Aw. Owen et Jw. Tilsley, AN ABNORMAL PYGIDIUM OF THE TRILOBITE BRACHYMETOPUS-ORNATUS WOODWARD FROM THE LOWER CARBONIFEROUS OF DERBYSHIRE, Geological journal, 31(4), 1996, pp. 389-392
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.