Devonian extinctions

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Just to open up a new area of discussion (all right, muddy the water), 
here are some observations on extinctions in Upper Devonian fish faunas.
     I work with the "continental" (in the broadest sense; some are 
probably marginal marine) fish assemblages of the Upper Old Red 
Sandstone. These are typically of rather moderate diversity, and 
dominated by a few cosmopolitan groups (placoderms, porolepiforms, 
osteolepiforms) which are each represented by a few very widespread 
genera (Bothriolepis, Asterolepis, Remigolepis Groenlandaspis, 
Holoptychius, Eusthenodon and a few others). During the latest 
Frasnian and the Famennian you also start getting stem-group 
tetrapods such as Ichthyostega appearing in these faunas.
     As far as I can tell there is no sign of the F-F extinction among 
the UORS fishes. One or two groups such as the psammosteid 
heterostracans do go extinct at the Frasnian-Fammenian boundary, but 
there is no dramatic change in faunal composition and the majority of 
genera cross the boundary quite happily. By contrast, the Famennian-
Tournaisian boundary marks a dramatic change with virtually no survival 
at the generic level and the complete extinction of both placoderms and 
porolepiforms. Lower Carboniferous continental fish faunas have a very 
different aspect from those fo the Upper Devonian. 
     This seems to raise some interesting questions, such as why the F-F 
event should leave vertebrates untouched, and whether the end-Famennian 
fish extinction correlates with a marine invertebrate event (I plead 
ignorance here!). Any thoughts on the subject would be interesting.

Cheers,    Per


Per Erik Ahlberg
Senior Research Fellow
Department of Palaeontology
The Natural History Museum
Cromwell Road, London SW7 5BD, UK
E-MAIL ADDRESS: pea@nhm.ac.uk