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

D.K. Elliott, E. Mark-Kurik, E.B. Daeschler. A revision of Obruchevia
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the Early Devonian (Pragian) up to the Late Devonian (Frasnian), and are particularly
characteristic of the Middle Devonian where they have been used as zonal indicators
(Mark-Kurik 2000; Weddige 2000). Species of some genera (Tartuosteus, Pycnosteus)
reached almost 2 m in length and breadth making them by far the largest heterostracans
known; however, only the Early Devonian forms are preserved in articulation (Gross
1963), while later forms are known only from isolated plates. Early Devonian psammosteids
are found in Germany, South West England, and Luxembourg (one genus) and Poland
(two genera); in the Middle Devonian they are particularly common in the Baltic area
(Estonia, Latvia) and the adjacent parts of Russia, the Leningrad and Pskov regions
(seven genera). In the Middle and Late Devonian psammosteids also occur in Scotland,
the Timan region and the Severnaya Zemlya Archipelago (Russia), Spitsbergen, Greenland,
and Arctic Canada. In the Late Devonian their numbers significantly diminished
(represented mainly by two genera) and at the end of the Frasnian psammosteids died
out over almost their entire range, the last representatives occurring in late Frasnian
deposits in the Canadian arctic.
Psammosteids have been studied since the first half of the 19 th century. However,
the most important work on them was carried out in the 1960’s resulting in two monographs
being published practically at the same time (Halstead Tarlo 1964, 1965; Obruchev
and Mark-Kurik 1965). The work by Obruchev and Mark-Kurik contains the description
of well preserved psammosteid material from the territory of the former Soviet Union,
particularly from the Baltic area. Halstead Tarlo’s monograph is an overview of the
entire suborder and includes a short overview of the taxa described by Obruchev and
Mark-Kurik. Since the publication of the above monographs, a number of papers has
been published on psammosteid morphology and taxonomy (Halstead Tarlo 1967a,
Halstead 1974; Lyarskaya 1971; Mark-Kurik 1968, 1984, 1993, 1999; Obruchev 1967);
nevertheless, the overall taxonomy of the group has not been reviewed since it was
presented in the monograph by Obruchev and Mark-Kurik (1965). It has been generally
accepted that psammosteids were derived from another heterostracan taxon, the
Pteraspidida (Elliott 1984; Blieck et al. 1991); however, this view has yet to be tested by
modern phylogenetic analysis using computer-assisted methods (Janvier 1996).
The Canadian Arctic record of psammosteids has been based on a small collection
from southern Ellesmere Island along Goose Fiord. This area was visited first during the
explorations of The Second Norwegian Arctic Expedition in the Fram (1898-1902) under
the command of Otto Sverdrup. At that time fossil vertebrates, including a few
psammosteid fragments, were collected by Per Schei, geologist to the expedition, from
strata referred to as “Series E,” but now designated as the Fram Formation within the
Okse Bay Group (Mayr et al. 1994). The psammosteid fragments were described as two
species of Psammosteus by Kiaer (1915) and revised by Halstead Tarlo (1965), but no
further work was carried out until 1999, 2000, and 2002, when expeditions led by one of
us (Daeschler) made a large collection of vertebrates, including psammosteids, from the
Devonian clastic wedge that stretches from Melville across Bathurst, Devon, and
Ellesmere islands. The psammosteid material ranges in age from the Frasnian (Fram
Formation, Okse Bay Group) into possibly the early Famennian (Parry Islands Formation)
(Mayr et al. 1994, 1998; Trettin 1978) although the Famennian age attribution
seems doubtful as little stratigraphic control is available. Regardless, the collection
appears to include some of the youngest known psammosteids (and heterostracans) as