Late Caradoc - early Ashgill trilobite distribution in the
central Oslo Region, Norway
DAVID L. BRUTON & ALAN W. OWEN
Bruton, D. L. & Owen, A. W.: Late Caradoc- early Ashgill trilobite distribution in the central Oslo
Region, Norway. Norsk Geologisk Tidsskrift, Vol. 59, pp. 213-222. Oslo 1979. ISSN 0029-I%X.
Detailed collections from profiles in the Upper Chasmops Limestone (Oslo-Asker) and Solvang
Formation (Ringerike) have yielded a total trilobite fauna of 45 taxa. The stratigraphical distribution of
these is presented in the form of faunal logs which show a progressive immigration of forms and increase
in diversity. The effect of this is emphasised by a major fauna! shift at the base of overlying shales which
are markedly diachronous westwards from Oslo. The trilobite fauna provides the best available means of
correlation so far with standard British late Caradoc and early AshgiU sheUy assemblages.
D. L. Bruton, Paleontologisk museum, Sars gate/, Oslo 5, Norway.
A.
W. Owen, Paleontologisk museum, Sars gate l, Oslo 5, Norway. Present address: Dept. of
Geology, The University, Dundee DDI 4HN, Scotland.
Since the early 1800's, the richly fossiliferous
Temeholmen (Asker), were recollected at var­
Lower Palaeozoic rocks in the neighbourhood of
ious intervals between 1968-1970 and, apart from
Oslo have yielded large collections, among them
Temeholmen,
those from the Ordovician Chasmops Beds. The
GunnbjØrg Qvale (Qvale 1977) in her search for
latter formed the basis for a research project
ostracodes. In the meantime Owen had started a
were
also
recollected
by
started in 1948 by the late Professor Leif StØrmer
study on the trilobite faunas from equivalent
(Størmer 1953) which aimed at describing the
beds in Ringerike and Hadeland in the north
various fossil groups and refining interregional
west of the Oslo Region (Owen 1977, 1978, 1979)
correlation. In that project the geographical area
and his data on trilobite distribution in these
for study was widened to include what Størmer
areas, together with that subsequently assem­
(1953:51) defined as the Oslo Region. In order to
bled jointly from the Oslo area, are presented
describe the total fauna, a group of specialists
here. A monograph of the Upper Chasmops
was invited to cooperate and, by 1977, 27 publi­
Limestone trilobites which is to form another
cations had appeared containing descriptions of
part of this joint study will appear later.
some of the major fossil groups. The bulk of
material used for these studies, however, was
mainly
from
collections
at
Paleontologisk
museum, Oslo, and others of 'bag-stratigraphi­
OSLO
cal' type (Jaanusson 1976: 302), and little or no
t
information was presented as to the spatial dis­
tribution of taxa within units.
With this in mind, one of us (D.L.B.) decided
to make detailed collections from several pro­
files in the Oslo-Asker area with a view to
describing trilobite
(and
other)
distributions.
With the help of cand. real. J. Fredrik Bockelie
and a number of students, sites were chosen and
detailed collections made at Bygdøy (Oslo) and
Fornebu (Bærum), while Bockelie alone took
Raudskjer (Asker) (Fig. l). All sites offer profiles
through the Upper Chasmops Limestone, the
most fossiliferous unit, and the subject of this
account. The profiles, together with beds on
OSLOFJORD
Fig. l. Map showing the localities in the Oslo-Asker district
referred to herein with the location of the measured profil es (p)
indicated. The Lower Palaeozoic succession is tightly folded
with a regional strike of 060.
214 D. L. Bruton & A. W. Owen
NORSK GEOLOGISK TIDSSKRIFT 3 (1979)
B. FORNEBU
A. BYGDØY
:;:
o
l
2
- z
f
'
'
'
'
3
l
4
5
'
'
6
'
'
'
r
!I
8
9
'
10
11
12
'
...
' '
... ...
l
... "'
CD .".
CD <>o
.... ..........
....
"'
.....
Fig. 2. The distribution of trilobites in the Upper Chasmops Limestone on A. Bygdøy and B. Fornebu. Metre intervals in the
sections measured from either the top (BygdØy) or base (Fornebu) of the Chasmops beds and samples in Paleontologisk museum,
Oslo from each profile are documented in terms of these datum lines. Most trilobite occurrences are illustrated as bars
representing 10 cm of the section and no break is shown between bars from successive horizons. In cases where the sample
locations are less precise, they are illustrated as dashed lines between the known limits of coUection. Vertical lines represent the
range of each species in the Upper Chasmops Limestone and when extended below the lowest sample indicate their occurrence in
the underlying Upper Chasmops Shale. In the case of the Fornebu profile most of this unit is not exposed (Toni 1975, fig. l). The
highest part of the Upper Chasmops Limestone may be missing in the Bygdøy profile. Apianurus thorslundi Bruton and
Platylichas laxatus (McCoy) are known from old collections from the Upper Chasmops Limestone on Bygdøy and Fornebu
respectively, but were not present in the samples on which this study is largely based.
NORSK GEOLOGISK TIDSSKRIFT 3 (1979)
Trilobite distribution in Oslo Region
OSTØYA
•
•
•
•
base
not
se en
.., o
Cl) ..,
Cl) "'
..... .....
N
Cl)
Cl)
.....
Fig. 3. The distribution of trilobites in the Upper Chasmops
Limestone on Ostøya. A strong eastward attenuation of the
shale horizons between 3.5-4.3 m in the profile may indicate
the presence of a strike fault cutting out part of the succession
here. New collections did not contain Tretaspis ceriodes and
Deacybele graci/is, but they are well represented in
Paleontel ogisk museum collections from the upper part of the
unit on the island.
Sampling
Each profile in the Oslo-Asker region was care­
fully measured from a fixed point either at the
exposed top of the succession or at the base, and
bulk sampling made to the nearest 30 cms or, in
15-
Geologisk Tidsskr. 3/79
215
most instances, less. Sample weights varied
from large (2-5 kgs) for limestones, to much
smaller samples (50 gs) for shales. There is thus a
sample size bias but the intervening shales are
singularly devoid of trilobites and have a very
restricted fauna of isolated bryozoa (Di­
plotrypa), many in growth position, and scat­
tered pockets of brachiopods often distorted and
cleaved. Each limestone sample was crushed in
the laboratory using a rock-jaw hydraulic cutter
and reduced to a size of about 5 cm or less.
Pieces were thereafter carefully searched under
the microscope, all fossil fragments ringed, and
where possible identified and separated accord­
ing to fossil group. In sorting the ostracodes,
many of the smaller trilobites were recognised at
the same time; this proved especially valuable .
Articulated specimens of trilobites were rare
in the samples, the majority being represented
by disarticulated elements. In many cases ex­
uviae were recognised and at certain horizons
specimens of Stenopareia made up coquinas.
Exuviae found in very fossiliferous pockets on
the soles of limestone beds showed exoskeletal
parts slightly displaced or not scattered far,
while other examples lay at all angles to the
bedding and bad clearly drifted or been trans­
ported in suspension to their present site. For
these reasons we have not attempted any
quantification of the trilobite fauna.
Jaanusson (1976) demonstrated that the
number of specimens of a species per sample
size as well as the number of samples in relation
to the stratigraphical thickness studied have a
profound effect on resultant fauna) logs of the
type presented here (Figs. 2 to 5). However, we
consider the size and frequency of the samples
under study to be sufficient for most forms to be
recovered at any given horizon and that the data
presented give a reasonably accurate picture of
trilobite distribution. Nevertheless, we are
aware of the fact that our samples are probably
not large enough to guarantee the presence of
forms which even now are known only from a
few specimens, among many thousands col­
lected and identified.
Stratigraphy
A revised lithostratigraphical scheme for the
Chasmops Beds in the Oslo-Asker area will be
described elsewhere. Reasons for dispensing
with the hitherto used numerical classification of
'etasjer' have already been presented by Owen
216
D. L. Bruton & A. W. Owen
NORSK GEOLOGISK TIDSSKRIFT
3 (1979)
RAUDSKJER
Il'
I
lt
-
. I I
tt-
-
T
Pj
i
J
�It t it
i
L
�
I
...
"'
"'
....
Fig.
4. The distribution of trilo­
bites in the
Upper
Chasmops
Limestone on Raudskjer. Sample
horizons between
3.6 and 3.8 m
below the top of the unit refer to
collections from west Raudskjer,
all the remainder are from east
Raudskjer.
(1978,
1979),
who
has
introduced
new
li­
thostratigraphical terms for equivalent strata in
transition between the dominant shale sequence
containing ellipsoidal nodules of the
Upper
These are followed
Chasmops Shale, and the succeeding nodular to
here. For the Oslo-Asker Region, we have fol­
well-bedded Iimestones and subordinate shale
Ringerike and Hadeland.
lowed Størmer (1953:65) in taking as the base of
succession (Brøgger 1887, Toni 1975). This py­
the Upper Chasmops Limestone the conspicu­
rite band is an excellent marker horizon in the
ous nodular pyrite band which occurs at the
Oslo-Asker area and can be recognised in all our
NORSK GEOLOGISK TIDSSKRIFT 3 (1979)
Trilobite distribution in Oslo Region
217
NORDERHOV
I
jI
base
not
seen
...
OJ
OJ
...
"'
o
...
"'
....
....
....
"'
N
OJ
OJ
....
Fig. 5. The distribution of trilo­
bites in the Solvang Formation at
Norderhov, Ringerike. A meas­
ured section is given by Owen
(1979, fig. 4). Decoroproetus sol­
enotus is also known from here.
profiles except Ostøya, where a fault obscures
the base of the unit. The thickness of the Upper
Chasmops Limestone between this marker hori­
zon and the overlying black Tretaspis Shales,
however, varies and there is a broad increase
westwards from Oslo to Asker.
Trilobite distributions
In an unpublished study based entirely on
museum collections, Owen (1977, fig. 42) li sted
the trilobite species occurring in the Upper
Chasmops Limestone and found a marked in­
crease in the total number of forms westwards
from Oslo through Bærum to Asker. Although
based on 'bag -stratigraphical' samples, the data
confirm the overall geographical distribution of
species shown by the present work (Fig. 6), but
it is now possible to assess the relative
stratigraphical distribution of these taxa over the
whole area. Our study has revealed 8 new
species and 13 other taxa not previously re­
corded from Norway. Where precise horizon
details are available on labels of studied museum
specimens, these correspond well with the levels
218 D. L. Bruton & A. W. Owen
NORSK GEOLOGISK TIDSSKR!Ff 3 (1979)
l
lllllllll l1f
Top Frognøya -
Top Raudskjer &
l
l
l
l
l
l l
Top Ostøya
Top Fornebu -
l
l
sha
TopBygdøy-
l l
l
l
UJ
::;
;:::
�
l
'
�
"
.:?..
�
�
.'l
�
�
l!
Fig. 6. The relative time distributions of trilobite species occurring in the Upper Chasmops Litnestone in Oslo-Asker and the
Solvang Formation in Ringerike. The approximate time levels for the tops of the limestone units in the measured profiles are
shown. This distribution is consistent with all available data from the region.
known from the measured sections as do co-oc­
currences of taxa on the same hand specimen.
Two genera Carrickia sp. (one specimen) and
Primaspis sp. (two specimens) occurring in the
museum collections have not been identified in
new collections. Both are from the uppermost
bed of the Upper Chasmops Limestone on
Skjærholmen and occur with other forms which,
however, are found in our collections.
Fig. 6 shows the order of appearance of
species and their relative stratigraphical ranges.
The scaling of the relative positions of first
appearances is based largely on stratigraphical
thicknesses in the measured sections (Figs. 2 to
5). However, this assumes a similar and constant
deposition rate over the whole area along with
subsequent uniform compaction, especially of
the shale horizons. Lacking reliable data on
these factors, this approach at best gives only a
close approximation to a time scale, the degree
of accuracy of which can be judged from the
following examples.
In Oslo-Asker, Ampyxella aculeata and
Ampyxina? sp. first occur either together or
within no more than l m of each other in each
profile. Both are restricted to the highest ex-
posed limestone bed (ll.8 m above base) on
BygdØy and the uppermost bed further east on
Nakholmen. The two forms first occur at 9.8 m
above the base of the Fornebu profile and within
60 cm, 11.9-12.5 m above the base on east
Raudskjer. This similarity in height above the
base of each profile suggests that the first oc­
currence of these species is broadly synchronous
throughout the whole area. Furthermore, all but
two of the forms restricted to localities west of
Fornebu first appear above this level. Similarly,
Tretaspis ceriodes (s.
1.),
Deacybele gracilis,
Illaenus (Parillaenus) aff. fallax, and Stygina
minor occur only in the uppermost part of the
Upper Chasmops Limestone on Kalvøya and
Ostøya whilst forms known only from further
west occur above the first appearance of these
species.
Størmer (1953:68) noted that the upper part of
the Upper Chasmops Limestone in Asker is
younger than in Oslo. Thus Tretaspis ceriodes
occurs in the uppermost horizons in the western
part of Bærum and in Asker, but further east is
known only in the lower part of the overlying
Lower Tretaspis Shale in a dark limestone 0.851.02 m above the Upper Chasmops Limestone.
NORSK GEOLOGISK TIDSSKRIFT 3 (1979)
Owen (1979) argues that this diachronism of the
base of the shale unit is seen also in Ringerike to
the north west, where limestones and shales of
the HØgberg Member of the Solvang Formation
on Frognøya are younger than the Solvang For­
mation at Norderhov and the Upper Chasmops
Limestone in Asker. The uppermost part of the
Norderhov profile (Fig. 5) contains a very simi­
lar trilobite fauna, including T. ceriodes, to that
of the upper beds on Raudskjer (Fig. 4).
The Solvang Formation in Hadeland (Owen
1978) contains a trilobite succession similar to
that found at Norderhov and in Asker, but Jack
of sufficiently complete profiles in Hadeland
prevents us illustrating this distribution in the
way we have done for the Oslo-Asker and Ringe­
rike sections.
Interpretation of trilobite
distribution
A survey of the underlying Upper Chasmops
Shale shows this unit to have a fairly restricted
trilobite fauna including Broeggerolithus discors
(Angelin, 1854) known only in Oslo but not in
Bærum and Asker. Others such as Chasmops
extensa, Harpide/la ( s. l.) sp. A, Remopleurides
sp. A, and Stenopareia glaber are less restricted
and persist into the Upper Chasmops Limestone
where they all have long vertical ranges.
The distribution of species within the Upper
Chasmops Limestone and the Solvang Forma­
tion indicates a gradual incoming of species, the
later stages of which were accompanied by the
initiation of dominantly shale deposition to the
east. The diachronous base of these shales
marks a very pronounced fauna! change and thus
the large!y carbonate-restricted fauna of the up­
per parts of the limestone units shows a progres­
sive westward increase in endemicity. As this
change is approximately at right angles to the
regional strike it is not necessary to produce a
palinspastic
reconstruction
in
order
to
appreciate the scale of these lateral changes.
Few trilobites from the Upper Chasmops
Limestone and Solvang Formation are known
from the overlying, and part!y coeval, shale units
which in Oslo-Asker and Ringerike have a fairly
low-diversity fauna dominated by members of
the Tretaspis seticornis group (of Ingham 1970)
and species of Flexicalymene, Decoroproetus,
and Primaspis.
Most species at any one time were probably
Trilobite distribution in Oslo Region
219
distributed throughout the area of dominantly
limestone deposition. This distribution may have
been somewhat patchy. Thus Calyptaulax aff.
norvegicus and Harpidella (s. l.) sp. A are not
known from BygdØy but occur in all other Oslo­
Asker profiles described here. A few of the older
species are known only from the east or west of
the Oslo-Asker district. Pseudosphaerexochus
bulbosus and Decoroproetus sp. A are known
from the lower part of the Upper Chasmops
Limestone in the Raudskjer profile but not from
further east. Similarly Carrickia sp. and
Primaspis sp. are known from Oslo in the east
but not Bærum or Asker. All these forms are
known from only a few specimens and no signifi­
cance is attached to their presently known dis­
tributions.
The distribution of species of Flexicalymene,
however, reflects some major ecological control,
possibly depth, in the Oslo-Asker district. F.
scabustula occurs at various horizons in the
BygdØy profile and the limited amount of availa­
ble information suggests the same is true on
Nakholmen. At Fornebu to the west, it is known
on!y from the middle part of the U pper
Chasmops Limestone. The species is not known
further west where the closely related F. jemt­
landica occurs in the upper part of the unit in
Bærum and Asker and also in the Solvang For­
mation at Norderhov. The lower part of the
stratigraphical range of F. jemtlandica slightly
overlaps with the range ofF. scabustula. Some
form of ecological control at the generic leve! is
shown by the distribution of Illaenus and
Stenopareia. I.(Parillaenus) fallax and S. glaber
coexist in the Upper Chasmops Shale but only
the latter persists into the succeeding Upper
Chasmops Limestone where it is abundant
throughout most of the unit in all areas. At the
top of the unit on OstØya, Raudskjer and
Terneholmen and in the Solvang Formation at
Norderhov and on Frognøya, S. glaber is dis­
placed by/. (Pari/laenus) aff. fallax and the two
appear to be mutually exclusive at this leve!.
Both genera coexist again in younger units.
Jaanusson (1976) distinguished two types of
fauna! changes:
fauna/ shifts associated with changes in local
physical conditions such as lithofacies, and
immigrations from outside the region induced
by large-scale environmental changes such as
temperature.
He acknowledged that there is not always a
clear distinction between these types of change
220
D. L. Bruton & A. W. Owen
NORSK GEOLOGISK TIDSSKRIFT
3 (1979)
and that in considering Ordovician successions
Dean (1960, 1963a) has commented on the
in the Balto-Scandinavian region, the uncer­
strong simi1arity between trilobite faunas in the
tainty has been compounded by insufficient data
Upper Chasmops Limestone with those of the
on geographical and vertical ranges of taxa. This
Actonian and Onnian stages in the type Caradoc
is still true, and, in view of such uncertainties,
succession of Salop (Shropshire), England. This
Jaanusson used the term faunal shift for all the
similarity applies to both the generic and specific
changes he described in
association. The following species from the type
Middle
Ordovician
(0) stages occur in the
benthonic faunas which are demonstrably as­
Actonian (A) and Onnian
sociated with lithological changes. The abrupt
Upper Chasmops Limestone in Oslo-Asker and
change in the trilobite fauna associated with the
the Solvang Formation at Norderhov, Ringerike,
change to dominantly shale deposition at the top
or are closely related to forms which do occur
of the Upper Chasmops Limestone and Solvang
there:
Formation, is clearly a fauna) shift. The available
Ampyxella edgelli (Reed, 1932) (A &O)
evidence suggests that no such shift in the trilo­
Calyptaulax actonensis Dean, 196 1 (A)
bite fauna occurred at the base of the Upper
Chasmops extensa (Boeck, 1838) (A &O)
Chasmops Limestone.
Lonchodomas pennatus (LaTouche, 1884) (A
Fauna) changes described as immigrations by
Jaanusson are characterized by the appearance
&O)
Platylichas laxatus (McCoy, 1846) (A &O)
of forms not known from older strata in the
Tretaspis
Balto-Scandinavian region and such changes are
1963a (A &O)
not associated with distinct lithological changes.
The gradual incoming of trilobite species seen in
ceriodes
(Angelin) favus
Remopleurella burmeisteri (Bancroft,
Dean,
1949)
(0)
the Upper Chasmops Limestone and the Solvang
Species of Flexicalymene from the Norwegian
Formation almost certainly represents an immi­
units are likewise closely related to those as­
gration in Jaanusson's sense. All the species and
signed to Onnicalymene from Salop by Dean
most of the genera whose first appearance in the
(1962, see Siveter 1977). The genus Gravica­
Upper Chasmops Limestone and Solvang For­
lymene Shir1ey, 1936 occurs in the type Actonian
mation lies above the bases of these units (Fig. 6)
and Onnian but its niche in the Upper Chasmops
are not known from lower horizons in the Oslo
Limestone fauna
Region. Moreover, the available evidence sug­
Prionocheilus Rouault, 1847.
was occupied probab1y by
gests that the same is true for the whole Balto­
The range of Tretaspis ceriodes favus in Salop
Scandinavian region. Three of the new genera
may be slightly different from that given by Dean
(Ampyxina?,
Carrickia,
and
Tretaspis)
are
(l963a), who described the subspecies from just
known only from earlier horizons in association
above and be1ow the Actonian-Onnian boundary
with North American faunas.
in the banks of the River Onny and from sup­
posed Actonian strata near Cardington. Speci­
mens collected by one of us (A. W. O.) from the
latter 1oca1ity are associated with Onnia gracilis
Remarks on correlation
Correlation
between
areas
(Bancroft, 1929), the supposed index fossil for
in
the
Balto­
the middle part of the Onnian Stage. Species of
Scandinavian region is still hampered by faunas
Onnia are a characteristic feature of Onnian
elsewhere being in need of modem documenta­
assemblages in
tion and description. Nevertheless among trilo­
(Dean 1960, 1962, 1963a, lngham 1966, 1974) but
bites described by Thorslund (1940) from the
the genus is unknown in Scandinavia (Owen in
Upper Chasmops Limestone in the autochthon­
Hughes, lngham &Addison 1975:597).
Salop and northern
England
ous sequence in Jemtland, Sweden, the follow­
The olenid Triarthrus linnarssoni Thorslund,
ing forms are the same as Norwegian species:
1940 described by Dean (1963) from the up­
Tretaspis ceriodes (Angelin, 1854), Triarthrus
permost part of the Onnian Stage, is not known
linnarssoni Thorslund, 1940 and probably Flex­
from the Upper Chasmops Limestone, but we
icalymene scabustula
have identified it ( = Triarthrus cf. skutensis of
pareia
'avus',
and
Siveter,
1977?,
Ampyxella
Steno­
aculeata
Nikolaisen
1965)
together
with
Tretaspis
(Angelin, 1854)?. Pseudosphaerexochus tvaer­
ceriodes (sensu lata) in the lowest part of the
ensis Thorslund, 1940 is closely re1ated to the
Norwegian form P. bulbosus.
Lower Tretaspis Shale on Nakholmen, Oslo.
The fauna) distribution data presented herein
NORSK GEOLOGISK TIDSSKR!Ff 3 ( 1979)
help further to explain the relationship between
the younger parts of the succession in Oslo­
Asker and Ringerike. Previously Kiær (I 921) and
Størmer (1953:87) believed, on the basis of the
succession of Tretaspis species, that in Ringe­
rike the occurrence of T. kiaeri StØrmer, 1930
indicated a hiatus in Oslo-Asker where it is
absent. Owen (1979) argued that the fauna of the
T. kiaeri- bearing Høgberg Member of the Sol­
vang Formation on Frognøya (essentially '4b82'
of StØrmer) simply represents a later phase in the
immigration of largely limestone-restricted
species than is seen in the Upper Chasmops
Limestone. The correlation of the HØgberg
Member with the standard British succession
has been discussed by Dean ( 1971: 14, 47-48),
Bruton (1976: 712), and Owen ( 1979). The latter
has shown that it is probably lowest Pusgillian in
age because it contains T. seticornis seticornis
(Hisinger, 1840). Members of this group (sensu
Ingham 1970) succeed T. ceriodes without over­
lap and independently of lithology in Scan­
dinavia and Britain. In the case of the lat­
ter, they occur at the Onnian-Pusgillian (i.e.
Caradoc-Ashgill) boundary in northern England.
Also occurring in the Høgberg Member and
confirming its age are Phillipsinella pre c/ara,
known also from the highest part of the Pusgillian
at Cross Fell (Bruton 1976), and Pseudosphaer­
exochus densigranulatus Nikolaisen, 1965 which
is the same as P. aff. tectus of Ingham ( 1974)
from Pusgillian horizons at Cross Fell and in the
Cautley area.
In the absence of graptolites from the Upper
Chasmops Limestone no precise correlation can
be made with the standard British graptolite
sequence. Nevertheless the trilobite faunas de­
tailed here provide the best available means of
correlation so far with British late Caradoc and
Ashgill shelly assemblages and hopefully this
may be refined further when results of other
groups, among them the brachiopods currently
being studied by Dr. M. G. Bassett, are made
known.
Acknowledgements. - The bulk of the material used for this
study was collected with the help of students and others: in
particular, Jo Gjessing, Gunnbjiirg Qvale, Arve Tunstad, J.
Fredrik Bockelie, and Leif Koch. Help with sorling specimens
was received from Leena M. Klaveness, Mette Havrevold,
and Liza Kravik. We thank Bockelie and Valdar Jaanusson for
discussing our work and reading the manuscript, and Gill
Owen for catologuing specimens and typing the text. Bruton's
work was partly financed by NAVF grants D.40.30.2 and
D.40.38. 6 and Owen was supported by a NATO Postdoctoral
Fellowship for 1977-1978.
August 1978
Trilobite distribution in Oslo Region
221
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