AN ABNORMAL PYGIDIUM OF THE TRILOBITE BRACHYMETOPUS-ORNATUS WOODWARD FROM THE LOWER CARBONIFEROUS OF DERBYSHIRE

A pygidium of Brachymetopus or natus Woodward shows an abnormally deve loped left side affecting the whole of the pleural area and its rib de velopment. Such an extensive abnormality, which does not extend beyond the axial furrow and shows none of the signs of resulting from injury , is currently unique. It is interpreted as a teratological condition in which a genetic or developmental malfunction resulted in the partia l mimicking of thoracic pleurae rather than the construction of the no rmal pygidial pleural ribs. The effects of this are progressively grea ter from the posterior to the anterior parts of the affected pleural r egion. (C) 1996 by John Wiley & Sons, Ltd.