Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ...

24 ADVANCES IN PALAEOICHTHYOLOGY
well as greatly extending the area from which these animals are known. The collection is
currently being described by two of us (Elliott and Mark-Kurik) and includes some of
the same species that occur in the Baltic (Elliott et al. 2003), thus allowing the development
of a correlation scheme (Obruchev and Mark-Kurik 1968).
Included within the collection are plate fragments from the Late Devonian Clastic
Wedge that are extremely thick and that do not have a surface ornament of dentine
tubercles. This feature is otherwise only described in the aberrant psammosteid
Obruchevia (Obruchev 1936) from the Lovat´ River, northwest Russia. In this paper
we describe the new Canadian material and also add to the knowledge of Obruchevia
by the description of additional material from the collections of the Latvian Natural
History Museum, Riga, and the Palaeontological Institute, Russian Academy of Sciences,
Moscow.
Stratigraphy
Most of the Canadian Arctic obrucheviid material in this report was collected from a
single site near Okse Bay on southern Ellesmere Island (Fig. 1). The site, designated as
field site NV2K11, is within the lower to middle part of the Nordstrand Point Formation
exposed on the northern limb of the Schei Syncline. The fossil site is located
within 2km of the type section of the Nordstrand Point Formation as designated by
Embry and Klovan (1976). Most of the Nordstrand Point Formation is middle Frasnian
in age based on palynomorph samples, although the upper 100 to 200 meters of the
675-meter-thick type section are late Frasnian (Embry and Klovan 1976). The
Nordstrand Point Formation is the uppermost unit of the Okse Bay Group. On southern
Ellesmere Island the Okse Bay Group is a 3000-meter-thick succession of fluvial
sediments derived from tectonic highlands to the east that fed several fluvial systems
prograding generally to the southwest (Mayr et al. 1994).
The fossiliferous horizon at the NV2K11 site is a dark, carbonaceous siltstone. The
obrucheviid and other fossil material is abundant in the fissile siltstone as well as an
underlying ironstone layer. The total thickness of this fossiliferous zone is 120 to 150
mm. The sediments suggest a low energy overbank depositional setting rich in organic
material. Fossil plant material occurs as large compressed stems and as three-dimensional
stems up to 100 mm in diameter. Associated vertebrate fauna from the NV2K11
site includes Psammosteus sp., Bothriolepis sp., and Holoptychius sp.
An additional fragment of obrucheviid plate was recovered in 1999 from the sequence
of Late Devonian clastic sediments near the southern coast of Ile Vanier (Bathurst
Island Group). This locality is designated NV9918, and is 400 km west of the NV2K11
site (Fig. 1). The stratigraphic position of the NV9918 site is problematic, although
currently mapped as within the Cape Fortune Member of the Parry Islands Formation
by Harrison and de Freitas (1998). The Cape Fortune Formation is mainly early Famennian
in age (Embry and Klovan 1976). Harrison’s mapping was based on lithostratigraphic
markers; however, the obrucheviid specimen, as well as psammosteid material from
other localities on Ile Vanier, suggest that it is in error. It is more likely that the
psammosteids on Ile Vanier are from the upper part of the Beverly Inlet Formation and
therefore Frasnian in age. The Beverly Inlet Formation is considered to be distal facies