puzzling, but they all are younger than the Climacograptus

pygmaeus Zone. Barnes (1974:233, 234) noted that conodont collections from the
lower part of the Allen Bay and Cape Phillips Formations on Ellesmere, Somerset,
and Bathurst islands (including samples from P. arcticus-bearing beds) belong to
conodont Fauna 12 of late Maysvillian to Richmondian age.

Discussion

The following features distinguish Pseudogygites arcticus n. sp. from P.
latimarginatus (Hall).

CEPHALON

The preglabellar field is shorter and the glabella expands farther laterally in front of
the eyes. A firmly impressed sagittal furrow is present on the front part of the
glabella. The lateral glabellar furrows are fainter and the median glabellar tubercle
appears transversely stretched. The palpebral lobes are located farther forward. A
median connective suture crosses the doublure.

HYPOSTOME

The posterior projections are longer and more pointed, the lateral shoulders are not as
rounded, and the central body is more inflated.

PYGIDIUM

The pygidium is semicircular in outline. The pleural lobes are inflated, so that they
stand as high as the axis. The axial ring furrows are faintly impressed. The axis is
relatively shorter (sag.). The pleurae are not very distinct and the pleural furrows are
continuous from the lateral border furrow to the axial furrow. The lateral border is
narrower.

The cranidium of Pseudogygites arcticus is rather similar to that of P. hudsoni in
having a short preglabellar field and a glabella that expands markedly in front of the
eyes and which contains a short sagittal furrow on its front part. The pygidium and
hypostome of P. arcticus and P. hudsoni differ markedly.

Fig. 8 A Pseudogygites arcticus Nn. sp.
Dorsal view of small pygidium, Gsc 47511, x 4, lower Cape Phillips Formation, Driftwood
Bay, Bathurst Island (Gsc loc. 67001).
B-G Pseudogygites hudsoni n. sp. All from Boas River Shale, Southampton Island.
Ventral view of hypostome, Gsc 47512, x 2 (Gsc loc. 84653).
Dorsal view of latex impression of external mould of pygidium, Gsc 47513, x 2.4 (Gsc loc.
84651).
Dorsal view of two pygidia, Gsc 47514, x 2.4 (Gsc loc. C—26369).
Dorsal view of exfoliated pygidium, Gsc 47515, x 2.8 (Gsc loc. 84653).
F Dorsal view of dismembered specimen, probably a moult association, Gsc 47516, x 2 (Gsc
loc. 84651).
G_ View of bedding plane with a number of cranidia and pygidia. Holotype is cranidium in lower
left corner marked by white dot, Gsc 47517, x 2.6 (Gsc loc. C—26369).

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Pseudogygites akpatokensis n. sp.
Fig. 105

Pseudogygites sp., Workum, Bolton, and Barnes, 1976:pl. 3, fig. 1.

Diagnosis

A species of Pseudogygites with a relatively long preglabellar field (= 19% of
sagittal cephalic length) and a subtriangular pygidium with 10 very faint pleurae,
well-defined border furrow, and relatively broad border.

Holotype

An external mould of a cranidium (GSC 41187a) from the bituminous limestone unit
on the westcentral coast of Akpatok Island, Ungava Bay collected by R.H. Workum
(Section II, 9.1 m above sea level; Workum, Bolton, and Barnes, 1976).

Occurrence

AKPATOK ISLAND

Same locality and collector as the holotype.

Age

The graptolites of the bituminous limestone unit on Akpatok Island were assigned to
the Climacograptus manitoulinensis Zone by Riva (in Workum, Bolton, and Barnes,
1976). This zone is herein considered late Maysvillian in age.

Discussion

Pseudogygites akpatokensis n. sp. differs from P. latimarginatus (Hall) in having a
more waisted glabella, fainter lateral glabellar furrows, eyes located farther back on

Fig. 9 Pseudogygites arcticus n. sp.

A Dorsal view of cranidium, Gsc 47518, x 2.1, lower Cape Phillips Formation, Driftwood Bay,
Bathurst Island (Gsc loc. 67001).

B_ Dorsal view of pygidium, Gsc 47519, x 1.7, lower Cape Phillips Formation, Driftwood Bay,
Bathurst Island (Gsc loc. 67001).

Cc Dorsal view of latex impression of external mould of cranidium, holotype, ROM 35388a,
x 2.3, Cape Phillips tongue in lower Allen Bay Formation, Grinnell Peninsula, Devon Island.

D Dorsal view of pygidium, ROM 35389, x 2.5, Cape Phillips tongue in lower Allen Bay
Formation, Grinnell Peninsula, Devon Island.

E Dorsal view of free cheek, ROM 35391, x 3.0, Cape Phillips tongue in lower Allen Bay
Formation, Grinnell Peninsula, Devon Island.

F Ventral view of hypostome, ROM 35392, x 3.0, Cape Phillips tongue in lower Allen Bay
Formation, Grinnell Peninsula, Devon Island.

G_ Dorsal view of exfoliated pygidium, ROM 35393, x 3.0, Cape Phillips tongue in lower Allen
Bay Formation, Grinnell Peninsula, Devon Island.

H Dorsal view of dismembered specimen, ROM 35390, x 1.4, Cape Phillips tongue in lower
Allen Bay Formation, central Cornwallis Island.

x
NS
SG
. SS

the cephalon, stout and bladelike genal spines, and a narrower (tr.) pygidium with
very faint pleural, interpleural, and axial ring furrows.

The pygidium of P. akpatokensis is most similar to that of P. arcticus, but it has a
narrower axis, shallower furrows, and a broader border.

Moulting of Pseudogygites latimarginatus

The prevalent occurrence of Pseudogygites latimarginatus in the dark brown
bituminous shales of the lower Whitby Formation is as disarticulated exoskeletal
pieces. Such accumulations often comprise enormous numbers of specimens (see
Levi-Setti, 1975, pl. 3 for a typical occurrence). Entire or nearly entire exoskeletons
are not uncommon. These do not occur in direct association with the massed
specimens mentioned above but, instead, occur in sparsely fossiliferous layers (see
Ludvigsen, 1979a, fig. 37 for an unusual concentration of such specimens). Entire
specimens are preserved in two main configurations. The first configuration is that
shown in Figs. 3A, 4D, 5G, and 7B which consists of the entire exoskeleton preserved
intact with no displacement of exoskeletal parts (other than that resulting from
compaction). In these specimens the hypostome is attached along the hypostomal
suture. In compressed specimens, the crescent-shaped imprint of the upturned portion
of the notched posterior margin of the hypostome may be seen in the central part of
the glabella between the eyes. In order to verify that this imprint actually represents
the hypostome, the glabella and underlying matrix were removed from a few
specimens to expose the hypostome on the ventral surface (Fig. 4D). The anterior
parts of the facial sutures are slightly opened in some of these specimens. This is not
an ecdysal sutural gape (Henningsmoen, 1975), but is merely the result of slight
splaying of the cephalic units during compaction. I conclude that this configuration
indicates the remains of a dead individual (a carcass). The second configuration is
best shown in Figs. 3B, 4B, E, and 5c. This configuration consists of the yoked
cheeks lying immediately in front of, and at an angle to, the conjoined thorax and
pygidium. In most cases, the posterior edge of one side of the yoked cheeks lies on

Fig. 10 A Pseudogygites hudsoni, n. sp.
Dorsal view of latex impression of external mould of cranidium, Gsc 47473, x 2.6, Boas
River Shale, Southampton Island (Gsc ioc. 84653).
B-D Pseudogygites arcticus n. sp.

B Ventral view of latex impression of external mould of free cheek (note median connective
suture), ROM 35388b, x 2.6, Cape Phillips tongue in lower Allen Bay Formation, Grinnell
Peninsula, Devon Island.

C_ Dorsal view of latex impression of external mould of cranidium, Gsc 49475, x 2.6, lower
Cape Phillips Formation, Driftwood Bay, Bathurst Island (Gsc loc. 67001).

D Dorsal view of crushed cranidium, Gsc 49474, x 2.1, lower Cape Phillips Formation,
Driftwood Bay, Bathurst Island (Gsc loc. 67001).

E Pseudogygites akpatokensis n. sp.

Latex impression of slab with fragmentary cranidia, pygidia, and a free cheek, Gsc 41187,
x 1.3. Holotype cranidium is in the lower third of photograph, marked by white dot.
Bituminous limestone and shale unit on westcentral coast of Akpatok Island, Ungava Bay
(Section II, 9.1 m above high sea level; Workum, Bolton, and Barnes, 1976).

top of the first few thoracic segments. The cranidium may be present and on one
specimen (Fig. 3B) it is displaced laterally and lies at an angle to the yoked cheeks.
The hypostome is often attached to the yoked cheeks, but in the specimen described
above, it is overturned and shifted sideways relative to the cheeks. In most of these
specimens, the thorax is not displaced or distorted, but in a single specimen (Fig. 5c)
it is slightly telescoped. Such persistent configurations of exoskeletal units strongly
suggest that they are exuviae of Pseudogygites latimarginatus and that the exuvial
units have not been moved from where they were shed.

With the two end members of the exuvial cycle preserved (that is, a carcass, Fig.
3A, and a complete exuvia, Fig. 3B) it is now possible to make an attempt at
reconstruction of the moulting behaviour of Pseudogygites latimarginatus. A
hypothetical sequence is summarized below (see also Ludvigsen, 1979a: fig. 14).

1. In preparation for moulting, the trilobite severed the connection between the key
exuvial units—the cephalic unit and the thoracopygon.

2. Further separation of these two exuvial units occurred as the trilobite anchored one
genal spine in the mud and crawled backward. This caused the opposite side of the
cephalic unit to rotate back and across the front part of the thoracopygon.

3. When the trilobite had backed up sufficiently so that the front part of the head was
behind the facial suture of the cephalic unit, it separated this unit along the facial
suture into an upper unit (cranidium) and a lower unit (yoked cheeks plus
hypostome) and nudged aside the upper cephalic unit with its head.

4. The trilobite then crawled forward and sideways through the gape of the facial
suture while shoving aside the cranidium. The thoracopygon became caught
against the edges of the upper and lower cephalic units and slid off the trilobite.

5. The trilobite continued its forward motion until it was clear of the thoracopygon
and the upper cephalic unit. The thrashing appendages dislodged and overturned
the hypostome.

The preceding is a possible moulting scenario which accounts for the constellation
shown in Fig. 3B. The absence of the lower cephalic unit in one specimen (Fig. 4B)
seems to indicate that occasionally the moulting trilobite carried away portions of the
cephalic unit.

An example of abnormal moulting is possibly shown by the specimen in Fig. 7A.
The crescent-shaped bulge between the eyes indicates that the hypostome is still in
place and because the sutures are tightly sealed, this specimen is interpreted as a
carcass. The first four segments on the left side are sharply bent backward and two of
these are broken near the axial furrow. If the moulting sequence outline above is
correct, then this specimen appears to represent an individual that perished during an
incomplete moult. The deformation is consistent with an unsuccessful attempt by the
trilobite at severing the cephalic unit from the thoracopygon.

The ability to complete a moult sequence quickly and without inflicting self-injury
was obviously of paramount importance to a trilobite and the inherent functional
requirements should be reflected in its morphology. Two features of Pseudogygites
latimarginatus—the yoked cheeks and the genal spines—may be explained, at least
in part, by reference to their role in facilitating ecdysis. The possession of yoked
cheeks would allow the trilobite to discharge the lower cephalic unit in one smooth

motion by applying pressure to just one side of the cephalon. In addition, the danger
of inflicting self-injury to the ventral surface would be reduced if the sharp edges of
the median connective suture were eliminated. Possession of spikelike genal spines
appears to have been significant because a rounded genal angle, such as that in mature
Isotelus, would probably have provided insufficient grip in soft mud.

Origin of Pseudogygites latimarginatus

Pseudogygites is an endemic North American trilobite whose numerical acme is
consistently confined to a narrow interval near the base of certain Upper Ordovician
bituminous shales where they overlie carbonates in eastcentral and arctic North
America (Fig. 11).

Previous workers have suggested that Pseudogygites belongs in the Asaphinae
(Raymond, 1912), the Ogygiocaridtinae (Whittington, 1966), or the Isotelinae
(Jaanusson, 1959). Because the morphology of this taxon clearly indicates an
assignment to the Isotelinae, the ancestry of Pseudogygites should be sought among
members of this typical North American subfamily. The following discussion centres
on the origin of P. latimarginatus because it is the best known of the four species, and
because more is known about the age and stratigraphic setting of the lower Whitby
Formation in southern Ontario than about the bituminous shale units in northern
Canada.

Isotelus, the most abundant and widely distributed of the Isotelinae in North
America, shares a number of general cephalic, pygidial, and hypostomal features
with Pseudogygites latimarginatus (compare Figs. 12D and 12H). The similarity
between mature /sotelus and mature Pseudogygites is not great, but immature
specimens of /sotelus (Fig. 12E) are strikingly similar to immature specimens of
Pseudogygites (Fig. 12A). In addition, many immature specimens of Jsotelus are very
similar to mature specimens of Pseudogygites (compare the immature /sotelus spp.
illustrated by Whittington [1941:pl. 75, figs. 27-29, 34-36], Hu [1975:pl. 4, figs.
12, 13, 20, 28, 29], and Chatterton and Ludvigsen [1976: pl. 2, figs. 38-41] with the
mature specimens of P. latimarginatus illustrated in this paper).

The evidence seems to indicate a derivation of P. latimarginatus from a species of
Isotelus—which species of Jsotelus is difficult (or perhaps impossible) to determine.
The presence of a novel shared-derived (synapomorphic) character may point to the
identity of the P. latimarginatus ancestor. The fused median connective suture is,
perhaps, such a character. Of the North American species of Jsotelus and
Pseudogygites, only I. gigas and P. latimarginatus definitely possess this character
(compare Fig. 6H and 4c). The cranidial microsculpture of the two species is nearly
identical (compare Fig. 6D and 6F). Because the remaining morphological characters
of the two species are compatible with such a phyletic connection, I suggest that P.
latimarginatus was derived from /. gigas during the early Late Ordovician of eastern
North America.

It is now becoming clear that many of the morphological characters that serve to
distinguish an immature /sotelus from a mature Jsotelus will equally well serve to
distinguish a mature Pseudogygites from a mature Jsotelus. Some of these characters
are listed below:

1. Virtually all dorsal furrows are deeper. These include the axial, lateral glabellar,
and border furrows on the cephalon and the axial, border, articulatory, axial ring,
interpleural, and pleural furrows on the pygidium.

. The preglabellar field is longer (sag.).

. The facial suture curves farther laterally in front of the eyes.
The eyes are located farther back on the cephalon.

. The free cheeks are wider (tr.).

. The cephalon and pygidium are rounded in outline.

. The glabella is more waisted.

. The axis is relatively narrower.

. The median glabellar tubercle is more prominent.

—
i)

. Long, slim genal spines are present.

—
—

. Vincular sockets are present.

—
N

. The hypostome is relatively longer.

Such a list of features implies that heterochrony played an important role in the
derivation of Pseudogygites latimarginatus from Isotelus gigas. Although critical
data on age and timing of maturation are not available, the juvenilized morphology of
P. latimarginatus, coupled with its large size, suggest that paedomorphosis by
retardation of somatic features (neoteny) was the evolutionary mechanism (see
Gould, 1977). P. latimarginatus is a large trilobite. In a sample of 79 entire
specimens from the lower Whitby Formation of southern Ontario, 20 per cent have
lengths exceeding 7 cm (Fig. 13). In a sample of 46 entire specimens of /. gigas from
the middle Trenton Group at Trenton Falls, New York, only 15 per cent are longer
than 7 cm (Fig. 13). P. latimarginatus, therefore, is as large and maybe somewhat
larger than /. gigas.

A comparison of the ontogenies of /sotelus gigas and Pseudogygites latimarginatus
is hampered by the availability of only a few meraspid specimens of either species.
Two meraspid specimens of P. latimarginatus are known; a degree 3 specimen (Fig.
12A) and a degree 5 specimen (Fig. 12B). Only a single meraspid specimen of
indeterminate degree, but probably early meraspid (Fig. 12E), occurs in the large
Walcott Collection of entire /. gigas from the middle Trenton Group kept in the
Museum of Comparative Zoology at Harvard University. The speciment of /. gigas in
this collection that Raymond (1914: fig. 1) reconstructed as a degree 4 meraspid is
probably a small holaspid, as Whittington (1957 : 445) has already pointed out. Figure
12 summarizes the main points of the ontogenies of the two species and this figure
clearly shows that P. /atimarginatus changes much less during ontogeny than does /.
gigas. If the morphologic information of early ontogenetic stages available for other

Fig. 11 Correlation chart of Late Ordovician successions in eastern and arctic North America.
Stratigraphic intervals with significant amounts of bituminous shale and limestone are indicated
by diagonal shading. Occurrences of species of Pseudogygites are indicated by circled dots. P.
latimarginatus (Hall) occurs in northern Michigan, Manitoulin Island, Craigleith, Oshawa, New
York, and Ottawa; P. hudsoni n. sp. occurs on Southampton Island; P. akpatokensis n. sp.
occurs on Akpatok Island; and P. arcticus n. sp. occurs on Bathurst, Cornwallis, and Devon
islands.

BATHURST,

o| NORTHERN |MANITOULIN|CRAIGLEITH| OSHAWA
MICHIGAN ISLAND

NEW YORK
( RODMAN )

OTTAWA

ISLAND

DEVON
ISLANDS

American
Stage

CARADOCIAN

COBOURG

UNNAMED
LIMESTONE
GROOS COBOURG COBOURG HILLIER CACHE
ARR RAPIDS
pie ? COBOURG

EDENIAN

31]

NSS

NS KG
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WN N

ONG
SSS ¥

SSSA

Re
~

species of /sotelus (see Whittington, 1941; Hu, 1975; and Chatterton and Ludvigsen,
1976) is used to augment the meagre data on meraspid ontogeny of /. gigas, then
tentative shape correlations can be drawn between the meraspid stage of P.
latimarginatus and the early meraspid stage of J. gigas and between the holaspid
stage of P. latimarginatus and the late meraspid and, possibly, the early holaspid
stage of J. gigas (Fig. 13).

In terms of Gould’s (1977: fig. 35) clock model of heterochrony, the neoteny of P.
latimarginatus is expressed by shape remaining in the juvenile domain of the
ancestor, size being slightly increased, and, possibly, maturation being delayed.

This is not the first heterochronous interpretation of J/sotelus gigas and
Pseudogygites latimarginatus. Raymond (1914) presented an account of the ontogeny
of J. gigas which, in the spirit of the times, he interpreted in a recapitulatory
framework. That is, in the course of its development /. gigas passes through stages
which represent the adults of its ancestors and that this sequence of stages constitutes
a phylogeny. Raymond named these stages, a Basilicus stage (for Basiliella
barrandei), an Ogygites stage (for Pseudogygites latimarginatus), an Isotelus
maximus stage, an /. iowensis stage, and an /. gigas stage. The first two stages are
meraspids, the last three are holaspids. Raymond’s phylogeny finds little support in
modern stratigraphic palaeontology. It is difficult to accept P. latimarginatus as an
ancestor of J. gigas because the *‘ancestor’’ appeared much later (early Maysvillian)
than the ‘‘descendant’’ (Blackriveran; Ludvigsen, 1978). Although Raymond’s
observations and heterochronic correlations appear to be valid, the similarity between
juvenile /. gigas and adult P. latimarginatus is better explained as the result of
paedomorphosis (by retardation) than of recapitulation (by acceleration).

A question remains. Why should P. latimarginatus suddenly become so abundant
immediately above the unconformable Cobourg-Whitby contact? The answer, I
believe, lies in chance preadaptation. The neotenous features of this trilobite became
advantageous in the new ecologic setting above the base of the Whitby Formation.

Isotelus gigas is one of the most abundant trilobites in the Trenton Group and, in
the upper Cobourg Formation and its correlatives, it is associated with relatively rare
Pseudogygites latimarginatus. The reverse is the case in the lower Whitby Formation
and its correlatives. Here, P. latimarginatus is by far the most common trilobite and
Isotelus (including J. gigas) follows distantly, well behind Triarthrus and
Flexicalymene. It seems reasonable to suggest that the environmental shift associated

Fig. 12 Comparative ontogeny of Pseudogygites latimarginatus (Hall) from the lower Whitby
Formation, southern Ontario and /sotelus gigas DeKay from the middle Trenton Group, Trenton
Falls, New York.
AD Pseudogygites latimarginatus (Hall)
A Meraspid (M.3), ROM 27759, x 11.5, Craigleith.
B Meraspid (M.5), ROM 37829, x 5.5, Bowmanville.
C Holaspid, ROM 37830, x 3.3, Bowmanville.
D Holaspid, ROM 35029, x 1.5, Craigleith.
E-H TIsotelus gigas DeKay
E Meraspid (degree indeterminate), Mcz 45, x 10.8 (original of Raymond, 1914: pl. 1, fig. 1).
F Holaspid, Mcz 48, x 5.3 (original of Raymond, 1914:pl. 1, fig. 2).
G Holaspid, Mcz 38, x 2.6 (original of Raymond, 1914: pl. 2, fig. 3).
H Holaspid, mcz 41, x 1.0 (original of Raymond, 1914:pl. 3, fig. 3).
Note that P. latimarginatus changes less during ontogeny than does /. gigas.

FREQUENCY FREQUENCY

|
\
Pe
X
Ns
\
)
MERASPID

possible |
/sotelus isomorphology Pseudogygites
gigas epirel ations latimarginatus

Fig. 13 Model of neotenous derivation of Pseudogygites latimarginatus. Tentative shape correlations are
drawn with its ancestor, /sotelus gigas. Also shown are frequency distributions of sagittal lengths
of entire holaspid specimens of /. gigas (46 specimens from the middle Trenton Group, Trenton
Falls, New York; data extracted from Whittington’s [1957] fig. 25; a single large specimen
measuring 180 mm is not shown) and P. latimarginatus (79 specimens from the lower Whitby
Formation, Craigleith; Royal Ontario Museum and University of Toronto collections). Both
species become holaspids at length of 8 or 9 mm. Compare with Fig. 12 and see discussion in
text.

with the change of sedimentary regime from predominantly carbonate mud deposition
in the Cobourg, to predominantly organic-rich mud deposition in the lower Whitby
was also responsible for the change in relative dominance of /. gigas and its
neotenate.

The lithologic and faunal attributes of the upper Cobourg Formation point to a
shallow, well-lit, well-oxygenated, and warm shelf environment. This environment is
clearly indicated by the abundance of organisms with thick calcareous shells (notably
bryozoans, gastropods, brachiopods, cephalopods, and echinoderms), the ubiquity of
carbonate mud, and the common occurrence of lingulid brachiopods in burrowing
position. The total species diversity is moderately high; Liberty (1969) tallied 93
species from the Cobourg Formation (his Lindsey Formation) in the Craigleith area.

The lower Whitby Formation, which disconformably overlies the Cobourg

Formation, is lithologically and faunally unique. The outstanding and most obvious
feature of the lower Whitby is its bituminous nature. The petroleum content of this
unit is considerable and Logan (1863) reported that in the 1860s condensate was
distilled from these shales near Craigleith (yields were 7 gallons of petroleum per ton
of shale). Pyrite is very common throughout the lower Whitby, occurring both as
finely disseminated crystals and as partial replacement of many of the fossils. The
fauna of the lower Whitby differs markedly from that of the Cobourg, even though
many of the same genera and some species occur in both units. Trilobites (P.
latimarginatus and Triarthrus eatoni); graptolites (climacograptids); small smooth
ostracods; cephalopods (Geisonoceras); and strophomenid, zygospirid, and lingulid
brachiopods dominate the fauna. Most of the calcareous fossils of the lower Whitby
possess thin shells.

Liberty (1969:72) suggested that ‘‘the lower member [of the Whitby Formation]
was probably deposited under reducing conditions in stagnant, non-aerated water’’.
The significant organic carbon content of the lower Whitby and the presence of iron
in reduced form are clearly indicative of the ‘‘oxygen debt’’ of these sediments
(Fischer and Arthur, 1977), but some faunal and lithologic aspects suggest that other
factors besides reducing conditions were important. Byers (1977:8) predicted that
when dissolved oxygen is lowered, ‘‘the benthos becomes less diverse, less
abundant, smaller in body size, less heavily calcified, and dominated by infauna’’.
The lower Whitby fauna is, by no means, impoverished or stunted and the fine
continuous laminae of the shales demonstrate that it lacks a burrowing infaunal
component. Furthermore, faunal lists compiled by Liberty (1969) show a total
diversity of 54 species for the lower Whitby in the Craigleith area.

The abrupt appearance of the olenid trilobite Triarthrus in the lower Whitby
(Ludvigsen, 1979a: figs. 38, 39) suggests that water temperature was an important
contributory factor governing the structure and composition of these black shale
communities. Olenids are characteristic trilobites of the slope biofacies that occupied
relatively deep and cold settings around the North American continent during the
Ordovician (Titus and Cameron, 1976; Ludvigsen, 1979b) and their abundance in the
lower Whitby suggests the influence of low water temperatures as well as low oxygen
levels. There is no evidence to suggest that the lower Whitby represents an
environment that is significantly deeper than that of the underlying Cobourg (Liberty,
1969:72) even though the deposition of these shales did coincide with the near
maximum extent of the late Ordovician transgressing seas.

If the Cobourg Formation represents a shallow, well-oxygenated, and warm
epicontinental shelf environment and the overlying lower Whitby Formation a
moderately shallow, oxygen-poor, and cold epicontinental shelf environment, then
the Cobourg-Whitby contact should indicate the time when a permanent thermocline
became established on the shelf seas. The establishment of a thermocline over the
epicontinental sea would have resulted in the spread of cold and oxygen-poor oceanic
waters well into shallow sites (Heckel, 1977: fig. 5). The biota inhabiting this new
ecologic setting would have been a mixture of those elements of the slope biofacies
that were able to follow the shifting oceanic water masses into shallow settings,
certain eurytopic elements of the shelf biofacies, and a few shelf taxa that, by chance,
happened to have been preadapted for the new conditions. Pseudogygites
latimarginatus could well fall into the third category.

Gould (1977:289, 290) emphasized that although the adaptive significance of

heterochrony has most commonly been expressed in terms of morphologic advantage
alone, certain life history strategies such as timing of reproduction, fecundity, and
longevity are adaptations in themselves and because such strategies are the very
processes of heterochrony, they provide additional and highly significant indications
of the expression of heterochrony in ecologic contexts.

Temperature profoundly affects most marine invertebrates, influencing their
morphology, distribution, physiology, growth, and reproduction. Valentine
(1973: 123) has noted that growth tends to be slower, the onset of reproduction later,
and death later among cold-water marine invertebrates than among warm-water
marine invertebrates. Despite their slower growth, animals in cold water often attain
larger dimensions than those in warm water because of postponed reproduction and
increased longevity.

The twin pathways of paedomorphism, that is progenesis (precocious sexual
development of an organism in a juvenile state) and neoteny (retention of juvenile
characters of ancestor by adult descendant) (Gould, 1977), and their possible
macroevolutionary roles should be examined in terms of temperature because
temperature is known to influence the timing of sexual maturation in insects (Gould,
1977:305) and, of course, timing of maturation is the very core of paedomorphism.

McNamara (1978) suggested that the Early Cambrian trilobite Olenellus
(Olenelloides) armatus was derived from Olenellus (Olenellus) by progenesis and
that the paedomorphic derivation may have been triggered by elevated water
temperatures in shallow sites.

I have here suggested that Pseudogygites latimarginatus was derived from I[sotelus
gigas by neoteny. It is possible that this paedomorphic derivation was triggered by
low water temperatures in deeper sites near the facies edge of the upper Trenton
Group, but such a theory presupposes that the relatively rare specimens of P.
latimarginatus that are known to occur in warm and shallow settings in the upper
Cobourg Formation of southern Ontario are merely peripheral members of larger
populations occupying deeper and colder environments during deposition of the upper
Cobourg. There is no evidence that such populations existed in the deeper water
Cobourg equivalents, such as the Utica and upper Canajoharie Shales of New York
State, and the first appearance of P. /atimarginatus in this area is synchronous with its
abundant appearance in the lower Whitby in Ontario (Fig. 11). One must conclude,
therefore, that the neotenous derivation of P. latimarginatus took place in the shallow
and warm Cobourg environment and that /sotelus gigas was sympatric with the initial
members of its neotenate.

Water temperature cannot be used to explain the paedomorphic derivation of
Pseudogygites latimarginatus, but it may offer an explanation for the great abundance
of this trilobite in the lower Whitby Formation. The convergence of the suite of
characteristics of cold-water invertebrates and the heterochronous characteristics of P.
latimarginatus is significant and suggests that this species was equipped with the
developmental and ecologic features of a cold-water species as a direct result of its
neotenous derivation. It remained relatively rare during deposition of the Cobourg,
but its preadaptation permitted explosive increase at the onset of declining water
temperatures in the lower Whitby.

The lower Whitby Formation may be viewed as a slope litho- and biotope that has
been shifted to a shelf position. This shift certainly necessitated a number of
lithologic and biotic changes, but the basic ecologic attributes of the predictable and

stable slope environment probably remained intact. In terms of the environmental
spectrum associated with a continuum of r to K selection strategies (Pianka, 1970),
the lower Whitby environment should fall closer to the K end member. The
abundance of Pseudogygites latimarginatus in this setting thereby supports Gould’s
(1977 :293) prediction that “‘progenesis will be associated with r strategies and
neoteny with K strategies’’.

The preceding is a possible explanation for the origin and localized abundance of
Pseudogygites latimarginatus. It originated as a neotenate of /sotelus gigas and its
great abundance in the lower Whitby is attributed to a fortuitous preadaptation to a
new ecologic setting. The origin of the remaining three species of Pseudogygites is
unclear. Each is somewhat younger than P. latimarginatus and each occurs in a
separate bituminous unit where it directly overlies a carbonate unit (Fig. 11). The
establishment of a uniform lithotope (brown bituminous shales and limestones) and
biotope (the Leptobolus-Triarthrus-Pseudogygites-Geisonoceras fauna of Workum,
Bolton and Barnes, 1976) over vast areas of eastern and arctic North America is one
result of the extensive Late Ordovician transgression (Sanford, in Bolton et al.,
1977). These shales and limestones apparently record the flooding of large parts of
the craton by cold and oxygen-poor waters. It is possible that the four species of
Pseudogygites constitute a single phyletic sequence (that is, P. latimarginatus to P.
hudsoni to P. akpatokensis to P. arcticus), but because the similarity between these
species of Pseudogygites are those features that could well indicate separate
neotenous derivations, it is equally possible that they are iterative paedomorphs of
different species of Jsotelus. The possible /sotelus ancestors of the northern species of
Pseudogygites, however, have not been studied.

Acknowledgements

I am greatly indebted to John Riva, Université Laval, Quebec for loan of specimens
of Pseudogygites from Rodman, New York and for detailed information about the
graptolite biostratigraphy of Pseudogygites-bearing beds in southern Ontario, New
York, Michigan, and the Canadian arctic. In addition, I thank G.S. Nowlan, C.R.
Barnes, A.C. Lenz, and J. Arengi for making available specimens of Pseudogygites
from Devon and Cornwallis islands and S.J. Nelson and B.V. Sanford for
information about Southampton Island stratigraphy. The photographs are by Brian
O’Donovan and the drafting by Subhash Shanbhag, both of the Department of
Geology, University of Toronto. The manuscript has benefited from critical reviews
by Brian Chatterton and John Riva. This study was supported by the National Science
and Engineering Research Council and the Department of Energy, Mines and
Resources.

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