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ACTA UNIVERSITATIS LATVIENSIS, 2004, Vol. 679, pp. 22-45 A revision of Obruchevia (Psammosteida: Heterostraci) and a description of a new obrucheviid from the Late Devonian of the Canadian Arctic DAVID K. ELLIOTT, ELGA MARK-KURIK, and EDWARD B. DAESCHLER D.K. Elliott, Department of Geology, Northern Arizona University, Flagstaff, Arizona 86001, USA, David.Elliott@nau.edu; E. Mark-Kurik, Institute of Geology, Tallinn University of Technology, Estonia Ave. 7, 10143 Tallinn, Estonia , Kurik@gi.ee; E.B. Daeschler, Academy of Natural Sciences, 1900 Benjamin Franklin Parkway, Philadelphia, Pennsylvania 19103, USA , Daeschler@acnatsci.org Psammosteids are the youngest heterostracans, surviving until the end of the Frasnian in western Europe where their faunal succession is well known. Recent collections made by the 1999-2002 Nunavut Paleontological Expeditions from the Devonian clastic wedge across Melville, Bathurst, Devon, and Ellesmere islands now show a similar psammosteid faunal succession in the Canadian Arctic. Some very thick psammosteid plates from southern Ellesmere lack dentine tubercles but do have an increased amount of the hard tissue pleromin infilling the spongy aspidin at the surface. This feature is otherwise known only in the psammosteid Obruchevia, described from the Lovat´ River, Novgorod District, northwestern Russia. The dorsal plates of Obruchevia are large, notably thick, and cardiform and appear to have grown by the addition of lateral flanges that developed from the lower surface of the margins. The surface is ornamented with radial furrows and pits. The branchial plates have a vertically directed lateral margin that would have functioned as a runner. Previously undescribed specimens from the Lovat´ River, housed in the collections of the Natural History Museum of Latvia, Riga, confirm the structure of the branchial plates and show that the ventral plate, not known before in Obruchevia, had a deep posterior notch similar to that found in Schizosteus, Pycnolepis, Pycnosteus, Ganosteus, and Tartuosteus. The almost complete specimens of the branchial plates from the Palaeontological Institute, Russian Academy of Sciences, Moscow, allow this plate to be more fully described. Although the obrucheviid from the Canadian Arctic is incomplete and shows an ornament of large elongated blisters and irregular ridges rather than pits and grooves, it also possessed a ventral plate with a well-developed posterior notch. In addition a well-developed dorsal sensory canal system is present as open surface grooves, an unusual feature in psammosteids. This species is clearly related to Obruchevia within the Obrucheviidae. Key words: Heterostraci, Psammosteida, Arctic Canada, Late Devonian, Obruchevia. Introduction Psammosteids (Suborder Psammosteida) are a group of heterostracans, extinct jawless vertebrates (Agnatha) in which the head and body are covered by a series of plates that form a bony carapace. Heterostracans are characterized by a pair of common branchial openings on either side of the head armor and are known to range from the Wenlock (Lower Silurian) to the Late Frasnian (Late Devonian). Psammosteids are known from
D.K. Elliott, E. Mark-Kurik, E.B. Daeschler. A revision of Obruchevia 23 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
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Page 5 and 6: Contents Ervīns Lukševičs and Ģ
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Page 9 and 10: Ervīns Lukševičs, Ģirts Stinkul
Page 15 and 16: Olga Afanassieva. Microrelief on th
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Page 59 and 60: H.-P. Schultze, T. Marss. Revisitin
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H.-P. Schultze, T. Marss. Revisitin
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O.A. Lebedev. A new tetrapod from R
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Ž. Žigaite. New telodont from Tuv
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