Giant genera and species occur in virtually every phylum of the Kingdom Animalia.  For
    the most part, their existence has been viewed as an illustration of Cope's Rule: the ten-
    dency toward phylogenetic size increase among groups of organisms.  Giant Mesozoic am-
    monites are frequently cited examples of this trend, and the giants are t ically treated as
    discrete taxonomic entities.  In contrast, pathologic gigantism is a rare condition that re-
    sults in abnormal growth of an individual beyond the normal size limits of its species, and
    does not reflect Cope's Rule.  That condition is little known among invertebrates, although
    pathologic gigantism has been reported in prosobranch „nd pulmonate gastropods, where
    it is associated with infestation by larval trematodes that caused parasitic castration.
          In Middle Carboniferous strata of the southern midcontinent, United States, cephalo-
    pod occurrences are dominated by ammonoids in unusual abundance.  These assemblages
    represent single horizons and localities where most individuals of a species are of strik-
    ingly similar size, apparently mature, and seem to reflect mass mortality, possibly related
    to reproduction (semelparity).  Associated with these assemblages are rare, conspecific in-
    dividuals thought to be pathologic giants that became abnormally large because they
    failed to achieve sexual maturity that would have caused their growth to cease.

   [Advancing Research on Living and Fossil Cephalopods, edited by Olóriz and
    Rodriguez-Tovar. Kluwer Academic / Plenum Publishers, New York, 1999.]