1 2 3 4 5 6 7 8 9 10 11 12
During August-September, Ahlberg spent two weeks in Latvia and Estonia, working with the collections at the Latvian Museum of Natural
History in Riga and the Geological Institute of the Estonian Academy of Sciences in Tallinn. This trip also included a one-day visit to Lode Quarry in Latvia (Gauja Formation, Upper Givetian). Although not yet fully analysed, the initial results are very promising. Several new taxa have been discovered, including a primitive coelacanth and a tristichopterid (= eusthenopterid) from Lode (Lode Member, Gauja Formation), a fragmentary proto-tetrapod from Latvia and Estonia (Sietini Member, Gauja Formation), protitanichthyids and holonematids from Estonia (Vadja Member, Narva Formation, Eifelian), and a possible petalichthyid fragment from Scotland (Alves Beds, Middle Frasnian). Tarlo's (1965) identifications of Scottish and Baltic psammosteids have all been confirmed by our study, with the exception of a branchial plate from Whitemire (RMS G 1904.2.10) which does not belong to Psammosteus megalopteryx as claimed. The supposed psammosteid Traquairosteus from Scat Craig shows a range of unique features and may not belong to this group; histology will be investigated by Ivanov.
Several Scottish arthrodires require reassessment. "Plourdosteus" magnus from the Nairn Beds seems to be a coccosteid, but genuine plourdosteids occur at Edenkillie (Whitemire Beds, Frasnian) and Scat Craig. The Edenkillie form appears similar to Plourdosteus mironovi (PlavinasRegional Stage, Lower Frasnian). An undescribed Plourdosteus also occurs in tbe Upper Plateau Beds of the Brecon Beacons, Wales. An unexpected addition to the British arthrodire fauna is the so-called "acanthodian" Cosmacanthus from Scat Craig, Whitemire and Poolymore (all Frasnian), which appears to be a groenlandaspid.
As regards antiarchs, Asterolepis maxima from the Nairn Beds of Scotland has proved to be almost identical to Asterolepis radiata (Amata Formation), and not to Asterolepis ornata (Gauja Formation) as previously thought. The Scottish material attributed to Bothriolepis alvesensis containsseveral different species, while Bothriolepis curonica (Eleja Formation, Lower Famennian, Latvia) closely resembles B. leptochira from Heads of Ayr, Ayrshire, Scotland. The supposed records of Grossilepis from Scotland cannot be substantiated.
The porolepiform sarcopterygians of the Main Devonian Field have revealed unexpected taxonomic complexity, with large and small forms coexisting at least from the Arukula Formation through to the Gauja Formation (Eifelian to Upper Givetian). The small forms all belong to Glyptolepis, though they represent several different species. However, the large forms belong to a sequence of distinct and apparently short-lived genera, which do not seem to be closely related. The last of these (from the Abava Member of the Burtnieki Formation, and the Gauja and Amata Formations) is the familiar Laccognathus; the older genera from the Burtnieki and Arukula formations have yet to be described. In Scotland, we have found that Brown (1978) was correct in identifying "Holoptychius decoratus" (Nairn Beds) as Laccognathus.
A set of tentative stratigraphic correlations can be put forward on the basis of our preliminary results: 1) The Eday Flags / John'o Groats Sandstones seem to be equivalent to the Abava Beds (Upper Givetian). 2) The Scottish sequence is missing an equivalent of the Gauja Formation. 3) The Nairn Beds are probably
equivalent to the Amata Formation (uppermost Givetian or lowermost Frasnian).
4) The Whitemire Beds may correlate with the Plavinas Regional Stage (Lower Frasnian). 5) The Alves Beds probably correlate with the Dubniki and Daugava Formations (Middle Frasnian). 6) Scat Craig seems to correlate with the Snezha or Pamusis Regional Stages (Upper Frasnian). 7) Rosebrae and Dura Den seem to be equivalent, and probably correlate with the Muri Formation - Svete
Formation interval (Middle Famennian).
During the visit to the Natural History Museum, London, a brief Greenland workshop was held. The participants were Ivanov, Luksevics and Mark-Kurik, together with John Marshall (University of Southampton) and Timothy Astin (University of Reading); Ahlberg was unable to attend. Marshall and Astin brought several fossil fishes from Greenland to be identified. Luksevics took the Bothriolepis material to Riga for further study, whereas several unidentified specimens were left at the NHM. A preliminary stratigraphic chart of the Middle and lowermost Upper Devonian of the Baltic region (Estonia and Latvia) was presented by Mark-Kurik, as a basis for the discussion of correlation problems. This chart was based on placoderms and some psammosteids. It should be mentioned in this context that an exoskeletal arthrodire plate described by Stensiö in 1938 under the name Heterostius groenlandicus actually belongs to Homostius.
It was suggested at the workshop that the Vilddal and Nathorst Fjord Groups may correspond approximately to the interval from the Narva to the lower Burtnieki Formations in the Baltic region, and to the Lower and Upper Caithness Flagstone Groups in Scotland. The Abava Member and its Scottish age equivalents (see above) with their distinctive fish fauna clearly differ from these units. The position of the base of the Givetian falls in different places according to miospore and fish data; the discussions on this topic will continue. Next year, Ahlberg will be visiting St.Petersburg and Moscow during two weeks to continue working on
sarcopterygian material. References:
Brown, A. K. 1978. The Scottish Devonian crossopterygian fish Holoptychius. Unpublished PhD thesis, University of Newcastle upon
Tyne.
Tarlo, L. B. 1961. Psammosteids from the Middle and Upper Devonian of Scotland. Quarterly Journal of the Geological Society 117,
193-213. Dr Per Erik Ahlberg
The Natural History Museum, London
IGCP 406 French Working Group Report 1997
Pierre-Yves Gagnier (Paris) continued work on Arctic acanthodians, and in particular on the samples of the 1995 IGCP 328 Canadian Arctic expedition (Prince of Wales Island). The specimens from the Drake Bay 4 locality (Lochkovian) include dentigerous bones, spines, and scales: publications see WWW.
11
